EFFECTS OF HYPERCARBIA ON AUTOREGULATION OF BRAIN BLOOD-FLOW AND CEREBRAL METABOLISM IN NEWBORN PIGLETS

We tested the hypothesis that, in newborn piglets, hypercarbia impairs autoregulation of total and regional brain blood flow at the lower li mb of the autoregulatory curve. Cerebral oxygen metabolism was measure d in the same piglets to relate changes in metabolism to blood flow. I nstrumented hypercarbic (n = 9) and normocarbic (n = 8) newborn piglet s exposed to phlebotomy were studied during normotension and graded hy potension with mean arterial blood pressures of 55-41, 40-31 and <30 m mHg. In the hypercarbic piglets, total brain blood flow decreased (P < 0.01) from the hypercarbic-normotensive value of 187+/-15 mt min(-1) 100g(-1) to 139+/-18, 66+/-11 and 34+/-6 at mean arterial blood pressu res of 55-41, 40-31 and <30 mmHg, respectively; in the normocarbic pig lets, total brain blood flow did not change from the normotensive valu e (70+/-11 mL min(-1) 100 g(-1)) until the mean arterial blood pressur e was (30 mmHg, when brain blood flow had decreased (P < 0.01) to 49+/ -8 mL min(-1) 100 g(-1). In the hypercarbic piglets, all brain regions (cerebrum, caudate nucleus, cerebellum, brainstem and medulla) demons trated similar response patterns to that of total brain blood flow dur ing hypotension. Thus, during hypercarbia, none of the brain regions d emonstrated autoregulation. In the normocarbic piglets, cerebral blood flow decreased (P < 0.01) from the normocarbic-normotensive value of 74+/-6 mL min(-1) 100 g(-1) to 51+/-8 and 37+/-7 at mean arterial bloo d pressures of 40-31 and <30 mmHg, respectively, and blood flow to the caudate nucleus, cerebellum and brainstem did not decrease significan tly, and in fact increased (P <0.01) to the medulla during hypotension . Although cerebral oxygen metabolism was compromised in the hypercarb ic and normocarbic piglets, the relationship between metabolism and bl ood flow was altered such that the cerebral metabolic rate of oxygen p er unit of blood flow was lower in the hypercarbic than the normocarbi c piglets. We conclude that hypercarbia impairs total and regional bra in blood flow autoregulation in newborn piglets.