[Interaction of quinidine and digoxin in humans (author's transl)]

After full digitalisation, 11 healthy subjects received 0.375 mg digoxin daily as maintenance dose. Under steady-state conditions and determination of serum concentration and renal clearance of digoxin, they were then given 500 or 1000 mg quinidine daily in addition to the digoxin. While serum concentration of digoxin rose significantly from 0.75 +/- 0.2 ng/ml after one week on 500 mg quinidine, and to 1.8 +/- 0.6 ng/ml after 1000 mg of quinidine, renal digoxin clearance fell from 186.2 +/- 67.4 to 125.4 +/- 61.8 ml/min after 500 mg of quinidine. Raising quinidine dosage to 1000 mg daily caused no further digoxin clearance reduction. During the total experimental period endogenous creatinine clearance remained unchanged. The results indicate that the rise in serum digoxin concentration on simultaneous quinidine administration is largely due to reduction in renal digoxin clearance. A clinical observation confirms the considerable practical importance of this interaction.