OXYGEN-TENSIONS MEASURED IN CAT CEREBRAL-CORTEX UNDER HYPERBARIC CONDITIONS

Pyrenebutyric acid (PBA), the intracellular fluorescent indicator, was used to measure the partial pressure of oxygen (PO2) in the exposed cerebral cortex of anesthetized cats at hyperbaric pressures up to 4 ATA. The validity of the PBA method for determining cortical PO2 was confirmed by demonstrating a precise linear relationship between PaO2 and the reciprocal of the fluorescence of PBA in the brain as the cat was ventilated with sequentially greater oxygen pressures while holding the PaCO2 nearly constant. Increments in the PaCO2 level resulted in stepwise greater oxygen tensions in the brain until an oxygenation end point was reached with a PaCO2 averaging near 122 Torr. Greater amounts of CO2 did not bring the mean PO2 of the brain, 1.017 Torr, closer to 2,000 Torr. During normocapnia the cortical PO2 was greater than the PO2 of cerebral venous blood collected from the superior sagittal sinus; however, in hypercapnia (PaCO2 > 45 Torr), the PO2 of the sinus blood exceeded the value determined in the cortex. This latter observation is taken as evidence for convective shunting of cerebral arterial blood to venous circulation when hypercapnia is present.