Congenital Cardiac Anomalies

Recognition of congenital heart defects in foals is important both in determining the cause of clinical signs such as fatigue, tachypnea, weakness or cyanosis, and in evaluating the potential future utility of a young horse.

I. Congenital Cardiac Defects

Congenital cardiac defects reported in the horse include [1-4]:

1. Simple defects, including ventricular septal defect [5], patent ductus arteriosus [6,7], patent foramen ovale or atrial septal defect, and pulmonic stenosis.
2. Complex defects [8], including tetralogy of Fallot, tricuspid atresia, truncus arteriosus, pentalogy of Fallot (tetralogy + PDA), double outlet right ventricle, Eisenmenger VSD or PDA, single ventricle and others.
3. Vascular anomalies, including persistent right aortic arch and anomalous coronary arteries.

II. Diagnostic Studies

Accurate diagnosis almost always requires the combination of findings from more than one examination.

1. Physical examination - Jugular pulse, arterial pulse, percussion, timing and location of heart murmur(s).
2. Electrocardiograms - Evidence of atrial or ventricular enlargement.
3. Thoracic radiographs - Degree of cardiomegaly, left atrial enlargement, pulmonary vascularity, and pulmonary/pleural signs of left or right heart failure.
4. Echocardiography - Atrial/ventricular dilation, ventricular hypertrophy, atrial and ventricular septa, heart valves, position of great vessels.
5. Cardiac catheterization/angiography - Systolic and diastolic pressures, blood oximetry, and abnormal blood flow patterns including valvular regurgitation and shunts.

III. Clinical Features of Common Congenital Cardiac Defects

1. Ventricular septal defect (VSD) [5].

1. Physical exam - A loud, harsh holosystolic murmur is usually heard best on the right cranial precordium and equally well or softer over the left heart base.
2. ECG - Often normal with small defects. Larger defects may cause increased QRS voltages.
3. Radiographs - Depending on defect size, heart size may be normal to moderately increased. Left atrium and pulmonary vasculature may be enlarged with large left-to-right shunts.
4. Echo - May be near normal with small defects. Larger defects produce increased diastolic dimension/volume of the left atrium and both left and right ventricles, and increased LV SF% due to volume overload. Large defect may be recognized by M-mode, but 2D is superior, often showing aortic-septal discontinuity even with small defects. LV saline injection can prove L-R shunt.
5. Cath/angiography - Pressures may be normal until heart failure increases atrial and ventricular diastolic pressures. Increased oxygen saturation/content occurs between the RA/RV apex and RV outflow/pulmonary artery. Confirmation of anatomy usually requires LV contrast injection to demonstrate L-R shunt.

2. Patent ductus arteriosus (PDA)

1. Systemic NSAIDs may be helpful but side effects in foals may preclude frequent use.

1. Continuous murmur localized or loudest over the left heart base. Diastolic portion of the murmur is often heard poorly elsewhere.
2. With large L-R shunt the arterial pulse is hyperkinetic (bounding).
3. If pulmonary hypertension develops, the murmur shortens into a transystolic or purely systolic murmur with a normal arterial pulse.
4. The continuous or transystolic murmur of a slightly patent ductus arteriosus may be present in normal foals for at least 3-4 days, and occasionally up to 7-8 days of age [6,7].

2. ECG - Normal unless shunt is large, causing increasing QRS amplitudes due to LV enlargement.
3. Radiographs - Mild to moderate cardiomegaly, enlarged LA and increased pulmonary vascularity
4. Echo - Increased LA and LV diastolic dimension/volume and hyperdynamic septal and LV wall systolic motion (increased SF%) commensurate with size of the L-R shunt. If pulmonary hypertension develops, the RV becomes dilated and the RV wall thickens. The ductus may be visible by 2D imaging from the left caudal transducer location in some foals.
5. Cath/angiography - Wide arterial pulse pressure, slightly to moderately increased LV diastolic pressure, increased PA oxygen saturation/content, LR shunt visible following LV or aortic root injection. With pulmonary hypertension the RV and PA systolic pressures are increased, sometime equivalent to arterial pressure, R-L (PA -->Aorta) shunt following IV, RV or PA injection.

3. Tetralogy of Fallot

1. Physical exam - Systolic ejection murmur of pulmonic stenosis is heard at the left heart base. Normal arterial pulse, normal to slight jugular venous pulse. Symmetrical cyanosis may be present at rest or following mild exercise.
2. ECG - RV hypertrophy may be indicated by negative QRS complexes in leads I, II, and aVF.
3. Radiographs - Mild to moderate cardiomegaly, rounding of the silhouette, and decreased pulmonary vascularity may be seen.
4. Echo - Thickened RV wall, septal echo dropout in the area of the VSD, rightward displacement of the aortic root, and abnormal pulmonary outflow region (2D). Injection of saline IV demonstrates R-L flow from RV to LV or aorta.
5. Cath/angiography - Systolic pressure gradient across pulmonary outflow region (pulmonic stenosis), increased RV systolic pressure equivalent to LV or aortic pressure. Decreased oxygen saturation/content between LV apex and ascending aorta. RV contrast injection shows pulmonic stenosis and R-L shunt from RV to LV/aorta.

4. Tricuspid atresia

1. Physical exam - Symmetrical cyanosis at rest or following very mild exertion (due to obligatory presence of atrial septal defect), holosystolic murmur loudest at the left heart base, normal to weak arterial pulse, increased jugular venous pulse.
2. ECG - Increased P wave amplitude and duration are common, increased QRS amplitudes due to LV enlargement may occur.
3. Radiographs - May be deceptively normal or show mild cardiomegaly and decreased pulmonary vascularity.
4. Atretic tricuspid valve, atrial and/or ventricular septal defects are best seen by 2D echo examination. The R-L interatrial shunt can be easily demonstrated by IV saline injection.
5. Cath/angiography - Decreased oxygen saturation/content in the left heart, equal LV and RV systolic and diastolic pressures, RA contrast injection shows small RV and R-L shunt.

5. Other types of complex malformations may occur.

1. Accurate diagnosis requires echocardiography and/or cardiac catheterization/angiography
2. Total anomalous pulmonary venous connection in a foal [9] seen in an Arabian-Morgan cross foal at 8 days with weakness and murmur.
3. Parachute left atrioventricular valve causing stenosis and regurgitation in a Thoroughbred foal [10].
4. Giant right atrial diverticulum in a foal [11].