Pharmacokinetic and pharmacodynamic effects of diltiazem

The pharmacokinetic and pharmacodynamic effects of diltiazem were studied in 8 patients after a short intravenous infusion (20 mg over 10 minutes), a single oral dose (60 or 90 mg), and repeated oral administration (60 or 90 mg every 6 hours for 16 doses). Diltiazem levels decreased in a triexponential manner after intravenous infusion. Terminal half-lives after intravenous, single oral, and repeated oral administration were not significantly different (4.5 +/- 1.3, 3.7 +/- 0.6, and 4.9 +/- 0.4 hours, respectively). The kinetic effects of oral diltiazem were nonlinear. With repeated oral administration, there was accumulation of both diltiazem and its metabolite, deacetyldiltiazem. The diltiazem area under the time versus concentration curve increased by a factor of 2.39 +/- 0.42 (p = 0.00002). Most patients showed a double peaked time versus concentration curve after oral administration, indicating possible enterohepatic recirculation. After intravenous administration, there was a substantial increase in the P-R interval (14.3 +/- 5.4%). Although only small changes in P-R interval were seen with a single oral dose, with chronic administration there was persistent P-R interval prolongation, peaking at 17.3 +/- 5.6% over control. Counterclockwise hysteresis was present in the P-R interval versus plasma diltiazem concentration curve after intravenous administration. Only small changes were seen in heart rate and blood pressure.