Cerebral circulation during acute microgravity induced by free drop in anesthetized rats

To evaluate changes in the cerebral circulation during acute microgravity (microG), we measured intracranial pressure (ICP), aortic pressure at the diaphragm level, and cerebral flow velocity (CFV) in anesthetized rats (n = 5) during 4.5 s of microG induced by free drop, then calculated arterial pressure at the eye level (AP(eye)) and cerebral perfusion pressure (CPP = AP(eye)-ICP), and estimated CPP-CFV relationship. The rats were placed in the flat and the 30 degrees head-up positions. In the head-up position, ICP, AP(eye), and CPP were significantly increased by 2.2 +/- 0.4, 12.3 +/- 2.0, and 10.1 +/- 1.7 mmHg respectively during microG, whereas the CFV did not change significantly. In the flat position, none of these variables were significantly affected by microG. The slope of the CPP-CFV relationship was decreased only in the head-up position, suggesting that the cerebrovascular resistance was increased by microG. These findings indicate that the change in gravitational (hydrostatic) pressure is a key factor in understanding the changes in cerebral circulation during acute microG.