In this investigation, we propose to perform dobutamine stress transesophageal echocardiography in patients with aortic regurgitation and in patients with mitral regurgitation in order to assess myocardial contractile reserve. The purpose of the study is to determine whether the contractile reserve of the myocardium measured during dobutamine stress echocardiography is a predictor of the development of subsequent left ventricular dysfunction and left ventricular dilatation, as well as recovery of left ventricular function after surgery, in these patients.