Exercise intolerance, due to cardiovascular disease in horses, may be
caused by cardiac arrhythmias, valvular regurgitation, congenital abno
rmalities, myocardial dysfunction, pericardial disease, and vascular t
hrombosis. The most common cardiovascular cause of exercise intoleranc
e in horses is atrial fibrillation. Cardiovascular abnormalities such
as cardiac arrhythmias or murmurs, however, are common in athletic hor
ses and are not always associated with exercise intolerance. Use of an
electrocardiography (during rest and exercise) and echocardiography m
ay be necessary to better determine the significance of the cardiovasc
ular abnormality.