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Examining the horse's heart
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Summary
The talk will focus on the relevant aspects of the examination of horses with cardiovascular disease. The most common abnormality detected in these examinations (murmurs) will be briefly discussed.
History
Presenting complaints that are common in the horse with cardiovascular disease are: exercise intolerance/poor performance/delayed recovery, murmur or arrhythmia on examination, syncope, collapse or “seizure”, weakness, failure to grow and thrive, increased respiratory rate or effort, cough, exercise induced pulmonary hemorrhage
When collecting the history of a horse with suspected cardiovascular disease useful questions to ask the owner are: length of ownership, previous illnesses, vaccination, deworming history, use or intended use, resting heart rate (horses in heart failure have a high resting rate and horses with a normal heart rate are not in heart failure), last successful performance or last time a normal cardiovascular examination was recorded.
Physical Examination
The general examination is often the key to the diagnosis. Aspects of the physical examination that are particularly relevant to the diagnosis of cardiovascular disease are:
Mucous membranes- color, capillary refill time are examined. This is particularly useful to recognize the presence of systemic disease as the cause or in association with the cardiovascular disease
Peripheral veins- Jugular veins are the main peripheral veins examined in horses. Lateral thoracic veins, saphenous or cephalic veins are other useful veins to assess Common abnormalities are:
- Decreased distension or refill that can be caused by abnormalities in venous return or thrombi distal to the site.
- Increased distension that can be caused by right sided heart failure, thrombi proximal to the distension, masses decreasing venous return (most commonly mediastinal masses or abscesses) or pericardial effusion.
- Jugular pulsations: With the neck in a neutral position normal horses can have visible jugular pulses in the lower third of the neck and with the head lower the pulses can extend more cranially. Arrhythmias (most commonly ventricular tachycardia) or severe tricuspid regurgitation are the most common causes of jugular pulsation in horses.
Peripheral pulses- The facial or transverse facial arteries are the easiest locations to feel peripheral pulses in adult horses. The medial artery is a useful site to feel peripheral pulses in recumbent neonates. The digital pulses are peripheral pulses like any other so it will be altered in the same way than other pulses with one exception. The exception is that peripheral pulses are also affected by disease of the foot like laminitis, foot abscesses etc. it is important to recognize that peripheral pulses represent the difference between systolic and diastolic pressure (the pulse pressure) and the absolute blood pressure The four most common abnormalities in peripheral pulses are.
- Weak pulses: Weak pulses are present in horses that are dehydrated or in horses with heart failure.
- Strong or bounding pulses: Pulses are strong or bounding in horses with moderate or severe aortic regurgitation. This is due to the lower diastolic pressure (‘the aortic valve does not hold the pressure in the arterial tree during diastole’) widening the pulse pressure.
- Pulses of variable intensity: Atrial fibrillation is the most common and almost only cause of pulses of variable intensity in horses
- Pulse deficits: pulse deficits are caused by arrhythmias like ventricular tachycardia or blocks.
Edema- limbs, abdomen, pectorals, prepuce/scrotum are the best areas to check for edema. Edema has 4 basis physio pathological origins: 1-decreased oncotic pressure, 2-increased capillary hydrostatic pressure, 3- increased capillary permeability (most common cause in the horse is vasculitis) and 4- obstruction of the lymphatic system. The most applicable mechanism to the discussion of heart disease is the increase in hydrostatic pressure in horses with right side of the heart failure. However, it would be a mistake to assign heart disease to a horse with edema as other causes are more likely in the absence of other signs of heart disease.
Respiratory system- Respiratory rate and effort is a critical monitoring tool in horses with left sided congestive heart failure. This increase occurs due to pulmonary edema in horses with left side of the heart disease. It is paramount to know that many horses with severe left side of the heart disease will not cough or show overt signs of disease that a horse owner without medical training is able to recognize. Horses with significant left side of the heart disease must have the respiratory rate and effort monitored and recorded. This will avoid that the first sign of progression of the disease that the owner notices is foam coming out of the nostrils or mouth. It is important to explain owners that pulmonary edema is a distressing condition and that dying of left side of the heart disease without treatment or euthanasia has to be avoided.
Cardiac auscultation- Two heart sounds can be heard in all horses and in many normal horses 3 or 4 heart sounds can be heart. These 4 heart sounds are:
- S1 – closure of atrioventricular valves (mitral and tricuspid)
- S2 – closure of aortic and pulmonic valves
- S3 – rapid ventricular filling
- S4 – atrial contraction
The 4 cardiac valves are associated with points of maximal intensity (PMI) during auscultation. It is important to recognize that an abnormality in auscultation on a PMI does not necessarily mean that this valve is the origin of the disease. See the discussion of murmurs for more details about this. The PMIs have been described as follows:
- Mitral area – 5th left ICS one-half distance between point of shoulder to sternum
- Aortic area – 4th left ICS just below point of shoulder
- Pulmonic area – 3rd or 4th left ICS slightly below level of aortic valve
- Tricuspid area – 3rd to 4th right ICS halfway between point of shoulder and sternum
Physiologically it makes sense to discuss right side of the heart vs. left side of the heart, anatomically horses have a cranial side of the heart (right side) and a caudal side of the heart (left side). Where we intuitively place the stethoscope to get a heart rate is the mitral valve area. This is the only location where the head of the stethoscope is visible. Moving the stethoscope one intercostal space cranial and dorsal will place it over the aortic valve area. In this location the head of the stethoscope is no longer visible. To place the stethoscope over the pulmonic valve area we have to make the horse place the left front limb forward and push under the triceps muscle. When the horse becomes bothered by you pushing you push harder and then you will place the head of the stethoscope over the pulmonic valve area. Placing the stethoscope over the pulmonic valve area of a large animal requires a conscious physical effort. Many veterinarians finish their careers without having ever placed a stethoscope over the pulmonic valve of a large animal. Palpation is useful to characterize murmurs as it allows to feel for a thrill. The palm of the left hand is placed over the different PMIs to palpate for a thrill. The 2 most common signs detected in horses with cardiac disease are murmurs and arrhythmias. We will discuss briefly how to evaluate murmurs during this session and will discuss arrhythmias in these next session. The proceedings give more comprehensive information than what we will be able to discuss during the meeting.
Murmurs
This part of the talk will discuss an easy to follow mental chart for field practitioners that have to interpret heart murmurs using their stethoscopes and their hands. A murmur is a sound associated with the cardiac cycle and generated by turbulent blood flow. There are three main broad reasons for turbulent flows: regurgitations, stenosis and shunts. Another relevant group of murmurs in horses are the ‘physiologic = ejection = flow murmurs’.
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