Horse Heart Anatomy

horse heart anatomy

Horses may be large animals, but their heart is not much bigger than that of a human. In fact, the average adult horse’s heart weighs between seven and nine pounds. And, despite its size, the horse’s heart is very efficient. The frog in each hoof acts as a pump and sends roughly a liter of blood back up the leg with each step. That’s a lot of work for one heart!

The horse’s heart is located within the thoracic cavity and is protected from direct contact with major blood vessels by a muscular sac called the pericardium. The pericardium is held in place by the ribs from the third through sixth intercostal spaces (Reece, Erickson, Goffson, & Uemura, 2015). The lateral aspect of the heart lies against the sternum, while the base or ventral portion is horizontal to the middle of the first rib.

Like other mammals, the equine heart has four valves—two atrioventricular and two semilunar. Each valve has a thick-walled ring around its orifice that expands and contracts to create a pulse of accelerated blood flow during ventricular systole. The rapid acceleration and deceleration of blood in the arteries produce vibrations, known as heart sounds, that can be heard using a stethoscope. Normally, three heart sounds—S1, S2, and S3—can be heard in the healthy horse. These heart sounds are associated with the opening and closing of the valves and can be used to evaluate the status of the valves.

A heart that does not sound normal is often referred to as having a murmur. A murmur is a low-frequency sound that may be caused by a leaking or abnormal valve. It can also be caused by abnormal blood flow or by disease of the vascular system.

Heart murmurs are often associated with poor performance. They can also indicate an underlying condition that can lead to sudden cardiac arrest. It is important to determine the cause of the murmur and treat it if possible to improve performance.

When evaluating the heart, it is important to consider whether the horse has an enlarged heart. The heart enlargement can result in an increase in the blood volume that is pumped, which can affect the horse’s gait and endurance.

Another heart problem that occurs in horses is aortic regurgitation (AR). AR causes the aortic valve to leak blood back into the thoracic cavity. It can be a serious problem for horses, so accurate diagnosis is essential. 3-D echocardiography has been shown to be an excellent tool for diagnosing and monitoring AR. In a recent study, I compared the thickness of the aortic valve cusps in horses with and without AR. I found a linear correlation between the AR score and the thickness of the aortic cusps. This suggests that measuring aortic cusp thicknesses may be a valuable new method for assessing AR severity and diagnosing AR-associated complications such as coronary artery disease. The results suggest that this measurement could help to identify those horses at the highest risk of sudden death from AR.