Treatment of acute mountain sickness by simulated descent: a randomised controlled trial.

OBJECTIVE To evaluate the therapeutic efficacy of a portable hyperbaric chamber for treatment of acute mountain sickness. DESIGN Controlled randomised trial over two mountaineering seasons. SETTING High altitude research laboratory at 4559 m above sea level. SUBJECTS 64 climbers with acute mountain sickness randomly allocated to different treatments. INTERVENTIONS One hour of treatment in the hyperbaric chamber at a pressure of 193 mbar or 20 mbar as control or bed rest. MAIN OUTCOME MEASURES Symptoms of acute mountain sickness before, immediately after, and 12 hours after treatment. Permitted intake of analgesic and antiemetic drugs in the follow up period. RESULTS Treatment with 193 mbar caused greater relief of symptoms than did control treatment or bed rest. During the 12 hour follow up period intake of analgesics was similar (58-80% of subjects in each group). Symptom scores had improved in all subjects after 12 hours with no significant differences between groups. CONCLUSIONS One hour of treatment with 193 mbar in a portable hyperbaric chamber, corresponding to a descent of 2250 m, leads to a short term improvement in symptoms of acute mountain sickness but has no beneficial long term effects attributable to pressurisation.