Reentry: Slow Conduction, Summation and Inhibition

p REMATURE systoles may arise through activity of an automatic focus of the sort normally present in the sinoatrial node or they may arise through reentry. A reentrant extrasystole is assumed to arise when the impulse that caused the previous systole lingers somewhere in the heart long enough to be able to emerge and reexcite the heart after the end of the refractory period. The familiar extrasystole with fixed coupling has long been regarded as possibly reentrant since it only appears following a previous impulse and appears at a fixed time after that impulse; whether reentrant or not, such an extrasystole certainly seems to be the direct result of the previous impulse. The difficulty in regarding this or any arrhythmia as reentrant stems from the fact that the refractory period of cardiac tissue is so long. In the human heart over 0.3 sec must elapse after excitation before ventricular muscle is again excitable. The normal wave of excitation travels between 1 and 4 m/sec in the ventricular conducting system and between 0.5 and 1.0 m/sec in the myocardium. Were the impulse responsible for reentry to travel at such speeds it would have to find a path between 15 cm and 1 m long in which to travel to be able to reexcite after the refractory period. That so long a path, however circuitous, could exist in functional isolation from the rest of the heart has never seemed likely. The possibility of reentry only demands, however, that the impulse survive in some manner until the refractory period is over. Travel at normal velocity is not the only way in which an impulse might occupy itself during the refractory period; nor is the refractory period fixed in length. The refractory period might be shortened, either locally