Heart disease represents a relatively low percentage of problems in horses. However, it is important to fully assess problems to determine the significance to the horse; as well as the consequences for athletic performance and risk to the rider.
The heart acts as a pump to distribute blood. The horse’s heart is about the size of a melon, weighing 4-5kg. It is an exceptionally strong muscle, responsible for pumping up to 25-40 litres of blood around the body every minute, and up to 240 litres/min during maximal exercise. The amount of blood ejected during each contraction is influenced by the heart’s structural integrity, the strength of contraction, resistance to blood flow and ability to fill normally.
The heart has four chambers: two receiving chambers (atria), and two pumping chambers (ventricles). Oxygen-rich blood from the lungs enters the left side and is pumped around the rest of the body, providing oxygen and nutrients to the tissues. Oxygen-depleted blood returns to the right side and is then delivered to the lungs for gas exchange. The time during which the heart contracts is known as systole; and filling of the heart occurs during diastole, when the heart relaxes.
Abnormalities affecting heart structure or function reduce the amount of blood, hence oxygen, delivered to the body tissues. This may only affect the horse’s maximal exercise ability initially, but severe problems can affect the horse at rest. Abnormal sounds or rhythms are often an incidental finding detected at routine or pre-purchase examination with no outward signs of illness.
Tests may be recommended to determine the problem’s origin, and its significance.
Heart problems commonly occur from irregular rhythms or structural abnormalities causing a murmur.
The normal equine heart beats regularly, 28-40 times per minute. Disruption to the regular rhythm is termed an arrhythmia. Arrhythmias are audible, but ECG is necessary to identify the specific problem. Different kinds of arrhythmias occur, but not all are problematic.
For example, pauses in the resting heart rhythm are frequently heard, called ‘dropped beats’ (second degree AV block). This arrhythmia arises due to the horse’s large heart size and disappears at exercise, so is considered normal.
Atrial fibrillation (AF) is the most common performance-limiting arrhythmia in the horse. AF is often an isolated problem, but may be associated with underlying structural heart problems causing atrial enlargement. Therefore, careful evaluation of the heart is required.
Normally, electrical waves cause regular, co-ordinated atrial muscle contractions, helping blood move through the heart. In AF these electrical waves become chaotic and the atria fails to contract. The ventricles continue to pump blood around the body at random intervals, the outcome being an irregular rhythm.
AF is treatable, but treatment depends on the presence of underlying disease and duration of the AF among other factors. Horses may return to previous levels of activity with successful treatment, although AF can recur. Horses with underlying heart problems are not good candidates for treatment and may need to be retired.
Just as a river with smooth banks runs quietly, blood normally creates little noise flowing through the heart. Disruptions or increases in flow cause turbulence which may be audible using a stethoscope. This is similar to increased noise heard when a river becomes a waterfall. This turbulent blood flow is called a murmur.
Murmurs are characterised using specific terminology to reflect where they are heard, when they occur and describe the sound; as well as being graded to assess loudness.
Not all murmurs represent a pathological problem: some short, quiet murmurs are termed ‘physiological murmurs’, which means they are not associated with a disease and do not cause a problem to the horse. Pathological murmurs are caused by structural abnormalities, for example valve leaks or abnormal communications within the heart. To identify the murmur’s origin and significance echocardiography is needed.
Congenital problems are present at birth. Ventricular septal defects (VSD) are the commonest congenital structural heart problem in horses. Foals which fail to grow normally, tire easily or collapse should be examined for congenital heart problems. A soft, continuous
murmur after the foal is born is normal, however this should resolve by a week of age.
A VSD is a hole between the two ventricles. During systole, oxygen-rich blood from the higher pressure left side flows through to the lower pressure right side via the VSD. The horse tires quickly as less oxygen-rich blood is delivered around the body per heartbeat. The right side of the heart now contains a greater blood volume requiring it to work harder.
Fortunately, most septal defects are small; but in severe cases, where a significant proportion of blood flows left to right, the extra blood volume stretches the heart chambers, which become unable to contract normally, resulting in heart failure. VSDs are identified by a loud systolic right-sided heart murmur and often found incidentally. Elite athletes may exhibit poor performance, but many horses are capable of working safely.
There are four valves within the heart, comprised by individual leaflets. When closed, valves prevent retrograde blood flow. Valvular problems predominantly originate from degeneration of the valve leaflets, thus problems increase with age. Despite this valvular problems are relatively uncommon compared to humans and domestic animals.
Degenerative changes cause the valve leaflets to thicken and distort. Early changes means the valve seals poorly, resulting in leakage and a murmur from increasingly turbulent blood flow. Often there are no signs of illness with mild to moderate valvular problems and provided they are carefully monitored, horses can continue working safely.
Blood which leaks backwards is known as ‘regurgitant’. In advanced valvular degeneration regurgitant blood accumulates in the chamber behind the affected valve, which enlarges. As the heart struggles to cope, the blood backs up even further and free fluid (oedema) collects in the horse’s tissues. Affected valves on the left side of the heart cause oedema in the lungs and a cough may develop; whereas right-sided heart problems have oedema in the abdomen or under the skin, often between the front legs.
At this stage the horse is deemed to be in heart failure. Severe valvular degeneration, especially if associated with exercise intolerance, irregular heart rhythms, oedema and/or heart enlargement, carries a poor prognosis and the horse should be retired.
Myocarditis is inflammation of the heart muscle. Inflammation can occur from bacterial or viral infections, for example strangles or influenza; or from toxins. Oleander or monensin ingestion; or severe vitamin E/Selenium deficiency can cause toxic damage. Myocarditis disrupts the electrical pathways, causing potentially fatal irregular heart rhythms to occur. Fortunately, it is exceptionally rare in the horse.