Pharmacokinetics of digoxin in patients subjected to the quinidine-digoxin interaction.

1 This study was designed to evaluate pharmacokinetically the digoxin-quinidine interaction in patients with atrial fibrillation. 2 Five patients on maintenance digoxin therapy were given [3H]-digoxin as a single i.v. dose before and during quinidine therapy and the elimination of [3H]-digoxin from plasma and excretion in urine were determined. 3 The mean steady state plasma concentration of digoxin increased from 0.7 to 1.3 nmol/l after quinidine administration. 4 The apparent volume of distribution of digoxin decreased on the average 38%. Renal clearance and the total body clearance of digoxin decreased 51 and 56% respectively (mean values). Also non renal clearance was reduced. The fraction of digoxin excreted unmetabolised in urine did not change during quinidine treatment. The mean elimination half life of digoxin increased from 49 to 72 h during quinidine. 5 In two patients the DC-shock did not cause a conversion to sinus rhythm. However, the quinidine induced changes in the pharmacokinetics of digoxin in these patients did not differ from the others. 6 Quinidine appears to decrease the amount of digoxin distributed to body tissue(s). In addition, the reduction of renal clearance of digoxin and the observed unchanged clearance of creatinine suggests an inhibition of the renal secretion of digoxin.