This study examines the efficacy of various doses of intravenous diltiazem to control the ventricular response during atrial fibrillation or atrial flutter. Control of the ventricular response of patients with atrial fibrillation and a rapid ventricular response can provide patients with relief of symptoms and improve hemodynamics. Eighty-four consecutive patients with atrial fibrillation or atrial flutter, or both, received an intravenous bolus dose of diltiazem followed by a continuous infusion of diltiazem at 5, 10, and 15 mg/hour. The mean ventricular response and blood pressure were monitored. Overall, 94% of patients (79 of 84) responded to the bolus dose with a > 20% reduction in heart rate from baseline, a conversion to sinus rhythm, or a heart rate < 100 beats/min. Seventy-eight patients received the continuous infusion. After 10 hours of infusion, 47% of patients (confidence interval [CI]: 36%, 59%) had maintained response with the 5 mg/hour infusion, 68% (CI: 57%, 79%) maintained response after the infusion was titrated to 10 mg/hour, and 76% (CI: 66%, 85%) after titration from the 5 and 10 mg/hour infusion to the 15 mg/hour dose. For the 3 diltiazem infusions studied, mean (+/- SD) heart rate was reduced from a baseline value of 144 +/- 14 beats/min to 98 +/- 19, 107 +/- 25, 107 +/- 22, 101 +/- 22, 91 +/- 17, and 88 +/- 18 beats/min at infusion times 0, 1, 2, 4, 8, and 10 hours, respectively. By the end of the infusion, 18% of patients (14 of 78) had conversion to sinus rhythm.(ABSTRACT TRUNCATED AT 250 WORDS)