Aortic insufficiency (AI), also referred to as aortic regurgitation (AR), is the leaking of the aortic valve of the heart that causes blood to flow in the reverse direction during ventricular diastole, from the aorta into the left ventricleis.

AI is most commonly caused by degenerative valve thickening and aortic valve prolapse. Other causes include congenital malformations, leaflet tearing, infective endocarditis, valvulitis, fenestrations, and aortic root disease.

The aortic valve regulates the blood flow from the heart's lower-left chamber (the left ventricle) into the aorta. The aorta is the main vessel that supplies blood to the rest of the body. With AI, the aortic valve in the heart does not close tightly, which causes blood to flow backwards from a widened or weakened aortic valve into the heart's lower chamber (the left ventricle). This means the left ventricle never fully empties of blood before the next load of blood arrives from the left atrium. As a result, the left ventricle of the heart expands its capacity to accommodate the leftover blood and the new blood; this means the heart must work extra hard and keeps trying to pump out all of the blood, even though it can't; this can cause the horse to become tired faster and over time can lead to heart failure. The most serious form of aortic regurgitation is caused by endocarditis, an infection that leaves holes in the valve leaflets.

Since AI tends to affect older horses, many times the condition goes unnoticed by the owner, since the expectation of performance level in older horses generally decreases with age.