FACTORS MAINTAINING CEREBRAL CIRCULATION DURING GRAVITATIONAL STRESS

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Abstract
Consciousness is usually lost when the mean cerebral blood pressure falls to 25 mm. Hg, but during exposure to high accelerations such as those producing aviator''s black-out, alertness may be retained in spite of such mean pressures. Arterial and cerebral venous oxygen saturation and arterial pressure at head level and cerebral venous pressures were measured in 3 subjects during acceleration. The venous saturation remained almost unchanged in spite of great falls in cerebral arterial pressure. This suggested that some factor was maintaining cerebral blood flow. Pressures ranging from 20-60 mm. Hg below ambient were found in the jugular bulb, This effect could maintain consciousness by sustaining the arterio-venous pressure differential, or by producing a combination of such an effect with passive cerebral vasodilation. There is also evidence that active cerebral vasodilation may occur during prolonged exposure to gravitational stress in the erect posture.