Use of arterial PO₂ to study convective and diffusive gas mixing in the lungs

During ventilation, O₂ is transported through the lung gas by convective, diffusive, and cardiac mixing. To investigate the relative importance of each, the authors ventilated anesthetized, paralyzed dogs with mixtures of 40% O₂ in He or N₂ while a fast responding, Clark type O₂ electrode, exposed to a constant flow of blood from a common carotid artery, provided a continuous record of PaO₂. Because PaO₂ fell 14% more rapidly during a breath hold on N₂ than on He, it is concluded that a diffusion limitation to O₂ transport exists in the lung gas in the absence of the convection due to ventilation. When convection was included by resuming ventilation, the washin of O₂ as judged by the rate of rise of PaO₂, was 13% faster with N₂ than with He. Switching from He to N₂ during the breath hold resulted in an even faster washin and the opposite switch produced the slowest O₂ washin of all. Finally, the steady state difference pO₂ between inspired air and arterial blood was 6% smaller with N₂. It is concluded that Taylor diffusion plays a role in O₂ penetration through the airways, but in the alveolar spaces ordinary molecular diffusion influences further transport to the alveolar wall.