The effect of altitude pre-acclimatization on acute mountain sickness during reexposure

Background: Acclimatization to high altitude appears to prevent acute mountain sickness (AMS), as evidenced by a decline in AMS symptoms as acclimatization progresses. Hypothesis: We hypothesized that partial retention of acclimatization would attenuate the incidence and/or severity of AMS upon reinduction to altitude. Methods: To test this hypothesis 6 male lowlanders returned to sea level after acclimatizing for 16 d at 4300 m (HA). After 8 d at sea level (PA), they were reexposed to 4300 m in a hypobaric chamber for 30 h (RA). AMS symptom severity was determined by the AMS- cerebral (AMS-C) scores calculated from the daily administration of the Environmental Symptoms Questionnaire during HA and RA. Results: The mean AMS- C scores were reduced from 0.6 on HA day 1 (HA1) to 0.1 during RA (p < 0.05). Four subjects were 'sick' (AMS-C > 0.7) during HA1, while only one was 'sick' during RA. The % oxyhemoglobin, hemoglobin concentration and hematocrit were higher during RA compared to HA1. Conclusions: These results suggest that the retention of acclimatization after 8 d at low altitude is sufficient to attenuate AMS upon reinduction to high altitude.