Alveolar gas exchange during breath holding with air

¹ Department of Physiology, State University of New York at Buffalo, Buffalo, New York

Four normal subjects used a partial rebreathing procedure to deliver frequent alveolar gas samples during breath holding with air. In breath holding during mild exertion, O₂ uptake declined steadily while CO₂ transfer dropped abruptly and later ceased entirely as alveolar Pco₂ reached a virtual plateau. Application of estimated instantaneous values of R provided an indication of mixed venous blood values and permitted interpretation in terms of the changing venous-alveolar-arterial relationships. Arterial Pco₂ very early equaled mixed venous Pco₂ and then exceeded it increasingly. The plateau of alveolar Pco₂ was due in part to the fall in alveolar Po₂ since decreasing O₂ uptake lessened the CO₂-concentrating effect of lung-volume reduction while the Pco₂-elevating effect of blood oxygenation was also diminishing. However, an important slowing of the rise in mixed venous CO₂ content and Pco₂ was also noted and remains unexplained.

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