Arterial-expired PCO2 differences in the dog during acute hypercapnia

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Abstract
To determine if the suggestion that negative blood gas CO2 partial pressure (PCO2) differences exist in the dog during hypercapnia, as mean expired PCO2 exceeded arterial PCO2 by more than 10 Torr when the CO2 fraction in inspired gas (FICO2) was 0.1, this problem was reinvestigated in anesthetized dogs breathing spontaneously room air or hypercapnic mixtures (FICO2 = 0.05 or 0.10). During steady state arterial blood samples were analyzed with electrodes, care being taken to keep the electrode temperature within .+-. 0.2.degree. C at the actual aortic temperature of the animal. Respired gas was measured at the tracheostomy by a sensitive low-noise respiratory mass spectrometer. During room air breathing the arterial-end-expired PCO2 difference, P(a-E'')CO2, averaged +5 Torr and decreased to +0.9 Torr and to +0.1 Torr with FICO2 = 0.05 and 0.1, respectively. Hypoxia (FIO2 = 0.10) had no apparent effect on the P(a-E'')CO2 difference. The decrease in P(a-E'')CO2 with hypercapnia is ascribed to the diminishing effects of alveolar dead space, whereby end-expired PCO2 approaches arterial PCO2. In blood gas equilibration in lungs, PCO2 in end-capillary blood comes close to alveolar PCO2. The negative blood gas PCO2 differences reported earlier are probably caused by deficiencies in the techniques.

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