Autoregulation of brain blood flow in the newborn piglet: regional differences in flow reduction during hypotension

The potential use of a piglet as a model for investigation of brain blood flow was evaluated by assessing the presence of autoregulation in 11 spontaneously breathing newborn piglets. Blood pressure was altered by phlebotomy. When the mean arterial blood pressure was greater than 50 mm Hg, no significant change in brain blood flow (microsphere technique) occurred (r = 0.04), indicating the presence of autoregulation. When the animals became hypotensive a pressure passive relationship exists between brain blood flow and mean arterial blood pressure. However, since the piglets breathed spontaneously and hyperventilated during hypotension, both the mean arterial blood pressure and PaCO2 fell and both correlated with brain blood flow. Thus, it cannot be determined which factor is responsible for the reduction in flow. The blood flow to the specific regions of the brain (cerebrum, cerebellum, brainstem) and mean arterial blood pressure also showed no correlation when the latter was greater than 50 mm Hg. During hypotension, each region demonstrates pressure passive relationships, but the reduction in blood flow is most pronounced in the cerebrum, less in the cerebellum, and least in the brainstem (mean +/- S.E., 64 +/- 8%, 41 +/- 13%, 32 +/- 13% reductions from control respectively, P less than 0.05). The study indicates that a newborn piglet may serve as an appropriate model for the study of brain hemodynamics particularly with regard to autoregulation. Furthermore, during hypotension, preferential protection of vital regions of the brain (cerebellum and brainstem) occur which may have important implications in interpreting the effect of hypotension on the newborn central nervous system.