Alterations in Distribution of Blood Flow to the Lung's Diffusion Surfaces During Exercise

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Abstract
We measured simultaneously, by single breath methods, pulmonary capillary blood flow (Q̇c), carbon monoxide diffusing capacity (DLCO), and isotopic oxygen (18O18O) diffusing capacity (DL18O2) in five normal males during conditions of rest and moderate exercise at mixed venous O2 tensions (PO2 33-44 mm Hg). During moderate exercise at a work load of 100 W. pulmonary capillary blood flow increased from 6.9±1.5 to 12.9±3.4 min-1 and DL18O2 increased from 25±4 to 43±3 ml·min-1·mm Hg-1, whereas DLCO showed no significant change (45±5 to 49±10 ml·min-1·mm Hg-1). DL18O2 increased proportionally to Q̇c (r = 0.74), where DLCO did not (r = 0.08). The greater increase in DL18O2 during exercise can be explained by a more homogeneous diffusion/perfusion (DLO2/Q̇c) distribution in the individual respiratory exchange units during exercise. This improved distribution of DLO2/Q̇c acts to help prevent an increase in alveolar-arterial O2 tension difference from developing despite the decrease in pulmonary erythrocyte transit times that occur during exercise. The insignificant rise in DLCO with exercise under these hypoxic breathholding conditions may result from pulmonary vasomotor responses to short-term hypoxia or from relative insensitivity of DLCO to moderate levels of exercise.