Cardiac output in the cold-stressed swimming rat

Abstract

Cardiac output was measured by the thermal dilution technique in rats forced to swim in water of various temperatures: 2, 16, 22, and 37 C. Observations were made relating cardiac output to body temperature, heart rate, and blood pressure during the swimming exercise. In water below thermoneutrality the core temperature decreased at a rate depending on the water temperature. Exhaustion occurred when body temperatures approximated 26 C. The swimming time to exhaustion in the 2, 16, and 22 C swims was approximately 5, 11, and 18 min., respectively. In 37 C water, where no decrease in core temperature occurred, the rats could swim easily for 30 min. with no signs of exhaustion. Cardiac output remained at about 50% above the resting level throughout the 37 C swims. In water below thermo-neutrality the dependence of cardiac output and heart rate on body temperature became evident. After an initial increase to approximately 50% above the resting level, the cardiac output fell progressively with the falling core temperature until it was some 20% below the resting value near exhaustion. A small increase in stroke volume occurred as the core temperature decreased. However, this increase was not sufficient to maintain the cardiac output as the heart rate decreased. Since the blood pressure remained at a fairly constant level throughout the swims in all water temperatures, a progressive increase in total peripheral resistance occurred in the cold swim.