Heart and Great Vessels

Introduction

Patent ductus arteriosus is the most common congenital heart defect diagnosed in dogs. In cats, ventricular septal defects and pulmonic stenosis are more common cardiac defects. Physical findings include a continuous murmur auscultated at the left heart base and a hyperkinetic pulse. Thoracic radiographs show dilation of the descending aorta, the left atrium, and the pulmonary artery. Pulmonary overcirculation is also present. Surgical correction of the defect should be performed as soon as possible after diagnosis. Most animals with untreated patent ductus arteriosus will die within 1 year from congestive heart failure. Pulmonary hypertension may cause reversal of flow through the ductus arteriosus in a few cases. Dogs presenting with pulmonary edema should be treated with furosemide prior to surgery.

Surgical Technique

PDA ligation is accomplished through a left 4th intercostal thoracotomy in dogs, or a 4th or 5th left intercostal thoracotomy in cats. The left cranial lung lobe is reflected caudally and packed with a moistened laparotomy sponge or 4x4 gauze in smaller animals. The vagus nerve courses over the ductus arteriosus and can be used as a landmark to locate the ductus arteriosus. The vagus nerve is elevated from the mediastinum by sharp dissection and retracted gently with a suture. The recurrent laryngeal nerve should be identified as it passes caudal to the ductus. Dissection of the vagus nerve should be performed outside of the pericardial sac with a right angle forceps. Dissection of the patent ductus arteriosus starts on its caudal aspect (Figure 42-1). The forceps should be kept parallel to the transverse plane during this part of the dissection. Dissection of the cranial portion of the ductus is performed at an angle of approximately 45° to the transverse plane in a triangle delineated by the aortic arch, pulmonary artery, and patent ductus arteriosus (See Figure 42-1). Careful sharp dissection with scissors is sometimes necessary to reflect the attachment of the pericardium ventrally from the aorta to expose this triangle. The dissection of the medial aspect of the patent ductus arteriosus is performed by passing the right angle forceps from caudal to cranial (Figure 42-2). Dissection should be as gentle as possible with small movements of the right angle forceps to avoid tearing the medial wall of the ductus. When the tip of the right angle forceps is clear of tissue, a #1 or 0 silk suture is grasped by the forceps and passed around the ductus. A second suture is passed around the ductus in the same manner. Alternatively, some surgeons pass a doubled strand of suture and cut the suture in the middle thus reducing the number of passes on the medial aspect of the ductus. The ligature closest to the aorta is slowly tightened and tied first (Figure 42-3). The second ligature is then tightened and tied. The palpable thrill in the pulmonary artery present prior to ligation should be completely eliminated after ligation. If the medial wall of the ductus is ruptured during dissection light pressure should be applied to control the bleeding. If the tear is not too large the bleeding will stop. However, continuing the dissection may worsen the tear and lead to uncontrollable hemorrhage. At this point, the options depend on the experience of the surgeon and on the availability of vascular instruments. One option is to abort the surgery and refer the case to a surgeon experienced in cardiovascular surgery for latter closure. Another option is to divide the ductus between two vascular forceps and close both ends with 4-0 polypropylene suture using a continuous mattress pattern. Intravenous injection of nitroprusside has been recommended to decrease arterial pressure after tearing a ductus arteriosus. Clamping of the aorta and the pulmonary artery to control bleeding has also been recommended. At the conclusion of intrathoracic surgery, the cranial lung lobe is unpacked, replaced in its normal position, and reinflated. A thoracostomy tube is placed and the thoracotomy is closed in a routine fashion. The thoracostomy tube is usually removed in the immediate postoperative period after negative intrathoracic pressure is attained.
 

Suggested Readings

Orton, E.C.: Congenital heart defect. Small animal thoracic surgery. Edited by E.C. Orton. Baltimore. Williams & Wilkins, 1995, pp 203-227.

Orton, E.C.: Cardiac surgery. Textbook of small animal surgery. Edited by D. Slatter. Philadelphia. W.B.Saunders, 2002, pp 955-986.

Eyster, G.E., Probst, M.R.: Basic cardiovascular surgery and procedures. In Canine and feline cardiology. Edited by P.R. Fox. New York. Churchill Livingston, 1988, pp 605-624.

Birchard, S.J., Bonagura, J.D., Fingland, R.B.: Results of ligation of patent ductus arteriosus in: 201 cases (1969-1988). J. Am. Vet. Med. Assoc., 196:2011, 1990.

Hunter, S. L., Culp, L. B., Muir, W. W., 3rd, et al. Sodium nitroprusside-induced deliberate hypotension to facilitate patent ductus arteriosus ligation in dogs Vet Surg, 32:336, 2003.

Hunt G.B., Simpson D.J., Beck J.A., et al. Intraoperative hemorrhage during patent ductus arteriosus ligation in dogs. Vet Surg, 30:58, 2001.
 

Introduction

Pulmonic stenosis is reported to be the third most common congenital heart disease in the dog with patent ductus arteriosus and aortic stenosis being first and second, respectively.¹ The English bulldog is the most common breed represented, however, other dogs at risk include the beagle, Samoyed, Chihuahua, schnauzer, Boykin spaniel, mastiff, and various terrier breeds.² The disease occurs equally between male and female except in the bulldog where the incidence in males predominates. The disease is rare in cats. Pulmonic stenosis has a genetic basis in dogs, although this is uncertain in the cat.³

The lesion may occur as a supravalvular, valvular, or subvalvular stenosis. With any of the three, an infundibular stenosis may occur in which the hypertrophied musculature obstructs the right ventricular outflow tract. The valvular site is by far the most common and is manifested by thickening, fibrosis, and hypoplasia of the valve leading to outflow obstruction.

Diagnosis

Many cases of pulmonic stenosis are asymptomatic early in their life; some remain asymptomatic indefinitely. More severe cases display exertional fatigue, dyspnea, and syncope. Signs of right heart failure including ascites, hepatomegaly, and arrhythmia may be present in advanced cases.⁴ Physical examination reveals a systolic ejection murmur heard over the pulmonic valve that often radiates along the sternum to both sides of the thorax. A holosystolic murmur of tricuspid insufficiency may sometimes be auscultated over the right hemithorax.

The ECG usually indicates right ventricular hypertrophy, including right axial deviation, S waves in leads I, II, III, and aVF. Thoracic radiographs reveal varying degrees of cardiomegaly. The right side of the heart predominates in the enlargement. A poststenotic dilatation of the main pulmonary artery is seen on the dorsoventral view. The pulmonary vessels appear normal or somewhat underperfused. Cardiac catheterization helps to locate the specific site of the stenosis and to measure pressure gradients for prognosis. Measuring gradients under anesthesia gives pressure readings that are usually much lower than actually exist. Angiographic features of pulmonic stenosis include thickened and dysplastic valve leaflets, narrowing of the outflow tract and valve orifice, poststenotic pulmonary artery dilatation, and right ventricular hypertrophy. In English bulldogs an anomalous left coronary artery may be seen crossing the ventricle at the level of the stenosis.

In many cases echocardiography and color flow Doppler echocardiography examination provide sufficient data making cardiac catheterization unnecessary. Typical findings are hypertrophy of the right ventricle, muscular narrowing of the right ventricular outflow tract, deformity and narrowing of the pulmonic valve, and post stenotic dilatation of the pulmonary artery. Pressure gradients measured by color flow Doppler echocardiography are more likely than catheterization to give an accurate assessment of the severity of disease because the examination does not require general anesthesia. Echocardiography and/or cardiac catheterization can usually determine the severity of the disease and locate the stenosis at the supravalvular, valvular, subvalvular, or infundibular site. This information is crucial in determining surgical candidates, selecting the correct surgical procedure, and giving prognosis. In some patients it is very difficult to delineate between a pure valvular stenosis and one that is both valvular and subvalvular. This makes selecting the proper surgical technique more difficult.

Surgical Guidelines

Nonanesthetized pressure gradients that are less than 50 mmHg are generally considered mild and do not require surgical intervention. Severe gradients exceeding 80 mmHg place the patient at risk of heart failure and death. These should have surgical intervention.^3,4 Dogs with moderate disease (gradients of 50 to 80 mmHg) may or may not require surgical correction depending on the progression of the disease. One author has recommended surgery when: 1) the right ventricular pressure exceeds 120 mmHg or a gradient exceeds 100 mmHg in a mature dog, or 2) the right ventricular pressure is 90 to 120 or a gradient of 70 to 100 in an immature dog.⁵ Others recommend surgery any time the gradient exceeds 50 mmHg and right ventricular hypertrophy is significant.⁶

Any animal not undergoing surgery should be re-evaluated at three month intervals to determine if the disease is progressing. Symptomatic animals should have surgical intervention regardless of their pressure gradients. A problem of waiting to see if a patient’s disease is progressive based on pressure measurements or disease signs is that they may become poorer surgical candidates with time. These animals may develop secondary infundibular muscular stenosis, worsening right ventricular hypertrophy, right ventricular fibrosis, and right heart failure. If possible surgery should be delayed until the animal is mature so the procedure is done on a fully developed heart that will not outgrow the correction.

Even though various authors have stated guidelines for surgical intervention, most of these are based upon personal observations. There have been no clinical trials in dogs with long-term follow-up to validate criteria for surgical intervention or to determine which corrective procedure gives the best results; however in a series of 72 cases of congenital pulmonic stenosis left untreated, only 65% of patients were alive after two years.⁷ Unfortunately in this series, the severity of the stenosis was not described.

Anesthesia for Pulmonic Stenosis

Nearly all anesthetic agents depress cardiopulmonary function directly or alter reflex regulatory mechanisms.⁸ Patients with cardiac disease may have little to no reserve for compensation; therefore, anesthetic agents must be administered carefully and in reduced dosages. Preanesthetic agents should be administered to relieve anxiety and to reduce the amount of depressant general anesthetic required. A combination of a benzodiazepine and an opioid are used for sedation. Opioid-induced respiratory depression may occur, therefore oxygen by mask should be provided during the induction process. Anticholinergics, especially atropine, are not used unless bradycardia occurs because of their propensity to induce tachycardia.

Administering low concentration isoflurane in oxygen until tracheal intubation can be accomplished completes anesthetic induction. Anesthetic maintenance is by continued low concentration of isoflurane supplemented with intermittent doses of an opioid. Intermittent positive-pressure ventilation is provided either manually or mechanically. Profound muscle relaxation can be produced by intravenous administration of atracurium, a nondepolarizing muscle relaxant.

Pulmonic stenosis patients must be closely monitored for cardiopulmonary function. Monitoring parameters should include heart rate, ECG, pulse quality, direct or indirect blood pressure, pulse oximetry, and central venous pressure. Assessment of blood volume and hemodilution is by serial determinations of packed cell volume and total plasma proteins. Measuring urine production assesses renal function. Blood pressure is maintained by a maintenance flow of intravenous crystalloids supplemented with colloids. Cross-matched whole blood must be available should major hemorrhage occur.

Surgical Procedures for Pulmonic Stenosis

Various surgical procedures have been described for correction of pulmonic stenosis.⁹ These include balloon dilatation, open valvulotomy/valvulectomy, closed valvulotomy/dilatation, open and closed patch grafting, by-pass conduit, and open-heart repair with cardiopulmonary bypass. The specific procedure depends upon the location of the stenosis, size of the patient, severity of the disease, expertise of the surgeon, and equipment available. Many of the procedures have been adapted from techniques used to correct pulmonic stenosis in children although direct application to animals may be erroneous. For example, valvular stenosis in children is commonly a fusion of the valve leaflets, whereas in dogs it is usually a fibrotic, thickened, dysplastic valve. Direct comparison of the techniques or the expected results between children and dogs should not be made.

Open Valvulotomy/Valvulectomy

This procedure is a modification of the technique developed by Swan¹⁰ using transient venous inflow occlusion and a pulmonary arteriotomy. The technique is used in patients with a valvular stenosis and minimal to no subvalvular component. The thorax is opened by a thoracotomy at the left fourth intercostal space. Dissecting between the thymus and the cranial aspect of the pericardial sac isolates the cranial vena cava. The cava is located on the right side of the thorax and ventral to the brachycephalic artery. A Rumel tourniquet of umbilical tape is placed on the vessel. Incising the caudal mediastinum immediately behind the pericardial sac and ventral to the phrenic nerve approaches the caudal vena cava. The vessel can be visualized deep in the mediastinal space to the right side of the thorax. Right angle forceps are used to place a Rumel tourniquet similar to the cranial cava. Dissection of the caudal cava may be impossible from the fourth intercostal space in dogs with severe cardiac enlargement. In these instances the caudal edge of the incised skin is retracted and a small thoracotomy incision is made at the sixth intercostal space. The cava is easily isolated from this position.

A third Rumel tourniquet is placed on the descending aorta just above the heart base. Tightening this tourniquet for 1 to 2 minutes after inflow occlusion maximizes blood flow to the heart and brain. It is released slowly as cardiac function returns to normal.

The pericardial sac is incised parallel and ventral to the phrenic nerve. Four to six stay sutures are placed in the pericardial sac and secured to the surgical drapes to “cradle” the heart (Figure 42-4). Lidocaine applied topically to the heart and an intravenous lidocaine drip help to minimize surgically induced arrhythmias. Stay sutures of 5-0 polypropylene are placed in the dilated pulmonary artery immediately distal to the pulmonary valve. Venous inflow occlusion is accomplished by tightening the caval tourniquets. After waiting a few seconds for the heart to partially empty, a 1 to 2 cm incision is made between the two stay sutures (Figure 42-5). A small retractor at the ventral end of the incision and the two stay sutures retract the arteriotomy site (Figure 42-6). Suction is used to empty the right ventricle and visualize the pulmonic valve. The dysplastic leaflets are grasped with forceps, and scissors or scalpel used to excise the valve (Figure 42-7). After all three leaflets have been excised or incised, a forceps is used to dilate the valve annulus. A “pop” can be felt as the annulus stretches. One finger is inserted into the outflow tract to assure that the stenosis is relieved. The cranial Rumel tourniquet is released and the heart and pulmonary artery permitted to fill to remove all intravascular air. The stay sutures are used to elevate the edges of the artery and a Satinsky clamp placed on the arteriotomy site (Figure 42-8). The second Rumel tourniquet is released. Cardiovascular resuscitation is aided by temporarily occluding the descending thoracic aorta with the third Rumel to increase coronary and cerebral blood flow. Cardiac massage and an intravenous infusion of dopamine may be required to reestablish normal cardiac function. Total inflow occlusion of a diseased heart should not exceed two minutes. If this is not sufficient time to complete the procedure, then inflow is terminated and 10 to 15 minutes of normal cardiac function is established. A brief second inflow occlusion can be utilized to complete the procedure. Normal hearts can tolerate four or more minutes of inflow occlusion; however diseased hearts often fibrillate and are difficult to defibrillate.
 

