REPEATED EXPOSURE TO 5-BAR NORMOXIC HE-N-2 CHANGES CEREBRAL BLOOD-FLOW DISTRIBUTION IN RATS

The regional cerebral blood flow (rCBF), arterial pressure (AP), heart rate, respiratory frequency, and arterial acid-base chemistry were me asured during control periods at 1 bar air and after 15 and 60 min at 5 bar normoxic He (4.0 bar)-N-2 (0.8 bar)(-) in two groups of awake ha bituated rats. Group 1 (10 control rats) were exposed 40 times while r estrained for 1 h in the pressure chamber at 1 bar air. Group 2 (10 ra ts) were restrained and exposed 40 times to normoxic 5 bar He-N-2 atmo sphere in the pressure chamber for 45 min. During the control period, the systolic and mean AP levels were higher (P < 0.05), whereas the av erage CBF and nine rCBF values were lower in the preexposed group. Dur ing 5-bar exposure, the systolic AP rose significantly in both groups, whereas the mean AP remained at the control level or was reduced. The arterial O-2, CO2, and HCO3 changed identically in both groups relati ve to hyperventilation. Generally, the total and local CBF values incr eased during the first 15 min in both groups and for 60 min in the pre exposed rats. After 60 min of exposure, the flow returned toward the c ontrol level in most regions in both groups, whereas the flow was stil l elevated in the cerebellum and mesencephalon in the control rats and in the bulbus olfactorius, mesencephalon, medulla oblongata, spinal c ord, and posterior part of cortex cerebri in preexposed animals. Hypot halamic rCBF in control rats was reduced after 60 min. The present fin dings indicate that repeated hyperbaric exposures induce decreased blo od flow in the central nervous system and that a normoxic hyperbaric H e-atmosphere increases the blood flow to most regions in the central n ervous system, suggesting a beneficial effect on inert gag washout in decompression sickness.