The effect of flecainide acetate, a new antiarrhythmic, on plasma digoxin levels

The possible effect of oral flecainide acetate on steady-state digoxin levels was assessed in 15 healthy men. Each volunteer received digoxin 0.25 mg daily (8 AM) for 22 consecutive days and flecainide 200 mg bid (8 AM and 8 PM) on days 11 through 15. Plasma digoxin and flecainide levels were measured by radioimmunoassay and gas-liquid chromatography methods, respectively. Flecainide levels were within the range associated with suppression of premature ventricular contractions in patients. Mean plasma digoxin levels just before the 8 AM dose were 0.46 ng/mL on days 9 and 10 (baseline), 0.57 ng/mL (P less than .05) on day 13, and 0.49 ng/mL (not significant [NS]) on day 15. Compared with a mean six-hour postdose baseline digoxin level of 0.58 ng/mL, postdose levels were 0.62 ng/mL (NS) and 0.65 ng/mL (P less than .05) on days 13 and 15, respectively. On an average for each subject, predose and six-hour postdose digoxin levels increased by 24 +/- 35% and 13 +/- 19%, respectively, during coadministration. The changes in electrocardiographic intervals and vital signs that occurred during concomitant drug administration were not clinically significant although a slight prolongation of the PR interval was noted in some subjects. Unless plasma digoxin levels are in the upper end of the therapeutic range, changes in magnitude as observed in this study should be clinically inconsequential for most patients.