Instability of lung units with low Va/Q ratios during O₂ breathing

¹ Department of Medicine, School of Medicine, University of California, San Diego, La Jolla, California 92037

Using a multiple inert gas infusion method, we have observed the development of shunts during oxygen breathing in lungs which contained areas of low ventilation-perfusion ratios while breathing air. This paper gives a theoretical analysis of the factors involved. When the inspired ventilation-perfusion ratio (Va_I/Q) of a lung unit is gradually reduced, a point is reached where the expired ventilation falls to zero. Such a unit will no longer eliminate gas but may continue gas uptake unless it becomes atelectatic. This critical Va_I/Q is determined by the net flux of O₂, CO₂, and N₂ from alveolar gas to capillary blood, and its value increases from about 0.001 to 0.1 as the inspired gas is changed from air to 100% O₂. The critical Va_I/Q at any inspired O₂ concentration is raised if the O₂ or N₂ content of mixed venous blood are reduced or if N₂ is replaced by a more soluble gas. In distributions of ventilation-perfusion ratios, the amount of shunt which develops during oxygen breathing depends on the degree of dispersion of the Va_I ratios. The release of hypoxic vasoconstruction following O₂ administration, in general, reduces the amount of shunt.

Submitted on August 2, 1974