Effects of heat load and hypobaric hypoxia on cognitive performance: a combined stressor approach

Figures & data

Table 1. Mean air temperature (°C), relative humidity (%), globe temperature (°C), and Wet Bulb Globe Temperature (WBGT) outside (°C) during the conditions and for each phase in the experiment.

Conditions	Phase	Air temperature °C (dry bulb temperature)	RH %	Globe temperature °C	WBGT outside °C
No stressors	Pre-heating	19.61	56.54	19.53	17.22
	Altitude	20.18	55.56	20.17	17.52
Heat load	Pre-heating	37.29	25.16	39.54	28.31
	Altitude	33.55	24.11	37.83	27.08
Hypobaric hypoxia	Pre-heating	20.53	49.93	20.62	17.51
	Altitude	19.61	38.67	19.12	15.39
Heat load and hypobaric hypoxia	Pre-heating	34.95	27.48	38.01	27.36
	Altitude	31.38	21.67	34.95	24.42

Figure 1. Overview of the four experimental conditions. Ascending from 0 to 13,000 ft in hypobaric hypoxia conditions took 13 min (1,000 ft per minute). In this figure, the ambient temperature (T_A) is an estimated value.

Four graphs with a schematic overview of the increase in simulated altitude and ambient temperature over time during each experimental condition. Ascending from 0 to 13,000 ft in hypobaric hypoxia conditions took 13 min. The ambient temperature is an estimated value increasing from 17 to 30 °C.

Figure 2. Timeline of the protocol. SpO₂: oxygen saturation; HR: heart rate; T_SK: mean skin temperature; T_C: core temperature. Gradient indicates potential ambient temperature.

Schematic overview of the timeline of the protocol. Participants ingested the pill 120 min before start potential ascending, followed by 30 min compliance phase, 30 min baseline measurements, 60 min potential pre-heating phase, 15 min potential ascending, 30 min potential altitude phase with measurements, and 10 min debriefing. Oxygen saturation, heart rate, mean skin temperature, and core temperature were measured during all measurement phases.

Table 2. Descriptives (mean ± SD) for each measure in each of the four conditions.

	No stressors	Heat load	Hypobaric hypoxia	Heat load + hypobaric hypoxia
SYNWIN
Composite score	130 ± 120.17	−69.14 ± 171.19	−7.93 ± 183.19	−69.77 ± 207.96
Memory task score	4 ± 30	−20 ± 43	−7 ± 28	1 ± 40
Arithmetic task score	72 ± 81	−35 ± 65	−14 ± 125	−80 ± 90
Arithmetic task: reaction time for correct responses	−1.91 ± 2.50	−.47 ± 2.68	−.05 ± 2.53	.83 ± 1.30
Arithmetic task: number of errors	−.53 ± 2.88	1.57 ± 1.99	.87 ± 5.36	2.42 ± 2.43
Fuel task score	16 ± 37	−14 ± 115	16 ± 128	−21 ± 82
Auditory task score	38 ± 50	0 ± 65	−3 ± 60	30 ± 55
VigTrack
Tracking error	−28.33 ± 129.6	51.92 ± 112.94	−6.36 ± 213.54	99.15 ± 151.82
% Stimuli missed	.03 ± 3.55	.87 ± 3.87	−.30 ± 5.53	5.33 ± 7.39
Reaction time (ms)	1.78 ± 31.52	4.61 ± 56.28	11.86 ± 39.46	22.81 ± 53.45
MATB-II
% Stimuli missed—system monitoring task	1.12 ± 6.49	−.39 ± 6.19	−2.02 ± 4.85	−3.10 ± 4.76
Reaction time (s)—system monitoring task	−.09 ± .35	.06 ± .34	−.12 ± .51	.18 ± .60
RMS tracking error—tracking task	−1.07 ± 3.91	.29 ± 5.61	.66 ± 4.89	1.50 ± 6.26
% Correct—radio communications task	.31 ± 7.70	.87 ± 8.20	.71 ± 8.03	−.56 ± 9.93
Mean absolute deviation—resource management task	−4.14 ± 48.88	.93 ± 88.98	−45.15 ± 123.20	33.64 ± 109.25
PVT as secondary task
During SYNWIN—reaction time (s)	1.60 ± .55	1.70 ± .55	1.63 ± .51	1.96 ± .63
During MATB-II—reaction time (s)	1.56 ± .41	1.52 ± .42	1.45 ± .37	1.64 ± .46
Physiological measures
HR	65.71 ± 7.56	78.2 ± 11.51	76.85 ± 7.75	82.65 ± 9.67
% SpO2	96.93 ± 2.08	96.81 ± 1.91	83.46 ± 2.87	84.4 ± 4.07
TSK	33.43 ± .51	35.87 ± .53	33.56 ± .70	35.31 ± .60
TC	36.93 ± .27	37.17 ± .34	36.97 ± .28	36.98 ± .37
MBT	35.67 ± .31	36.45 ± 1.10	35.74 ± .31	36.38 ± .44
TFOREARM–FINGER	3.11 ± 2.96	.39 ± 1.15	4.11 ± 3.63	.68 ± 2.14
Questionnaire
Thermal sensation	.20 ± .56	2.93 ± .88	.93 ± 1.10	2.33 ± .82
Thermal discomfort	.67 ± .52	2.33 ± 1.03	1.83 ± .75	2.17 ± .75

