Studies on digitalis. III. The influence of triiodothyronine on digitalis requirements.

T HERE HAS BEEN considerable interest in whether or not changes in the thyroid state modify the effectiveness of digitalis. A number of reports have suggested that patients with hyperthyroidism associated with congestive heart failure or atrial fibrillation are relatively unresponsive to the usual doses of digitalis.1-8 Others have indicated, however , that these patients may respond to the ordinary dose of digitalis.9"-" Some observers5 have advocated that great caution should be exercised in administering digitalis to patients with thyrotoxicosis, and the possibility that digitalis may result in myocardial necro-sis in hyperthyroidism has been suggested by experimental studies.12 These widely varied views are based primarily on clinical impressions. The present investigation was therefore designed to evaluate , in quantitative fashion, the changes in digitalis requirements that follow alteration of the thyroid state induced by triiodothy-ronine administration. In addition, an attempt was made to elucidate the mechanism involved in the observed changes in digitalis requirements. Portions of this study have been presented elsewhere in preliminary form.13 Methods Six patients with inactive rheumatic heart disease and one patient (F.G.) with arteriosclerotic heart disease were studied; these patients ranged in age from 20 to 60 years and all had chronic atrial fibrillation. None of the patients was in con-gestive heart failure at any time during the investigation. One patient (J.D.) had been rendered myxedematous by means of J131-indueed thyroid ablation for congestive heart failure, which had existed 2 years prior to this study. One patient had spontaneous myxedema. Alterations of the metabolic state were induced by the daily oral administration of 75 to 500 jug. of triiodothyronine (T3) in divided doses, or by a single intravenous injection of this drug. The effects of this medication on heart rate, basal metabolic rate, serum cholesterol, and protein-bound iodine were noted. In all patients in whom hyper-thyroidism was induced by means of oral T3, an increase in nervousness, fatigability, and sweating was observed. The ventricular rate was utilized to provide a quantitative expression of digitalis effect throughout these studies. In all but the first patient described below, the resting ventricular rates were determined each morning for a 2-minute period after the patient had been in the recumbent position for 30 minutes. Experimental Procedures and Results I. J.D., a myxedematous patient with rheumatic heart disease and chronic atrial fibril-lation, was studied by determining the manner in which the administration of T3 altered the amount of acetylstrophanthidin required …