Normobaric Hypoxia Exposure During Sleep Produces Sleep-Specific Ventilatory Acclimatization At High Altitude

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Sea-level (SL) residents have used repeated exposures to normobaric hypoxia (NH) during sleep to induce ventilatory acclimatization prior to rapid ascent to high altitude (HA). However, few reports have verified whether ventilatory acclimatization resulting from sleeping in NH is retained during subsequent HA exposure. PURPOSE: To determine if ventilatory acclimatization induced during NH is present during HA exposure. METHODS: Two groups of SL residents having similar resting end-tidal partial pressure of CO2 (PetCO2), peak oxygen uptake (∼47 ml/kg/min), and age (∼24 yr) slept for ∼6.5 hrs/night for 7 consecutive nights in portable rooms under blinded NH (n=14) or sham (n=8) conditions. Ambient % O2 for the NH group was progressively reduced by ∼0.30% O2 each night from ∼16.2% O2 (2200 m equivalent) on the 1st night to ∼14.4% O2 (3050 m) on the 7th night. The sham group remained at ≥20.5% O2 (150 m). Within 25 hrs of treatment end, all were exposed to HA (Pikes Peak, CO; 4300 m). At SL before and within 1 hr after treatment end, and in the first 24 hrs of HA, resting arterial oxygen saturation (SaO2) and PetCO2, and steady-state exercise and sleep SaO2 were measured. RESULTS: (Mean±SE) Over 7 nights during sleep, NH SaO2 decreased (p<0.05) from 92±1 to 88±1%, but sham SaO2 remained at 96±1%. From before to after treatment, SL resting PetCO2 decreased (p<0.001) for the NH group (39.1±0.8 to 34.9±0.7 mmHg) but not for the sham group (39.1±0.7 vs 38.3±1.0 mmHg). Within the first 5 hrs of HA, there were no between-group differences (NH vs sham) in resting PetCO2 (33.5±0.7 vs 33.1±1.0 mmHg), SaO2 (83±2 vs 83±1%) or exercise SaO2 (74±1 vs 73±2%). First night sleep SaO2 in HA was higher (p=0.031) for NH than for sham (78±1.3 vs 73±1.5%). CONCLUSION: These results indicate that ventilatory acclimatization induced by sleeping in NH is present at HA during sleep but not during the awake state. Thus, sleeping in NH at SL may not induce functionally useful ventilatory acclimatization for subsequent rapid ascent to high altitude. Funding: US Army MRMC ATO IV.MD.2006.01. Authors‘ views; not official U.S. Army or DoD policy.