Table 3. Overview of significant and non-significant results of heat load, hypobaric hypoxia and the interaction term using univariate multilevel analysis adjusted for repeated measurements within individuals. Fs and degrees of freedom for the fixed effects with corresponding p-values are shown. Grey highlighted statistic implies a significant result.

	Heat load	Hypobaric hypoxia	Heat load × hypobaric hypoxia
SYNWIN
Composite score	F (1, 53) = 8.15, p = .006*	F (1, 53) = 2.30, p = .135	F (1, 53) = 2.26, p = .139
Memory task score	F (1, 39.65) = .96, p = .334	F (1, 39.84) = .37, p = .544	F (1, 39.84) = 3.38, p = .075
Arithmetic task score	F (1, 53) = 12.20, p = .001*	F (1, 53) = 6.99, p = .011*	F (1, 53) = .69, p = .412
Arithmetic task: reaction time for correct responses	F (1, 52) = 3.35, p = .073	F (1, 52) = 6.25, p = .016*	F (1, 52) = .20, p = .660
Arithmetic task: number of errors	F (1, 52) = 3.80, p = .056	F (1, 52) = 1.44, p = .236	F (1, 52) = .09, p = .768
Fuel task score	F (1, 53) = 1.67, p = .202	F (1,53) = .02, p = .903	F (1,53) = .02, p = .901
Auditory task score	F (1, 53) = .02, p = .871	F (1, 53) = .13, p = .722	F (1, 53) = 5.46, p = .023*
VigTrack
Tracking error	F (1, 38.49) = 4.96, p = .032*	F (1, 39.98) = .72, p = .400	F (1, 38.49) = .09, p = .767
% Stimuli missed	F (1, 51) = 5.20, p = .027*	F (1,51) = 2.12, p = .152	F (1,51) = 2.85, p = .098
Reaction time	F (1, 51) = .31, p = .582	F (1, 51) = 1.29, p = .261	F (1, 51) = .11, p = .746
MATB-II
% Stimuli missed—system monitoring task	F (1, 40.09) = .921, p = .343	F (1, 41.25) = 4.63, p = .037*	F (1, 40.09) = .02, p = .877
Reaction time—system monitoring task	F (1, 40.04) = 4.37, p = .043*	F (1, 41.18) = .17, p = .686	F (1, 40.04) = .42, p = .521
RMS tracking error—tracking task	F (1, 40.11) = .64, p = .429	F (1, 41.45) = 1.14, p = .292	F (1, 40.11) = .036, p = .850
% Correct—radio communications task	F (1, 53) = .03, p = .875	F (1, 53) = .05, p = .819	F (1, 53) = .17, p = .684
Mean absolute deviation—resource management task	F (1, 53) = 2.73, p = .104	F (1, 53) = .03, p = .871	F (1, 53) = 2.11, p = .152
PVT as secondary task
During SYNWIN—reaction time	F (1, 37.96) = 5.81, p = .021*	F (1, 37.99) = 1.29, p = .263	F (1, 38.47) = .20, p = .655
During MATB-II—reaction time	F (1, 36.87) = .77, p = .385	F (1, 37.49) = .06, p = .807	F (1, 37.49) = 1.05, p = .313
Physiological measures
HR	F (1, 41.15) = 40.41, p = .001*	F (1, 41.15) = 29.34, p = .001*	F (1, 41.15) = 5.40, p = .025*
% SpO2	F (1, 41.41) = .41, p = .526	F (1, 41.41) = 415.22, p = .001*	F (1, 41.41) = .69, p = .410
TSK	F (1, 38.77) = 274.24, p < .001**	F (1, 38.89) = 3.41, p = .072	F (1, 38.89) = 8.38, p = .006*
TC	F (1, 42) = 5.58, p = .023*	F (1, 42) = 1.95, p = .170	F (1, 42) = 5.21, p = .028*
MBT	F (1, 40.50) = 23.54, p < .001**	F (1, 40.58) = .00, p = .986	F (1, 40.58) = .31, p = .583
TFOREARM-FINGER	F (1, 42) = 27.06, p < .001**	F (1, 42) = 1.17, p = .286	F (1, 42) = .36, p = .553
Questionnaire
Thermal sensation	F (1, 42) = .89.10, p <.001**	F (1, 42) = .09, p = .762	F (1, 42) = 9.27, p = .004*
Thermal discomfort	F (1, 15) = 13.17, p = .002*	F (1, 15) = 3.29, p = .090	F (1, 15) = 5.85, p = .029*

*p < .05, **p < .001.

