ENTRAINMENT OF REENTRANT VENTRICULAR-TACHYCARDIA IN ANISOTROPIC RINGS OF RABBIT MYOCARDIUM - MECHANISMS OF TERMINATION, CHANGES IN MORPHOLOGY, AND ACCELERATION

Background. Entrainment of ventricular tachycardia can either terminate or change the rate and/or morphology of ventricular tachycardia. The purpose of this study was to elucidate the underlying mechanisms by mapping of entrainment of ventricular tachycardia. Methods and Results. In 10 Langendorff-perfused rings of anisotropic rabbit left ventricular epicardium created by a cryoprocedure, ventricular tachycardia with a cycle length of 167+/-17 milliseconds was induced by incremental pacing. During transient entrainment (10 stimuli), the circulating wave was extinguished by collision with the paced antidromic wave, whereas ventricular tachycardia was reset by the paced orthodromic wave. At shorter pacing intervals, the site of collision shifted deeper into the circuit. Entrainment at high rates (104+/-11 milliseconds) resulted in either termination (n=54), a change in morphology (n=8), or acceleration (n=6) of ventricular tachycardia. Termination of ventricular tachycardia was due to complete (84%) or partial (16%) block of the paced orthodromic wave. Partial block induced microreentry within the circuit, resulting in a reflected echo wave that terminated ventricular tachycardia. A change in morphology of ventricular tachycardia was due to reversion of the direction of propagation of the circulating wave around the obstacle. Acceleration of ventricular tachycardia was caused by double-wave reentry induced by block of the paced antidromic wave. In 28 cases, the sequence of activation during entrainment was not stable but changed from beat to beat due to varying arcs of conduction block. Block occurred predominantly (86%) during slow transverse propagation. Before termination, local oscillations in interval occurred, resulting in a shortening of the last local interval at the site of block by 10+/-6 milliseconds. Conclusions. Termination of ventricular tachycardia by entrainment was due either to complete orthodromic block or to a reflected echo wave. A change in morphology occurred when the direction of the circulating wave reversed. Acceleration of ventricular tachycardia was due to initiation of double-wave reentry. All changes were preceded by conduction block during one or more stimuli at one or multiple sites in the circuit. Block occurred predominantly during slow transverse propagation and was preceded by local oscillations in interval at the site of block.