Ischemia-induced and reperfusion-induced arrhythmias: importance of heart rate

The relationship between heart rate and ischemia-induced and reperfusion-induced arrhythmias was studied using 573 isolated rat hearts. Hearts (12/group), subjected to 7 min of coronary occlusion and 10 min reperfusion, were paced at 300, 330, 360, 390, 420, 480, or 540 beats/min. Pacing either throughout the experiment or during ischemia alone led to a rate-dependent increase in the incidence of reperfusion-induced ventricular fibrillation (VF) from 25% in the unpaced hearts to greater than 90% when the rate was 420 beats/min or higher. However, pacing during reperfusion alone did not increase the incidence of reperfusion-induced VF. In separate hearts, the right atrium was removed to permit examination of both low and high rates (167 +/- 2, 240, 336 +/- 3, or 480 beats/min throughout the experiment) over a wide range of durations of occlusion (3, 5, 7, 10, 15, 20, or 40 min). Ischemia-induced VF incidence was critically dependent on heart rate, low rates being protective. During reperfusion, the incidence of VF was also highly rate dependent if reperfusion was initiated within 10 min of the onset of ischemia (ranging from 8% when rate was 167 +/- 2 beats/min to 100% when rate was 480 beats/min) but was unrelated to heart rate when reperfusion occurred at later times (ranging from 33 to 50% when ischemia duration was 40 min). Heart rate can therefore influence susceptibility to ischemia- and reperfusion-induced arrhythmias, probably as a result of an effect on the rate of development of ischemic injury.