Slow Conduction and Reentry in the Ventricular Conducting System

Depression of excitability and responsiveness provoked by the action of high K1 ^ and epinephrine on short bundles of excised canine Purkinje fibers yields reentrant excitation. An impulse entering a depressed area undergoes marked slowing of conduction; the impulse then may continue forward while also returning through the pathway by which the initiating impulse entered the depressed area. The impulse may be delayed or blocked in either the forward or the retrograde direction. The reentrant excitation can occur in the absence of premature excitation. Various methods of depression of excitability produce return extrasystoles in Purkinje fibers. The common factor is very slow conduc-tion which depends upon the abolition of the fast upstroke and the appearance of a low-voltage, slowly propagated action potential that is readily blocked. KEY WORDS echo beats ventricular bigeminy myocardial ischemia ventricular fibrillation coupled extrasystoles reciprocal rhythm mechanisms of arrhythmias myocardial infarction ventricular tachycardia • That reentry of excitation within the ventricle or the ventricular conducting system might cause extrasystoles and other distur-bances of ventricular rhythm has been a widely held hypothesis for many decades. Yet it has proved surprisingly difficult to evoke unequivocal reentry under experimental con-•ditions. Doubt has also been expressed tha