Control of cardiac function and venous return in thyrotoxic calves

The mechanisms responsible for maintenance of the high-output state associated with thyrotoxicosis have been investigated by measurement of cardiac-function curves and venous compliance during ganglionic blockade with trimethaphan. Thirteen calves were injected daily with L-thyroxine (200 micrograms/kg) for 12-14 days. Thyroxine treatment increased heart rate (70%), left ventricular systolic pressure (22%), cardiac output (120%), left ventricular maximum rate of pressure development (dP/dt) (56%), and total blood volume (18%) and decreased systemic vascular resistance (39%). These hemodynamic changes persisted during ganglionic blockade or autonomic blockade with atropine and propranolol. Cardiac-function curves in conscious thyrotoxic calves were displaced upward and to the left. Mean circulatory filling pressure (MCFP), measured during anesthesia, was increased from 8 +/- 1 to 12 +/- 1 mmHg. During autonomic and ganglionic blockade MCFP remained elevated after treatment with thyroxine. Venous compliance decreased from 2.1 +/- 0.2 to 1.3 +/- 0.1 ml X mmHg-1 X kg-1 after thyroxine. Unstressed vascular volume was increased from 52.3 +/- 1.1 to 67.1 +/- 0.9 ml/kg. Thus the elevated cardiac output and new cardiac-function curve in thyrotoxicosis are associated with a combination of increased inotropic state, increased blood volume, and decreased venous compliance. These effects are not the result of autonomic influences and may represent direct actions of thyroid hormone on the heart and peripheral venous circulation.