Subjects
The subjects, coming from a road construction company, were going to high altitude to build roads. After signing informed consent forms, subjects completed a structured questionnaire covering anthropometric variables, lifestyle questions, and medical history. Meanwhile, our research faculties followed them to the high altitude from Xi’an city (430 m) by vehicles in five days. The field working bases with hotel facilities were distributed into the four different altitudes (3700 m, 4000 m, 4700 m, and 5380 m). Accordingly, our research faculties were organized into four teams to carry out the experiment in specific altitude (Team 1–4).
At each altitude, the subjects were screened. Some were excluded when not suitable for testing. Exclusion criteria included cardiac illness, diuretic use, chronic medical conditions, or previous experience of AMS. All of the 88 qualified subjects were young males, of similar height (from 169 to 172 cm) and weight (from 68 to 72 kg), ranging in age from 20 to 24 years. They were divided into 5 groups, 34 at 3700 m (17 in each of Group 1 and 2), 18 at 4000 m (Group 3), 19 at 4700 m (Group 4), 17 at 5380 m (Group 5).
Additional 8 young men (22–24 years old) in Xi’an city were selected for the blood gas tests in hypobaric chamber.
The study was approved by the local institutional ethical review boards of all the participating institutions.
Instrument
To satisfy needs in high altitude, the OIR was invented (Figure 1, Chinese patent No. 200610104868.1, 200630090257.7, 200403262498.0). The total weight is 370 g, and the dimensions are 152 × 79 × 34 mm. As a simple, portable, durable, and convenient device, the OIR increases atmospheric pressure and therefore oxygen partial pressure in a nasal mask by means of a centrifugal fan with a rotational speed of 10000 rpm. Magnetic suspension bearing was adopted to reduce noise and improve power consumption ratio. Conveniently, it uses internal rechargeable Li-ion batteries (11.1 V, 1800 mAh) which, under extreme cold environment, can be displaced under the user’s clothes with an extended wire connecting to the machine body to be kept warm and working.
The instrument does not change the percentage of oxygen (approximately 21%) in the compressed atmosphere. There are 12 small holes (2.5 mm in diameters) in the mask allowing air exchanges. The output pressure in the mask, as measured using a U-tube manometer, is 3 mmHg above the local atmospheric pressure; flow rate of atmosphere is approximately 50 L/min. The edge of the nasal mask is made of polysiloxanes, which can fit different faces and comfort the wearers.
Experiment design
The details of the experimental design are shown in Figure 2. The base at 3700 m has better facilities and two Groups (Group 1 and 2) were assigned with more tests performed than other groups. In Group 1, heart rate, SpO2, MDA, SOD and BLA were tested. On day one, subjects wore the OIR. Team 1 recorded the subjects’ heart rates and SpO2 at rest by a Multi-parameter patient monitor (IntelliVue MP70, Philips, Eindhoven, The Netherlands). Next, all subjects exercised using Harvard Step Test designed to induce fatigue [12]. The height of each step is 40 cm; subjects went up and down at a rate of 20 steps per minute for 3 minutes. The heart rate and SpO2 were recorded at the end of exercise, and the recovery of heart rate was tracked during a rest period of 5 minutes. Immediately after the excise, the vein blood was drawn from the antecubital fossa to assess the concentrations of MDA [13], SOD [14], and BLA with three diagnostic kits (Jiancheng Bioengineering Institute, Nanjing, China) [15]. The experimental protocol was repeated without OIR on day two after a rest period of 24 hours.
PWC-170 [16] were tested from another 17 subjects in Group 2. They performed consecutive workloads on a cycle ergometer (EGM-II, Yueyang Electronic Ins., Co., Hunan, China) with OIR. Subjects were asked to keep the tachometer at around 60 rpm. Loaded power of the ergometer was initially set to 50 W, then increased by a step of 50 W per 3 minutes until reaching 200 W. Heart rate was monitored until a steady read-out was achieved. Each steady-state heart rate and workload were graphed, with the line of best fit for the three points extrapolated to estimate the power out that would elicit a heart rate of 170 beats per minute [16, 17]. The experimental protocol was repeated without OIR after a rest of 24 hours.
At higher altitude (4,000 m, 4,700 m and 5,380 m for Group 3, 4, 5 respectively), only heart rate and SpO2 were measured from subjects exercising Harvard Step Test with the same type of patient monitor. Subjects performed the experiments first with OIR and then repeated without OIR after a rest of 24 hours.
An additional experiment was conducted to verify the exact arterial partial pressure of oxygen in blood (PaO2) and arterial oxygen saturation (SaO2) by measuring arterial blood gas with a portable clinical analyzer (i-STAT 200, Abbott Point of Care Inc., USA). Eight young men were exposed in the hypobaric chamber simulating high altitude of 4000 m. Blood samples were collected from each subject after 2 hours exposure (without OIR). Afterwards, OIRs were used for 15 minutes and blood samples were collected (with OIR).
Statistical analysis
All of the data are shown as means ± S.D. We used SPSS 13.0 software (SPSS Inc., Chicago, IL, USA) to perform Shapiro-Wilk normality test and paired t-test. P<0.05 was considered statistically significant.