The arteriotomy is closed with a double row of continuous 5-0 polypropylene sutures, and the stay sutures are removed (Figure 42-9). The pericardial sac is loosely closed with 3-0 absorbable suture. Closing the sac tightly could result in tamponade if the arteriotomy site leaks. The tourniquets are removed, and the thorax lavaged with warm saline to remove all blood. The intercostal nerves are blocked with local anesthetic for analgesia, a thoracic tube placed, and the thorax closed in a routine manner.
 

Closed Valvulotomy/Dilatation

This procedure is used in patients who likely cannot tolerate even brief in-flow occlusion. The surgical approach is as described above but without inflow occlusion. A purse-string suture is placed in the right ventricular outflow tract just below the pulmonic valve or in the dilated pulmonary artery above the valve. The suture ends are placed through a piece of tubing similar to the Rumel tourniquet. A stab incision is made through the purse string and into the lumen of either the right ventricle or the pulmonary artery. A blunt tipped bistoury or valvulotome is passed through the valve and several blind cuts are made through the stenotic valve by cutting against backpressure applied by the surgeon’s finger (Figure 42-10). A forceps is then placed through the purse string and the valve annulus dilated to completely break down the stenotic ring. In dogs with severe muscular hypertrophy and a narrow outflow tract, this procedure is more easily accomplished through a purse string in the dilated pulmonary artery. Simple dilatation without first cutting the stenotic valve may only provide temporary relief since the torn and stretched tissue may heal with scar tissue resulting in a new stenosis.
 

Patch Grafting

The use of patch grafting for repair of pulmonic stenosis in the dog was first reported in 1976.¹¹ The graft extends over the pulmonary artery to the right ventricle outflow tract and is effective in correcting valvular, supravalvular, and subvalvular stenosis while alleviating infundibular lesions. Patch grafting may be performed by either a closed or open technique. The closed patch graft technique^11,12 relies upon the placement of a cutting wire across the stenotic lesion under the applied patch. Unfortunately the technique does not allow excision of the dysplastic valve and relies upon the surgeon’s ability to place a cutting wire blindly across the defect.

An open technique for patch grafting has been described^13,14 The authors prefer a modification of this technique, which is performed through a left lateral thoracotomy at the fourth intercostal space. The lungs are retracted to expose the pericardial sac. Rumel tourniquets are placed around the cranial and caudal vena cava, and the thoracic aorta as described for open valvulotomy/valvulectomy. The pericardium is incised parallel and ventral to the phrenic nerve, with an extension ventral and perpendicular. Pericardial basket sutures are placed. An elliptical shaped polytetrafluoroethylene (PTFE; Gortex, W.L. Gore and Assoc.) patch is cut so that the graft will extend both proximal and distal to the stenotic lesion. The patch is sutured to the outflow tract and pulmonary artery using 4-0 polypropylene suture and a double-armed taper point needle. Suturing is started at the ventral tip of the patch, which is placed on the ventricle with an interrupted suture. The opposite end of the patch is sutured to the pulmonary artery above the stenotic site. The margins of the patch are sutured in a continuous fashion to the ventricle and onto the pulmonary artery. It is critical that the patch is sutured in a “tented” fashion over the stenotic area. This extra graft allows for expansion of the stenotic area. Once the patch has been applied, it is incised longitudinally at an equal distance between the cranial and caudal margins (Figure 42-11A and B). The caval tourniquets are tightened to accomplish venous inflow occlusion. A stab incision with a #11 scalpel blade is made into the pulmonary artery and extended to the dorsal and ventral margins of the patch with Metzenbaum scissors. The valve is inspected and the leaflets excised (Figure 42-12). A forceps may be used to further dilate the valve and annulus. A finger is inserted into the annulus to insure that the stenosis has been relieved. Air is evacuated from the heart by releasing the cranial Rumel tourniquet. The incised patch graft is clamped using Satinsky tangential vascular occlusion clamps. The caudal Rumel tourniquet is then released. The patch graft incision is sutured with 4-0 polypropylene in a continuous pattern (Figure 42-13A and B). Total inflow occlusion time should not exceed two minutes. The Rumel on the aorta is used temporarily to improve heart and brain perfusion. The pericardium is closed loosely with interrupted sutures. A thoracostomy tube is placed and the thoracotomy incision is closed in routine fashion.

Open patch grafting is effective in young animals with severe valvular, but preferably supravalvular, subvalvular, or infundibular stenosis. Care must be taken in identifying an aberrant coronary artery, which crosses the right ventricular outflow tract occasionally in boxers and bulldogs, negating the use of this procedure.

A case series of nine dogs undergoing closed patch grafting has described significant morbidity and mortality associated with the closed patch procedure.¹⁵ There was one intra-operative death, and blood transfusions were required to treat life-threatening hemorrhage in six of the nine dogs. The clinical signs improved in five of the six dogs that survived in this study. Progression of right ventricular hypertrophy was delayed, but not prevented by the procedure.
 

Conduits

Vascular grafts or conduits have been used to repair supravalvular pulmonic stenosis in the dog.¹⁶ The use of conduits from the pulmonary artery to the right ventricle may be used to bypass the stenotic pulmonary valve in animals with an aberrant coronary artery. The technique is performed through a left lateral thoracotomy at the fifth intercostal space. The pericardium is opened and sutured as previously described. The stenotic region is observed and an appropriately sized Dacron or PTFE conduit chosen. A Satinsky partially occluding vascular clamp is applied to the pulmonary artery above the site of the lesion. An arteriotomy is made with a #11 scalpel blade and extended with Potts scissors. The conduit is cut at an oblique angle and sutured end-to-side to the pulmonary artery with continuous 5-0 to 6-0 polypropylene suture on a double-armed taper point needle. The conduit is anastomosed to the ventricular wall in end-to-side fashion following coring a hole in the ventricular wall. Closures of the pericardium and thoracotomy incisions are routine.

Conduits, with the exception of those used in supravalvular stenosis (pulmonary artery to pulmonary artery) have been met with limited success in veterinary medicine. The procedure may be best applied under cardiopulmonary bypass.

Cardiopulmonary Bypass

Pulmonic stenosis can be repaired effectively utilizing cardiopulmonary bypass. This technique permits direct visualization and repair of the lesion without the time constraints of inflow occlusion. Valvuloplasties, patches, and conduits can all be performed with cardiopulmonary bypass permitting the surgeon to do precise surgical repairs.¹⁷

References

1. Buchanan JW: Causes and prevalence of cardiovascular disease. In: Kirk RW, Bonagura JD, eds.: Current Veterinary Therapy XI. Philadelphia: WB Saunders, 1992, p 647.
2. Buchanan JW: Changing breed predispositions in canine heart disease. In: Proceedings of the 10th ACVIM Forum, 1992, p 213.
3. Bonagura JD, Darke PG: Congenital heart disease. In: Ettinger SJ, Feldman EE, eds.: Textbook of Veterinary Internal Medicine. Philadelphia: WB Saunders, 1995, p 892.
4. Thomas WP: Therapy in congenital pulmonic stenosis. In: Kirk RW, Bonagura JD, eds.: Current Veterinary Therapy XII. Philadelphia: WB Saunders, 1995, p 817.
5. Eyster GE: Basic cardiac surgical procedure. In: Slater DH, ed.: Textbook of Small Animal Surgery. Philadelphia: WB Saunders, 1993, p 462.
6. Orton EC: Pulmonic stenosis and subvalvular aortic stenosis: surgical options. Sem Vet Med Surg 9:221, 1994.
7. Ewey DM, Pion PD, Hird, DW: Survival in treated and untreated dogs with pulmonic stenosis. J Vet Intern Med 2:114 (abstract),1992.
8. Hellyer PW: Anesthesia in patients with cardiovascular disease. In: Kirk RW, Bonagura JD, eds.: Current Veterinary Therapy XI. Philadelphia: WB Saunders, 1992, p 655.
9. Breznock EM: Surgical relief of pulmonic stenosis. In: Bojrab MJ, ed.: Current Techniques in Small Animal Surgery. Philadelphia: Lea & Febiger, 1990, p 513.
10. Swan H: Surgery by direct vision in the open heart during hypothermia. J Am Med Assoc 153:1081, 1953.
11. Breznock EM, Wood GL: A patch-graft technique for correction of pulmonic stenosis in dogs. J Am Vet Med Assoc 169:1090, 1976.
12. Shores A, Weirick WE: A modified pericardial patch graft technique for correction of pulmonic stenosis in the dog. J Am Anim Hosp Assoc 21:809, 1985.
13. Orton EC, Bruecker KA, McCracken TO: An open patch graft technique for correction of pulmonic stenosis in the dog. Vet Surg 19:148, 1990.
14. Hunt GB, Pearson MRB, Bellenger CR, Malik R: Use of a modified open patch-graft technique and valvulectomy for correction of severe pulmonic stenosis in dogs: eight consecutive cases. Aust Vet J 70:244, 1993.
15. Staudte KL, Gibson NR, Read RA, Edwards GA: Evaluation of closed pericardial patch grafting for management of severe pulmonic stenosis. Aust Vet J 82:33, 2004.
16. Ford RB, Spaulding GL, Eyster GE: Use of an extra cardiac conduit in the repair of supravalvular pulmonic stenosis in a dog. J Am Vet Med Assoc 172:922, 1978.
17. Orton EC: Cardiopulmonary bypass for small animals. Sem Vet Med Surg 9:210, 1994.
     

Introduction

For much of its early history, the technique of cardiac catheterization was utilized exclusively for diagnosis. However, beginning in the 1960’s, resourceful pediatric cardiologists developed methods of transcatheter therapeutic intervention. Since that time, the indications for interventional catheterization in both pediatric and adult cardiovascular medicine have expanded remarkably. Obstructive lesions are addressed by balloon dilation, pathologic shunts are occluded by transcatheter techniques, stents have been used to maintain patency of vessels and conduits and more recently, percutaneous methods of valve replacement and repair have been investigated. In veterinary medicine, transcatheter therapy has been confined primarily to balloon dilation of outflow tract obstruction and occlusion of patent ductus arteriosus (PDA). This chapter reviews the current applications of transcatheter intervention in veterinary patients with congenital cardiovascular disease.

Transcatheter Occlusion of Patent Ductus Arteriosus (PDA)

Etiopathogenesis of PDA

The ductus arteriosus connects the ventral aspect of the proximal descending aorta with the dorsal aspect of the pulmonary artery bifurcation. The histology of the normal duct is distinct from that of the aorta and this is relevant to normal closure as well as to the angiographic appearance of the persistently patent duct. The tunica media of the aorta consists primarily of elastic fibers. In contrast, the media of the duct is comprised of smooth muscle fibers in both circumferential and spiral orientations.¹ During fetal life, pulmonary vascular resistance exceeds systemic vascular resistance and the ductus diverts the majority of the right ventricular output to the systemic circulation. Maintenance of fetal ductal patency primarily depends on production of prostaglandin-E.²

In normal, term neonates, closure of the ductus begins shortly after birth and initially results from contraction of ductal smooth muscle. The mechanism of ductal closure is complex and likely has a multifactorial basis. However, increases in oxygen tension associated with parturition limit the production of dilative prostaglandins, initiate a vasoconstrictive prostaglandin cascade and decrease the sensitivity of ductal smooth muscle to dilative stimuli.² The time required for functional closure of the duct is species-dependent but generally is within 3 to 5 days of birth. Anatomic ductal closure occurs later and is associated with the development of intimal edema, cellular degeneration and necrosis or apoptosis; the result is the arterial ligament.^1,3

The cause of post-natal ductal patency in most if not all affected dogs is a deficiency of ductus-specific smooth muscle.⁴ In dogs that ultimately develop a patent duct, ductal smooth muscle is replaced by elastic tissue which generally extends from the aortic side of the duct toward the pulmonary artery. In the most severely affected individuals, the media of the entire duct is replaced by elastic tissue. The result is a tubular, large diameter duct that is associated with neonatal pulmonary hypertension and a bidirectional or right-to-left shunt.⁵ In less severely affected individuals, the elastic tissue extends a variable distance from the aortic-ductal junction. Because of persistence of functional smooth muscle in the more distal aspect of the ductus, most left-to-right shunting PDA have a conical shape and are narrowest where the ductus joins the pulmonary artery. The duct is widest at the attachment of the aorta and the flask-shaped dilation is known as the ductal ampulla. The cranial aspect of the ampulla is partially roofed by a shelf of tissue, known as the crista (or plica) reunions, which extends caudally from the ventral wall of the proximal descending aorta.¹ PDA is heritable in miniature poodles⁵ and, based on breed predispositions, PDA likely has a genetic basis in other purebred dogs.

Pathophysiology

The ductus provides a communication between the pulmonary and systemic circulations. The size of the shunt is primarily determined by ductal diameter and the relationship between pulmonary and systemic vascular resistance. When pulmonary vascular resistance is less than that of the systemic circulation, blood shunts from aorta to pulmonary artery. The resultant increase in pulmonary venous return imposes a volume load on the left atrium and ventricle. Most canine PDAs provide resistance to ductal flow so that aortic pressure exceeds pulmonary artery pressure. Even then, the shunt volume can be considerable resulting in left ventricular dilation and hypertrophy, left atrial enlargement, functional mitral valve regurgitation and potentially the development of systolic myocardial dysfunction.

A large, non-restrictive duct necessarily results in systemic pulmonary artery pressures. In this setting, the development of obstructive vascular disease potentially results in suprasystemic pulmonary vascular resistance and shunt reversal. However, shunt reversal is uncommon in dogs and generally occurs in neonates. Patients with right-to-left shunting PDA are not candidates for operative therapy and are treated medically. Shunt direction associated with canine PDA is most commonly left-to-right. The remainder of this discussion relates to the diagnosis and management of left-to-right shunting PDA in the dog.

Clinical Findings

In many, if not most cases, the PDA does not cause clinical signs before the age of 4 to 6 months and the lesion is detected incidentally during routine physical examination. There is a distinct gender predisposition; about two thirds of the affected population is female. Distinct breed predispositions are also evident; Maltese, Pomeranian, miniature poodles, Bichon Frise and Shetland sheepdogs are more likely to have PDA than are mix breed dogs.⁶ PDA has been reported in cats but it is uncommon.

