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

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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 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 VaI/Q at any inspired O2 concentration is raised if the O2 or N2content 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 O2 administration, in general, reduces the amount of shunt.

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