A new indirect method for measuring arteriovenous O2 content difference and cardiac output from O2 and CO2 concentrations by rebreathing air.

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Abstract
A new indirect method for measuring the arteriovenous O2 content difference (avDO2) was developed. The avDO2 was calculated by dividing the gradient of the CO2 dissociation curve by that of a gas exchange ratio against PCO2. The latter slope was obtained from O2 and CO2 concentrations in rebreathing air. The validity of the method was tested preliminarily in human subjects by comparing the cardiac output calculated from avDO2 and O2 uptake (.ovrhdot.VO2) with that measured hitherto by other authors and then in dogs by comparing the calculated avDO2 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 avDO2. For each rebreathing period the avDO2 was calculated by using the CO2 dissociation curve obtained in the individual dogs. The correlation coefficient between the measured and calculated avDO2 was 0.87, demonstrating reasonable validity of the method. The .ovrhdot.VO2 was further measured from the time interval during which a known amount of pure O2 was consumed. Then, the cardiac output was calculated by dividing the .ovrhdot.VO2 by the measured and calculated avDO2. The correlation coefficient between the respective cardiac output values was 0.88, indicating the reliability of using the calculated avDO2.

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