A left-to-right PDA results in a continuous murmur; the murmur begins during systole, peaks in intensity at the time of the second heart sound, and persists through at least a portion of diastole. When the heart rate is very slow, or there is pulmonary hypertension related to a large shunt and high pulmonary vein pressures, the murmur may be inaudible during late diastole. More often, the murmur persists through the entire cardiac cycle and has a typical aorticopulmonary or “machinery” quality. The intensity of the murmur generally correlates with the size of the shunt. Very soft and focal murmurs are usually associated with a small shunt while moderate or large shunts typically result in a loud murmur that radiates widely. In patients with large shunts, a distinct systolic murmur due to functional mitral valve regurgitation sometimes can be heard. The third heart sound is audible in some patients; generally this finding reflects a large shunt and high left atrial pressures. When the shunt is moderate or large, the decrease in diastolic arterial pressure widens the pulse pressure and results in a hyperkinetic, or “bounding”, arterial pulse.

Diagnostic Evaluation

In the absence of cardiac arrhythmias, the electrocardiogram (EKG) contributes little to diagnosis although most patients do have electrocardiographic evidence of left ventricular hypertrophy.⁷ Thoracic radiographs of patients with PDA typically have distinctive, if not diagnostic, features. Usually, there is cardiomegaly with left-sided emphasis. Evaluation of the pulmonary vessels may provide evidence of pulmonary hyperperfusion. Prominence of the proximal descending aorta is perhaps the most consistent radiographic feature. In some patients, the main pulmonary artery and left atrial appendage are also prominent resulting in the appearance of three closely associated bulges in the dorosoventral or ventrodorsal projection.

Echocardiography demonstrates variable degrees of left ventricular and left atrial enlargement. Echocardiographic measures of cardiac performance such as fractional shortening, usually are normal or mildly depressed. However, ventricular loading conditions are altered by the shunt and often, by concurrent mitral valve regurgitation which complicates interpretation of functional indices such as fractional shortening. Indeed, evaluation of the end-systolic ventricular dimension provides echocardiographic evidence of myocardial dysfunction in most patients with long-standing, uncorrected PDA. Doppler studies confirm the presence of continuous, disturbed flow within the main pulmonary artery. Although it is sometimes technically difficult to do so, the pulmonary-ductal junction, if not the entire duct, can be echocardiographically identified in the vast majority of patients (Figure 42-14). Transesophageal echocardiography may have a particular utility for more completely defining the dimensions and morphology of the PDA (Figure 42-15).
 

Other than PDA, there are few disorders that result in a continuous murmur. When it is certain that there is a single continuous murmur and not distinct systolic and diastolic murmurs as can result from ventricular septal defect complicated by aortic insufficiency, the diagnosis is generally assured and the need for further evaluation can be debated. However, echocardiography is recommended in order to confirm the diagnosis before intervention, evaluate myocardial function and identify concurrent malformations which occasionally can complicate the presentation. The need for pre-procedural echocardiography is particularly acute when transcatheter ductal occlusion is contemplated because echocardiographic data can be used to provide a preliminary assessment of the ductal size and morphology.

Management of PDA - General Statements

It is accepted that mortality for canine patients with uncorrected PDA is high and that the risk associated with operative correction is relatively low. Therefore, occlusion of the duct, either by transcatheter methods or surgical ligation is advisable for nearly all patients with PDA. Mortality in uncorrected PDA is primarily due to congestive heart failure; other complications such as ductal endocarditis and progressive vascular disease are uncommon. Because of this, watchful waiting that includes echocardiographic surveillance is probably appropriate for the occasional patient that has a small ductus and minimal or no ventricular enlargement. With respect to treatment decisions, it is relevant that PDA is most common in small breed dogs. Dogs of this signalment are predisposed to the development of geriatric mitral valve degeneration (MR) and conceivably, the development of MR might result in clinical decompensation in older individuals with a previously tolerated ductus. Although there is a small risk associated with correction of PDA, the ratio of risk and benefit is in favor of repair for nearly all patients. The only exception to this general principle is the patient with a complex cyanotic malformation such as tetralogy of Fallot in which the ductus contributes to pulmonary perfusion.

Transcatheter Occlusion

Transcatheter PDA occlusion using different devices and subtly different techniques has been reported.^8-19 Initially, thrombotic Gianturco coils were used most commonly in veterinary medicine, but use of the recently developed, purpose-designed Amplatz^® canine ductal occluder (ACDO) has, to a great extent, superseded that of Gianturco coils in veterinary practice.

Basic Technique – Transcatheter Occlusion

Numerous variations on the basic technique of transcatheter ductal occlusion have been reported. The retrograde trans-arterial approach is described here. After induction of general anesthesia, access to the femoral artery is percutaneously obtained using the modified Seldinger technique or is obtained by arteriorotomy after a small inguinal incision. In the former technique, a needle or short catheter is used to puncture the femoral artery while the patient is dorsally recumbent. When blood pulses from the entry needle, a wire-guide is introduced through the needle into the vessel lumen. The needle is then removed while hemostasis is maintained by digital pressure. A catheter or more often, a vessel sheath-introducer system with hemostasis port, is advanced along the wire into the femoral arterial system. The percutaneous technique is in almost universal use in pediatric and adult catheterization laboratories and has advantages with respect to vessel preservation. However, dogs tolerate post-procedural ligation of the femoral artery without apparent long-term sequlae and furthermore, complications of the percutaneous approach can be catastrophic.²⁰ If the femoral artery is entered proximal to the inguinal ligament, post-procedural attempts at hemostasis may be ineffective resulting in potentially fatal intra-abdominal hemorrhage. Additionally, severe subcutaneous hemorrhage can result despite appropriate vessel entry site and post-procedural hemostasis.¹⁷ For these reasons, surgical isolation and arteriotomy may be the superior method of arterial access.

Regardless of technique used for vessel access, the use of a hemostasis sheath is advisable to facilitate catheter exchange and the movement of catheters through the femoral artery. The patient is positioned in lateral recumbency and the fluoroscopic image intensifier is centered over the cardiac shadow. Using fluoroscopic guidance, an angiographic catheter such as a pigtail is advanced to the ascending aorta. Central aortic pressures are evaluated and an angiogram is recorded after injection of contrast material in the proximal descending aorta. It is important that the angiogram clearly delineates the entire ductus including the pulmonary-ductal junction (Figure 42-16). The angiographic appearance can be classified according to ductal morphology.^14,21
 

Amplatz^® Canine Ductal Occluder - Technique

In collaboration with a manufacturer of cardiovascular devices, two veterinary cardiologists, Ngyuenba and Tobias, developed a metallic plug that was specifically designed to occlude the canine ductus.²² This device, the ACDO, became commercially available in 2007. The ACDO is constructed from 2 to 3 layers of a fine nitinol mesh. The device is both collapsible and self-expanding; in its unstressed state, a waist separates a flat distal disk from a larger proximal, cupped disk. The ACDO is available in a range of sizes that are defined by diameter of the waist. The device is provided by the manufacturer within a tubular cartridge and attached to a delivery cable. The ACDO is deployed using a retrograde approach after angiographic delineation of the ductus. Femoral arterial access is routine but because some ACDO require relatively large delivery catheters, exteriorization of the artery after inguinal incision is probably the most appropriate technique. The size of the ACDO is selected based on the smallest angiographic diameter of the duct and therefore, careful, quantitative assessment of ductal size is crucial. A device with a waist that is approximately twice the diameter of the minimum ductal dimension is said to be optimal.^19,22 After angiographic evaluation, the duct is crossed with a curved catheter such as an MPA which is then exchanged over a wire-guide for a long sheath or guiding catheter. Alternatively, contrast material for angiography can be injected through a long sheath such as a Mullins, Ansel or CHB type, and if it is possible to cross the duct with a wire-guide advanced through the sheath, this technique obviates the need for a catheter exchange.²³ Predictably, larger devices must be deployed through larger catheters and this must be taken into account not only as the delivery catheter is advanced, but also initially, when a short, vascular access sheath is placed in the femoral artery. When the distal end of an appropriately sized guiding catheter or long vascular sheath is in place within the main pulmonary artery, the device is introduced into the hub of the catheter or sheath using the loading cartridge and then advanced using the delivery wire. The proximal disk is deployed within the pulmonary artery at which time the wire and catheter are withdrawn together until the disk is firmly apposed to the ductal orifice. Then, the catheter is retracted so that the remainder of the device is deployed within the ductus. Suitability of positioning is then evaluated through manipulation of the wire, injection of contrast material through the side-arm of the catheter and potentially, through transesophageal echocardiography (Figure 42-17). If positioning is inappropriate, the device can be withdrawn into the deliver catheter. When the device is properly positioned, and it has resumed it’s unstressed configuration, it is detached from the delivery wire (Figure 42-18).

The use of the ACDO is restricted to patients that have a femoral artery of sufficient caliber to accommodate the catheter or sheath required to deploy the device. This limits the use of the ACDO to relatively large patients but a modification of the basic technique of ACDO placement that can be used in patients as small as 2.5 or 3 kg has been described. Briefly, a 4F short vascular sheath is placed in the femoral artery and after angiography, a 4F curved, end-hole catheter is used to enter the pulmonary artery via the duct at which time, the catheter and sheath are removed over an exchange wire, the distal tip of which is left in the pulmonary artery. The outer diameter of a vascular sheath is generally 2F larger than the catheter that it will accommodate; that is, the outer diameter of a 4F sheath is 6 French units. Therefore, after the sheath has been removed, a 6F guiding catheter with hydrophilic coating can generally be advanced over the wire at which point, ACDO with waist diameters as great as 6 mm can be deployed within the duct.²²
 

Coil Occlusion - Technique

Gianturco coils are manufactured in numerous sizes and configurations but in all cases the device consists of a stainless steel or platinum wire that is tightly wound to produce a helix with a diameter between 0.014-0.043 inches. The wire made up of these primary windings is coiled to produce loops, the specific number of which depends on the length of wire and the diameter of the loop. Dacron tufts are attached to the wire and this makes the coil thrombogenic. The devices are packaged in tubular cartridges. Because the steel has structural memory, the loops reform when the coil is extruded from the cartridge or an intravascular catheter. The size and configuration of each coil is defined by three characteristics: wire diameter, loop diameter and wire length. Wire diameters of 0.035 in, 0.038 in and 0.052 in have all been used for coil occlusion of PDA in veterinary patients. Coils that form loops of numerous sizes ranging from 3 to 20 mm are available. In addition to characteristics that define loop size, number and deformability, some coils such as the Cook Detachable coil and the Cook Flipper are designed for controlled release into the circulation.

After angiographic evaluation, a curved, end-hole catheter such as a Judkins (right), JB-1, MPA or vertebral catheter is advanced to the ductus. Sometimes it is necessary to use a straight but floppy-tipped wire-guide to enter the ductal ampulla. It is useful to advance the catheter across the duct and into the pulmonary artery while monitoring intravascular pressures in order to identify fluoroscopic landmarks that relate to the pulmonary artery-ductal junction. Typically, this junction is close to the ventral border of the tracheal shadow. The dimensions of the coil to be deployed within the ductus are chosen based on measurements obtained from the angiogram or transesophageal echocardiogram. The loop diameter should be about twice the minimal ductal diameter and approximate the diameter of the ampulla. A wire-guide is used to extrude the coil from the cartridge and into the proximal end of the catheter. The coil then can be advanced through the catheter using the wire-guide until the more distal end exits the end of the catheter and begins to form a loop within the circulation. In pediatric practice, it is accepted that one or more loops of the device should be deployed in the pulmonary artery. When using non-detachable coils, most veterinary cardiologists deploy the entire coil within the ampulla of the duct. Provided that the coil forms sufficient number of loops, part of the coil can be deployed in the proximal aorta and then pushed into the ampulla. When coil position appears to be appropriate, the remainder of the coil is extruded from the catheter. When a single coil substantially occludes flow, the mean and diastolic artery pressures rise shortly after deployment but a Branham response generally is not observed. After about ten minutes the ductus is again evaluated angiographically or by transesophageal echocardiography (Figure 42-19). Ideally, the duct is completely occluded during the catheterization procedure although small residual shunts may resolve weeks or months after the procedure. If a substantial shunt persists, additional coils are placed within the first coil. A technique in which a biopsy device is used for controlled release of 0.052 in coils was described and then modified for use in veterinary patients by Miller.^24,25
 

Outcome/Complications of Transcatheter Occlusion

Of cases in which the procedure is attempted, about 80% are amenable to coil placement and occlusion although this figure likely depends on echocardiographic and angiographic criteria used to select candidates.^11,26 Of patients in which coils are deployed, complete ductal occlusion during the immediate post-procedural period has been reported to occur in 34 to 100% of cases.^11,12,14,16 Specific method, patient selection and perhaps operator experience are variables that likely affect immediate occlusion rates. In general, complete, acute resolution of the shunt can be achieved in 50 to 60% of cases. Delayed ductal closure occurring in the first months after the procedure occurs in about 30% of cases in which a residual shunt is evident in the immediate post-procedural period. Although residual shunting is relatively common it is not necessarily hemodynamically important and often is clinically silent. Indeed, coil occlusion is associated with a hemodynamically satisfactory result in the vast majority of patients subject to the procedure, such that fewer than 5% of cases require a second intervention.^14,26

The ACDO has filled an important niche in the practice of veterinary interventional cardiology. In contrast to coil occlusion, the rate of short-term occlusion is high and complications are rare. More specifically, Ngyuenba and Tobias reported the initial experience using a prototype of the ACDO.¹⁹ Eighteen dogs with PDA were subject to cardiac catheterization and angiographic characterization of the duct. Ultimately, ACDO were successfully deployed in all patients although in one case, the device, determined afterward to be inappropriately small relative to ductal diameter, migrated to the left main pulmonary artery. The errant device was not retrieved, adverse effects were not observed and later, during a separate procedure, an ACDO was placed without complications. Complete ductal occlusion was echocardiographically documented in 17 of 18 patients but in one, recurrent ductal patency was evident at one day and at three months after the procedure. Others subsequently confirmed the initial, encouraging results.²³ In a series of 41 canine patients with PDA, procedural success was documented in 40; the small size of one patient precluded placement of the sheath required to deploy a sufficiently large device. Complete ductal occlusion occurred within 24 hours of the procedure in all 40 patients.^19,23 Published results suggest that the ACDO is a device that can be used to successfully occlude PDA over the broad range of ductal size and morphologies. Presumably because the device firmly engages the duct and is attached to the delivery cable until the operator chooses to deploy, device embolization and other complications are rare although a single case of post-procedural device migration was recently reported.²⁷ Recently, patient outcomes after transcatheter occlusion by one of four different devices and techniques were retrospectively evaluated.²⁸ Procedural success was documented in 92% of cases but coil occlusion was associated with a greater number of complications than was placement of the ACDO. Patients were not randomized to device type and predictably, operators selected coil occlusion for the cohort for which body-size was smallest as coils can be delivered through relatively small diameter catheters.

