Regional cerebral blood flow and tissue oxygenation during hypocarbia in geese

Very low arterial CO2 tension (PaCO2) experienced by birds during high-altitude flight may result in cerebral vasoconstriction and reduced cerebral O2 delivery. To examine this possibility, we measured regional cerebral blood flow (CBF) and tissue PO2 in pentobarbital-anesthetized geese (Anser domesticus). Twenty-five-micrometer Teflon-coated platinum electrodes for H2-clearance measurements of local blood flow or tissue PO2 were implanted in the cerebral cortex in 11 geese. Tissue H2 and O2 were measured by voltage clamping the electrodes at +0.30 and -0.5 V, respectively. Washout kinetics of H2 gas administered via unidirectional lung ventilation was used to calculate local blood flow for those electrodes exhibiting one- or two-compartment washout kinetics of H2 (128 of 296 washouts in 31 electrodes). PaCO2 was controlled between 8 and 55 mmHg by altering pulmonary gas flow or by adjusting inspired PCO2. CBF decreased as PaCO2 fell from 50 to 20 mmHg but did not decrease further as PaCO2 was reduced below 20 mmHg. CBF was uniformly distributed in different regions of the cortex. Despite the plateau in CBF during severe hypocapnia, tissue PO2 continued to decline as PaCO2 fell below 20 mmHg. Severe alkalosis may limit cerebral O2 delivery in birds during high-altitude flight.