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The effects of triiodothyronine (T3) administration on digitalis requirements was studied in a group of patients with atrial fibrillation. The ventricular rate was utilized to provide a quantitative expression of digitalis effect. In one patient the amount of acetylstrophanthidin, administered as a constant infusion, required to slow the ventricular rate to 70 beats per minute rose from an average of 0.58 mg. while he was myxedematous, to 1.50 mg. when he was euthyroid. When mild thyrotoxicosis was induced in three euthyroid patients the daily dose of digoxin had to be increased approximately four-fold in order to maintain the ventricular rate at a control level. In three patients it was observed that reserpine or syrosingopine administered intramuscularly abolished the increased requirements for digoxin induced by T3. In two patients syrosingopine was found to prevent partially the tachycardia resulting from large doses of intravenous T3. The possible mechanisms responsible for these effects and their therapeutic implications are discussed.

(C) 1961 American Heart Association, Inc.