Major complications of transcatheter intervention for PDA include intra-operative death, incomplete occlusion, post-procedural hemolysis, and device migration. Mortality associated with transcatheter intervention for PDA generally is quite low, near 2%,^14,26 although higher mortality has been reported in small studies that specifically recruited high risk patients.²⁹ Post-procedural hemolysis is sometimes associated with persistence of ductal flow after coil occlusion. This complication is apparently uncommon but has been reported in the pediatric literature and in dogs.^26,30

Other Devices andTechniques

The Amplatzer^® ductal occluder (ADO) is a mushroom shaped device that consists of a nitinol framework that is enmeshed with fabric; it was designed for occlusion of the human ductus. The device is extruded from a delivery sheath that is first advanced from the femoral vein, through the ductus and into the aorta. The device is pulled into the duct and released from the delivery wire. Use of this device has been reported in veterinary patients.^16,17

The Grifka-Gianturco occlusion device consists of a nylon sac that contains Gianturco coils that are deposited in the ductus using a controlled delivery system. The use of this device in a dog has been reported in the veterinary literature.¹⁵

Some consideration of the relative merits of transcatheter intervention and surgical ligation is unavoidable in any discussion of the treatment of PDA. The advantages of transcatheter intervention are relatively obvious. It is a minimally invasive technique that is generally associated with low mortality. Because it is minimally invasive, morbidity and hospitalization is apt to be less than that associated with thoracotomy and surgical ligation. Certainly, ductal size and morphology are important determinants of procedural success for coil occlusion but the development of the ACDO has expanded the indications for transcatheter therapy to include PDA of diverse size and morphology. Still, patient size does have a bearing on the suitability of candidates for transcatheter intervention. A technique for transcatheter occlusion of PDA using 0.025 in coils in patients weighing less than 3 kg has been described,¹³ but in general, femoral arterial access can be problematic in very small patients. To some extent, this difficulty can be overcome if a venous approach is used and indeed, this technique has been used for transcatheter coil occlusion of PDA in cats and dogs.^31,32 However, the use of venous access without concurrent arterial access may pose a risk to the patient in the event of aortic embolization. In contrast, patient-size and ductal morphology likely have a limited effect on the outcome of surgical ligation; experienced operators can successfully ligate PDA in dogs that weigh less than 0.5 kg.³³

As discussed, there are numerous potentially serious complications of transcatheter intervention for PDA. Most of these complications do not result in patient mortality but they may require referral to a surgeon or additional catheterization procedures. While the clinical importance of hemodynamically inconsequential residual shunts has been not been defined, the prevalence of incomplete occlusion also deserves consideration in a comparison of surgical ligation and transcatheter intervention. When patients are subject to echocardiographic scrutiny after treatment of PDA, the prevalence of incomplete occlusion after surgical ligation varies but is as high as 53% when the Jackson-Henderson technique is used.³⁴ Furthermore, shunts that persist after coil occlusion are apt to become progressively smaller. In contrast, the mechanism of incomplete occlusion after ligation presumably relates to inadequate dissection of the periductal adventitia, an insufficiently tight ligature or loosening of knots; this being the case, late closure is not to be expected. Incomplete occlusion after placement of an ACDO is considerably less common than after coil occlusion.

Balloon Dilation of Obstructive Lesions
Pulmonic Stenosis

Etiolopathogenesis

Pulmonic (or pulmonary) stenosis (PS) refers to narrowing of the right ventricular outflow tract. PS is a common cardiac malformation in the dog but occurs infrequently in cats.⁶ Acquired PS is rare and this discussion will be concerned exclusively with congenital obstruction. The obstruction of the outflow tract most often results from narrowing of the pulmonary valve although subvalvular PS and supravalvular PS are occasionally observed. Subvavular, or infundibular PS, is seldom an isolated lesion and is more often associated with complex malformations such as Tetralogy of Fallot or is the result of right ventricular hypertrophy related to valvular PS.

The cause of PS is unknown although a heritable basis has been established in beagle hounds.³⁵ Pedigree analyses or planned breeding studies of dogs other than beagles have not been reported. However, the disproportionate occurrence of PS in certain purebred dogs provides indirect evidence that canine PS generally has a genetic basis. The English bulldog, Samoyed, miniature schnauzer and terrier breeds are predisposed to the development of PS.⁶ Interestingly, despite a proven genetic basis for pulmonary valve dysplasia in beagles, this breed is not overrepresented in epidemiological surveys. PS in English bulldogs requires specific mention. In dogs of this breed, PS has been associated with concurrent coronary artery anomalies.³⁶ The coronary anomaly that seemingly is most common is sometimes referred to as an “R2A”, and is characterized by a single right coronary ostium; the left coronary artery arises from the right main coronary artery and then encircles the infundibulum. It has been suggested that the mechanical effect of the abnormal course of the coronary artery is responsible for maldevelopment of the PV.³⁷ Certainly, detection of the R2A anomaly has clinical relevance because it is a contraindication for surgical patchgraft procedures and possibly a contraindication for transcatheter balloon dilation.^36,38 Minimally, detection of a circumpulmonary coronary branch requires use of a modified technique in which the diameter of the dilating balloon approximates, rather than exceeds, the diameter of the valve annulus. Recently, coronary anomalies other than the R2A have been reported which emphasizes the importance of angiographic assessment of the coronary anatomy prior to intervention.^39,40

The pathology of PS is clinically relevant because it is an important determinant of the efficacy of therapeutic intervention. The normal pulmonary valve is a trileaflet structure. Each of the leaflets has a semilunar attachment to the interior of the proximal pulmonary artery. A true fibrous valve annulus does not exist in normal specimens but the ventriculo-arterial ring is a clinically useful landmark which is generally known as the PV annulus.⁴¹ In the pediatric literature, PS characterized by commissural fusion of otherwise normal or mildly thick valve leaflets is known as typical PS. This form of PS is distinguished from more extensive malformation of the leaflets and annulus which is known as valvular dysplasia.⁴¹ In cases of PV dysplasia, the leaflets are abnormally thick and mobility of the cusps is limited by rigidity and abnormal attachment to the neighboring leaflets or pulmonary artery intima. Often, the annulus of the valve is narrow and together these abnormalities serve to narrow the PV orifice. Based on echocardiographic appearance, a similar scheme for classification of canine PS has been proposed; type A PS is primarily the result of commissural fusion while type B results from narrowing of the annulus and restricted mobility of abnormal valve leaflets.⁴² Post-mortem examination of beagles with hereditary PS demonstrated a continuum of lesions; some had features of typical PS while others were similar to the valvular dysplasia described in the pediatric literature. At least in beagles with heritable PS, differences in valvular morphology appear to reflect variable expression of a single disease process and therefore it may be that categories of PS are artificial. Despite this, the distinction between dogs with PS and a normal annulus from dogs with PS and annular hypoplasia is clinically useful because the two populations differ in response to transcatheter intervention.⁴²

Pathophysiology

Obstruction of the outflow tract increases the impedance to ventricular ejection. In consequence, the ventricle must generate supraphysiologic systolic pressures in order to maintain perfusion pressure distal to the stenosis. As a result, there is a pressure gradient (Δ P) across the obstruction. Peak systolic Δ P that are less than 40 mmHg are generally considered to be mild and those greater than 80 or 100 mmHg, severe.^43,44 Concentric hypertrophy – an increase in myocardial mass without concomitant chamber dilation – at least temporarily offsets the increase in ventricular wall stress that results from outflow obstruction. The mechanism by which compensatory hypertrophy progresses to ventricular failure has not been resolved. Both mechanical and neuroendocrine factors likely contribute. However, the right ventricle is not geometrically suited to the development of high systolic pressures and tricuspid valve regurgitation and myocardial dysfunction are potential sequelae of severe PS.

Clinical Presentation

Canine PS is usually first detected by auscultation when pups are subject to routine veterinary evaluation. At least in young pups, a history of clinical signs related to PS is the exception rather than the rule. PS causes a systolic ejection murmur that usually is heard best over the left heart base. When PS is severe, the murmur generally is loud and typically associated with a precordial thrill. In most cases, the arterial pulse is normal. Thoracic radiographs or EKG do not provide diagnostically specific information. However, most patients with severe PS have radiographic cardiomegaly with right-sided emphasis. Often, the proximal main pulmonary artery is prominent due to development of post-stenotic aneurysm. Pulmonary hypoperfusion is often radiographically evident in patients with severe PS. Electrocardiographic evidence of right ventricular hypertrophy is commonly observed in patients with severe PS although EKG abnormalities are typically absent in patients with mild obstruction. The diagnosis can be confirmed by cardiac catheterization although echocardiography generally has replaced invasive studies for diagnostic purposes. When PS is severe, two-dimensional echocardiographic studies reveal consequences of obstruction including right ventricular hypertrophy and right atrial enlargement. Usually, the valve leaflets are abnormally thick and doming of the leaflets is sometimes observed. This latter finding, which may also be evident angiographically, reflects commissural fusion of valve leaflets and generally predicts a favorable response to balloon dilation. In healthy individuals, the annulus diameter is similar to that of the aorta but in patients with PS, varying degrees of annulus hypoplasia are relatively common. Doppler echocardiography demonstrates abnormal acceleration within the right ventricular outflow tract – a velocity step-up – which is the Doppler correlate of obstruction. Peak velocity across the obstruction is related to Δ P by the modified Bernoulli equation. Agreement between Doppler estimates of Δ P and gradients measured by catheterization is excellent but there are factors that confound the relationship. Most importantly, invasively acquired gradients are generally obtained from veterinary patients who are anesthetized. Pressure gradients depend not only on the severity of stenosis – the degree to which the cross-sectional area of the orifice is diminished – but also on flow. Because cardiac output is decreased by anesthesia, Δ P obtained by cardiac catheterization may be as much as 50% of Δ P estimated by Doppler echocardiography in the awake or sedated patient.

Identification of Candidates for Intervention/Natural History

Little is known of the natural history of canine PS. The prognosis for children with mild PS is excellent without intervention and the same appears to be true in dogs. Canine patients in which Doppler-derived Δ P exceeds 80 mmHg are at risk of sudden death or death due to congestive heart failure; further, this risk increases incrementally in association with increasing gradient.⁴⁴ A history of exercise intolerance or collapse also predicts poor outcome in patients with severe PS.⁴⁴ The minimum gradient at which the benefit of therapeutic intervention exceeds the associated risks has not been established. Based partly on the approach adopted by pediatric cardiologists, it has become accepted that intervention is reasonable if Δ P exceeds 80 mmHg even if clinical signs are absent. Several surgical procedures for correction of PS have been described but, based on an apparently favorable risk/benefit ratio, transcatheter balloon dilation is generally recommended as the initial approach to severe PS.

Balloon Dilation

After induction of general anesthesia, access to the femoral vein or external jugular vein is obtained percutaneously or after a small skin incision. Complications of percutaneous venous access are infrequently observed and there are advantages to the percutaneous approach. The decision regarding choice of vessel is primarily one of operator preference.

It is a basic precept of catheterization technique that hemodynamic variables, including Δ P obtained by catheter pullback, are recorded before injection of contrast or attempted intervention. However in dogs, the efficacy of balloon dilation ultimately is judged by the effect on the awake, Doppler-derived Δ P not by the acute effect on measured gradients. Furthermore, in some patients with very severe PS, it can be difficult to cross the obstruction and therefore difficult to justify sacrifice of a therapeutically advantageous catheter placement for the sake of diagnostic completeness. In these difficult cases, a right ventriculogram is recorded at the outset of the procedure (Figure 42-20). Then, an end-hole catheter is fluroscopically guided to the pulmonary artery and is exchanged for a balloon dilation catheter over a long 200 to 260 cm wire guide. This catheter exchange is necessary because therapeutic balloons are carried by catheters that are too stiff to safely manipulate free in the circulation. The balloon is centered across the valve and is inflated with a mixture of saline and contrast material. The proportions of saline and contrast medium are not crucial. It is important that the inflated balloon is fluoroscopically visible but contrast material is quite viscous making rapid inflation and deflation difficult. Something less than 50% contrast material by volume likely is appropriate. The required number of inflations varies. When the dilation is successful, there is first the appearance of an indentation and then abrupt disappearance of this “waist” (Figure 42-21). During inflation, right ventricular stroke volume declines precipitously as does, shortly thereafter, systemic pressure and perfusion. In most cases, unassisted hemodynamic recovery occurs promptly after balloon deflation. The balloon catheter is removed over the wire guide. It is important to aspirate from the balloon port to maintain negative pressure and reduce the profile of the balloon while it is withdrawn through the heart and vessels.
 

Sometimes, perhaps most often in patients with dynamic, infundibular obstruction, the force of ventricular contraction causes the balloon to “pumpkin seed” through the valve orifice resulting in an ineffectual inflation. Patience and gentle tension on the catheter during inflation often will eliminate this difficulty although intravenous administration of acetylcholine immediately prior to inflation can be used to cause a brief period of ventricular asystole.

Balloon catheters are supplied by the manufacturer in numerous configurations. Generally, the balloon is constructed from a plastic polymer such as polyvinyl chloride and surrounds the distal catheter shaft. The catheter has two lumens; one that courses the length of catheter and a second that is used for inflation and deflation of the balloon. The clinically important characteristics of the catheter shaft are length, outer diameter, which is described using the French scale, and inner diameter which is measured in inches.⁴⁵ The latter property determines the diameter of guide wire which the catheter will accept. The balloon itself is described in terms of length, outer diameter, profile and material characteristics that determine burst pressure. The length of the balloon is chosen based on ventricular size which is generally related to body size. It can be difficult to maintain the position of a short balloon during inflation but overly long balloons can cause injuries including cardiac perforation and disruption of the tricuspid valve apparatus. A 3 cm balloon is appropriate for most canine patients. Two cm and 4 or 5 cm balloons are sometimes used for very small or very large patients. The outer diameter of the balloon is chosen based on echocardiographic or angiographic assessment of PV annulus diameter. Recommendations regarding balloon diameter have become more aggressive in the years since the introduction of the technique. A balloon diameter that is 120 to 150% of the valve annulus is believed to be optimal. Larger relative balloon sizes have been associated with cardiovascular injury in experimental models and with complications in children.^46,47 Profile is the term used to describe the increment in total catheter diameter which results from the structure of the balloon. Profile and physical characteristics that determine profile are inter-related. Balloons are best constructed of materials that exhibit low compliance and high burst pressures since this most effectively transmits radial force to the valve. However, balloons with those characteristics necessarily have a larger profile than do those with lower burst pressure. Profile is important because unnecessarily large balloons can result in intimal or valvular injury.

