Idiopathic hypertrophic subaortic stenosis and heart block: Cycle-to-cycle variation as a function of alterations in preload and afterload

Abstract

In two patients with idiopathic hypertrophic subaortic stenosis and heart block, right and left heart catheterization studies showed marked cycle-to-cycle variation in subvalvular gradient. The patient with complete heart block showed a marked decrease in gradient when atrial systole fortuitously preceded ventricular contraction by an appropriate interval. This decrease appeared to be due to augmented ventricular end-diastolic volume provided by atrial contraction. The second patient, who had second degree heart block with the Wenckebach phenomenon, demonstrated marked increases in gradient after blocked cycles in which ventricular volume appeared to be enhanced by a long diastolic filling period. Since aortic diastolic pressure (afterload) was also decreased in the latter patient, a comparison of the hemodynamics in these two unusual cases showed that decreased afterload was a more important variable affecting outflow tract gradient in idiopathic hypertrophic subaortic stenosis than the augmentation of preload by atrial systole.