Quinidine-Digoxin Interaction

Abstract

To the Editor.— We read the report by Leahey et al (240:533,1978) of a possible interference with serum digoxin concentrations by quinidine. Indeed, we as well as others have made similar observations. 1 However, based on the data presented, we have some reservations regarding the incidence of this interaction. Many of the serum digoxin concentrations reported as steady-state values before quinidine therapy were considerably lower than would have been expected based on the dose of digoxin supposedly being administered. We estimated the steady-state serum digoxin concentrations for hypothetical patients with creatinine clearances of 50, 75, and 100 mL/min to be 1.9, 1.5, and 1.2 ng/mL, respectively. 2 (To make these calculations, we assumed a lean body weight of 70 kg and daily administration of 0.25 mg of digoxin with 80% absorption.) Since these calculated serum digoxin concentration values are considerably higher than many of those reported by the authors, we question