Digoxin-quinidine interaction in the dog

In seven healthy dogs, digoxin was given as an oral loading dose (0.05 mg/kg/day) on the first day, followed by an oral maintenance dose (0.02 mg/kg/day) during the next 14 days. On the sixth day of digoxin treatment, oral quinidine (200 mg b.i.d.) was added until the tenth day. Plasma concentrations of digoxin and quinidine were measured; in three of the seven dogs ECG and physical signs of digitalis toxicity were evaluated. The average steady state plasma concentration of digoxin increased significantly (P less than 0.01) during quinidine administration (from 1.4 to 2.3 ng/ml). On the days that digoxin was administered without quinidine, none of the dogs vomited nor was anorectic; the PQ-interval increased significantly (P less than 0.01) between 0.01 and 0.03 s. When quinidine was added, vomiting and anorexia occurred but no further increases in the PQ-interval were seen.