Time of useful consciousness determination in aircrew members with reference to prior altitude chamber experience and age

Background: Hypoxia tolerance has been evaluated by the secondary responses to oxyhemoglobin deoxygenation: the time of useful consciousness (TUC). su bjective 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 monito red arterial oxygen saturation (SaO(2)) 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 (less than or equal to 39 yr , n = 108) and senior (greater than or equal to 40 yr, n = 44). The duratio n from mask-off to mask-on (TUC), the duration from mask-off to the time of SaO(2), of 90%(T-90), the duration from 90% to 70% of SaO(2)(T-70), and th e SaO(2) value at mask-on (bottom SaO(2)) were compared between the two gro ups. (In a separate, but related study, 10 medical variables were analyzed by stepwise regression to investigate the contributing factors to hypoxia t olerance in 77 other subjects, including 11 female subjects.) In study 2, 1 2 subjective hypoxic symptoms at 25,000 ft were compared between junior (n = 369) and senior (n = 160) groups. Results: TUC was significantly longer i n the junior group, but T-70 was longer and bottom SaO(2) was higher in the senior group. Age was adopted as a contributing variable in three out of s ix dependent parameters. Subjective symptoms were in the similar rank order for both groups. Conclusion: Pulse oximetry reconfirmed that age is a majo r influencing factor for acute hypoxia tolerance. This may be due not only to the physiological effect but also to the training experience to recogniz e hypoxic deterioration more quickly.