COMPARATIVE ELECTROPHYSIOLOGIC FINDINGS BETWEEN RESPONDERS AND NONRESPONDERS TO CLASS-III ANTIARRHYTHMIC DRUGS AMONG PATIENTS WITH VENTRICULAR TACHYARRHYTHMIA

Electrophysiologic testing was performed in 31 patients with ventricul ar tachycardia (21 cases) and fibrillation (10 cases) to characterize the electrophysiologic properties of patients responding or not respon ding to therapy with class III antiarrhythmic drugs. At the baseline, there were no differences among the patients in the monomorphic VT cyc le length (CL), block CL or the width of the zone of entrainment. Vent ricular tachyarrhythmias after the administration of class III drugs ( sotalol: 9, amiodarone: 15 and E-4031/MS-551: 7) were inducible (nonre sponders) in 17 patients and non-inducible (responders) in 14 (45%). T he class III drugs prolonged the sinus cycle length (SCL), QT interval and right ventricular effective refractory period (VERP), but had lit tle effect on ventricular conduction time in the responders and non-re sponders. The SCL, QT interval and VERP at the three drive cycle lengt hs of 600, 400 and 300 msec were significantly longer in the responder s than in the non-responders, but the class III drug action on VERP sh owed a reverse use-dependency. Isoproterenol administered to the respo nder did not fully reverse the class III antiarrhythmic drug-induced p rolongation of QT, QTc and VERP, which remained significantly prolonge d compared to the baseline values. Furthermore, when the VERP after th e administration of class III drugs were greater than 270, 250 and 240 msec at the three drive cycle lengths of 600, 400 and 300 msec, respe ctively, it was associated with the non-inducibility of VT/VF. Though the precise mechanism of the drug efficacy is not yet known, these obs ervations help to clarify the ability of class III drugs to prevent th e induction of ventricular tachyarrhythmia.