Pulmonary gas exchange was studied in five normal subjects both at rest and during moderate steady-state exercise on a bicycle ergometer while breathing a) room air and b) a mixture of 20.9% O2-balance sulfur hexafluoride (SF6). The alveolar-arterial oxygen pressure differences (AaDO2) widened significantly from rest to exercise. Breathing the O2-SF6 mixture reduced the AaDo2 significantly from 10.9 to 4.2 Torr at rest and from 15.5 to 10.1 Torr during exercise (P less than or equal to 0.01). There were no concurrent changes in metabolism, cardiac output, or heart rate during the SF6 breathing. Possible changes in the anatomic shunt fraction, alveolar-end-capillary equilibration, or the distribution of blood flow cannot account for these observations. We conclude that the AaDO2 increase during exercise reflects an inhomogeneity of ventilation-perfusion ratios (VA/Q) most probably arising within regions of the lung (intraregional inhomogeneity) rather than between regions (interregional inhomogeneity).
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