CONVERSION OF ATRIAL-FLUTTER BY IBUTILIDE IS ASSOCIATED WITH INCREASED ATRIAL CYCLE LENGTH VARIABILITY

Objectives. This study was designed to test the hypothesis that conver sion of atrial butter in humans by ibutilide, a new class III antiarrh ythmic agent, is characterized by an increase in atrial cycle length v ariability. Background. Conversion of tachyarrhythmias has been associ ated with increased oscillations of cycle length. Methods. Electrogram s and monophasic action potentials from the right atrium in 35 patient s,vith spontaneous, sustained atrial butter were recorded before, duri ng and after intravenous ibutilide (0.005 to 0.025 mg/kg body weight, n = 25) or placebo (n = 10). Atrial cycle length, cycle length variabi lity (coefficient of variation), diastolic interval and diastolic inte rval variability were measured from 10 consecutive cycles at baseline and 3 min before, 1 min before, 30 s before and immediately before con version. Similar measurements were made in patients who received ibuti lide or placebo but did not convert. Results. Ibutilide converted atri al flutter in 14 of 25 patients 25 +/- 16 min (mean +/- SD) after init iation of the infusion, whereas placebo converted no patients. Atrial cycle length was prolonged to the same extent in ibutilide converters and nonconverters (36 +/- 19 vs. 38 +/- 21 ms, p = NS) and was not aff ected by placebo. Beat-to-beat variability in atrial cycle length (bas eline 1.2 +/- 0.7 vs. preconversion 7.3 +/- 4.9, p < 0.01) and diastol ic interval (baseline 11 +/- 8 vs. preconversion 33 +/- 23, p < 0.05) increased significantly just before atrial flutter conversion and rema ined unchanged in ibutilide nonconverters and placebo group patients. Conclusions. Ibutilide prolongs atrial butter cycle length, but conver sion of atrial flutter by ibutilide is characterized by increased vari ability in atrial cycle length and diastolic interval.