Characterization of return cycle responses predictive of successful pacing-mediated termination of ventricular tachycardia

Objectives. The purpose of this study was to characterize response patterns during overdrive pacing that predict successful termination of ventricular tachycardia. Background. Overdrive pacing during ventricular tachycardi typically results in entrainment at slow pacing rates and in termination or acceleration at faster rates. The factors that determine the critical paced cycle length that results in tachycardi termination have not been extensively studied. Methods. Ventricular tachycardias in 14 patients with coronary artery disease were studied with overdrive pacing at several cycle lengths. Return cycles were measured after each additional paced beat at each paced cycle length. The return cycle responses during pacing trials that resulted in tachycardi termination and those that resulted in entrainment were compared. Results. Three return cycle responses were identified: flat, plateau and increasing. Twenty trials of overdrive pacing resulted in tachycardi termination; all were characterized by an increase in the return cycle with the delivery of each successive beat in the pacing drive until the tachycardi terminated (increasing response). Thirty-four pacing trials resulted in entrainment and not termination; these were characterized either by constant return cycle (flat response) or an initial increase in return cycle followed by longer, constant return cycle (plateau response) with the delivery of additional paced beats. The longest paced cycle length that resulted in tachycardi termination correlated with the relative refractory period of the circuit, defined as the tachycardi cycle length minus the fully excitable gap (r2 = 0.764, P = 0.0001). Tachycardi termination was not observed unless the paced cycle length was shorter than the relative refractory period of the circuit. Conclusions. The critical paced cycle length that causes termination of ventricular tachycardi depends on the relative refractory period of the circuit because this factor determines whether the nth + 1 beat of the pacing drive will encounter partially recovered tissue. These dat provide insights into the mechanism of pacing-mediated tachycardi termination and entrainment and are applicable to the development of improved antitachycardi pacing algorithms.