IMPEDANCE MEASUREMENTS OF CARDIAC OUTPUT DURING MODERATE HEAT EXPOSURE.

Abstract : Cardiac outputs of nude resting subjects were estimated from measurements of the intrathoracic impedance pulses as recorded with the four electrode system. Ambient temperature was increased from 28C to 43C. Increases in cardiac output were small, (about 1.0 L/sq.m./min) variable, and due to small increases in heart rate and stroke volume. The greater cardiac output during heat was probably due to the cutaneous vasodilatation as demonstrated in the cutaneous opacity pulses, but this relation was not demonstrated decisively. The ratio of increase in cardiac output to sweating was about the same as that shown previously for cutaneous blood flow and sweating. A decrease occured in intrathoracic impedance indicating a greater volume of electrolyte fluid in this area. Validation of the thoracic impedance pulses as a measure of right ventricular stroke volume was accomplished in human and canine subjects. (Author)