A new indirect method for measuring the arteriovenous O
2 content difference (av
DO2) was developed. The av
DO2 was calculated by dividing the gradient of the CO
2 dissociation curve by that of a gas exchange ratio against
PCO2. The latter slope was obtained from O
2 and CO
2 concentrations in rebreathing air. The validity of the method was tested preliminarily in human subjects by comparing the cardiac output calculated from av
DO2 and O
2 uptake (
VO2) with that measured hitherto by other authors, and then in dogs by comparing the calculated av
DO2 with the measured value. In the dog experiments, the rebreathing was performed 7 times in each of 7 dogs. Immediately after the rebreathing arterial and mixed venous blood were sampled and analyzed for av
DO2. For each rebreathing period the av
DO2 was calculated by using the CO
2 dissociation curve obtained in the individual dogs. The correlation coefficient between the measured and calculated av
DO2 was 0.87, demonstrating reasonable validity of the method. The V
O2 was further measured from the time interval during which a known amount of pure O
2 was consumed. Then, the cardiac output was calculated by dividing the V
O2 by the measured and calculated av
DO2. The correlation coefficient between the respective cardiac output values was 0.88, indicating the reliability of using the calculated av
DO2.
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