Pharmacokinetics and Pharmacodynamics of Intravenous Diltiazem in Patients With Atrial Fibrillation

Background. Diltiazem, a calcium channel blocker, has been shown to be safe and effective in the treatment of patients in atrial fibrillation and/or atrial flutter. However, there have been no pharmacokinetic/pharmacodynamic studies of diltiazem in these patients. Methods and Results. The pharmacokinetics and pharmacodynamics of intravenous diltiazem were determined in 32 patients with atrial fibrillation or atrial flutter (mean+SD age, 66±7 years; mean baseline heart rate, 131±+10 beats per minute) after 20 mg or 20 mg followed by 25-mg bolus doses and a 10 and 15 mg/hr infusion for 24 hours. After the 10 and 15 mg/hr infusions of diltiazem, mean±SD elimination half-life was 6.8± 1.8 and 6.9± 1.5 hours, volume of distribution was 411+ 151.8 and 299±70.8 1, and systemic clearance was 42±f12.4 and 31±8.3 1/hr, respectively. Percentages of the plasma concentrations of the principal metabolites desacetyldiltiazem and N-desmethyldiltiazem to diltiazem were <15% and <10%o, respectively. Thirty of 32 patients maintained response throughout the 24-hour infusion of diltiazem. Using a sigmoidal Em. pharmacodynamic model, a strong relation (mean+SD r'2, 0.78±0.2) was observed between plasma diltiazem concentration and percent heart rate reduction. Mean±SD Em.. (maximum percent reduction in heart rate from baseline) and EC.50 (plasma diltiazem concentration that achieves half Em..x) were 52±17% and 110±84 ng/ml, respectively. The model predicts that mean plasma diltiazem concentration of 79, 172, and 294 ng/ml are required to produce a 20%, 30%1, and 40%Yo reduction in heart rate, respectively. A relation between plasma diltiazem concentration and percent change in systolic blood pressure (SBP) or diastolic blood pressure (DBP) from baseline was not observed (mean±SD r2, SBP/DBP: 0.35±0.24/036±0.2). There were no untoward side effects observed. Conclusions. First, the pharmacokinetics of diltiazem in patients with atrial fibrillation or atrial flutter is nonlinear with an apparent dose-dependent decrease in systemic clearance with increasing infusion rate. Second, using a sigmoidal E., model, there is a strong relation between plasma diltiazem concentration and percent heart rate reduction. Third, the plasma concentrations of the principal metabolites desacetyldiltiazem and N-desmethyldiltiazem are low and are not expected to contribute significantly to the pharmacodynamics of intravenous diltiazem in these patients. (Circulation 1992;86:1421-1428)