DRUG-INDUCED NARROWING OF THE WIDTH OF THE ZONE OF ENTRAINMENT AS A PREDICTOR OF THE SUBSEQUENT NON-INDUCIBILITY OF REENTRANT VENTRICULAR-TACHYCARDIA AFTER AN ADDITIONAL DOSE OF AN ANTIARRHYTHMIC DRUG

Background-The efficacy of drugs used to treat inducible monomorphic s ustained ventricular tachycardia (VT) has been assessed by investigati ng their ability to suppress inducibility, but the mechanism of the dr ug action remains to be determined. Objectives-To determine electrophy siological variables that predict inducibility, divided doses of class I antiarrhythmic drugs were given and their effects were analysed, pa rticularly the ability of the final dose to suppress inducibility. Met hods-The excitable gap was estimated by the zone of entrainment, which was defined as the difference between the cycle length of VT and the longest paced cycle length that interrupted VT during entrainment of V T with rapid pacing at paced cycle lengths in decrements of 10 ms. The cycle length of VT, the block cycle length, and the zone of entrainme nt were measured in the drug free state and after intermediate and fin al doses of procainamide, disopyramide, cibenzoline, and mexiletine. R esults-Sustained monomorphic VT with a mean (SD) cycle length of 285 ( 43) ms was induced in 8 patients. It was entrained and interrupted at the block cycle length of 231 (31) ms. The width of the zone of entrai nment was 54 (23) ms. In 8 studies VT was not inducible at final doses of procainamide in 4, cibenzoline in 1, and mexiletine in 3. In anoth er 10 studies (procainamide in 4, disopyramide in 1, cibenzoline in 2, and mexiletine in 3), VT remained inducible at the intermediate dose and at the final dose. The cycle length of VT was prolonged to a simil ar degree in studies of effective and ineffective drugs, but the cycle length that blocked VT was longer at the intermediate dose of the eff ective drugs. Consequently, the width of the zone of entrainment was s ignificantly narrowed at the intermediate dose of effective drugs and the width of the zone of entrainment was narrower than when ineffectiv e drugs were given (22 (13) ms upsilon 76 (18) or 75 (37) ms at the in termediate and final doses respectively (P < 0.02). Conclusion-Drugs t hat narrowed the zone of entrainment were associated with non-inducibi lity of VT after the final dose of the drug was given. The baseline va riables did not predict the responses to class I antiarrhythmic drugs.