Digoxin — Quinidine Interaction

To the Editor:—1 read with interest the recent article on monitoring digoxin therapy in long-term facilities, by Carter et al.1 Although the authors emphasized the importance of recognition of drug interactions, their list of drugs known to interact with digoxin failed to include quinidine. This omission is unfortunate, not only because patients with atrial fibrillation, which was one of the two disorders evaluated by the authors for digoxin therapy, frequently receive quinidine, but because of the high prevalence of interactions between quinidine and digoxin, which is now well recognized. That quinidine displaces digoxin from tissue-binding sites is a major mechanism of the interaction.2 It has been shown that the serum digoxin levels doubled in patients receiving quinidine, and the prevalence of such interactions was greater than 90 per cent.3 No significant association was found between the magnitude of the serum digoxin increase and age, sex, type of heart disease, or initial serum digoxin level.3 Since no criteria are available to predict the serum digoxin increases in these patients, the physician's attempt to adjust the digoxin dose is hampered when quinidine is being used at the same time. Thus, careful monitoring of both the serum digoxin and quinidine levels is advised whenever both drugs are used, especially in the geriatric population.