Repeated exposure to 5 bar normoxic He-N₂ 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 measured during control periods at 1 bar air and after 15 and 60 min at 5 bar normoxic He (4.0 bar)-N₂ (0.8 bar) in two groups of awake habituated rats. Group 1 (10 control rats) were exposed 40 times while restrained for i h in the pressure chamber at i bar air. Group 2 (10 rats) were restrained and exposed 40 times to normoxic 5 bar HeN₂ atmosphere in the pressure chamber for 45 min. During the control period, the systolic and mean AP levels were higher (P < 0.05), whereas the average CBF and nine rCBF values were lower in the preexposed group. During 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 O₂, CO₂, and HCO₃ changed identically in both groups relative to hyperventilation. Generally, the total and local CBF values increased during the first 15 min in both groups and for 60 min in the preexposed rats. After 60 min of exposure, the flow returned toward the control level in most regions in both groups, whereas the flow was still elevated in the cerebellum and mesencephalon in the control rats and in the bulbus olfactorius, mesencephalon, medulla oblongata, spinal cord, and posterior part of cortex cerebri in preexposed animals. Hypothalamic rCBF in control rats was reduced after 60 min. The present findings indicate that repeated hyperbaric exposures induce decreased blood flow in the central nervous system and that a normoxic hyperbaric He-atmosphere increases the blood flow to most regions in the central nervous system, suggesting a beneficial effect on inert gas washout in decompression sickness.