Effect of lung inflation on pulmonary diffusing capacity at rest and exercise.

Abstract

Membrane diffusing capacity increases as the lung volume expands from functional residual capacity (FRC) to total lung capacity (TLC). The increase is proportional to the estimated increase in alveolar surface area and occurs at both rest and exercise in humans. Pulmonary capillary blood volume is little affected by expansion of the lung from FRC to TLC either at rest or exercise. Steady state lung diffusing capacity (Dl) values are compared with single breath Dl values taking lung volume into account. The resting steady state DlcO is lower than the DlCO measured during breath holding. During exercise the steady state and single breath DlCO are similar. Reported values for Dlo2 measured by the Lilienthal-Riley technique are lower than the DlO2 calculated from single breath data at rest, but the 2 methods give similar values for exercise. This implies that measurements of single breath DlCO at operational lung volume during exercise may be translated into terms of O2 transport. The discrepancy between the steady state and breath-holding Dl at rest and their similarity at exercise may be explained by postulating that the distribution of ventilation and perfusion with respect to diffusing capacity becomes more uniform from rest to exercise.