Objective To observe the changes in the symptoms and relevant physiological indicators in subjects after inhaling the hypoxic air produced by a hypoxic air generator at a low altitude prior to their entry into high-altitude environment, and to explore its early warning effect for acute mountain sickness (AMS) among the subjects upon their subsequent entry into high-altitude environment.

Methods A total of 50 subjects who were going to visit high-altitude regions were enrolled. All subjects were men, with an average age of (22.00±1.52) years. They continuously inhaled for 30 minutes hypoxic air (which simulated the air at the altitude of 5200 m, with an oxygen content 10.80%) generated by a hypoxic air generator. During this period fingertip oxygen saturation, heart rate, blood pressure, and symptoms of discomfort were observed and recorded. On the fourth day after living at an altitude of 4020 m, the subjects completed the evaluation for the symptom scores of acute mild altitude disease (AMAD). The subjects were divided into a maladjusted group (18 cases) and a well-adjusted group (32 cases) according to whether they experienced discomfort (including drowsiness, dizziness, chest tightness, cold sweating of the hands, etc.) during the inhalation of hypoxic air at a low altitude. After entry into the high-altitude environment, they were divided into an AMS group (28 cases) and a non-AMS group (22 cases) according to whether they experienced AMS after entering the he high-altitude environment. The primary indicator was the incidence of AMS, including the incidence of AMAD and severe acute mountain sickness (SAMS), and the incidence of AMS in the maladjusted group and the well-adjusted group after entering high-altitude environment. The secondary indicator was the relationship between the changes in fingertip oxygen saturation after inhaling hypoxic air at a low altitude and the incidence of AMS and the AMAD symptom scores.

Results All 50 subjects traveled by air to the target altitude of 4020 m above sea level at the same time. The AMS incidence among them was 56.0% (28/50), with the incidence of AMAD being 54.0% (27/50) and the incidence of SAMS being 2.0% (1/50). In the single case of SAMS, the patient had high-altitude pulmonary edema. The incidences of AMS after entering high-altitude environment in the maladjusted and well-adjusted groups were 88.9% (16/18) and 37.5% (12/32), respectively, and the difference was statistically significant (P<0.01). In the 50 subjects, fingertip oxygen saturation decreased rapidly in the first 11 minutes into the inhalation of hypoxic air at a low altitude, with a more pronounced decrease in the AMS group than that in the non-AMS group, and the differences between the groups were statistically significant after 5, 9, and 11 minutes (P<0.05). Fingertip oxygen saturation plateaued in the 50 subjects from the 12th to the 30th minute, with no significant differences between the AMS and non-AMS groups. The mean value of fingertip oxygen saturation within 30 minutes of hypoxic air inhalation was negatively correlated with the AMAD symptom scores after subjects' entry into high-altitude environment (r=－0.300).

Conclusions Those who experience symptoms of discomfort after exposure to hypoxic air produced by a hypoxic air generator at a low altitude are more likely to develop AMS and close attention should be paid to the decrease in fingertip oxygen saturation within the first 11 minutes.