PROCAINAMIDE INDUCED CHANGE OF THE WIDTH OF THE ZONE OF ENTRAINMENT AND ITS RELATION TO THE INDUCIBILITY OF REENTRANT VENTRICULAR-TACHYCARDIA

AIZAWA, Y., ET AL.: Procainamide Induced Change of the Width of the Zo ne of Entrainment and its Relation to the Inducibility of Reentrant Ve ntricular Tachycardia. Procainamide depresses conduction velocity and prolongs refractoriness in myocardium responsible for reentrant VT, bu t the mechanism by which the induction of VT is suppressed after proca inamide administration remains to be determined. In the present study, the relationship between electrophysiological parameters and the noni nducibility of VT was assessed during procainamide therapy with a spec ial reference to the change of an excitable gap. Clinically documented monomorphic sustained VT was induced in 30 patients and, utilizing th e phenomenon of transient entrainment, the zone of entrainment was mea sured as the difference between the cycle length of VT and the longest paced cycle length interrupting VT (block cycle length) which was det ermined as the paced cycle length decreased in steps of 10 ms, and use d as an index of the excitable gap. The effective refractory period wa s measured at the pacing site and the paced QRS duration was used as a n index of the global conduction time in the ventricle. The cycle leng th of VT, the block cycle length, and the width of the zone of entrain ment were determined and compared between the responders and nonrespon ders. In 15 patients, these parameters were determined at the intermed iate dose and related to subsequent noninducibility at the final dose. At the final doses of procainamide, VT was suppressed in 8 (26.7%) of 30 patients. However the cycle length of VT, the block cycle length, and the width of the zone of entrainment were unable to predict the dr ug efficacy i.e., noninducibility. The change in the effective refract ory period at the pacing site or the width of the paced QRS duration w as not different between the responders and nonresponders. Among the v ariables, only the width of the zone of entrainment showed a significa nt narrowing in the responders at the intermediate dose of procainamid e, and it was smaller than that of the nonresponders. The significant narrowing of the width of the zone of entrainment was associated with the subsequent noninducibility of TIT at the final dose. The present s tudy showed that the baseline cycle length of VT, the block cycle leng th, the drug induced change of the effective refractory period, or the paced QRS duration was not a predictor of the noninducibility after p rocainamide administration. However, a significant narrowing of the wi dth of the zone of entrainment at the intermediate dose was associated with the noninducibility of VT at the final dose.