['This study was designed to test the hypothesis that conversion of atrial flutter in humans by ibutilide, a new class III antiarrhythmic agent, is characterized by an increase in atrial cycle length variability.', 'Conversion of tachyarrhythmias has been associated with increased oscillations of cycle length.', 'Electrograms and monophasic action potentials from the right atrium in 35 patients with spontaneous, sustained atrial flutter were recorded before, during and after intravenous ibutilide (0.005 to 0.025 mg/kg body weight, n = 25) or placebo (n = 10). Atrial cycle length, cycle length variability (coefficient of variation), diastolic interval and diastolic interval variability were measured from 10 consecutive cycles at baseline and 3 min before, 1 min before, 30 s before and immediately before conversion. Similar measurements were made in patients who received ibutilide or placebo but did not convert.', 'Ibutilide converted atrial flutter in 14 of 25 patients 25 +/- 16 min (mean +/- SD) after initiation 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 affected by placebo. Beat-to-beat variability in atrial cycle length (baseline 1.2 +/- 0.7 vs. preconversion 7.3 +/- 4.9, p < 0.01) and diastolic interval (baseline 11 +/- 8 vs. preconversion 33 +/- 23, p < 0.05) increased significantly just before atrial flutter conversion and remained unchanged in ibutilide nonconverters and placebo group patients.'] Is conversion of atrial flutter by ibutilide associated with increased atrial cycle length variability?