Figure 3. Significant main and/or interaction effects of heat load and hypobaric hypoxia with mean composite (A), mean arithmetic (B), and auditory score (E) on the SYNWIN, mean arithmetic error (C), and arithmetic reaction time (RT) (D) on the SYNWIN, mean tracking error (F) and percentage stimuli missed (G) on the VigTrack, mean reaction time (RT) in seconds (H) and mean percentage stimuli missed (I) on the system monitoring task (SMT) of the MATB-II. Error bars are ±1 SE (standard error).

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Graphs with significant main and/or interaction effects of heat load and hypobaric hypoxia on cognitive performance measures. For the mean arithmetic score on the SYNWIN a main effect of both heat load and hypobaric hypoxia was found, with both factors decreasing performance. The interaction term was not significant for this task. Heat load significantly decreased performance on the mean composite score and mean arithmetic errors on the SYNWIN, the mean percentage stimuli missed on the VigTrack as well as the mean reaction time on the system monitoring task of the MATB-II. Hypobaric hypoxia increased mean arithmetic reaction times on the SYNWIN, while a higher percentage decrease of stimuli missed on the system monitoring task of the MATB-II was found in the presence of hypobaric hypoxia. The interaction term for heat load and hypobaric hypoxia was significant for the auditory monitoring task of the SYNWIN. However, pairwise comparisons were not significant.

Figure 4. Significant effect of heat load on mean reaction time (RT) on the PVT in milliseconds (s). Error bars are ±1 SE (standard error).

Graph with the significant increase of mean reaction time on the PVT during the SYNWIN in the presence of heat load compared to the absence of heat load.

Figure 5. Significant main and/or interaction effects of heat load and hypobaric hypoxia with mean HR in beats per minute (bpm) (A), percentage SpO₂ (B), mean skin temperature (T_SK) (C), mean core temperature (T_C) (D), mean body temperature (MBT) (E) and mean forearm-fingertip gradient (T_{FOREARM-FINGER}) (F). Error bars are ±1 SE (standard error).

Graphs with significant main and/or interaction effects of heat load and hypobaric hypoxia on physiological measures. Heat load significantly increased the mean skin temperature, mean core temperature, and mean body temperature and decreased the forearm-fingertip gradient. Hypobaric hypoxia increased the heart rate and decreased the percentage oxygen saturation. The interaction term for heat load and hypobaric hypoxia on heart rate, mean skin temperature and core temperature was significant. Pairwise comparisons showed that heat load increased heart rate, mean skin temperature, and core temperature more in the absence of hypobaric hypoxia.

Figure 6. Significant effects of heat load and hypobaric hypoxia on subjective temperature sensation (A) and subjective thermal discomfort (B). Error bars are ±1 SE (standard error).

Graphs with significant effects of heat load and hypobaric hypoxia on subjective temperature sensation and subjective thermal discomfort. Heat load significantly increased the thermal sensation and thermal discomfort ratings. The interaction term for heat load and hypobaric hypoxia was significant. Pairwise comparisons showed that heat load increased both thermal sensation and thermal discomfort more in the absence of hypobaric hypoxia.

Table 4. Median and range of reported hypoxia related symptoms on a scale from 0 (none) to 7 (extreme).

	Heat load		Hypobaric hypoxia		Heat load + hypobaric hypoxia
	Pre-altitude phase	Post-altitude	Pre-altitude phase	Post-altitude	Pre-altitude phase	Post-altitude
Heat	5 (2–7)	5 (1–6)	0 (0–4)	1 (0–4)	4 (1–5)	4 (1–6)
Fatigue	3 (0–6)	2 (0–5)	1 (0–4)	2 (0–6)	2 (1–6)	2 (0–7)
Dry mouth	1 (0–3)	1 (0–4)	0 (0–2)	1 (0–2)	1 (0–3)	1 (0–3)
Yawning	1 (0–6)	1 (0–3)	0 (0–4)	1 (0–5)	1 (0–4)	1 (0–6)
Sweating	3 (0–5)	3 (0–6)	0 (0–1)	0 (0–2)	2 (0–4)	2 (0–6)
Headache	1 (0–6)	1 (0–7)	1 (0–3)	1 (0–5)	1 (0–6)	1 (0–6)

Figure 7. Expected performance curves for conditions without stressors (most right line), with one stressor (middle line), and with two stressors (most left line) as function of increasing task load based on the results of the arithmetic subtask of the SYNWIN.

Expected performance curves for conditions without stressors (most right line) , with one stressor (middle line), and with two stressors (most left line) as a function of increased task load. When the task load increase, the relative performance start to decline. In the presence of one or two stressors the maximum task load that can be achieved is expected to shift to lower task load levels, indicated by the leftward shift of the performance curve.