Observations on seven cases of mitral insufficiency in the horse

Observations on seven cases of mitral insufficiency are described. Three of the horses had recent histories of cardiac failure and formed a separate group in which there were marked haemodynamic changes which varied in relation to heart rate but were associated with pulmonary hypertension, elevated right ventricular pressure and low peak left ventricular pressure. In the other four horses the heart appeared to be compensating at rest but to be unable to cope with severe exertion. On auscultation, a widespread pansystolic murmur and a prominent third heart sound were characteristic of the left ventricular volume overload. Microscopic changes were found in one left papillary muscle. The possibility that changes in the position of the papillary muscles consequent upon left ventricular dilatation may lead to or exaggerate mitral valve eversion is discussed.