Serial determination of cardiac output during prolonged work by a CO2-rebreathing technique

Cardiac outputs were calculated on eight male college students while walking on a treadmill at controlled heart rate levels. The heart rate levels were maintained through the use of a heart rate controller (Quinton Model AI-607), standard deviations for heart rate in beats/min ranged from +/-2.1 at the heart rate level 110 to +/-3.2 at 170. Test-retest correlations of the cardiac output values at the various heart rate levels were: at heart rate 110, 0.66 after 7 min and 0.85 at 30 min; at heart rate 150, 0.92 after 7 min and 0.68 at 30 min; at heart rate 130, 0.97 after 7 min, and at heart rate 170, 0.85 after 7 min. The calculated cardiac output values when plotted against oxygen uptake compare favorably with published reports. The data collected demonstrate that the CO2-rebreathing method used in the present investigation provides a reliable bloodless technique for determining multiple cardiac output during prolonged work of varied intensities.