Effects of Va and Va/Q distribution and of time on the alveolar plateau.

Abstract

These studies were undertaken in order to determine whether the changes in expired gas composition during the course of a single prolonged expiration are due to a difference in the time spent by the gas in the lungs or to uneven distribution of inspired gas and/or of ventilation-perfusion ratios. Insofar as the fraction of an inert gas (such as nitrogen when breathing air) is affected by the O2 and CO2 exchange, the ventilation effects (Va/Va) were studied using argon in the inspired gas as a tracer. Although both A and N2 are individually affected by alveolar gas exchange, their ratio is not, thus providing index of distribution of ventilation. The relative contribution of various zones of the lung to expired gas remains constant throughout expiration. Therefore changes in expired O2 and CO2 composition are due to a difference in the time spent in the lung. The results lead to the belief that in normal man only the central part of the respiratory units is ventilated by bulk flow, while gas in the peripheral areas is renewed only by diffusion.