Effects of hypercarbia on autoregulation of brain blood flow and cerebral metabolism in newborn piglets.

Abstract

We tested the hypothesis that, in newborn piglets, hypercarbia impairs autoregulation of total and regional brain blood flow at the lower limb of the autoregulatory curve. Cerebral oxygen metabolism was measured in the same piglets to relate changes in metabolism to blood flow. Instrumented hypercarbic (n = 9) and normocarbic (n = 8) newborn piglets exposed to phlebotomy were studied during normotension and graded hypotension with mean arterial blood pressures of 55-41, 40-31 and < 30 mmHg. In the hypercarbic piglets, total brain blood flow decreased (P < 0.01) from the hypercarbic-normotensive value of 187 +/- 15 mliter min-1 100 g-1 to 139 +/- 18, 66 +/- 11 and 34 +/- 6 at mean arterial blood pressures of 55-41, 40-31 and < 30 mmHg, respectively; in the normocarbic piglets, total brain blood flow did not change from the normotensive value (70 +/- 11 mliter min-1 100 g-1) until the mean arterial blood pressure was < 30 mmHg, when brain blood flow had decreased (P < 0.01) to 49 +/- 8 mliter min-1 100 g-1. In the hypercarbic piglets, all brain regions (cerebrum, caudate nucleus, cerebellum, brainstem and medulla) demonstrated similar response patterns to that of total brain blood flow during hypotension. Thus, during hypercarbia, none of the brain regions demonstrated autoregulation. In the normocarbic piglets, cerebral blood flow decreased (P < 0.01) from the normocarbic-normotensive value of 74 +/- 6 mloter min-1 100 g-1 to 51 +/- 8 and 37 +/- 7 at mean arterial blood pressures of 40-31 and < 30 mmHg, respectively, and blood flow to the caudate nucleus, cerebellum and brainstem did not decrease significantly, and in fact increased (P < 0.01) to the medulla during hypotension. Although cerebral oxygen metabolism was compromised in the hypercarbic and normocarbic piglets, the relationship between metabolism and blood flow was altered such that the cerebral metabolic rate of oxygen per unit of blood flow was lower in the hypercarbic than the normocarbic piglets. We conclude that hypercarbia impairs total and regional brain blood flow autoregulation in newborn piglets.