Alveolar CO2 During the Respiratory Cycle

Abstract

Alveolar PCO2 plotted against time for the first 20 sec. of breath holding follows a logarithmic curve whose asymptote is virtual venous PCO2 and whose negative exponential constant equals the slope of the dissociation curve of CO2 times the rate of pulmonary bloodflow times B-47 divided by the equivalent lung volume. The latter is greater than the volume of alveolar gas by virtue of the capacity of capillary blood of the lungs, pulmonary tissue, and tissue fluid for CO2- Factors that may disturb this curve are: alteration of the rate of pulmonary bloodflow, recirculation, diffusion of alveolar gases into the dead space and vice versa, inhomo-geneity of the alveolar gas, and diminution of the lung volume due to the R. Q. Expiration causes the PCO2 in the alveolar gas to increase more rapidly than one finds for breath holding. This is due to diminishing lung volume. Fluctuations of about 2 mm. Hg in alveolar CO2 are computed for a normal respiration using the Fick principle and the relationship "concentration equals amount divided by volume" for each 0.1 sec. of the cycle. Mean alveolar CO2 can be sampled shortly after the middle of the expiratory period.