Brain blood flow and O2 delivery during hemorrhagic hypotension in the piglet.

Abstract

Summary: We have previously demonstrated regional differences in brain blood flow reduction when hemorrhagic hypotension was induced in spontaneously breathing newborn piglets. Both the mean arterial blood pressure and Paco2 correlated with the changes in brain blood flow. To determine if changes in brain blood flow during hypotension can be attributed to alterations of mean arterial blood pressure alone, eight newborn piglets were studied under conditions of controlled ventilation. Regional brain blood flow was measured by microspheres and the blood pressure was varied by phlebotomy. After baseline determinations, three measurements of blood flow were performed during hypotension. Hypotension produced a significant reduction in blood brain flow confirming the independent effect of mean arterial blood pressure in autoregulation of cerebral circulation. Significantly different regional blood flow responses occurred in response to hypotension (P < 0.05). The % reduction from control blood flow was greatest to the cerebrum, less to the cerebellum, and least to the brainstem. Furthermore, the blood flow was consistently decreased in the cerebral hemispheres during the entire exposure to hypotension (mean ± S.E.: −36 ± 10%, −18 ± 11%, and −33 ± 11% change from control) whereas the brainstem demonstrated an intial reduction and then an increase in blood flow (mean ± SE: −14 ± 12%, +16 ± 15%, and +5.5 ± 13% change from control). The cerebellar responses were intermediate between those of the cerebrum and brainstem. O2 delivery was significantly decreased to all brain regions during hypotension, maintaining the same rank order of the regions as when blood flow reductions occurred. Speculation: Newborn piglets subjected to profound hypotension demonstrate regional differences in the reduction of brain blood flow. Cranial window studies of the microvasculature in adult cats clearly demonstrate that precapillary vessels in the brain do not respond in an identical manner to hypotension but rather react in a size dependent fashion. The observed differences in brain regional blood flow are possibly a manifestation of the relative composition (with respect to vessel size) of the regional parenchymal arterial beds.