Advances in catheter and guide-wire construction including the development of low-profile balloons, flow directed catheters, steerable guide-wires with soft, flexible tips and tip-deflecting wires have expanded the indication for balloon dilation to include patients of virtually any size. However, balloon dilation for PS can be technically difficult in patients that weigh less than 6 or 7 kg. In small patients with tight stenosis, directing a catheter into the right ventricular outflow tract and crossing the obstruction are often the most difficult aspects of the procedure. It may be necessary to make numerous attempts with different catheters and guide-wires. Flow directed catheters often can be used to atraumatically cross a stenotic pulmonary valve. Flow-directed (“wedge”) catheters are constructed of soft materials and are equipped with a small balloon near the distal catheter tip. The balloon is filled with room-air causing it to float in the circulation which carries the catheter tip in the direction of blood flow. However, marked tricuspid valve regurgitation makes it difficult to manipulate and advance these catheters. Wire-guides can be used to stiffen the catheter but sometimes this a liability in that flow-directed catheters do not generally accommodate large gauge wires. This difficulty can be circumvented by exchanging the flow-directed catheter for a thin-walled multipurpose catheter which is then, in turn exchanged for the balloon dilation catheter over a stiffer, larger gauge wire. However, this extra manipulation is time consuming and might represent a risk in a hemodynamically unstable patient. A multipurpose catheter can often be coaxed across the obstruction with or without a wire guide. In other cases, the use of specific catheter configurations such as the Judkins (right) coronary catheter or Berenstein catheter can be helpful in crossing the stenosis. Tip-deflecting wires can also be used to direct the tip of a straight catheter into the right ventricular outflow tract. Knowledge of the precise anatomical location of the catheter tip is important because tip-deflecting wires are rather stiff and cardiac perforation is a potential complication.

Results/Efficacy

The safety and efficacy of PBV in the management of PS in humans is well established. In fact, the only indication for surgical correction of isolated PS is failure of technically adequate balloon dilation to effectively decrease the associated ΔP. There are few published data that relate to the efficacy of balloon dilation in veterinary patients. Case reports and case series attest to short-term safety and efficacy of the procedure.^48-51 Recent retrospective cohort studies provide evidence that PBV is associated with a low rate of complications and generally decreases ΔP to a degree that is thought to be prognostically favorable.^42,44,52 In general, it can be stated that PBV decreases ΔP by 50% or more in roughly 75% of dogs with PS. Analysis of patient characteristics and outcome using a Cox multivariable regression model demonstrated that PBV confers a survival advantage in dogs with an initial ΔP that exceeds 80 mmHg.⁴⁴ PBV is apparently less effective in the management of canine PS than it is in the treatment of PS in people; possibly this is because of a greater prevalence of obviously dysplastic valves in affected dogs. Indeed, valve morphology is an important determinant of the efficacy of PBV in both humans and dogs. One year after PBV, the mean gradient reduction in dogs with PS normal annulus diameter was 63% while the reduction was only 39% in dogs with a small annulus and thick, immobile valve leaflets.⁴² Restenosis after PBV is uncommon. In fact, the gradient continues to decrease in the months after PBV in some dogs. Partly, this might be due to resolution of dynamic outflow gradients but it has been suggested that a progressive decrease in iatrogenic valvular edema might also be responsible.⁵³

Complications

Major complications of PBV including cardiac perforation and death associated with occlusion of ventricular outflow in hemodynamically unstable patients are relatively uncommon. Death due to coronary artery avulsion was previously mentioned in the context of the r2A coronary anomaly that has been observed in brachycephalic dogs.³⁸ Damage to the tricuspid valve during inflation or during transit through the heart is relatively common. This complication is generally well tolerated provided balloon dilation effectively enlarges the stenotic orifice. Pulmonary valve regurgitation commonly is associated with PS and fairly often, it is worsened by balloon dilation although this seldom has clinical consequences. Arrhythmias are relatively common during manipulation of catheters through the heart but they usually are transient and resolve when the catheter is withdrawn or its position adjusted.

Balloon Dilation of Other Lesions

Subvalvular aortic stenosis (SAS) is one of the most prevalent congenital cardiac malformations in dogs. In its severe form, the lesion most often consists of a fibrous or fibrocartilaginous ring that completely encircles the subvalvular left ventricular outflow tract.⁵⁴ In a few cases, there is more diffuse subvalvular narrowing resulting in tunnel stenosis. Severely affected patients are at risk for premature death that is usually the result of sudden, presumably arrhythmic, death.⁵⁵ Balloon dilation of SAS in dogs has been reported and the technique is similar to that use in treatment of right ventricular outflow tract obstruction.^56,57 A catheter is advanced to the left ventricular apex from the external carotid or femoral artery and then exchanged over a long wire guide for a suitably sized balloon catheter. The risk of iatrogenic and clinically important aortic valve regurgitation is believed to be high and the balloon is usually chosen to approximate the diameter of the aortic annulus. Short and mediumterm gradient reduction has been demonstrated but, relative to a group medically treated with atenolol, balloon dilation did not favourably affect survival.⁵⁷ This is not necessarily surprising given the nature of the lesion and the fact that successful surgical treatment failed to demonstrate a survival benefit.⁵⁸ Recently, an intervention in which the sequential use of a cutting balloon – one in which microsurgical blades are longitudinally oriented along the exterior of the balloon – and then a high-pressure balloon, has been reported but outcome data have not been published.⁵⁹

With variable success, other less common obstructive lesions have been addressed by transcatheter dilation. Palliation of tetralogy of Fallot using transcatheter balloon dilation has been attempted the dog.⁶⁰ Both tricuspid valve stenosis and caudal caval obstruction due to cor triatriatum dexter have been treated by balloon dilation.^61-63 Additionally, balloon dilation of a mid-right ventricular outflow tract obstruction – double-chambered right ventricle – also has been reported.⁶⁴

References

1. Buchanan J. Patent ductus arteriosus : mophology, pathogenesis, types and treatment. Journal of Veterinary Cardiology 2001;3:7-19.
2. Benson LN CK. The arterial duct: its persistence and its patency. In: Anderson RH BE, Macartney FH, Rigby ML, Shinebourne EA, Tynan M, ed. Paediatric Cardiology, 2nd ed. Edinburgh: Churchill Livingstone; 2002:1405-1460.
3. Gittenberger-de Groot AC, Strengers JL, Mentink M, et al. Histologic studies on normal and persistent ductus arteriosus in the dog. J Am Coll Cardiol 1985;6:394-404.
4. Buchanan JW, Patterson DF. Etiology of patent ductus arteriosus in dogs. J Vet Intern Med 2003;17:167-171.
5. Patterson DF, Pyle RL, Buchanan JW, et al. Hereditary patent ductus arteriosus and its sequelae in the dog. Circ Res 1971;29:1-13.
6. Buchanan J. Prevalence of cardiovascular disorders. In: Fox PR SD, Moise NS, ed. Textbook of Canine and Feline Cardiology - Principles and Clinical Practice., 2nd ed. Philadelphia: WB Saunders Co.; 1999:457-470.
7. Van Israel N, French AT, Dukes-McEwan J, et al. Review of left-to-right shunting patent ductus arteriosus and short term outcome in 98 dogs. J Small Anim Pract 2002;43:395-400.
8. Snaps FR, Mc Entee K, Saunders JH, et al. Treatment of patent ductus arteriosus by placement of intravascular coils in a pup. J Am Vet Med Assoc 1995;207:724-725.
9. Fox PR, Bond BR, Sommer RJ. Nonsurgical transcatheter coil occlusion of patent ductus arteriosus in two dogs using a preformed nitinol snare delivery technique. J Vet Intern Med 1998;12:182-185.
10. Saunders JH, Snaps FR, Peeters D, et al. Use of a balloon occlusion catheter to facilitate transarterial coil embolisation of a patent ductus arteriosus in two dogs. Vet Rec 1999;145:544-546.
11. Stokhof AA, Sreeram N, Wolvekamp WT. Transcatheter closure of patent ductus arteriosus using occluding spring coils. J Vet Intern Med 2000;14:452-455.
12. Schneider M, Hildebrandt N, Schweigl T, et al. Transvenous embolization of small patent ductus arteriosus with single detachable coils in dogs. J Vet Intern Med 2001;15:222-228.
13. Hogan DF, Green HW, III , Gordon S, et al. Transarterial coil embolization of patent ductus arteriosus in small dogs with 0.025-inch vascular occlusion coils: 10 cases. Journal of Veterinary Internal Medicine 2004;18:325-329.
14. Gordon SG, Miller MW. Transarterial Coil Embolization for Canine Patent Ductus Arteriosus Occlusion. Clinical Techniques in Small Animal Practice 2005;20:196-202.
15. Grifka RG, Miller MW, Frischmeyer KJ, et al. Transcatheter occlusion of a patent ductus arteriosus in a Newfoundland puppy using the Gianturco-Grifka vascular occlusion device. J Vet Intern Med 1996;10:42-44.
16. Glaus TM, Berger F, Ammann FW, et al. Closure of large patent ductus arteriosus with a self-expanding duct occluder in two dogs. J Small Anim Pract 2002;43:547-550.
17. Sisson D. Use of a self-expanding occluding stent for nonsurgical closure of patent ductus arteriosus in dogs. J Am Vet Med Assoc 2003;223:999-1005.
18. Hogan DF, Green HW, Sanders RA. Transcatheter closure of patent ductus arteriosus in a dog with a peripheral vascular occlusion device. Journal of Veterinary Cardiology 2006;8:139-143.
19. Nguyenba TP, Tobias AH. Minimally Invasive Per-Catheter Patent Ductus Arteriosus Occlusion in Dogs Using a Prototype Duct Occluder. Journal of Veterinary Internal Medicine 2008;22:129-134.
20. McEntee K, Snaps F, Clercx C, et al. Clinical vignette. Tetralogy of Fallot associated with a patent ductus arteriosus in a German shepherd dog. J Vet Intern Med 1998;12:53-55.
21. Krichenko A, Benson LN, Burrows P, et al. Angiographic classification of the isolated, persistently patent ductus arteriosus and implications for percutaneous catheter occlusion. Am J Cardiol 1989;63:877-880.
22. Nguyenba TP, Tobias AH. The Amplatz(R) canine duct occluder: A novel device for patent ductus arteriosus occlusion. Journal of Veterinary Cardiology 2007;9:109-117.
23. Gordon SG, Saunders AB, Achen SE, et al. Transarterial ductal occlusion using the Amplatz Canine Duct Occluder in 40 dogs. J Vet Cardiol 2010;12:85-92.
24. Ronald G. Grifka MD TKJ. Transcatheter closure of large PDA using 0.052? Gianturco coils: Controlled delivery using a bioptome catheter through a 4 French sheath. Catheterization and Cardiovascular Interventions 2000;49:301-306.
25. Miller M. Transarterial coil occlusion of patent ductus arteriosus: outcome in 120 cases. Proceedings of the 20th Annual Forum of the American College of Veterinary Internal Medicine 2002:102.
26. Campbell FE, Thomas WP, Miller SJ, et al. Immediate And Late Outcomes Of Transarterial Coil Occlusion Of Patent Ductus Arteriosus In Dogs. J Vet Intern Med 2006;20:83-96.
27. Carlson JA, Achen SA, Saunders AB, et al. Delayed embolization of an Amplatz((R)) canine duct occluder in a dog. J Vet Cardiol 2013;15:271-276.
28. Singh MK, Kittleson MD, Kass PH, et al. Occlusion Devices and Approaches in Canine Patent Ductus Arteriosus: Comparison of Outcomes. Journal of Veterinary Internal Medicine 2012;26:85-92.
29. Schneider M SI, Wehner M, Hildebrandt N. Transvenous embolization of large PDA (>4.0 mm) with stiff and multiple coils (abstract). J Vet Intern Med 2002;16:630.
30. Van Israel N, French AT, Wotton PR, et al. Hemolysis associated with patent ductus arteriosus coil embolization in a dog. J Vet Intern Med 2001;15:153-156.
31. Schneider M, Hildebrandt N. Transvenous embolization of the patent ductus arteriosus with detachable coils in 2 cats. J Vet Intern Med 2003;17:349-353.
32. Hildebrandt N, Schneider C, Schweigl T, et al. Long-Term Follow-Up after Transvenous Single Coil Embolization of Patent Ductus Arteriosus in Dogs. Journal of Veterinary Internal Medicine 24:1400-1406.
33. Buchanan JW, Soma LR, Patterson DF. Patent ductus arteriosus surgery in small dogs. J Am Vet Med Assoc 1967;151:701-707.
34. Stanley BJ, Luis-Fuentes V, Darke PG. Comparison of the incidence of residual shunting between two surgical techniques used for ligation of patent ductus arteriosus in the dog. Vet Surg 2003;32:231-237.
35. Patterson DF, Haskins ME, Schnarr WR. Hereditary dysplasia of the pulmonary valve in beagle dogs. Pathologic and genetic studies. Am J Cardiol 1981;47:631-641.
36. Buchanan JW. Pulmonic stenosis caused by single coronary artery in dogs: four cases (1965-1984). J Am Vet Med Assoc 1990;196:115-120.
37. Buchanan JW. Pathogenesis of single right coronary artery and pulmonic stenosis in English Bulldogs. J Vet Intern Med 2001;15:101-104.
38. Kittleson M, Thomas W, Loyer C, et al. Single coronary artery (type R2A). J Vet Intern Med 1992;6:250-251.
39. Visser LC, Scansen BA, Schober KE. Single left coronary ostium and an anomalous prepulmonic right coronary artery in 2 dogs with congenital pulmonary valve stenosis. Journal of Veterinary Cardiology 2013;15:161-169.
40. Waterman MI, Abbott JA. Novel Coronary Artery Anomaly in an English Bulldog with Pulmonic Stenosis. Journal of Veterinary Internal Medicine 2013;27:1256-1259.
41. Stamm MD C, Anderson MD F, Robert H., Ho PhD F, Siew Yen. Clinical Anatomy of the Normal Pulmonary Root Compared With That in Isolated Pulmonary Valvular Stenosis. Journal of the American College of Cardiology 1998;31:1420-1425.
42. Bussadori C, DeMadron E, Santilli RA, et al. Balloon valvuloplasty in 30 dogs with pulmonic stenosis: effect of valve morphology and annular size on initial and 1-year outcome. J Vet Intern Med 2001;15:553-558.
43. Fingland RB, Bonagura JD, Myer CW. Pulmonic stenosis in the dog: 29 cases (1975-1984). J Am Vet Med Assoc 1986;189:218-226.
44. Johnson MS, Martin M, Edwards D, et al. Pulmonic stenosis in dogs: balloon dilation improves clinical outcome. J Vet Intern Med 2004;18:656-662.
45. Yeager SB FM, Keane JF. Catheter intervention: Balloon valvotomy. In: Lock JE KJ, Perry SB, ed. Diagnostic and Interventional Catheterization in Congenital Heart Disease. Boston: Kluwer Academic; 2000:151-178.
46. Ring JC, Kulik TJ, Burke BA, et al. Morphologic changes induced by dilation of the pulmonary valve anulus with overlarge balloons in normal newborn lambs. Am J Cardiol 1985;55:210-214.
47. Berman W, Jr., Fripp RR, Raisher BD, et al. Significant pulmonary valve incompetence following oversize balloon pulmonary valveplasty in small infants: A long-term follow-up study. Catheterization and cardiovascular interventions : official journal of the Society for Cardiac Angiography & Interventions 1999;48:61-65; discussion 66.
48. Bright JM, Jennings J, Toal R, et al. Percutaneous balloon valvuloplasty for treatment of pulmonic stenosis in a dog. J Am Vet Med Assoc 1987;191:995-996.
49. Sisson DD, MacCoy DM. Treatment of congenital pulmonic stenosis in two dogs by balloon valvuloplasty. J Vet Intern Med 1988;2:92-99.
50. Madron Ed, Bussadori C. Five cases of valvuloplasty with a balloon catheter for stenotic pulmonary valves in dogs. Pratique Médicale & Chirurgicale de l’Animal de Compagnie 1994;29:383-391.
51. Martin MWS, Godman M, Fuentes VL, et al. Assessment of balloon pulmonary valvuloplasty in six dogs. Journal of Small Animal Practice 1992;33:443-449.
52. Johnson MS, Martin M. Results of balloon valvuloplasty in 40 dogs with pulmonic stenosis. J Small Anim Pract 2004;45:148-153.
53. Estrada A, Moise NS, Erb HN, et al. Prospective evaluation of the balloon-to-annulus ratio for valvuloplasty in the treatment of pulmonic stenosis in the dog. J Vet Intern Med 2006;20:862-872.
54. Pyle RL, Patterson DF, Chacko S. The genetics and pathology of discrete subaortic stenosis in the Newfoundland dog. Am Heart J 1976;92:324-334.
55. Kienle RD, Thomas WP, Pion PD. The natural clinical history of canine congenital subaortic stenosis. J Vet Intern Med 1994;8:423-431.
56. DeLellis LA, Thomas WP, Pion PD. Balloon dilation of congenital subaortic stenosis in the dog. J Vet Intern Med 1993;7:153-162.
57. Meurs KM, Lehmkuhl LB, Bonagura JD. Survival times in dogs with severe subvalvular aortic stenosis treated with balloon valvuloplasty or atenolol. J Am Vet Med Assoc 2005;227:420-424.
58. Orton EC, Herndon GD, Boon JA, et al. Influence of open surgical correction on intermediate-term outcome in dogs with subvalvular aortic stenosis: 44 cases (1991-1998). J Am Vet Med Assoc 2000;216:364-367.
59. Kleman ME, Estrada AH, Maisenbacher HW, 3rd, et al. How to perform combined cutting balloon and high pressure balloon valvuloplasty for dogs with subaortic stenosis. J Vet Cardiol 2012;14:351-361.
60. Oguchi Y, Matsumoto H, Masuda Y, et al. Balloon dilation of right ventricular outflow tract in a dog with tetralogy of Fallot. J Vet Med Sci 1999;61:1067-1069.
61. Adin DB, Thomas WP. Balloon dilation of cor triatriatum dexter in a dog. J Vet Intern Med 1999;13:617-619.
62. Brown WA, Thomas WP. Balloon valvuloplasty of tricuspid stenosis in a Labrador retriever. J Vet Intern Med 1995;9:419-424.
63. Kunze P, Abbott JA, Hamilton SM, et al. Balloon valvuloplasty for palliative treatment of tricuspid stenosis with right-to-left atrial-level shunting in a dog. J Am Vet Med Assoc 2002;220:491-496, 464.
64. MacLean HN, Abbott JA, Pyle RL. Balloon dilation of double-chambered right ventricle in a cat. J Vet Intern Med 2002;16:478-484.
     

