Slow Conduction and Reentry in the Ventricular Conducting System

Closed loops of fibers of the ventricular conducting system of canine or bovine hearts were used to study circus movement of excitation. Action potentials were recorded at three sites with intracellular microelectrodes. Discrete segments were depressed by application of K + -rich agar or the entire loop was depressed by modified Tyrode's solution containing 15-17 mM K + and 1 to 5 x 10 -6 M epinephrine. The loops were 12-35 mm long and the effective conduction velocity was 0.02-0.08 m/sec. Impulses entering some loops traveled in one direction only, circling around the loop and returning to produce a second response at one or more sites (single circus movement). In other loops the impulse traveled around the circuit repeatedly (sustained circus movement). Circus movement around short loops requires a low conduction velocity and must be initiated by an impulse that travels around the loop in only one direction. Single circus movement can cause extrasystoles. Sustained circus movement can cause idioventricular rhythms and ventricular tachycardia.