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1452 COMBINED EFFECT OF HYPOXIC HYPOXIA (HH) AND HYPERCAPNIA ON CEREBRAL BLOOD FLOW (CBF) IN LAMBS

CBF increases during both HH and hypercapnia. It is not clear, however, whether there is an interaction between these stimuli, so that the magnitude of the CBF response to HH is influenced by the prevailing PCO2, or whether they are independent. We examined this question in 7 anaesthetized, paralyzed, artificially ventilated lambs. Catheters were placed in the femoral artery for blood pressure monitoring, in the left ventricle for radioactive microsphere (MS) injection, in the brachiocephalic artery for withdrawal of the MS reference sample, and in the superior sagittal sinus for the withdrawal of cerebral venous blood. The animal was exposed to three levels of HH (Range PO2 79-18 mmHg) at a preset PCO2 of 22 ± 0.4 (±SEM) mmHg. PCO2 was then increased to 60 ± .8 mmHg and the protocol repeated. Arterial and cerebral venous blood was obtained for blood gas and oxygen content measurements at control PO2 and at each of 3 levels of hypoxia. MS CBF measurements were made at control PO2 and the lowest PO2. The slope of the CBF response and the slope of the CBF/CMRO2 (CBF ÷ cerebral O2 consumption) response to hypoxia were compared in each animal at each PCO2. The slopes increased at high PCO2, but not significantly. We conclude, therefore, that the effects of PCO2 and HH on CBF are independent.