Update on the influence of quinidine and other agents on digitalis glycosides

Abstract

Since this topic was last reviewed,’ additional data on previously described interactions have been published and certain new interactions have been described. This update follows the format of the earlier review by first discussing new data on the quinidine-digoxin interaction and then discussing related interactions by drug class or mechanism of interaction. QUINIDINE-DIGITALIS INTERACTIONS Ouinidine-digoxin interaction. As previously reviewed,’ quinidine therapy can be expected to double or triple the serum digoxin concentration (SDC) in most, if not all, patients who achieve a serum quinidine concentration (SQC) of 2 pg/ml or greater. This interaction occurs within a few hours after starting quinidine therapy but may require several days or even weeks before the new steady-state SDC is achieved. It appears that quinidine alters digoxin kinetics by reducing the apparent volume of distribution (Vd), renal clearance (Clr), and nonrenal clearance (Clnr). Although the degree of reduction of Clnr is probably not dependent on the SQC, the reduction in Vd and Clr is greater at higher SQC. Even though the data on the mechanisms of the interaction suggest that the interaction should occur in patients with renal failure, conflicting views exist. Doering erroneously suggested that the interaction should not occur in anuric patients, while others3’4 have demonstrated the interaction in such patients. Hirschberg et a1.3 found that 4 days of quinidine therapy produced a mean increase of 57% in the SDC. However, the graphic presentation of their data appears to suggest that the greatest increases in