A preliminary study on determining the time window of hypothermia cerebral protection in rat cortex by laser speckle flowmetry

Laser speckle imaging technique was used to characterize the spatiotemporal changes in cerebral blood flow (CBF) in rat cortex induced by the local ultraprofound hypothermia (0 degrees C) with the duration time of 1 min, 2 min, 5 min, 7 min and 10 min. The experimental results showed significant difference of the spatiotemporal. characteristics of changes in CBF between short term and long term of ultraprofound hypothermia. For the short duration of ultraprofound hypothennia (I min, 2 min and 5 min), the hypothermia cause the CBF decrease firstly, and then the CBF increase rapidly when the temperature is recovered to 37 'C, exceeding the baseline level and lasting 10 3 min, finally return to the baseline. This trend of changes in CBF is similar in the regions of artery, vein and parenchyma, but with different amplitude. For the duration time of 7 min, the changes in CBF also exhibit the similar decrease induced by ultraprofound hypothermia and the rapid increase induced by the temperature recovering, however the increase does not show the overshoot, but only reach around 75% of the baseline level. For the duration of 10 min of ultraprofound hypothermia, the CBF does not increase rapidly when the temperature is recovered to 37 degrees C, but remains at the low level of CBF for 12 +/- 2 min, and then increases gradually at artery sites, or increases rapidly and then decrease slightly later at the vein and parenchyma sites. Similar as the case in the duration time of 7 min, the final CBF only recovers to about 75% of the baseline level. The experimental results suggest that the CBF can not recover to the baseline after a long duration of ultraprofound hypothermia longer than 7 min.