The effect of inspired oxygen concentration on the ventilation-perfusion distribution in inhomogeneous lungs

The coupled conservation of mass equations for oxygen, carbon dioxide and n itrogen are written down for a lung model consisting of two homogeneous alv eolar compartments (with different ventilation-perfusion ratios) and a shun t compartment. As inspired oxygen concentration and oxygen consumption are varied, the flux of oxygen, carbon dioxide and nitrogen across the alveolar membrane in each compartment varies. The result of this is that the expire d ventilation-perfusion ratio for each compartment becomes a function of in spired oxygen concentration and oxygen consumption as well as parameters su ch as inspired ventilation and alveolar perfusion. Another result is that t he "inspired ventilation"-perfusion ratio and the "expired ventilation"-per fusion ratio differ significantly, under some conditions, for poorly ventil ated lung compartments. As a consequence, we need to distinguish between th e "inspired ventilation"-perfusion distribution, which is independent of in spired oxygen concentration and oxygen consumption, and the "expired ventil ation"-perfusion distribution, which we now show to be strongly dependent o n inspired oxygen concentration and less dependent oxygen consumption. Sinc e the multiple inert gas elimination technique (MIGET) estimates the "expir ed ventilation"-perfusion distribution, it follows that the distribution re covered by MIGET may be strongly dependent on inspired oxygen concentration . (C) 2000 Academic Press.