Quinidine-digoxin interaction: evidence for involvement of an extrarenal mechanism

The influence of quinidine 750mg per day for one week on serum digoxin concentration (SDC) was evaluated in digitalized anuric patients on chronic haemodialysis. During quinidine administration the SDC increased markedly, from 0.84 +/- 0.37 to 1.58 +/- 0.72 ng/ml (p less than 0.01), a comparable effect ot that reported previously in patients with normal renal function. Neither in vitro nor in vivo did quinidine alter the serum protein binding of digoxin. The increase in SDC in anuric patients indicates a decrease in the extrarenal clearance of digoxin, which means that mechanisms other than of renal origin are also involved in the interaction of quinidine and digoxin. There was great interindividual variability in the extent of the quinidine-induced rise in SDC. Regardless of the state of renal function, careful monitoring of digitalized patients seems mandatory once quinidine treatment is initiated.