Decrement in postural control during mild hypobaric hypoxia.

The effects of mild hypoxia on the postural control system of 39 naive subjects were examined by measuring the postural sway with a Kistler force platform, at ground level and at one of four simulated altitudes: 1,521 m (5,000 ft), 2,438 m (8,000 ft), 3,048 m (10,000 ft), or 3,658 m (12,000 ft). The total sway increased above the ground level controls for the 1,521 m, 2,438 m, and 3,048 m exposures (p less than or equal to 0.005) as did the sway at the lowest measurable frequency (p less than or equal to 0.002), but no change in sway was seen in those subjects exposed to 3,658 m as compared to ground level values. Significant interaction between altitude and exposure was observed at p less than or equal to 0.04, reflecting the definite effect at the lower altitudes and the lack of an effect at the higher altitude. The multiple comparison test indicated no difference between the responses at 1,524 m, 2,438 m, and 3,048 m. Both arterial oxygen saturation, SaO2, and the end-tidal oxygen partial pressure, PetO2, decreased in relation to the test altitudes with a statistically significant interaction between altitude and PetO2 (p less than or equal to 0.02), and SaO2 (p less than or equal to 0.005). There was no significant interaction between altitude and end-tidal carbon dioxide partial pressure (p = 0.4853). The postural control mechanisms, as an intergrative functional unit, are very sensitive to acute mild hypoxia. Arguments are advanced to indicate that intervention of compensatory mechanisms at higher altitudes may explain the recovery of postural stability at 3,658 m.