Title: Simulated Altitude During The Night Reduces Severity Of Acute Mountain Sickness
Abstract: Preacclimatization by intermittent exposure to artificial hypoxia is used to avoid acute mountain sickness (AMS). Effects so far have mainly been studied in uncontrolled or unblinded studies. PURPOSE: To examine the effect of exposure to normobaric hypoxia during the night on symptoms of AMS and ventilatory acclimatization in a randomized placebo-controlled double blinded study. METHODS: 75 healthy men (age 18-55 years) were exposed for 4 h to FiO2=12% (4500m), then acclimatized for 14 nights in commercially available "hypoxia-tents" at simulated altitudes from 2500m (FiO2 = 15.4%) to 3300m (FiO2 = 14.0%; HYP-group) or sea level (CON-group). Equipment and handling were identical for both groups. After a break of 4 days subjects were re-exposed for 20 h to FiO2 of 12%. AMS, ventilation and blood gases were assessed before and after 4 h of hypoxic exposure at preexamination and retest as well as at the end of the retest. RESULTS: For technical reason many subjects did not reach the desired sleeping altitude of 3043m at average. Therefore we analyzed the 21 "highest" (=HYP) and the 21 "lowest" (=CON). Mean sleeping altitude was 2599m in HYP compared to 325m in CON. Sleep-quality during the acclimatization period in the tents was not affected by hypoxia. The severity of AMS-symptoms were significantly reduced by sleeping in hypoxia, indicated by reduced AMS-scores (Lake-Louise-score: 3.1±2.2 vs. 5.6±3.6, P = 0.04; AMS-c: 0.38 ± 0.38 vs. 1.10 ± 1.23, P = 0,01). The incidence of AMS, defined as AMS-c > 0.7, was also significantly reduced from 14% in HYP compared to 52% in CON (Chi2-test P = 0.02). Compared to preexamination after acclimatization the drop of oxygen saturation by 4 h of hypoxia was significantly less in HYP (18.7±5.2 vs. 15.6 ± 4.2, P = 0,01) without any difference in CON (17.3 ± 7.9 vs. 17.0 ± 5.3, P = 0,89). Ventilatory parameters showed no significant difference between HYP and CON. CONCLUSIONS: This study indicates that preacclimatization during 14 nights of 8 h in normobaric hypoxia equivalent to 2600m reduces severity and incidence of AMS and therefore is useful to prevent AMS later at altitude if acclimatization in natural altitude is not possible. The effect can be expected to be more pronounced if a higher degree of hypoxia is attained in the tents.