Gas exchange data does not indicate the presence of right-to-left intrapulmonary shunt developing with exercise, despite recent data suggesting that large-diameter arteriovenous shunt vessels may be recruited with exercise. Inert gas data has shown that the increase in A- aDO 2 is explained by decreased ventilation-perfusion matching, and the development of a diffusion limitation for oxygen. While there is great individual variability, A- aDO 2 is generally largest at peak exercise in subjects with the highest peak oxygen consumption. In healthy subjects, gas exchange, as evaluated by the alveolar-to-arterial PO 2 difference ( A- aDO 2), worsens with incremental exercise, and typically reaches an A- aDO 2 of approximately 25 mmHg at peak exercise. ![]() As the first step in the oxygen-transport chain, the lung has a critical task: optimizing the exchange of respiratory gases to maintain delivery of oxygen and the elimination of carbon dioxide.
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