Effect of repeated normobaric hypoxia exposures during sleep on acute mountain sickness, exercise performance, and sleep during exposure to terrestrial altitude

Abstract

Fulco CS, Muza SR, Beidleman BA, Demes R, Staab JE, Jones JE, Cymerman A. Effect of repeated normobaric hypoxia exposures during sleep on acute mountain sickness, exercise performance, and sleep during exposure to terrestrial altitude. Am J Physiol Regul Integr Comp Physiol 300: R428 -R436, 2011. First published December 1, 2010; doi:10.1152/ajpregu.00633.2010.-There is an expectation that repeated daily exposures to normobaric hypoxia (NH) will induce ventilatory acclimatization and lessen acute mountain sickness (AMS) and the exercise performance decrement during subsequent hypobaric hypoxia (HH) exposure. However, this notion has not been tested objectively. Healthy, unacclimatized sea-level (SL) residents slept for 7.5 h each night for 7 consecutive nights in hypoxia rooms under NH [n ϭ 14, 24 Ϯ 5 (SD) yr] or "sham" (n ϭ 9, 25 Ϯ 6 yr) conditions. The ambient percent O2 for the NH group was progressively reduced by 0.3% [150 m equivalent (equiv)] each night from 16.2% (2,200 m equiv) on night 1 to 14.4% (3,100 m equiv) on night 7, while that for the ventilatory-and exercise-matched sham group remained at 20.9%. Beginning at 25 h after sham or NH treatment, all subjects ascended and lived for 5 days at HH (4,300 m). End-tidal PCO 2, O2 saturation (Sa O 2 ), AMS, and heart rate were measured repeatedly during daytime rest, sleep, or exercise (11.3-km treadmill time trial). From pre-to posttreatment at SL, resting end-tidal PCO 2 decreased (P Ͻ 0.01) for the NH (from 39 Ϯ 3 to 35 Ϯ 3 mmHg), but not for the sham (from 39 Ϯ 2 to 38 Ϯ 3 mmHg), group. Throughout HH, only sleep Sa O 2 was higher (80 Ϯ 1 vs. 76 Ϯ 1%, P Ͻ 0.05) and only AMS upon awakening was lower (0.34 Ϯ 0.12 vs. 0.83 Ϯ 0.14, P Ͻ 0.02) in the NH than the sham group; no other between-group rest, sleep, or exercise differences were observed at HH. These results indicate that the ventilatory acclimatization induced by NH sleep was primarily expressed during HH sleep. Under HH conditions, the higher sleep Sa O 2 may have contributed to a lessening of AMS upon awakening but had no impact on AMS or exercise performance for the remainder of each day. ventilatory acclimatization; physical performance; hypobaric hypoxia; arterial oxygen saturation ALTITUDE ACCLIMATIZATION RESULTS from numerous interrelated physiological adjustments that compensate for hypoxemia, with augmented ventilation being one of the most important and consistently reported (17,18,22,28). Ventilatory acclimatization (VEacc) can be characterized by the progressive decrease in the end-tidal PCO 2 (PET CO 2 ) that leads to an increase in arterial O 2 saturation (Sa O 2 ) during the first several days of moderate-to high-altitude residence [hypobaric hypoxia (HH), reduced barometric pressure (P B ) and 20.9% O 2 ] (7, 28). The enhanced oxygenation is closely linked with reduced acute mountain sickness (AMS) and improved exercise performance during HH residence (1,11,12,14). Some studies show that VEacc can also be induced by 1-4 h of HH exposure repeated daily at altitudes of 4,300 -4,500 m ...

Acute high-altitude sickness

“…Studies have reported conflicting results regarding intermittent normobaric or hypobaric hypoxic exposures, with some studies showing benefit [107] and others not demonstrating a clear effect [108,109]. One of the challenges in interpreting these discrepant results is that the hypoxic exposure protocols vary significantly between studies with regard to the magnitude and duration of the hypoxic exposures.…”

“…While these preacclimatisation strategies may carry benefit and probably pose little risk, implementation may be difficult for many travellers and the optimal approach remains unclear. Infrequent and/or short exposures to hypoxia are unlikely to reduce the incidence of AMS [109,110], while longer and/or more frequent exposures are necessary to reduce the risk of altitude illness [107,111]. Such exposures should be performed as close in time to the planned high-altitude travel as possible.…”

Effect of Intermittent Normobaric Hypoxia Exposures on Acute Mountain Sickness During Acute Ascent to 3500 m in Indian Military Personnel

“…The effect of intermittent hypoxia (normobaric) and its physiological outcome during subsequent exposure to actual field condition (hypobaric hypoxia) is scanty. Schommer 22 , et al studied the normobaric hypoxia exposure of 14 h -18 h in 12 per cent -16 per cent O 2 for 70-90 min per day at the rate of 3 days per week for four weeks along with an overnight stay at 3611 m on arterial blood gases or AMS during subsequent hypobaric hypoxia residence at 4559 m. The effect of repeated normobaric hypoxia exposure in un-acclimatised sea level residents during sleep for 7.5 h in each night for seven consecutive days on acute mountain sickness and sleep during subsequent exposure to hypobaric hypoxia at 4350 m altitude showed significantly higher SaO 2 and AMS upon awakening was lower 18 . Our study is also first to report where volunteers breathed normobaric hypoxia air (12 % F I O 2 ) for four hours per day for four consecutive days on acute mountain sickness at 3500 m high altitude.…”

“…The studies on the effect of (IHE) at sea level and its possible outcomes on reducing the susceptibility to AMS during subsequent high altitude sojourn are very limited. Only two laboratory based studies have showed a significant reduction in the incidence and severity of AMS after IHE at sea level in laboratory based condition 17,18 . The effect of intermittent hypoxia (normobaric) and its physiological outcome during subsequent exposure to actual field condition (hypobaric hypoxia) is scanty.…”

“…The training in intermittent normobaric hypoxia at sea level and its effect on AMS is very limited 16,17 . Today there is only one study that has reported the use of normobaric hypoxic exposure during sleep at sea level and its effect on AMS during subsequent exposure to hypobaric hypoxia 18 . None of the previous studies have assessed the effect of intermittent normobaric hypoxia exposure (IHE) at sea level and its effect on the incidence of AMS during acute exposure to 3500 m altitude in altitude in Indian military personnel.…”