Cardiovascular responses to beta-stimulation with isoproterenol in deep hypothermia

The aim of this study was to investigate the effects of beta-stimulation in deep (25 degrees C) hypothermia. Cardiac catheterization was performed on seven anesthetized beagle dogs. They were cooled between ice bags down to 25 degrees C and received isoproterenol administered intravenously three times: at the normal body temperature (37 degrees C) before cooling, after cooling at 25 degrees C, and after rewarming at 37 degrees C. Circulatory function was measured for every 1 degree C of temperature change. Isoproterenol infusion at 37 degrees C induced cardiac acceleration, including the increases of heart rate, cardiac output, and peak first derivative of the left ventricular pressure curve. Systemic vascular and mean outflow resistances and mean aortic pressure decreased. During cooling, shivering thermogenesis continued, even down to 25 degrees C. At 25 degrees C, cardiac acceleration after isoproterenol infusion did not exist but relaxation rate increased slightly. Systemic vascular and mean outflow resistances decreased, but left ventricular end-diastolic and filling pressures increased. beta-Stimulation at normal body temperature increases shivering thermogenesis during cooling. The venous return to the left ventricle at 25 degrees C increased after isoproterenol infusion while systemic vascular resistance decreased, indicating systemic vasodilatation. This increase in preload is probably due to vasoconstriction in pulmonary vessels, which may be mediated by prejunctional beta-adrenoceptors. For cardiac inotrophy, the isoproterenol had no physiologically significant effects at 25 degrees C. After rewarming at 37 degrees C, the effects of isoproterenol were physiologically similar to the effects at the same temperature before cooling.