Dr Leanne Begg BVSc Dip VCS MS MACVSc Dip ACVIM

http://randwickequine.com.au/contact

Phar Lap's heart was reported to weigh approximately 6.2kg, whilst a normal horse's heart weighs approximately 4kg. The heart has the important function of pumping approximately 40L of blood through the average adult horse's body. It is no surprise then, that when the heart is not functioning properly, performance quickly suffers. The most common problems we see are electrical conduction problems in the heart, which produce irregularities in heartbeat, or murmurs  caused by heart valve abnormalities.

When a normal horse is auscultated (listened to with a stethoscope) Up to four heart sounds are heard. These consist of two sounds that are heard in a human heart associated with the valve closure, but also two further sounds associated with blood flow in the big chambers of a horse's heart. Normally the beat is very regular, but some irregularities are also normal in the horse. A regular dropping of a beat (most commonly caused by second degree heart block) is normal in the horse, and can be abolished by exciting the horse. An irregularly dropped beat is more concerning and does not usually disappear with exercise. The most common cause of this is atrial fibrillation, when the atria, or top chambers in the heart contract on their own, out of synchrony with the rest of the heart. As the atria are not contracting properly, the ventricles (or bigger bottom chambers) do not fill properly and this affects the ability of the heart to pump blood effectively. Horses that develop this in a race tend to perform very poorly and tail the field home. This is one condition which is checked for by veterinarians in a post-race examination. Some horses with atrial fibrillation convert back to normal on their own, while others require hospitalisation at a veterinary practice and repeated administration of quinidine. The cause of atrial fibrillation can sometimes be defined, such as a leaky heart valve producing an extra big top chamber which pre­disposes to the condition, but most commonly no underlying problem is found. The big normal size of a horse's or cow's heart results in this problem occurring more commonly in these species compared to humans or small animals. Horses with no underlying problem often go on to race successfully once the heart is back in normal rhythm. In a rare instance, the arrhythmia can involve the ventricles (or  big bottom chambers) as opposed to the atria (top chambers). This can lead to sudden death or acute cardiovascular collapse. These horses most commonly have a frothy exudate in their airways at post mortem, due to the presence of pulmonary oedema (fluid exudation in their lungs).

An ECG (or electrocardiogram) is the tool for definitively diagnosing the source of an arrhythmia. It measures the electrical conductivity of the heart and can identify which part of the heart is not contracting property. As the innervation of the horse's heart is far more complex than in man or the dog, an ECG cannot be used to estimate heart size (i.e. a heart score). Heart size can only be measured in the horse with an ultrasonographic examination (an echocardiogram). With this examination. the size of all chambers and major vessels can be measured, as well as observing the appearance and function of the heart valves. This is the diagnostic tool of choice for diagnosing a murmur. A murmur is really just an abnormal noise in the heart that is most commonly caused by a leaking valve in horses. Very, few murmurs in horses are due to stenotic valves, where valves do not open properly. Occasionally in a young horse a murmur may be due to a hole in the heart, as can occur in man. For this reason auscultation of both sides of the chest is an important part of the pre-purchase examination of any young horse. Some murmurs are caused by valves that anatomically look normal. In this instance a Doppler (an ultrasono­graphic examination where direction of blood flow call be assessed) examination is required.

As horses are herbivores, elevated blood cholesterol and subsequent coronary artery disease do not occur. In man, this is the most common cause of heart disease and `heart attacks'. Thankfully, this is a disease we don't have to deal with in our equine patients.