Dr. Eugene Braunwald: The region of the aortic valve is the last point passed by the blood in its course through the heart and lungs. As long as the functions of the left ventricle and left atrium are well maintained, obstruction to left ventricular outflow may be extremely severe without the clinical manifestations of heart failure, which tend to occur earlier in the course of the disease, when the obstruction exists at a point upstream to the aortic valve. As a consequence, the relationships between the physiological and clinical abnormalities that exist in patients with obstruction to left ventricular outflow