Although heart disease is rare, heart murmurs and arrhythmias (irregular heart rhythm) are commonly detected in horses, and their significance must be determined. In many cases a thorough examination of the cardiovascular system is all that is required to assess the relevance of an abnormal finding, but sometimes more advanced diagnostic procedures such as electrocardiography (ECG) or echocardiography may be necessary.
It is thought that around 3 horses in a hundred suffer with heart abnormalities from birth. Congenital heart disease is often suspected when a foal, weanling or immature horse is identified with a loud heart murmur or signs of heart failure, such as exercise intolerance or accumulation of fluid on the legs and under the chest and abdomen.
The most common acquired cardiac abnormalities in horses tend to affect the valves in the heart, either between heart chambers (atrioventricular valves) or between the heart and the vessels that exit it (semilunar valves). The function of these valves is to avoid back flow of blood. Horses have an amazing cardiac reserve which means they can tolerate mild to moderate alterations in their cardiac function without showing any signs; this means however, that when signs of heart failure become evident it is usually too late for any treatment to be effective.