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A young man with IHSS who developed complete heart block was successfully treated with a permanent pacemaker. Echocardiography and other noninvasive techniques showed marked cycle-to-cycle variation in the evidence of subvalvular obstruction which decreased markedly when atrial systole preceded the ensuing paced complex by an appropriate interval. Because cycle length and therefore afterload were constant, it is concluded that diminished obstruction resulted from augmented ventricular end-diastolic volume produced by atrial contraction. The mitral valve echocardiogram showed unusual movements in diastole dependent upon the timing of atrial systole. Early reopening of the leaflets was a direct result of atrial contraction when the P waves were appropriately timed in presystole, whereas late reopening was passive and a result of ventricular filling in mid-diastole. Variations in intensity of the first heart sound correlated with the position of the mitral valve leaflets at the onset of ventricular systole.

Complete heart block in a case of idiopathic hypertrophic subaortic stenosis: noninvasive correlates with the timing of atrial systole.