The effect of hyperbaric oxygen on experimentally increased intracranial pressure.

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✓ The effect of breathing high partial pressures of oxygen on intracranial pressure (ICP) was assessed in 10 anesthetized, ventilated dogs in which ICP had been increased by slow inflation of extradural balloons. Cerebrovascular responsiveness was tested by the addition of carbon dioxide (CO2) to the inspired gases. Inhalation of 100% oxygen reduced ICP by 23% when delivered at normal atmospheric pressure (arterial pO2, 585 mm Hg) and by 37% when delivered at 2 atmospheres absolute (ATA) in a hyperbaric chamber (arterial pO2, 1076 mm Hg). This reduction in ICP, which was present only during oxygen administration, was not associated with any change in arterial blood pressure or pCO2. Hyperbaric oxygen (OHP) reduced ICP only at the stage when the cerebral circulation was still responsive to CO2, as manifested by a rise in ICP; at high levels of ICP, responsiveness to both CO2 and OHP was lost. When the ipsilateral pupil became dilated following balloon inflation, a reduction in size was observed during administration of OHP in 11 out of 13 instances. During the responsive phase, when OHP reduced ICP, cerebral venous PO2 which was 41.7 ± 2.3 mm Hg (S.E.M.) on air breathing was modestly increased by OHP to 58.3 ± 2.5 mm Hg; when ICP no longer responded to OHP, cerebral venous PO2 was higher, on both air breathing (52.0 ± 4.1 mm Hg) and OHP (161.4 ± 14.7 mm Hg).