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

Abstract

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 (VaI/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 VaI/Q is determined by the net flux of O 2 , CO 2 , 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% O 2 . The critical VaI/Q at any inspired O 2 concentration is raised if the O 2 or N 2 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 VaI ratios. The release of hypoxic vasoconstruction following O 2 administration, in general, reduces the amount of shunt.