Heart disease

Although the presence of heart murmurs and arrhythmias (irregular heart rhythm) are not uncommon in horses, they rarely cause any signs of actual heart disease in most horses. The most common type of abnormalities detected are abnormalities of the valves between the heart chambers or between the heart and the arteries, and some arrhythmias.

The prevalence of congenital heart abnormalities in the overall equine population is unknown but it is estimated to be around 3.5%. Congenital heart disease is often suspected when a foal, weanling or immature horse is identified with a prominent heart murmur, cyanosis (blue mucous membranes) or signs of heart failure (exercise intolerance, accumulation of fluid on the legs and under the chest and abdomen, etc).
It is important to note at this stage that heart murmurs are very common in neonatal foals and the majority are not pathologic in origin and will probably disappear in the first 48 hours following birth.
A ventricular septal defect is the most common abnormality in the horses heart. This means that due to a lack of complete closure of the wall between the left and right ventricles of the heart during fetal development, there is a connection between these two chambers that allows oxygenated and non-oxygenated blood to mix, which does not happen in the normal heart.
These septal defects can sometimes be so big that they are not compatible with life and a foal will show severe signs of heart disease form birth. Sometimes the defect is small enough to allow a comfortable life, although they are likely to affect the athletic performance of the horse in the future.
If your vet suspects your horse has a ventricular septal defect he may want to perform an ultrasound scan study of the heart to confirm the diagnosis and measure the size of the defect in the ventricular wall.

Currently there is no treatment for ventricular septal defects, but as previously explained, foals with small defects may not need medical intervention to live a comfortable life.

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 into the preceding chamber.
There are two atrioventricular valves, the tricuspid valve on the right side of the heart and the mitral valve on the left side of the heart. These valves can start to leak blood when they should be completely closed giving origin to a murmur that can be detected on auscultation during contraction (systole) of the heart.
There also two semilunar valves, the pulmonic valve between the pulmonary artery and the right side of the heart, and the aortic valve between the aorta and the left side of the heart. These valves can also leak, in which case they will produce a murmur audible during relaxation (diastole) of the heart.
The specific valve affected may be identified with detailed auscultation of the heart, but sometimes an echocardiogram or ultrasound scan of the heart may be necessary to assess the valve(s) involved and the degree of regurgitation (amount of blood leaking). Echocardiography will also allow measurement of the heart chambers, as leakage through a valve means that the preceding chamber has to increase in size to accommodate the extra blood, causing further problems.
Most of the time these valves will start to leak because of ageing of the normal structures. Although not very frequent, a valve can also leak because it gets damaged by bacteria on its surface, a condition known as bacterial endocarditis.
Conditions affecting the pericardium, the outer layer of the heart, are also seen, but are not very common. Sometimes fluid can accumulate in the pericardium, which stops the heart from beating properly.

Horses have an amazing cardiac reserve which means they can tolerate mild to moderate alterations in their cardiac function. This means, however, that when signs of heart failure become evident it is usually too late for any treatment to be effective.
Your vet may elect to treat your horse with diuretics, to prevent accumulation of fluid in the lungs. Also different drugs can be given to try to make the way the heart pumps blood more effective.
In cases when the problem is infection of a heart valve, your horse will require long-term therapy with antibiotics, but again, it is likely that the damage to the valve will be too advanced by the time signs are noted.
When fluid accumulates in the pericardium, drainage of this fluid can be attempted.

Arrhythmias or irregularities of the heart rhythm are common in horses. Most of the time these are of no clinical significance, they may even be a sign of an extremely healthy and strong heart.
However, sometimes these arrhythmias can affect the performance of the horse and in some cases they can even be life-threatening. The exact type of arrhythmia can often be identified by auscultation, but in most situations performing an electrocardiogram (ECG) will be necessary.
The most common pathologic arrhythmia in horses is atrial fibrillation. In most cases it is detected as an incidental finding, but it can affect performance in horses that do high intensity exercise. It is sometimes associated with nose bleeds, and can be quite commonly detected in larger sized horses.
If atrial fibrillation is confirmed, a full ultrasound scan study of the heart should be performed to address the cause of the arrhythmia as sometimes, particularly if it is associated with a heart murmur, it can be caused because of atrial enlargement due to mitral or tricuspid regurgitation.  However, in multiple occasions this cannot be achieved and the arrhythmia is classed as idiopathic.

Treatment of arrhythmias can be attempted although the potential complications of the therapy should be discussed with your vet. The treatment may be done by administration of oral anti-arrhythmic drugs or by passage of an electric current through wires placed in the heart. In both cases this is specialised treatment that should be performed at a referral hospital.