Stratification of ventilation and blood flow in the normal lung.

Abstract

The rate of change of gas tensions during breath holding was examined at 3 fixed points on expired gas tension plateaus following single inspirations of an argon-oxygen-nitrogen mixture. At each plateau point PA [Argon pressure] rose and PO2 [O2 pressure] fell linearly with time, and PcOo [CO2 pressure] followed an exponential course towards mixed venous PCO2- Rates of gas tension change were different at each plateau point. Expired gas tension plateaus corrected to 0 time, following different inspired test-breath volumes, were consistent with a lung model in which 2 alveolar populations were arranged in series with upstream: downstream volume and ventilation ratios in the range 1:1 to 3:1. Previously reported miltibreath and "argon bolus" studies of gas distribution in the normal lung were also consistent with this series model. Studies of expired gas plateaus when the preceding test breath also contained nitrous oxide allowed gas exchange phenomena to be separated from diffusive mixing effects. A gradient of diminishing Qc/Va [blood flow/unit alveolar volume] accompanied the effective gradient of diminishing Va/Va [alveolar ventilation/alveolar volume] down through the alveolar populations arranged in series, but P02 and PCOo gradients due to residual effective Va/Qc [ventilation/perfusion ratio] disparities persisted.