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

Steady state measurements of the diffusing capacity of the lung are made at operational lung volume, whereas measurements by the breath-holding method are customarily made at full inspiration. The effect of lung inflation on diffusing capacity must be taken into account before the steady state and breath-holding methods can be validly compared. At rest the single breath diffusing capacity of the lung for CO (DLCO) increases as the lung is inflated, presumably due to expansion of the alveolar-capillary surface; the volume of blood in the pulmonary capillary bed does not increase (1-5). During exercise the diffusing capacity, measured at full inspiration, increases with pulmonary blood flow, and this increase is associated with expansion of the pulmonary capillary blood volume (1, 2, 6, 7). It is not known how changing the lung inflation affects the expansion of the pulmonary capillary surface and blood volume during exercise. To investigate this problem we measured DLCO by the breath-holding technique at different lung volumes at rest and exercise in five healthy adults. Measurements were made at two different alveolar oxygen tensions to calculate membrane diffusing capacity for carbon monoxide (DMco) and pulmonary capillary blood volume (Vc) by the method of Roughton and Forster (8). We have compared our measurements of diffusing capacity obtained by the breath-holding method with steady state measurements from other laboratories.