Analysis of cerebral blood flow and oxygen metabolism under mild brain hypothermia

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To assess the impact and physiological mechanisms of mild hypothermic therapy for brain ischemia, the changes in cerebral blood flow and cerebral oxygen tension during and after bilateral ischemia were measured for 30 min. in gerbil rats under normothermic (-37ﾟC) and hypothermic (-33.5ﾟC) conditions. Brain ischemia was induced by bilateral occlusion of internal carotid arteries of the animals. Cerebral blood flow (CBF) and tissue oxygen tension (PTO_2) were measured by Laser-Doppler flowmetry and by polarographic oxymetry, respectively. CBF fell rapidly to background level of the Laser-Doppler flow meter by -15 sec after the start of ICA occlusion, but PTO_2 showed slower reduction. Also, the restoration of PTO_2 to pre-ischemic level was retarded than that of CBF.The obtained profile of PTO_2/CBF ratio during recovery from ischemia showed that the oxygen metabolism in the brain was better maintained in the mildly hypothermic condition.Together with these studies, a new system was developed to visualize the changes in oxygen tension on the cortical surface by using an oxygen-quenching fluorescent membrane technique, recently developed by us for in vivo visualization of PO_2 distribution in the rat mesenteric microcirculation, as epifluorescent microscopic patterns on the oxygen-sensitive membrane. This visualization technique was considered to be a novel potent tool for the study of oxygen transport and metabolism in the ischemic brain.Also, to further explore the effect of mild hypothermia on cerebral oxygen metabolism, rheological (blood viscosity in microcirculation) and physical (diffusion coefficient of oxygen in viscous model solutions) factors were assessed as a function of temperature.