Alveolar-to-arterial PCO2 differences

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Abstract
Animals can be maintained in an apneic state for long periods of time by using a Sci-Med membrane oxygenator to clear CO2 from blood when a source of pure O2 is attached to the trachea. Under these circumstances alveolar CO2 partial pressure (PaCO2) rapidly comes into a steady state with blood and uniformity of alveolar compositon occurs. Because the lung acts as a tonometer for blood, adding O2 but not exchaning CO2, differences in regional blood flow affect the rate of change of PaCO2 but not the steady-state PCO2 level. After .apprx. 20 min of apnea, the PCO2 of the expirate was uniform after dead space clearance. Since this [dog] preparation eliminates the normal factors causing alveolar-to-arterial (A-a) PCO2 gradients, it seemed ideal to test for the presence of the positive A-a differences previously observed. A-a PCO2 differences ranging from 2-25 torr were found. These differences increased as a function of PCO2 and decreased as a function of arterial pH. Although these experiments do not shed light on the mechanism of this PCO2 difference, they give additional experimental support to the existence of the phenomenon.

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