Incidence

Persistent right aortic arch (PRAA) accounts for an estimated 95 percent of all clinically significant vascular ring anomalies in the dog. PRAA is the fourth most common cardiovascular malformation in dogs; only patent ductus arteriosus, pulmonic stenosis, and aortic stenosis have a higher incidence. Other vascular ring anomalies that are less commonly seen include double aortic arch, aberrant right and left subclavian artery, persistent right ligamentum arteriosum and left aortic arch with an anomalous right patent ductus arteriosus.

Purebred dogs are more susceptible than mongrels to PRAA. The condition is likely heritable with German Shepherds, Irish setters and Boston Terriers having a higher incidence than the general canine population. Increased numbers of offspring with PRAA have been observed in certain family lines and were seen in related Greyhounds in a kennel. Two German shepherd littermates with left aortic arch and anomalous right sided PDA are also reported. Single or multiple recessive genes appear to be responsible for the trait and breeding of affected animals is discouraged.

In cats, the exact incidence of PRAA is unknown, but it appears to be less common than in dogs. About one half of the feline cases occur in Siamese and Persian cats, although the absolute numbers are insufficient to make conclusions on breed predisposition.

Animals with PRAA usually are diagnosed shortly after weaning with the vast majority of cases diagnosed before six months of age. Exceptions sometimes occur, however, with dogs as old as 10 years being reported. Virtually all cases of PRAA in the dog and cat involve some degree of esophageal constriction and obstruction resulting in oral or nasal regurgitation.

Surgical Embryology

Persistent right aortic arch occurs when the right fourth arch, instead of the left develops into the functional adult aorta. The right ductus arteriosus degenerates and the left ductus arteriosus remains forming a strap that constricts the esophagus between the left pulmonary artery and the anomalous right aorta. The esophagus is thus constricted by the aorta on the right, the ligamentum arteriosum (LA) on the left dorsolaterally, the pulmonary trunk on the left, and the base of the heart ventrally. Persistent left cranial vena cava occurs concurrently with PRAA about 40 percent of the time however the left cranial vena cava is not clinically significant, as it empties into the right atrium and 

does not act as an esophageal constricting vascular ring. Its presence may complicate surgical dissection of the LA because it passes over the pulmonary artery and limits visualization of the surgical field. PRAA is associated with concurrent patent ductus arteriosus only about 10 percent of the time. When a patent ductus is present, blood flow through the ductus is minimal and insufficient turbulence is produced to create a murmur.

Clinical Presentaion and Diagnosis

Dogs and cats affected with PRAA are often asymptomatic before weaning but regurgitation of solid food may be observed as early as four to eight weeks of age. A ravenous appetite is typically reported, but the animal usually lags behind litter mates in size and body weight. Regurgitation may occur shortly after eating or may be delayed for several hours. The regurgitated food usually is undigested, covered by mucus, and has a neutral pH. A cough may be present, indicating the presence of aspiration pneumonia.

Auscultation of the heart is usually normal; even in cases of PRAA with patent ductus arteriosus. When present, diagnosis of the patent ductus arteriosus is usually made at the time of surgery. Lung sounds can be normal or rales can be heard if aspiration pneumonia is present. Food retained in the dilated esophagus may produce a gurgling sound upon auscultation. If dilation extends up into the central esophagus, a characteristic postprandial bulge may be seen or palpated at the thoracic inlet. Simultaneous closing of the mouth and external nares while gently squeezing the abdomen may produce bulging of the cervical esophagus.

Abnormal radiographic signs seen on survey radiographs include moderate or marked focal left curvature of the trachea near the cranial border of the heart on a VD or DV view. Ventral tracheal displacement, mediastinal widening, and occasionally a right sided descending aortic shadow may be seen on the lateral view. Ventral tracheal displacement and narrowing is caused by the dilated esophagus. If only the cranial thoracic esophagus is dilated, the trachea returns to a normal position at the tracheal bifurcation over the heart base and the trachea and theheart will be displaced ventrally.

An esophagram should be performed to confirm the diagnosis. Cranial thoracic esophageal dilation is associated with an abrupt esophageal narrowing over the heart base at the fourth or fifth rib. On the ventral dorsal view, the esophagus may be displaced to the left just proximal to the esophageal constriction with an indentation into the right side of the esophagus. The presence of a poststenotic esophageal dilatation is thought to indicate a more guarded prognosis for return to normal esophageal function. Fluoroscopic swallowing studies may be used to evaluate the quality of esophageal peristalsis in the dilated esophagus both pre and postoperatively. Esophageal endoscopy can be useful in evaluating the magnitude of esophageal dilation and also ruling out other causes of constriction of the intrathoracic esophagus. Occasionally, angiography may be needed to diagnose more complex vascular ring anomalies other than PRAA.

Presurgical Considerations

Definitive treatment for PRAA involves surgical ligation and division of the ligamentum arteriosum as soon after weaning as possible. Feeding of slurries alone without relieving the esophageal constriction is not effective since the pre-stenotic esophageal dilation often enlarges with time. Animals with PRAA are often presented in a debilitated, cachectic, and dehydrated state that requires special presurgical considerations. Fluid or electrolyte imbalances should be corrected before surgery. Aspiration pneumonia, if present, compromises the patient’s ability to effectively ventilate the lungs. Placement of gastric feeding tubes to establish esophageal bypass in combination with broad-spectrum antibiotic therapy may be indicated preoperatively in patients with severe aspiration pneumonia.

We use propofol for rapid intravenous induction and tracheal intubation. Immediately after induction, the patient should be assisted in its ventilatory effort. Anesthesia is maintained with inhalant anesthesia. The dilated esophagus should be evacuated with suction prior to surgery since a grossly enlarged cranial esophagus may inhibit the ability to inflate the cranial and middle lung lobes during thoracotomy.

Surgical Technique

Surgical ligation of the LA is best accomplished through a left fourth thoracotomy. The cranial lung lobe is packed caudally with moistened surgical sponges. The esophagus, aorta, main pulmonary artery, and left vagus nerve are identified. The mediastinal pleura is transected longitudinally and the vagus nerve is reflected dorsally with 2-0 silk. The LA is usually longer than normal and is often difficult to visualize within the fibrous ring. It is most easily located cranial to the recurrent laryngeal nerve. If a persistent left cranial vena cava is present, it may have a hemizygous branch that obscures the LA. This structure can be double ligated, transected and reflected ventrally. If an aberrant left or right subclavian artery is present, it can be ignored if the vessel is not compressing the esophagus. If esophageal constriction is present, the subclavian vessel may be elevated and divided between ligatures. Adequate collateral circulation will be provided by the vertebral arteries.

The LA is carefully elevated off the esophagus from its left lateral aspect. Blunt dissection of the LA is performed in a caudal to cranial direction with right-angle Mixter or Lahey forceps (Figure 42-22A). Care must be taken during dissection near the pulmonary artery, as this vessel is easily torn. When the ligament is successfully freed and isolated, two ligatures of 0 surgical silk are placed as close to the aorta and pulmonary artery respectively as possible (Figure 42-22B). The LA is then transected between the ligatures. Traction then is placed on the ligatures, and the esophagus is dissected free of any residual fibrous bands between the aorta and pulmonary artery (Figure 42-22C). A 22 French Foley catheter is then introduced through the mouth into the esophagus and passed to the esophageal constriction. Inflation of the cuff at the constriction helps visualize any residual fibrous constricting bands and facilitates their dissection and removal. Extreme care is necessary during this dissection because the esophagus is thin and easily perforated. Passage of the inflated cuff back and forth at the stricture site will help further dilate the constriction. (Figure 42-22D).

With moderate esophageal dilation, passage of food improves once the constriction is relieved. Plication or resection of a dilated esophagus only reduces redundant tissue and does not restore normal esophageal peristalsis. If severe chronic dilation is present, plication of a redundant esophagus with Lembert-type gathering sutures of 4-0 nylon or polypropylene can be attempted but is of questionable benefit. If plication is attempted, care must be taken to not penetrate the mucosa of the esophagus, as leakage around the sutures may occur and postoperative pleuritis or pyothorax may result. Hand-sewn resection of the dilated cranial esophagus is not recommended because of its thin wall and inherent tendency for leakage. For intractable regurgitation, resection of a dilated esophagus with TA55 autostapling equipment has been attempted but with only fair results. Plication or resection of a dilated esophagus only reduces redundant tissue and does not restore normal esophageal peristalsis.

After ligation and division of the LA is completed and the esophagus freed of constricting fibrous bands, a thoracostomy tube is placed and routine thoracic closure is performed. Postoperative antibiotics are continued if aspiration pneumonia is present. I use combinations of bupivacaine rib blocks, intramuscular opioids or continuous rate infusion of opioids or ketamine and injectable NSAIDS to manage postsurgical pain (See Chapter 9). Blood glucose levels are closely monitored during recovery from anesthesia particularly in small breeds of dogs.
 

Postoperative Feeding

Elevated feedings of small quantities of semisolid food are provided three or more times daily starting the day following surgery. Feeding of liquid diets should be avoided. The semisolid solid food usually does not pocket in the cervical esophagus and will not reflux into the trachea as easily as liquid diets if regurgitated. The animal is held upright by the owner or is fed from a stool or platform that requires the forelimbs to be elevated off the ground. Holding the patient upright while rocking it slowly back and forth may also facilitate passage of the food. Gradually, over several days, the food is increased in consistency until feeding of solid food is attempted. If regurgitation subsides, elevated feedings are continued for at least eight weeks before horizontal feedings are attempted. Some animals will resume regurgitation with horizontal feedings, requiring that vertical feedings be adopted as a lifelong procedure.

Prognosis

Morbidity and mortality associated with persistent right aortic arch that is seen in the perioperative period is usually due to aspiration pneumonia. Animals surviving the postoperative period and leaving the hospital regurgitate less frequently following surgery and demonstrate good body weight gain with time. Those that survive at least six months do particularly well. In one study of 25 dogs, 70 percent of animals followed for two to four weeks had no regurgitation; but in those animals followed for 6 months 92 percent did not regurgitate after eating. Less than 10 percent of the cases failed to respond to surgery and were euthanized. Conversely, it is thought but not proven that dogs or cats with post-cardiac esophageal dilation tend to continue regurgitation after surgery and respond less favorably to surgery.

