STUDIES ON DIGITALIS. VII. INFLUENCE OF HYPER- AND HYPOTHYROIDISM ON THE MYOCARDIAL RESPONSE TO OUABAIN

¹ Cardiology Branch, National Heart Institute, Bethesda, Maryland

The effect of a continuous, intravenous infusion of ouabain on right ventricular contractile force, ventricular automaticity, and the functional refractory period of the atrioventricular conduction system (AVFRP) was determined in three groups of anesthetized open-chest dogs. Observations were made in euthyroid animals, in animals rendered hyperthyroid with thyroid feeding and in animals which were rendered hypothyroid by pretreatment with I¹³¹.

Cumulative doses of ouabain in excess of 12 µg/kg were found to produce a significantly greater increment in the ventricular contractile force in the hypothyroid animals than in the hyperthyroid and euthyroid dogs. The dose of ouabain required to produce multiple premature ventricular contractions was significantly higher in the hyperthyroid dogs than in the other two groups of animals. A cumulative dose of ouabain of 42 µg/kg produced a significantly greater prolongation of the AVFRP in both the euthyroid and hypothyroid dogs than in the hyperthyroid animals. These observations indicate that while hyper- and hypothyroidism alter the response to ouabain, the three myocardial effects of ouabain which were studied were not altered in a uniform manner by changes in the thyroid state.