Alveolar gas exchange during breath holding with air

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Abstract
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. Submitted on October 17, 1962

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