A contrast esophagram performed 24 to 72 hours after surgery will serve as a baseline and demonstrate adequate release of the constriction. Contrast studies performed three to four months postoperatively are recommended to evaluate the esophagus for decreasing dilation. Esophageal dilation usually decreases with time, but is not reversible. Likewise, esophageal peristalsis also usually improves with time, but never returns to normal. The exact cause of postcardial esophageal dilation associated with PRAA is unknown, but interference with the vagus nerves at the esophageal constriction may play a role in decreasing esophageal peristalsis.

At the present time, early surgical ligation and division of the ligamentum arteriosum offers patients with PRAA a reasonable long-term prognosis. Reversal of clinical signs is less likely as the age of the animal increases. In addition to surgical management, prolonged upright feeding may be required. Although some degree of esophageal dilation remains after surgery the frequency and severity of regurgitation is usually reduced over time.

Thorascopic Correction of Praa

Since the last edition of this text veterinary surgeons with an interest in minimally invasive surgery have thorascopically ligated and divided the LA in dogs with persistent right aortic arch. The reported advantages of this technique are 1) improved visualization of the LA during surgery 2) less postoperative patient discomfort, and 3) minimal intraoperative hypothermia. Disadvantages include equipment costs, technical expertise required, and the need for selective and specialized anesthesia techniques. This appears to be a promising method for surgical management of PRAA and larger clinical studies should be forthcoming.

Suggested Readings

Buchanon JW: Tracheal signs and associated vascular anomalies in dogs with persistent right aortic arch. J Vet Intern Med 18:510, 2004.

Ellison GW: Vascular ring anomalies in the dog and cat. Comp Cont Ed 2:693, 1980.

Gunby JM, Hardie RJ, Bjorling DE: Investigation of the potential heritability of persistent right aortic arch in Greyhounds. J Am Vet Med Assoc 224:1120, 2004.

Helphrey ML: Vascular ring anomalies in the dog. Vet Clin N Am 9:207, 1979.

Holt D, Heldman E, Mikel K, et al: Esophageal obstruction caused by a left aortic arch and an anomalous right patent ductus arteriosus in two German shepherd littermates. Vet Surg 29:264, 2000.

Macphail CM, Monnet E, Twedt DC. Thorascopic corrections of persistant right aortic arch in a dog. J Am Anim Hosp Assoc 37:577, 2001.

Muldoon MM, Birchard SJ, Ellison GW: Long-term results of surgical correction of persistent right aortic arch in dogs: 25 cases. J Am Vet Med Assoc 210:1761, 1997.

Shires PK: Persistent right aortic arch in dogs: a long-term follow-up after surgical correction. J Am Anim Hosp Assoc 17:773, 1981.

Van Gundy T: Vascular ring anomalies. Comp Cont Educ Pract Vet 11:36, 1989.

Vianna ML, Krahwinkel DJ: Double aortic arch in a dog. J Am Vet Med Assoc 225:1222, 2004.

Wheaton LG: Persistent right aortic arch associated with other vascular anomalies in two cats. J Am Vet Med Assoc 184:848, 1984.
 

Diseases affecting the canine pericardium can result in either pericardial effusion or pericardial constriction, both of which can be managed surgically. Antemortem diagnosis of feline pericardial disease is rare.

Anatomy and Physiology of the Pericardium

The pericardium is a fibrous sac composed of an outer fibrous layer and an inner serous layer. The serous layer is divided into the visceral pericardium (epicardium), which adheres firmly to the surface of the heart, and the parietal pericardium, which lines the interior surface of the fibrous pericardium. The pericardial cavity lies between the serous layers and normally contains a small quantity of clear fluid.

The fibrous pericardium forms a tough, thick sac that blends with the adventitia of the great vessels at the base of the heart. It is attached to the diaphragm in the xiphoid region by the sternopericardiac ligament ventrally and by pleural reflections caudally. The phrenic nerves course across the dorsal third of the pericardium on the left and right sides.

The functions of the pericardium are not completely understood, and its physiologic significance has been debated in literature. The following functions have been attributed to the pericardium: prevention of overdilation of the heart, protection of the heart from infection and from formation of adhesions to surrounding tissues, maintenance of the heart in a relatively fixed position within the chest, regulation of the interrelation between the stroke volumes of the two ventricles, and prevention of right ventricular regurgitation when ventricular diastolic pressure is increased.

Suggestions that the pericardium serves no vital functions have arisen from observations that humans and animals can live normally after pericardiectomy. Studies in animals suggest that the heart probably undergoes some minor dilation after pericardiectomy, although significant impairment of cardiac function has not been demonstrated.

Pericardial Effusion

Pathophysiology

Pericardial effusion is an abnormal accumulation of fluid within the pericardial sac. Severe pericardial effusion may result in cardiac tamponade, a potentially life-threatening compression of the heart in which intrapericardial pressure rises sufficiently to affect cardiac function. Cardiac tamponade occurs when enough pericardial fluid accumulates to exhaust the limits of pericardial elasticity. Once the pericardium can no longer stretch to accommodate additional fluid, the addition of small amounts of fluid begins to produce rapid increases in intrapericardial pressure.

Cardiac tamponade primarily affects cardiac function during diastole and has little effect on systolic function. Because intra-pericardial pressure is transmitted directly through the ventricular wall, diastolic filling pressures rise until the diastolic pressures within each ventricle are equal to one another and to intra-pericardial pressure. The right atrium and ventricle are more thin walled than the left, and are more susceptible to compression, so that signs of cardiac tamponade mimic signs of right heart failure. As predicted by the Frank-Starling law, decreased diastolic filling results in decreased myocardial stretching, force of contraction, and cardiac output.

The cardiovascular system attempts to compensate for falling cardiac output through peripheral arterial and venous vasoconstriction and increased heart rate.

However, these compensatory mechanisms may themselves stress the heart. The catecholamines responsible for vasoconstriction increase myocardial oxygen consumption, and tachycardia decreases coronary blood flow by decreasing the proportion of the cardiac cycle spent in diastole, when coronary flow occurs. Coronary flow is further compromised by low cardiac output and pressure on the coronary vessels produced by the pericardial fluid. These factors may produce myocardial ischemia and can eventually lead to cardiac decompensation.

Causes

The most common causes of pericardial effusion in the dog are neoplasia and idiopathic hemorrhagic pericardial effusion. Most neoplastic effusions are hemorrhagic and result from acute or chronic hemorrhage from the tumor surface. Intra-pericardial cysts, pericardial effusions caused by bacterial or fungal infections, and other less common causes of pericardial effusion have also been reported.

The most common neoplastic cause of pericardial effusion is right atrial hemangiosarcoma. This tumor generally arises from the right auricular appendage, although the right atrial wall may be involved. German Shepard dogs and other large breeds are predisposed. The tumor is highly metastatic and almost always spreads to other organs such as the liver or lungs before it is discovered in the heart.

Chemodectomas arise from the aortic bodies located around the aorta at the heart base. The aortic bodies are composed of chemoreceptor tissue sensitive to blood pH, carbon dioxide content, and oxygen tension, and they are involved in the regulation of ventilation. Chemodectomas vary in their location around the aorta and in their degree of local invasiveness. The metastatic rate of this tumor is unknown. Although chemodectomas may occur in any breed, brachycephalic breeds may be predisposed, suggesting that chronic hypoxia may be an underlying cause. Anecdotally, the apparently high incidence of chemodectoma among dogs in Colorado further implicates chronic hypoxia in the pathogenesis of the tumor.

Other neoplastic causes of pericardial effusion are much less common. Malignant diseases that may metastasize to the heart or pericardium include hemangiosarcoma, lymphosarcoma, melanoma, and mammary adenocarcinoma. Mesothelioma can occasionally cause pericardial effusion, either alone or in combination with pleural or peritoneal effusion.

Idiopathic hemorrhagic pericardial effusion, a poorly understood syndrome, is also a common cause of pericardial effusion in the dog. It occurs predominately in large and giant breeds, has a distinct male predilection, and affects dogs of all ages. Patients have signs of acute or chronic cardiac tamponade, which may respond to either conservative treatment or surgical management. Although the cause of this syndrome is unknown, a similar syndrome in humans is suspected to be either viral or immune-mediated. Histologically, blood vessels of the canine parietal (and possibly visceral) pericardium appear to be the targets of the disease process and are the source of pericardial hemorrhage.

Intra-pericardial cysts are large, benign mass lesions that occasionally cause effusion and cardiac tamponade in young dogs. The cysts arise from the apex of the pericardial sac and resemble acquired cystic hematomas grossly and histologically. Although the cause of intra-pericardial cysts is unknown, it is possible that they develop from herniated omental or falciform fat in dogs born with small peritoneopericardial diaphragmatic hernias. Intra-pericardial cysts usually are diagnosed in dogs between 6 months and 3 years of age, although they occasionally are identified later in life.

Infectious pericardial effusion is reported to be caused most commonly by migrating grass awns. Many different species of bacteria have been cultured from the pericardial fluid of effected dogs. Pericardial effusion caused by infection with Coccidioides immitus has been reported in geographic areas, such as the south-western United States, where the fungal agent is endemic. Young, large breed dogs are usually affected, and dogs may or may not have chronic histories of coccidioidomycosis. The pericardial disease is usually both effusive and constrictive.

Other potential causes of pericardial effusion include congenital peritoneopericardial hernias, left atrial rupture secondary to mitral insufficiency, blunt or penetrating trauma, congestive heart failure, and uremia. Pericardial effusion resulting from the latter two conditions is usually inconsequential and tends to be a postmortem finding only.

History and Clinical Signs

Dogs with cardiac tamponade are usually presented with acute or chronic histories of nonspecific signs suggestive of right-sided heart failure. These include lethargy, dyspnea, cough, abdominal distension, anorexia, weight loss, and exercise intolerance. Acute collapse with no prior signs is seen occasionally. In general, the history is not helpful in differentiating neoplastic from idiopathic hemorrhagic pericardial effusion; signs may be acute or chronic in either condition.

Several physical findings may suggest cardiac tamponade as the cause of right-sided heart failure. These include muffled heart sounds, pronounced jugular pulses and jugular distension, and weak arterial pulses. Hepatomegaly, ascites and peripheral edema may also be present. Pulsus paradoxus is an exaggerated pattern of change in arterial pressure with respiration, characterized by a weak pulse during inspiration and a stronger pulse during expiration. The sign is often present but overlooked in dogs with pericardial effusion, and may be best appreciated in dogs breathing slowly while lying in lateral recumbency.

Diagnostic Evaluation

The diagnostic evaluation of dogs with signs compatible with cardiac tamponade should be aimed at demonstrating pericardial effusion and determining its underlying cause. Pericardial effusion can be demonstrated in most cases using a combination of electrocardiography, thoracic radiography, and M-mode or 2-dimensional echocardiography. Diminished QRS voltages and electrical alternans are seen in a significant proportion of electrocardiograms. Diminished QRS amplitudes are likely caused by decreased conduction of electrical impulses through fluid media, although decreased ventricular filling may be involved. Pleural effusion as well as pericardial effusion can produce decreased QRS voltages. Electrical alternans is a beat-to-beat variation in QRS amplitude produced by a swinging motion of the heart within the pericardial sac.

Thoracic radiography demonstrates pericardial effusion if the volume of effusion is substantial. Generalized heart enlargement is seen, and the heart may have a characteristic globoid appearance, which is best demonstrated on dorsoventral views. Pleural effusion, ascites, hepatomegaly and distension of the caudal vena cava may also be present.

M-mode echocardiography is the most sensitive test available for detecting pericardial effusion and differentiating pericardial fluid from cardiomegaly and peritoneopericardial hernias. Effusions are demonstrated in approximately 90% of cases, and volumes as small as 75 ml can be detected.

Because pericardial effusions caused by neoplasia have a distinctly poorer prognosis than idiopathic hemorrhagic and other effusions, the detection of cardiac masses, particularly right atrial hemangiosarcomas, is an important part of the diagnostic evaluation. Cytologic examination of fluid obtained by pericardiocentesis (discussed later) generally does not differentiate neoplastic from idiopathic hemorrhagic effusions. In both cases, the fluid is hemorrhagic and non-clotting, and it contains predominately red blood cells, macrophages, and reactive mesothelial cells. Demonstration of neoplastic cells is extremely rare, and care must be exercised in cytologic interpretation because reactive mesothelial cells can have neoplastic characteristics. Exudative effusions are usually caused by bacterial or fungal infection; the causative organism may be visible on cytologic examination or identified by bacterial or fungal culture.

Two-dimensional echocardiography is the most sensitive test available for detecting cardiac masses and for determining preoperatively whether a mass is likely to be surgically resectable. In the hands of experienced cardiologists, echocardiography is highly sensitive and highly specific for both right atrial masses and heart base masses. Examination from both sides of the thorax allows accurate localization of cardiac masses. Because right atrial hemangiosarcomas are often small (1 to 2 cm in diameter), they occasionally escape detection. Involvement of the right atrial wall, which increases the difficulty of surgical excision, often can be detected echocardiographically. Chemodectomas often can be visualized in association with the ascending aorta. Small, discrete chemodectomas confined to the aortic area may prove resectable, whereas larger, more invasive masses are less likely to be resectable. Chemodectomas may be situated on either the right or left side of the aorta, and ultrasonography can assist in the selection of a surgical approach. Mesotheliomas have a diffuse growth pattern and usually are not detected with ultrasonography. Intrapericardial cysts are large lesions that are detected easily by echocardiography.

Routine laboratory tests may occasionally be useful in determining the cause of pericardial effusion. A complete blood count may show neutrophilia with a left shift in dogs with infectious effusions. Increased numbers of nucleated red blood cells or schistocytes are suggestive of right atrial or splenic hemangiosarcoma. Serum fungal titers are usually elevated in dogs with pericardial effusion caused by Coccidioides immitus infection. Marked elevations in serum levels of certain cardiac troponins, which are markers of myocardial ischemia and necrosis, may suggest that right atrial hemangiosarcoma rather than idiopathic effusion is present. Cardiac troponin assays are not routinely available to veterinarians at the current time.

Unfortunately, on rare occasions, it may be difficult to make a definitive diagnosis, and particularly to differentiate right atrial hemangiosarcoma from idiopathic hemorrhagic pericardial effusion, without exploratory thoracotomy.

