Isometric cardiac contraction. a possible cause of the disorganized myocardial pattern of idiopathic hypertrophic subaortic stenosis.

The bizarre septal architecture of idiopathic hypertrophic subaortic stenosis (IHSS) may be secondary to a small systolic cavity with late systolic isometric contraction. We examined ventricular muscle for IHSS-type muscle-fiber disarray in infant hearts in which isometric contraction would occur during development-namely, pulmonary or aortic-valve atresia with intact ventricular septum and normal atrio-ventricular valves. Fifteen patients with aortic atresia and 10 with pulmonary atresia were compared to 25 normal controls of matched age and heart weight. Aortic atresia showed disorganization of muscle-fiber alignment of left ventricle, particularly septum, and intramural coronary-artery changes virtually identical to IHSS. Pulmonic atresia had similar right ventricular disarray and vessel changes, again most marked in the septum. Thus, cardiac muscle-cell disorientation similar to IHSS occurs in infant ventricles with outflow-tract obstruction. This IHSS-type myocardial-fiber disarray may result from altered wall stresses related to isometric systolic contraction.