The quinidine-digoxin interaction in patients with impaired renal function.

Quinidine has been reported to reduce clearance and the distribution volume of digoxin. Data are presented indicating that serum digoxin concentration (SDC) is increased throughout the coadministration of quinidine. This strongly suggests that the quinidine-induced reduction of digoxin clearance is the main mechanism underlying this drug interaction. It has been suggested that beside renal clearance quinidine also reduces non-renal clearance of digoxin. Direct evidence is provided by a study in patients with impaired renal function. Irrespective of the degree of renal impairment, quinidine increases SDC to about the same amount as found in patients with normal renal function. Since quinidine does not interfere with plasma protein binding of digoxin, this implies a decrease in non-renal clearance. In all patient groups the incidence of this drug interaction is rather high; however, pronounced interindividual differences occur as regards the extent of the increase in SDC. Regardless of the state of renal function careful monitoring of digitalized patients is mandatory once quinidine therapy is initiated. Since it may take a week or more until a new steady state is established in patients with impaired renal function, this period of close monitoring should be extended correspondingly.