Treatment

Pericardiocentesis

Indications: Pericardiocentesis is preformed for both diagnostic and therapeutic purposes. The removal of small volumes of pericardial fluid in patients with cardiac tamponade can result in rapid and dramatic decreases in intra-pericardial pressure and is often a lifesaving measure. Approximately 50% of dogs with idiopathic hemorrhagic pericardial effusion can be successfully treated by periodic pericardiocentesis, performed when necessary to relieve cardiac tamponade. Multiple pericardiocenteses, days to weeks apart, may be necessary to produce a resolution, and recurrence of pericardial effusion is reported to occur as late as 4 years after pericardiocentesis. Owners of dogs treated by pericardiocentesis alone should be made aware of the potential for sudden recurrence of cardiac tampondade. The advantages of pericardiectomy over pericardiocentesis are discussed below.

Technique: Pericardiocentesis is performed at the right third, fourth, or fifth intercostal space near the costochondral junction. Excellent descriptions of this procedure are available elsewhere.

Pericardiectomy

Indications: Pericardiectomy is used most often to treat idiopathic pericardial effusions and effusions caused by neoplasia, intrapericardial cysts, infection, and penetrating foreign bodies. Effusions caused by congestive heart failure or uremia usually are treated medically. The specific goals of pericardiectomy depend on the primary disease being treated. Pericardiectomy may be performed either by open thoracotomy or with thoracoscopy.

In pericardial effusion caused by neoplasia, the pericardium is often excised to allow surgical exploration of the heart. Pericardiectomy alone, without excision of the neoplastic mass, traditionally has been thought to be of little or no value. However, a study of dogs undergoing thorascopic partial pericardiectomy without mass excision showed that all dogs with neoplastic effusions experienced palliation of signs of cardiac tamponade. Median survival of treated dogs was only 1 month; however, some dogs survived beyond 1 year. In addition, 2 separate studies have shown that dogs with chemodectoma can have prolonged survival after open pericardiectomy alone: median survival times were greater than 2 years in both studies, and were significantly longer than survival times of dogs that did not undergo pericardiectomy. Although it seems reasonable to assume that pericardiectomy combined with excision of neoplastic masses should produce superior results to pericardiectomy alone, this has not yet been proven in controlled trials. Excision of right atrial hemangiosarcomas, chemodectomas, and intrapericardial cysts is discussed below.

Idiopathic hemorrhagic pericardial effusion can be treated successfully by creation of a pericardial window or by partial pericardiectomy below the level of phrenic nerves which allow any persistent effusion to be removed by the large absorptive area of the pleural space. Although the condition often is manageable by periodic pericardiocentesis, early pericardiectomy has some advantages. Treatment by pericardiocentesis alone risks a sudden recurrence of life-threatening cardiac tamponade. Pericardiectomy does not entail long term risks for the patient, and, unlike pericardiocentesis, eliminates most of the tissue responsible for the effusion. Some evidence suggests that idiopathic pericardial effusion may progress to pericardial constriction, although this appears to be uncommon; surgery is technically simpler, and is associated with a better prognosis, for pericardial effusion than for pericardial constriction. Finally, early surgical exploration may allow identification of small tumors that were not revealed by echocardiography, and may offer the best chance for their removal.

The indications for open versus thorascopic pericardiectomy are not firmly established. Advantages of thoracotomy include its wide availability, the ability to more thoroughly explore the thorax, and its potential to permit resection of neoplastic mass lesions. The major advantages of thorascopic pericardiectomy are reduced postoperative pain and morbidity, and a more rapid recovery time. In thorascopic pericaridectomy, a small pericardial window is usually created, whereas with thoracotomy, partial pericardiectomy below the level of the phrenic nerves may be performed. Traditionally, creation of a pericardial window has been thought to be associated with a risk that residual pericardium would adhere to the surface of the heart, resulting in recurrent pericardial effusion. Early experience with thorascopic pericardiectomy and with percutaneous balloon pericardiotomy suggests that the risk of this complication is quite low. Minimally invasive approaches are most often indicated when there is a high index of suspicion that the effusion is idiopathic, eg. in dogs with no echocardiographic evidence of a cardiac mass who have developed recurrent effusion months after pericardiocentesis. Small masses involving the tip of the right auricular appendage may be removed with minimally invasive techniques. Thoracotomy should be chosen for resection of auricular masses not deemed amenable to minimally invasive surgery, and for heart base masses. In circumstances other than these, the choice between thorascopic and open pericardiectomy is a matter of the surgeon’s and owner’s preferences. The technique for thorascopic pericardiectomy is discussed elsewhere.

Surgical approach: When the cause of pericardial effusion is unknown, either a right fifth intercostal thoracotomy or a median sternotomy may be performed. Excision of right atrial tumors may be accomplished with similar ease through either approach. For the majority of dogs with idiopathic effusions, a right sided approach, followed by creation of a pericardial window and inspection of the right atrial appendage to rule out hemangiosarcoma, is a reasonable approach. When partial pericardiectomy below the level of the phrenic nerves is deemed necessary, either a median sternotomy or an intercostal approach may be used. Subtotal pericardiectomy is somewhat easier to perform through a median sternotomy, because an intercostal approach does not permit good visualization of the opposite side of the thorax. In addition, if an intercostal approach is used, the heart must be elevated as the far side of the pericardial sac is excised, a maneuver that temporarily impairs venous return. Chemodectomas are approached through either a right or left fourth intercostal thoracotomy, depending on the location of the tumor as determined by ultrasonography. Intrapericardial cysts are best approached through a median sternotomy, which facilitates subtotal pericardiectomy and allows inspection of the diaphragm for a peritoneopericardial hernia.

Technique: Once the thoracotomy is completed, the phrenic and vagus nerves are identified. The phrenic nerve may be isolated and gently retracted with a Penrose drain, although retraction of the nerve usually is unnecessary. The vagus nerve is located more dorsally and is unlikely to be damaged during pericardiectomy. To create a pericardial window, a controlled stab incision is made in the pericardium ventral to the phrenic nerve with a scalpel blade, and pericardial fluid is removed by suction. The incision is then extended with Metzenbaum scissors or electrocautery to create a window several centimeters in diameter ventral to the phrenic nerve. The right atrial appendage is inspected to rule out the presence of a right atrial mass by carefully retracting the cranial and dorsal edges of the window. If partial pericardiectomy below the level of the phrenic nerves is to be performed, the initial incision is continued cranially and caudally until it is completed circumferentially (Figures 42-23 and 4-24). If an intercostal approach has been used, completion of the pericardiectomy on the left side requires elevation of the heart. An assistant should cradle the patient’s heart in one hand and gently rotate the apex of the heart laterally and dorsally to permit incision of the pericardium below the level of the left phrenic nerve. Because elevation of the heart impairs venous return, this maneuver should be performed as quickly as possible. Diseased pericardia are often thickened and extremely vascular, and care must be taken to limit hemorrhage with electrocautery. Once the sternopericardiac ligament is divided, either with electrocautery or between ligatures, the pericardial sac can be removed and submitted for histopathology. A thoracostomy tube is placed before closure, and postoperative management generally is uncomplicated. The thoracostomy tube may be removed after 12 hours if it is unproductive.

For dogs with bacterial pericardial effusion, long-term antibiotics, selected on the basis of culture and sensitively testing, should be administered postoperatively. The prognosis for these dogs is generally excellent.
 

Excision of Intrapericardial Cysts

Intrapericardial cysts are usually located at the apex of the pericardial sac and can be excised readily by routine subtotal pericardiectomy (Figure 42-25). If the patient has an associated peritoneopericardial hernia, the edges of the hernia are incised, and the defect is closed with a row of simple continuous sutures.
 

Excision of Right Atrial Hemangiosarcoma

Indications: Excision of right atrial hemangiosarcomas should be considered palliative, because the tumor almost invariably metastasizes prior to detection. The goal of surgery is to prevent a recurrence of cardiac tamponade. Many hemangiosarcomas are confined to the right auricular appendage, and are therefore amenable to surgical excision as described below. Inflow occlusion or cardiopulmonary bypass are required to excise tumors with significant right atrial wall involvement.

Technique: After median sternotomy or right fifth intercostal thoracotomy, an incision is made in the pericardial sac approximately 1 cm below and parallel to the phrenic nerve. The pericardial incision is extended cranially and caudally as far as necessary with Metzenbaum scissors or electrocautery to fully expose the auricular appendage. Exposure may be improved by using stay sutures or Babcock forceps to retract the incised edges of the pericardial sac.

Either conventional suturing or surgical stapling equipment may be used to remove right atrial masses. If conventional suturing is elected, a tangential vascular clamp is placed across the base of the auricular appendage. The appendage is transected immediately distal to the clamp, leaving a cuff of auricular tissue. The margin of the excised tumor should be inspected to ensure that excision was complete; if possible, at least 1 cm of normal auricular tissue should be removed with the tumor. The auricle is then oversewn with two rows of simple continuous sutures, with rows oriented perpendicularly to each other (Figures 42-26 and 42-27). 3-0 or 4-0 polypropylene suture on a tapered needle may be used.
 

Surgical stapling is faster and less technically demanding than hand suturing. A 55 mm thoracoabdominal stapler is used, with 3.5 mm staples (Kendall-Tyco Corp, Norwalk, CT). The stapler should be positioned to provide a 1 cm resection margin (Figure 42-28). If there is room, a tangential vascular or other noncrushing clamp should be placed across the base of the auricle before releasing the stapler; the clamp should be slowly released as the staple line is inspected for bleeding. If necessary, the staple line may be oversewn with a layer of simple continuous suture. After tumor excision, a partial pericardiectomy should be performed.
 

Prognosis: Because right atrial hemangiosarcoma is a highly metastatic tumor, surgical excision is purely palliative. Mean survival time after surgery is reported to be approximately 4 months. Euthanasia is performed in most affected dogs because of distant metastases, usually to the liver or lungs, within a few months of surgery. Unfortunately, no compelling evidence yet exists to suggest that survival times in dogs with either splenic or right atrial hemangiosarcoma can be significantly prolonged with adjuvant chemotherapy, and prospective controlled clinical trials are needed.

Excision of Chemodectomas

Because of the difficult location of chemodectomas, their highly vascular nature, and the excellent survival times reported following pericardiectomy alone, pericardiectomy without tumor excision should be considered a viable alternative to tumor excision for this disease.

Technique: Control of hemorrhage is the major difficulty encountered during attempts at tumor excision. Because of their location, chemodectomas must be marginally excised at the gross limits of the tumor; wide margins are impossible to provide. Excision is best accomplished by slow, meticulous, sharp dissection with the help of electrocoagulation. Care must be taken to avoid perforating the aorta or pulmonary artery; cottontipped swabs are useful for slowly dissecting the tumor away from these structures. Before closure, the tumor bed should be closely inspected, and residual points of hemorrhage should be controlled with precise electrocoagulation.

Prognosis: Chemodectomas seem to be slow-growing tumors, and limited experience suggests that dogs undergoing excision of small chemodectomas can have prolonged survival postoperatively. Whether surgical excision improves survival beyond that associated with pericardiectomy alone is unknown. Studies investigating the efficacy of chemotherapy or radiation therapy have not yet been reported.

Pericardical Constriction

Pathophysiology and Causes

As in cardiac tamponade resulting from pericardial effusion, pericardial constriction restricts diastolic volume. Diastolic filling is limited by the fibrotic pericardium, which acts as a noncompliant shell around the heart.

Pericardial constriction in dogs usually is idiopathic. Like idiopathic hemorrhagic pericardial effusion, the condition occurs predominately in medium-size and large breeds, although no evidence of male sex predilection exists. Some evidence based on isolated case reports indicates that idiopathic hemorrhagic pericardial effusion can progress to pericardial constriction, although this seems uncommon. Whether idiopathic pericardial constriction and idiopathic hemorrhagic effusion are different manifestations of the same syndrome, or are separate disease entities, is unknown.

Dogs with pericardial disease caused by Coccidioides immitus infection most commonly have a combination of effusive and constrictive pericarditis. This condition should be considered in any dog with pericardial disease in geographic regions where the fungus is endemic.

History, Clinical Signs, and Diagnosis

Dogs with constrictive pericardial disease are usually presented with signs of chronic right-sided heart failure. Abdominal distension, dyspnea, weakness or syncope, exercise intolerance, and weight loss are common signs. Typical physical examination findings are ascites, jugular distension, and weak arterial pulses. Poorly auscultable heart sounds are also common. A “pericardial knock,” produced as blood is rapidly compressed against the rigid ventricular wall, may be heard on auscultation. Approximately half of dogs with pericardial effusion-constriction caused by C immitus infection have chronic histories of coccidioidomycosis, producing signs such as lameness, dermatopathy, and uveitis.

Definitive diagnosis of pericardial constriction may require surgical exploration, although a presumptive diagnosis can often be made preoperatively based on a combination of physical, electrocardiographic, imaging, and hemodynamic findings. One or more abnormalities may be present on electrocardiographic examination. Decreased QRS amplitudes and increased P-wave duration are the most common findings. Radiographs may reveal free pleural fluid and mild to moderate cardiomegaly. Echocardiographic findings that support the diagnosis include decreased end-diastolic diameter, decreased fractional shortening, flattening of left ventricular free-wall motion during late diastole, and rapid premature diastolic closure of the mitral valve. Dogs with pericardial effusion-constriction caused by Coccidioides immitus infection usually have elevated serum titers for antibodies against the organism. In general, a diagnosis of pericardial constriction should be considered in dogs with signs of right-sided heart failure that cannot be explained by pericardial effusion, congenital or acquired heart disease, or pulmonary hypertension. Surgical exploration should be performed if the condition is suspected.

Subtotal Pericardiectomy for Pericardial Constriction

Technique: Because significant epicardial fibrosis usually is not present in dogs with idiopathic pericardial constriction, most dogs can be treated successfully by subtotal pericardiectomy, as previously described. Median sternotomy is the preferred approach because it allows visualization and division of any epicardial adhesions that may be present. In dogs with significant epicardial fibrosis, epicardial decortication may be necessary. This is a difficult procedure that may require partial removal of myocardial tissue. Caution is necessary to avoid inadvertent damage to coronary vessels. Epicardial decortication is associated with significant perioperative morbidity and mortality.

In dogs with effusion-constriction caused by C immitus infection, extensive mature adhesions to the epicardial surface of the heart are likely to be present, and pericardiectomy may be significantly complicated by hemorrhage. Adhesions may be disrupted manually and by careful instrument dissection. Strips of pericardium overlying the coronary vessels may be left in place if there are firm adhesions to the vessels. If fibrosed, the epicardium may be removed from areas distant from the coronary vessels, using scissors or a periosteal elevator to carefully lift the epicardium from the myocardium. The perioperative mortality rate in a series of dogs with C immitus pericarditis undergoing partial pericardiectomy was 23.5%, and among dogs that were discharged from the hospital, the 2 year survival rate was 82%.

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