Elevated alveolar PCO2 relative to predicted values during normal gas exchange

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Abstract
A negative aADCO2 [arterial alveolar CO2 gradient] was demonstrated during ventilation with hypercarbic gas mixtures and during rebreathing, but was never demonstrated during normal gas exchange. This anomalous behavior of CO2 was studied by comparing it to the behavior of 5 infused inert gases during normal gas exchange in 10 anesthetized mongrel dogs. The distribution of .ovrhdot.VA/.ovrhdot.Q [ventilation perfusion ratio] heterogeneity and the respiratory dead space in the animals was uantitated using excretion-solubility data from the 5 infused inert gases. The predicted excretion fraction (PACO2/P.hivin.VCO2) [alveolar CO2 partial pressure/mixed venous CO2 partial pressure] for CO2 was obtained from the inert gas excretion-solubility curve, using a measured solubility for CO2. The measured excretion fraction for CO2 (PACO2/P.hivin.VCO2), even after correction for Haldane effect, was significantly greater than the predicted fraction (P < 0.001). This corresponded to an alveolar PCO2 that exceeded the predicted value by a mean of 5.0 Torr.

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