Idiopathic Hypertrophic Subaortic Stenosis: III. Intraoperative Studies of the Mechanism of Obstruction and Its Hemodynamic Consequences

Pressures in the left ventricle and central aorta were recorded together with instantaneous aortic blood flow at the time of operation in 9 patients. Four patients had IHSS, 4 had valvular or discrete subvalvular aortic stenosis, and 1 was considered to have a normal aortic valve and left ventricular outflow tract. In each of the 4 patients with IHSS, the pattern of aortic blood flow was distinctly abnormal when significant obstruction was present and nearly 80% of total flow took place in the first half of systole. In contrast, [see figure in the PDF file] the distribution of flow was symmetrical in all patients with discrete aortic stenosis. When flow and the pressure gradient were related at brief intervals during systole, and the orifice size calculated, it was found that the orifice was always greatest at the onset of contraction, diminished rapidly in size, and often became totally obliterated toward the end of the ejection period in patients with IHSS. The size of the orifice remained constant during systole in the patients with discrete obstruction. The abnormalities of instantaneous aortic blood flow, and the rate of systolic narrowing of the subvalvular orifice were exaggerated following premature ventricular contractions, after the administration of pharmacologic agents with positive inotropic effects (isoproterenol and calcium chloride), and after reduction of the circulating blood volume. With valvular and discrete subaortic stenosis, however, these interventions did not change the symmetrical pattern of blood flow and the orifice area was constant. Augmentation of the circulating blood volume or partial occlusion of the ascending aorta decreased the severity of outflow obstruction in every patient with IHSS and converted the abnormal flow pattern to one characteristic of discrete stenosis. These studies provide the first evidence of the instantaneous changes which occur within the outflow tract of the left ventricle in patients with IHSS, and provide information concerning the various mechanisms by which the intensity of obstruction may be modified.