Effects of Changes in Body Position on the Severity of Obstruction to Left Ventricular Outflow in Idiopathic Hypertrophic Subaortic Stenosis

Abstract

The circulatory responses to alterations in posture were determined in 10 patients with idiopathic hypertrophic subaortic stenosis. Tilting to a 45° head-up position augmented the subaortic gradient by an average of 35 mm Hg and intensified the obstruction to left ventricular outflow. In addition, the post-extrasystolic decrease of the arterial pulse pressure became more prominent. Tilting to a 20° head-down position reduced the gradient by an average of 11 mm Hg and tended to reduce the severity of the obstruction. Elevation of the lower extremities also resulted in a reduction of the intraventricular systolic pressure gradient, the reduction averaging 28 mm Hg, and apparently increased the dimensions of the obstructing orifice. It is postulated that these effects on the obstruction to outflow induced by changes in posture result primarily from alterations in left ventricular volume secondary to changes in venous return. As a consequence, in the upright position, the size of the left ventricular outflow tract is reduced and the intraventricular pressure gradient is augmented while the cardiac output is reduced, resulting in an intensification of the obstruction. The opposite changes occur during the head-down tilt or during elevation of the legs. These hemodynamic findings provide an explanation for the intensification of symptoms commonly experienced by patients with idiopathic hypertrophic subaortic stenosis in the erect position, and for the discrepancies between the symptoms and the severity of obstruction determined by cardiac catheterization carried out in the supine position. Assumption of the head-down, legs-up position is suggested as an emergency therapeutic maneuver for patients with hypertrophic subaortic stenosis.