Effect of diltiazem on renal clearance and serum concentration of digoxin in patients with cardiac disease.

The effect of diltiazem on digoxin serum concentration was evaluated in 9 patients who had been treated chronically for heart disease with digoxin, 0.25 mg/day. The indications for digoxin therapy were arrhythmias in 5 patients and mild heart failure in the other 4. Renal digoxin clearance was also evaluated in 8 of these patients. Serum digoxin concentration was measured at control, 7 +/- 2 days after initiation of 120 mg/day of diltiazem and 11 +/- 5 days after increasing the dose of diltiazem to 240 mg/day. Serum digoxin concentration was 0.9 +/- 0.4 ng/ml at control, 0.8 +/- 0.4 ng/ml with 120 mg/day of diltiazem, and 0.8 +/- 0.3 ng/ml during therapy with 240 mg/day. The differences between these values were not significant. Renal digoxin clearance also did not show a significant change after diltiazem therapy (44 +/- 15 ml/min before diltiazem and 46 +/- 13 ml/min with 240 mg/day of diltiazem). This study shows no effect of diltiazem in doses of 120 to 240 mg/day on serum digoxin concentration or renal digoxin clearance in patients who are treated chronically for heart disease with digoxin. In this dose range, diltiazem has advantages over verapamil, which markedly elevates digoxin levels.