The effect of head-down tilt and water immersion on intracranial pressure in nonhuman primates.

A headward fluid shift occurs in humans exposed to space-flight. This shift is thought to be the stimulus for the observed reduction in plasma volume, and may produce a sustained rise in intracranial pressure (ICP). In order to determine the effect of head-down tilt (HDT) and water immersion on intracranial pressure (ICP), five anesthetized rhesus monkeys were fitted with intracranial pressure-monitoring transducers and subjected to -6 degrees HDT followed by head-out thermoneutral (34.7 degrees) water immersion. ICP was elevated from 3.8 +/- 1.1 mm Hg in the horizontal control period to 5.3 +/- 1.3 mm Hg (p less than 0.05) during the 15 min pre-immersion HDT. When seated in the empty immersion tank, ICP stabilized at -6.3 +/- 1.3 mm Hg for the control period and increased to -2.2 +/- 1.9 mm Hg (p less than 0.05) when the water level was maintained for 30 min at the sternal notch. The ICP returned toward pre-immersion levels (-5.5 +/- 1.4 mm Hg) as the tank was drained. Mean blood pressure (MBP) remained constant during the pre-immersion tilt test. In the pre-immersion control period, MBP was 91 +/- 3 mm Hg compared to 90 +/- 4 mm Hg (p greater than 0.05) during immersion and 82 +/- 1 mm Hg (p less than 0.05) in the post-immersion recovery period. In summary, exposure of rhesus monkeys to either head-down tilt or water immersion produced a prompt rise in ICP during the stimulus period.(ABSTRACT TRUNCATED AT 250 WORDS)