Effects of quinidine and disopyramide on serum digoxin concentrations.

Although quinidine and digoxin are frequently given together, it has only recently become apparent that serum digoxin concentration may rise during quinidine treatment. A prospective study was performed to compare the effects of quinidine and disopyramide in patients receiving maintenance digoxin therapy. During quinidine administration serum digoxin concentration rose by more than 50% in seven of nine patients (the mean concentration rising from 1.43 +/- 0.20 to 2.61 +/- 0.43 nmol/l, P < 0.005). During the disopyramide treatment a small rise in serum digoxin was noted (mean 1.3 +/- 0.16 to 1.5 +/- 0.19 nmol/l, P < 0.05). We suggest that digoxin doses should be reduced immediately prior to commencing quinidine therapy in patients already receiving adequate maintenance digoxin, and patients should be followed carefully for evidence of digoxin toxicity. Disopyramide appears a suitable alternative anti-arrhythmic drug to quinidine in patients on maintenance digoxin.