Hypoxia and hyperoxia both transiently affect distribution of pulmonary perfusion but not ventilation in awake sheep

Despite a remarkable gravity independent heterogeneity in both local pulmonary ventilation and perfusion, the two are closely correlated at rest and during exercise in the normal lung. These observations strongly indicate that there is a mechanism for coupling of the two so that local V˙/Q˙‐ratio is kept fairly uniform throughout the lung. This is also necessary to achieve adequate gas exchange in the lung. It was recently suggested that oxygen‐induced vasoconstriction has a slow and intense component that might contribute to the matching of ventilation and perfusion also under normal conditions ( Vejlstrup & Dorrington 1993). We therefore simultaneously determined distribution of local (≈ 1½ cm3 lung pieces) ventilation and perfusion in eight sheep at normoxia (FiO2 21%) and after 10 min and 2½ h exposure to hypoxia (FiO2 12%; four sheep) or hyperoxia (FiO2 40%; four sheep). We used a ≈ 1 μm wet fluorescent aerosol and 15 μm radioactive microspheres i.v. to measure local ventilation and perfusion, respectively. Neither hypoxia nor hyperoxia caused changes in the distribution of ventilation. After 10 min exposure to hypoxia or hyperoxia, distribution of perfusion was altered so that the correlation between values for local ventilation and perfusion decreased. After 2½ h exposure to either hypoxia or hyperoxia, distributions of perfusion and v˙/Q˙‐ratio had returned to baseline. These results show that distribution of perfusion is influenced by acute changes in oxygen tension, so that local matching of ventilation and perfusion is affected. Apparently, some mechanism restores the matching during extended exposure to the altered oxygen tension.