A system and method for determining indicators of left ventricular function, and for determining the effectiveness of a cardiac therapy are disclosed. The indicators of left ventricular function can include isovolumetric relaxation time and the negative slope of a V wave obtained from measuring changes in left atrial pressure during diastole. The apparatus may include a catheter comprising an inflatable balloon for insertion into the esophagus of an individual, a compressor device for pressurizing the balloon, and a means for sensing and recording pressure changes on the balloon.