Abstract: AIM: To compare the effects of 600 mg·d^-1 and 800 mg·d^-1 amiodarone on heart rate and rhythm in healthy volunteers. METHODS: 16 subjects were randomly assigned to two groups: amiodarone 800 mg·d^-1 and 600mg·d ^-1 group. The drug was withdrawal when bradycardia arrhythmia occurred. The ECG and Holter were detected during procedure. RESULTS: Mean HR in 800 mg·d ^-1 and 600 mg·d ^-1 amiodarone group was significantly lower than that in the baseline after drugs were administrated 1 day (65 ±7.4 vs 75 ± 15.6 bpm, P <0.05) and 2 days, respectively, but both return to normal at 3 days afterwithdrawal. The earliest manifestation of ECG was prolongation of PR interval which was dose-related. QTc was prolonged after administration. Mean HR and total HR of 24 hours recorded by Holter significantly decreased at 3rd day in 800 mg·d ^-1, while they were not decreased in 600 mg·d ^-1 .HR restored to normal at 1 week after withdrawal. The reasons for withdrawal were prolongation of PR interval, paroxysmal sinus arrest, conjuncular /ventricular escape, II degree I /II atrioventricular block and sinus bradycardia. CONCLUSION: The earliest manifestation of amiodarone’s pro-arrhythmia is prolongation of PR interval which is dose-related. So the monitoring of PR interval is necessary after administration.

Key words: amiodarone, heart rate, rhythm, clinical trial, phase I