Effects of diltiazem on PR intervals in healthy young adults.

The effects of diltiazem on atrioventricular conduction were evaluated in 20 healthy young men. PR intervals of surface ECG were used as markers of atrioventricular conduction. Two hours after the oral administration of 60 mg of diltiazem, the PR interval was prolonged by 30 msec or more (range, 30 to 160 msec) in four subjects, all of whom had long PR intervals (200 to 230 msec) before receiving diltiazem. Changes in the axis of the P wave suggesting the shift of pacemaker from sinus to atrium were observed in one subject, who was excluded from the analysis of PR intervals. The remaining 15 subjects showed no statistically significant changes in PR intervals. None of these 15 subjects had abnormally prolonged PR intervals before taking diltiazem. A significant correlation between PR interval at baseline and the degree of its prolongation by diltiazem was observed. Plasma concentrations of diltiazem ranged from 16.3 to 80.5 ng/ml (mean +/- SD, 29.7 +/- 14.2 ng/ml) two hours after drug administration and did not correlate with changes in PR intervals. The heart rate did not change after diltiazem administration, even in five subjects with sinus bradycardia. We conclude that diltiazem may delay atrioventricular conduction in healthy young adults with long PR intervals.