Background: Hypoxia tolerance has been evaluated by the secondary responses to oxyhemoglobin deoxygenation: the time of useful consciousness (TUC), subjective symptoms, ventilatory responses, and cardiovascular changes. It is not clear whether the deoxygenation rate per se reflects hypoxia tolerance or how it relates to the former parameters.
Methods: In Study 1, we monitored arterial oxygen saturation (SaO2) by pulse oximetry in the nonsmoking subjects experiencing hypoxia at the simulated altitude of 25,000 ft. Male subjects were divided into two groups: junior (< or =39 yr, n = 108) and senior (> or =40 yr, n = 44). The duration from mask-off to mask-on (TUC), the duration from mask-off to the time of SaO2 of 90%(T90), the duration from 90% to 70% of SaO2(T70), and the SaO2 value at mask-on (bottom SaO2) were compared between the two groups. (In a separate, but related study, 10 medical variables were analyzed by stepwise regression to investigate the contributing factors to hypoxia tolerance in 77 other subjects, including 11 female subjects.) In study 2, 12 subjective hypoxic symptoms at 25,000 ft were compared between junior (n = 369) and senior (n = 160) groups.
Results: TUC was significantly longer in the junior group, but T70 was longer and bottom SaO2 was higher in the senior group. Age was adopted as a contributing variable in three out of six dependent parameters. Subjective symptoms were in the similar rank order for both groups.
Conclusion: Pulse oximetry reconfirmed that age is a major influencing factor for acute hypoxia tolerance. This may be due not only to the physiological effect but also to the training experience to recognize hypoxic deterioration more quickly.