Estimates of the CO 2 pressures in systemic arterial blood during rebreathing on exercise

Abstract

Blood was sampled from the brachial arteries of 3 subjects on 120 occasions before or during the prolonged rebreathing of CO 2 in O 2 mixtures. Studies were made at rest and during 3 grades of exercise (300–900 kpm/min). The CO 2 tension in rebreathed gas was consistently higher than that measured in arterial blood and the difference increased with the level of exertion but at steady loads it did not change with time. The CO 2 contents of 45 samples were measured on the same occasions. Their tension-content coordinates fell on the in vitro dissociation curves of normal blood. Previous studies on the same subjects had shown a similar discrepancy between rebreathed gas and pulmonary arterial blood. In spite of these findings the indirect Fick method, based on rebreathing estimates of the mixed venous P CO 2 , and end-tidal estimates of the arterial P CO 2 , yields values for cardiac output which closely agree with those obtained in direct Fick and dye dilution studies published by others. The interpretation of oxygen based estimates of cardiac output is also discussed.