Effects of hypoxemia on coronary blood flow and cardiac output in normal and hypothyroid dogs

Abstract

Simultaneous determinations of coronary blood flow by the nitrous oxide desaturation method and of cardiac output by the direct Fick arteriovenous oxygen difference method were made on dogs breathing 100%, 10% and 5% oxygen-nitrogen mixtures under morphine chloralose anesthesia. The observations were repeated with 100% oxygen and 10% oxygen- 90% nitrogen mixtures after a hypothyroid state had been induced by administration of 35 mc. of I 131. Parallel increases in coronary blood flow and cardiac index occurred in the normal dog with progressive hypoxcmia. In the hypothyroid dog the cardiac index did not change significantly, and the increase in coronary blood flow was only about 50 per cent of the value observed in the normal group. The failure of the hypothyroid dog to demonstrate an increased cardiac index in response to hypoxemia is attributed to the removal of the potentiating action of the thyroid hormones upon the catecholamines of the sympathoadrenal system. The reduced but still significant increase in coronary blood flow in the hypothyroid group is brought about by hypoxic stimuli that act independently of hormonal influences.