In a relatively recent survey on medical problems of geriatric horses that were referred to a tertiary care center, only 7 percent of the horses had primary cardio-vascular disease. Thus, compared to other geriatric species, serious cardiac disease leading to congestive heart failure is relatively less common in the horse.

Photo 1: Myxomatous degeneration of the mitral valve leaflets, causing thickening and insufficiency.(Photos: Dr. Michelle Henry Barton)

Despite the fact that the valve becomes thicker, and thus one might suspect that stenosis would ensue, myxomatous degeneration of cardiac valves almost invariably results in failure of coaptation and therefore insufficiency or regurgitation across the valve into the preceding chamber.

Aortic valve insufficiency

It has been estimated that about half of horses over the age of 10 that are evaluated for cardiac disease have some degree of aortic valve insufficiency. In the majority of cases, aortic valve insufficiency remains subclinical and progresses slowly over several years, if it progresses at all. With subclinical aortic valve insufficiency, the owner typically does not report any concerns.

As aortic insufficiency progresses, owners may report weight loss, reduced performance, tachypnea and prolonged recovery time after exercise, ataxia or, in severe cases, collapse. If pulmonary hypertension develops, respiratory distress, coughing and serous or foamy nasal discharge may be present.

Aortic valve insufficiency is readily detected on cardiac auscultation as a decrescendo diastolic murmur. Diastolic murmurs can be distinguished from systolic murmurs by simultaneously auscultating the heart while feeling the pulse of the facial artery as it crosses the ventral edge of the mandible.

Diastolic murmurs will be audible after the pulse is felt. The murmur will be loudest at the site of origin at the aortic valve, located at approximately the left fourth intercostal space, just below the level of the shoulder. However, as aortic insufficiency progresses, the murmur will radiate over the left heart base and toward the left apex and may even be audible on the right side of the heart.

Vibration of the valves, septum or myocardium during the turbulent backward flow of regurgitation may produce a bizarre-sounding honk, squeak or "musical" quality to the murmur. Volume overload and myocardial disease should be suspected if tachycardia, a prominent S3 or an arrhythmia are present at rest.

It is particularly important to evaluate the pulse quality in horses with aortic insufficiency, because a sharp bounding or "water hammer" pulse is indicative of left-sided volume overload and is a helpful prognostic indicator. Pulse strength is determined by the difference between the systolic and diastolic blood pressure.

In patients with aortic insufficiency, the backward flow of blood from the aorta into the left ventricle during diastole lowers the diastolic pressure. As insufficiency progresses, left ventricular volume overload increases the systolic pressure. Combined, these two events expand the difference between the systolic and diastolic pressure, generating a "stronger" or more prominent bounding pulse quality.

Photo 2: This horse has a nodule on the left coronary cusp (arrow), a common finding of myxomatous degeneration of the aortic valve. LV=left ventricle, AO=aorta, RV=right ventricle.