Effect of exercise on dilution estimates of extravascular lung water and on the carbon monoxide diffusing capacity in normal adults.

Previous studies in exercising animals have demonstrated that the extravascular lung water accessible to measurement by dilution methodology increases in the transition from rest to low-level exercise and thereafter does not change with progress to high-level exercise. In normal humans, similar systematic examination is essential to provide a background for the interpretation of changes in measured extravascular lung water in pathophysiological states. Moreover, such an examination might provide new insight into the mechanisms underlying the change in the pulmonary diffusing capacity with exercise. We therefore measured both the pulmonary extravascular lung water (by use of the triple indicator-dilution technique) and the diffusing capacity for carbon monoxide in 11 subjects, seated on an exercise bicycle, at rest and usually during two levels of exercise. The central blood volume increased by 50% with a tripling of the cardiac output. The accessible lung water increased from an average of 2.16 g/kg to 2.55 g/kg in the transition from rest to low-level exercise, but it did not increase further at the higher level of exercise. The simultaneously measured diffusing capacity for carbon monoxide (single breath and steady state) continued to increase over the whole range of cardiac outputs. We infer that the proportion of the pulmonary parenchyma perfused by blood flow increases slightly in the transition from rest to low-level exercise but increases no further at the higher level of exercise. The continued increase in the pulmonary diffusing capacity over the range in which the estimated lung water values do not change appears to imply that part of this increase may be blood flow dependent rather than dependent on the recruitment of additional surface for exchange.