OBJECTIVETo explore the effects of hyperbaric oxygen preconditioning (HBOP) on human stress responses during acute exposure to high altitude and the possible mechanism.

METHODSEight male subjects were treated with HBOP for 3, 5, and 7 days, followed by acute exposure to hypoxia simulating an altitude of 4,000 m. Subjects at rest were divided into sea-level control group, simulated high-altitude group, and 5-day HBOP intervention group, while subjects after physical load were divided into sea-level control group, simulated high-altitude group, 3-day HBOP intervention group, and 7-day HBOP intervention group. The physical load test was performed for each subject before and after HBOP, and the plasma levels of dopamine (DA), epinephrine (E), norepinephrine (NE), and adrenocorticotropic hormone (ACTH) were determined before and after exercise. The physical load test was performed by stepping up on to a 30 cm-high stepping stool at a rate of 25/min for 5 minutes, which was a type of moderate physical exercise. The stepping rate and timing were controlled by a metronome.

RESULTSThe levels of DA, E, NE, and ACTH at rest and after physical load were significantly higher in subjects acutely exposed to high altitude than in the sea-level control groups (all P<0.05). Moreover, the levels of DA, E, NE, and ACTH at rest were significantly higher after acute exposure to high altitude in the 5-day HBOP intervention group than in the simulated high-altitude group (all P<0.01). Except for the ACTH level in the 3-day HBOP intervention group, the levels of DA, E, NE, and ACTH after physical load were significantly higher after acute exposure to high altitude in the 3-day and 7-day HBOP intervention groups than in the simulated high-altitude group (all P<0.01).

CONCLUSIONHBOP can elevate the plasma expression of DA, E, NE, and ACTH, and then speed up the establishment of a new balance of homeostasis to adapt to the acute hypoxia at high altitude.