Cardiac memory: a mechanical and electrical phenomenon.

Alterations in repolarization following prolonged periods of ventricular pacing, termed "cardiac memory," have been well documented. Postpacing changes in cardiac function have also been noted in hypertrophic cardiomyopathy. This study was designed to evaluate the effects of ventricular pacing on postpacing diastolic function and its relationship to repolarization changes. Eight subjects (mean age, 76 yr) with permanent pacemakers were enrolled in this study. Each subject was evaluated at a fixed pacing rate with recording of electrocardiographic and echocardiographic data. Seven sets of measurements were performed in the same sequential pattern: 1) after 1 wk of atrial pacing, 2) within 10 min after initiation of atrioventricular sequential pacing (ventricular pacing), 3) within 10 min after termination of 1 h of ventricular pacing, 4) after 1 wk of ventricular pacing, and 5) within 10 min, at 1 h, and at 24 h after termination of ventricular pacing. All subjects had repolarization changes characteristic of cardiac memory only after 1 wk of ventricular pacing. Changes in repolarization parameters were accompanied by changes in peak left ventricular filling rate (dD/dt/D; P = 0.02) and isovolumic relaxation time (IVRT; P = 0.03) that at 24 h approached baseline values. Correlations existed between changes in the Q-T interval and IVRT (r = 0.53, P = 0.007) and between changes in T wave amplitude and dD/dt/D (r = 0.48, P = 0.018) after long-term ventricular pacing. Thus changes in both repolarization and diastolic function persist after cessation of ventricular pacing and lend support to the concept of electrical and mechanical cardiac memory.