Effect of Acute, Subacute, and Repeated Exposure to High Altitude (5050 m) on Psychomotor Vigilance

Abstract

Aim: High altitude (HA) hypoxia may affect cognitive performance and sleep quality. Further, vigilance is reduced following sleep deprivation. We investigated the effect on vigilance, actigraphic sleep indices, and their relationships with acute mountain sickness (AMS) during very HA exposure, acclimatization, and re-exposure. Methods: A total of 21 healthy altitude-naive individuals (25 ± 4 years; 13 females) completed 2 cycles of altitude exposure separated by 7 days at low altitude (LA, 520 m). Participants slept at 2900 m and spent the day at HA, (5050 m). We report acute altitude exposure on Day 1 (LA vs. HA1) and after 6 days of acclimatization (HA1 vs. HA6). Vigilance was quantified by reaction speed in the 10-min psychomotor vigilance test reaction speed (PVT-RS). AMS was evaluated using the Environmental Symptoms Questionnaire Cerebral Score (AMS-C score). Nocturnal rest/activity was recorded to estimate sleep duration using actigraphy. Results: In Cycle 1, PVT-RS was slower at HA1 compared to LA (4.1 ± 0.8 vs. 4.5 ± 0.6 s^-1, respectively, p = 0.029), but not at HA6 (4.6 ± 0.7; p > 0.05). In Cycle 2, PVT-RS at HA1 (4.6 ± 0.7) and HA6 (4.8 ± 0.6) were not different from LA (4.8 ± 0.6, p > 0.05) and significantly greater than corresponding values in Cycle 1. In both cycles, AMS scores were higher at HA1 than at LA and HA6 (p < 0.05). Estimated sleep durations (TST) at LA, 1st and 5th nights were 431.3 ± 28.7, 418.1 ± 48.6, and 379.7 ± 51.4 min, respectively, in Cycle 1 and they were significantly reduced during acclimatization exposures (LA vs. 1st night, p > 0.05; LA vs. 5th night, p = 0.012; and 1st vs. 5th night, p = 0.054). LA, 1st and 5th nights TST in Cycle 2 were 477.5 ± 96.9, 430.9 ± 34, and 341.4 ± 32.2, respectively, and we observed similar deteriorations in TST as in Cycle 1 (LA vs. 1st night, p > 0.05; LA vs. 5th night, p = 0.001; and 1st vs. 5th night, p < 0.0001). At HA1, subjects who reported higher AMS-C scores exhibited slower PVT-RS (r = -0.56; p < 0.01). Subjects with higher AMS-C scores took longer time to react to the stimuli during acute exposure (r = 0.62, p < 0.01) during HA1 of Cycle 1. Conclusion: Acute exposure to HA reduces the PVT-RS. Altitude acclimatization over 6 days recovers the reaction speed and prevents impairments during subsequent altitude re-exposure after 1 week spent near sea level. However, acclimatization does not lead to improvement in total sleep time during acute and subacute exposures.

Keywords: actigraphy; altitude; brain; hypoxia; psychomotor vigilance task; sleep.

FIGURE 1

Expedition schedule and data collection time points. The Y-axis depicts altitude (meters), and the X-axis depicts the days of the entire expedition with two cycles at high altitude interspersed with 1 week at LA. The numbers within the large black circles depict time points of data collection. The small black circles with checkmark indicate the sleep nights with actigraphy, i.e., actigraphic data collection time points. The gray shaded horizontal part at the base of each cycle indicates Cycle 1 and Cycle 2. “1-Week” with a dashed line connecting two cycles indicates time spent at LA. HA1, high altitude exposure at 5050 m asl on Day 1; HA6, high altitude exposure at 5050 m asl on Day 6 on the respective cycles of expedition; FL, familiarization; BL, baseline; REC, recovery; m, meters; n, numbers.

FIGURE 2

Effect of altitude exposure on PVT parameters during acute and acclimatization exposures. It illustrates changes in different PVT parameters during high altitude exposure. Four important time points of the expedition are illustrated (BL, baseline at LA; HA1, acute high altitude exposure; HA6, acclimatization exposure of both cycles; REC, recovery). The Y-axis depicts sleep parameters with the mean and standard deviation (Mean ± SD) of the PVT parameters. The X-axis depicts different expedition time points (BL1, HA1, HA6, and REC of Cycle 1 and Cycle 2). Solid bars represent Cycle 1 exposure while empty bars represent Cycle 2 exposure. Here, panel A illustrates number of errors/lapses while panel E represents PVT reaction speed. Panels B and F represent mean and median of reaction times respectively. Panels C and G show fastest and slowest 10% reaction times while D and H depict reciprocals of them in respective orders. The significance shown with dashed line indicates baseline comparisons of two cycles. The comparison with solid lines is between different time points of respective cycles. n, number; RT, reaction time; ms, milliseconds; s^-1, per second; PVT-RS, psychomotor vigilance test reaction speed; p, level of significance; p,^∗ level of significance value with less than 0.05.

FIGURE 3

Effect of altitude on actigraphy sleep indices during acute and acclimatization exposure. It has two panels as Cycle 1 (Left, A,B) and Cycle 2 (Right, C,D). The Y-axis depicts sleep parameters with the mean ± standard deviation (Mean ± SD) at baseline, acute exposure (1st night altitude), and acclimatization night (5th night at altitude). The X-axis depicts different sleep parameters as reported from actigraphy during the first expedition of Cycle 1 and Cycle 2. Black bars, baseline sleep at Santiago, Chile (520 m asl); gray bars, acute exposure to altitude, i.e., 1st night sleep at high altitude (2900 m asl) and empty bars, sleep after acclimatization at high altitude, i.e., 5th night sleep at high altitude in each cycle. The significance shown with dashed line indicates baseline comparisons of two cycles. The comparison with solid lines is between different time points of respective cycles. TIB, time in bed; TST, total sleep time; SL, sleep latency; WASO, wake after sleep onset; Awak, awakenings; SE, sleep efficiency; SD, standard deviation; min, minute; p, level of significance; n, number; %, percentage; ^∗p, level of significance value with less than 0.05.

FIGURE 4

Inverse correlation between reaction speed vs. AMS-C score and positive association with changes in reaction time (HA1 – BL) vs. AMS-C score. It has two panels, left panel (A) shows negative correlation between PVT-RS vs. AMS score and the right panel (B) shows positive correlation between changes in reaction time (HA1 – BL) vs. AMS score. The Y-axis depicts reaction speed during the first exposure at altitude in A and the changes in reaction times (HA1, reaction time of high altitude at Day 1 – BL, reaction time at baseline) in B during acute exposure at altitude during Cycle 1. In both panels, the X-axis depicts AMS-C score during the acute exposure Cycle 1. The vertical dashed red lines in both panels separates no AMS subjects from AMS with the cut-off of a ≥ 0.7 AMS-C score. In B, the horizontal dashed line passing through zero of Y-axis is a reference line separating negative (i.e., subjects with decreased reaction time at high altitude) and positive (i.e., subjects with increased reaction time at high altitude) values of changes in reaction times. PVT-RS, psychomotor vigilance test reaction speed; AMS, acute mountain sickness; AMS-C, acute mountain sickness – cerebral; BL, baseline at low altitude; HA1, acute high altitude exposure; s, seconds; ms, milliseconds; r, Pearson’s correlation; p, level of significance.