Clearance of infusion edema fluid at high and low levels of intracranial pressure.

We found that under conditions of sustained edema production, the rate of edema resolution by the CSF is not appreciably enhanced with lowering of the ICP. Although the entrance of edema fluid into CSF for the high-pressure series was notably slower, the amount of edema remaining in the brain was not appreciably affected. This appears somewhat contradictory to the principle that increasing the bulk flow pressure gradient should increase flow into the CSF. However, simultaneous steady-state measurements of the change in brain tissue pressure for a corresponding change in CSF pressure showed that the two pressures are identical. As a result, when ICP is lowered, the brain tissue pressure follows with no gradient increase. This also suggests that lowering of ICP and the concomitant reduction of tissue pressure may increase the intravascular tissue pressure gradient, which may act to increase the edema production. More work is required to resolve this issue.