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

为探讨亚低温治疗对脑缺血的影响及其生理机制，在常温（-37 ℃）和低温（-33.5 ℃）条件下，测定了沙土鼠双侧脑缺血30 min时脑血流量和脑氧分压的变化。采用双侧颈内动脉阻断法制备脑缺血模型。分别用激光多普勒血流仪和极谱法测定脑血流量（CBF）和组织氧分压（PTO_2）。伊卡阻断后15秒CBF迅速下降至激光多普勒血流仪的背景水平，而PTO_2下降较慢。脑缺血恢复过程中脑组织的氧代谢在轻度低温条件下得到了较好的维持。与此同时，我们还建立了一种新的系统，用氧猝灭荧光膜技术观察脑皮层表面氧分压的变化。本文介绍了我们最近研制的一种在体观察大鼠肠系膜微循环氧敏感膜上PO_2分布的显微荧光图象。此外，为了进一步探讨亚低温对脑氧代谢的影响，本实验还研究了亚低温对脑氧代谢的影响，并对脑氧代谢的流变学（微循环中血液粘度）和物理学（粘性模型溶液中氧的扩散系数）进行了研究。

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.