EFFECTS OF OVERDRIVE STIMULATION ON FUNCTIONAL REENTRANT CIRCUITS CAUSING VENTRICULAR-TACHYCARDIA IN THE CANINE HEART - MECHANISMS FOR RESUMPTION OR ALTERATION OF TACHYCARDIA

Introduction: The purpose of the experiments described in this article was to investigate the effects of overdrive stimulation on functional anisotropic reentrant circuits causing ventricular tachycardia in inf arcted canine ventricles. We determined how overdrive stimuli affect r eentrant circuits to alter characteristics of the tachycardia. Methods and Results: Activation patterns were determined by mapping excitatio n with a 192 bipolar electrode array. We found that overdrive stimuli could activate the circuits with the same pattern as the reentrant wav efront and that after overdrive stopped either the last or the next to last stimulated wavefront continued propagating through the circuit a s a new reentrant impulse and tachycardia continued. When the circuit was not altered after overdrive, the exit route that the stimulated wa vefront took from the circuit to activate the rest of the ventricles w as also not altered and the tachycardia after overdrive had the same c ycle length and QRS morphology as prior to overdrive. In some experime nts, however, the overdrive stimuli did not follow the original reentr ant pathway but led to the formation of a different circuit with a dif ferent exit route to the ventricles. As a result, after overdrive stim ulation tachycardia had a different QRS morphology and cycle length th an prior to stimulation. When the new circuit after overdrive was smal l and the revolution time of the reentrant impulse around the circuit was short, fibrillation occurred. Conclusion: Functional reentrant cir cuits can either be maintained or altered after a period of overdrive stimulation. The results explain many of the effects that have been se en during overdrive stimulation of clinical ventricular tachycardia.