Previous studies have suggested that, during prolonged cold exposure, shivering thermogenesis may gradually be attenuated, supposedly reflecting a state of central fatigue (aka ‘thermoregulatory fatigue’) provoked by extended shivering activity, that precipitates hypothermia. The purpose of this study was to revisit the validity of this notion. Twelve noncold-acclimatized men participated in three ∼10-h sessions, during which they performed repeatedly three 120-min cold-water immersions. To induce discrete amounts of heat-producing thermoeffector output, presumptively leading to distinct levels of fatigue during each session, subjects were submersed, within each session, in either severely (15°C), moderately (20°C), or slightly (28°C) cold water. The cold-induced elevation in thermogenic rate was similar across the three repeated immersions performed within the 15°C (∼130 W·m2) and 20°C (∼100 W·m2) sessions (P ≥ 0.43). In the 28°C-session, the metabolic heat production was augmented by ∼7% in the second and third immersions compared with the first immersion (P = 0.01). No intrasession differences were noted with regards to the body-core cooling rate, the cold-induced drop in skin temperature and forearm cutaneous vascular conductance, or the stress-hormone (salivary α-amylase and cortisol concentrations) and thermoperceptual responses (P > 0.05). The present findings, therefore, demonstrate that the ability to generate heat remains intact during prolonged iterative exposure to a high-heat loss environment in a single day, regardless of the severity of cold stressor. The intermittent application of slight cold stress (i.e., 28°C water) appears to mediate metabolic sensitization, reflecting either the circadian rhythmicity of heat-producing thermoeffector activity, or perhaps the rapid induction of metabolic adaptation.

When entering a coordinated flight turn without visual references, the perception of roll-angular displacement is determined by vestibular cues, and/or probably by assessment of the gravitoinertial (G) load (G magnitude) and its translation into the corresponding bank angle. Herein, we examined whether repeated exposures to hypergravity (G training) in a centrifuge, would advance, not only the ability to accurately assess the G load but also the capacity to detect or estimate the corresponding roll inclination of the centrifuge gondola. To this end, in nine men without piloting experience, the subjective estimation of G load and roll tilt were assessed, in complete darkness, during 5-min coordinated turns in the centrifuge, performed at 1.1 G (25 degrees roll-tilt angle) and 2.0 G (60 degrees roll tilt angle). These trials were conducted before and after 5 wk of G training {3 x 40-min sessions<middle dot>wk-1; protocol: 20 x 1 min at G levels close to the individual relaxed G-level tolerance [range: similar to 2.6 G (similar to 67 degrees)-3.6 G (74 degrees)], separated by 1-min intervals at idle speed (1.4 G)}, whereas continual feedback to the subjects was limited to the G load. As expected, G training improved subjects' capacity to assess G load, especially at 2.0 G (P = 0.006). The perception of roll tilt, however, was consistently underestimated (by similar to 70%-80%), and not enhanced by G training (P >= 0.51). The present findings demonstrate that prolonged repeated G-induced roll-tilts in a centrifuge gondola, while external feedback is restricted to graviception, enhance the capacity to perceive G load, but fail to advance the ability to detect or consciously estimate the magnitude of roll-angular displacement during a coordinated turn. NEW & NOTEWORTHY During a coordinated flight turn without external visual references, the pilot typically underestimates the aircraft bank angle, because unreliable information of roll tilt is conveyed by the vestibular system. The present results demonstrate that prolonged repeated gravitoinertial (G)-induced roll-tilts in a centrifuge gondola, while external feedback is restricted to graviception, enhance the capacity to perceive G load, but fail to advance the ability to consciously estimate the magnitude of roll angular displacement.

Divers are at enhanced risk of suffering from acute cognitive deteriorations, due to the low ambient temperatures, and the narcotic action of inert gases inspired at high pressures. Yet, the behavioral effects of cold and inert-gas narcosis have commonly been assessed in isolation, and during short-term provocations. We, therefore, evaluated the interactive influence of mild hypothermia and narcosis engendered by a subanaesthetic dose of nitrous oxide (N2O; a normobaric intervention analogue of hyperbaric nitrogen) on cognitive function during prolonged iterative exposure. Fourteen men partook in two ~12-h sessions (separated by ≥4 days), wherein they performed sequentially three 120-min cold (20°C) water immersions (CWIs), while inhaling, in a single-blinded manner, either normal air, or a normoxic gas mixture containing 30% N2O. CWIs were separated by 120-min rewarming in room-air breathing conditions. Prior to the first CWI and during each CWI, subjects performed a finger dexterity test, and the Spaceflight Cognitive Assessment Tool for Windows (WinSCAT) test assessing aspects of attention, memory, learning and visuo-spatial ability. Rectal and skin temperatures were, on average, reduced by ~1.2°C and ~8°C, respectively (P<0.001). Cooling per se impaired (P≤0.01) only short-term memory (~37%) and learning (~18%); the impairments were limited to the first CWI. N2O also attenuated (P≤0.02) short-term memory (~37%) and learning (~35%), but the reductions occurred in all CWIs. Further, N2O invariably compromised finger dexterity, attention, concentration, working memory and spatial processing (P<0.05). Present results demonstrate that inert-gas narcosis aggravates, in a persistent manner, basic and higher-order cognitive abilities during protracted cold exposure.

Breath-holding preceded by either an overnight fast or hyperventilation has been shown to potentiate the risk of a hypoxic blackout. However, no study has explored the combined effects of fasting and hyperventilation on apneic performance and associated physiological responses. Nine nondivers (8 males) attended the laboratory on two separate occasions (≥48 h apart), both after a 12-h overnight fast. During each visit, a hyperoxic rebreathing trial was performed followed by three repeated maximal static apneas preceded by either normal breathing (NORM) or a 30-s hyperventilation (HYPER). Splenic volume, hematology, cardiovascular, and respiratory variables were monitored. There were no interprotocol differences at rest or during hyperoxic rebreathing for any variable (P ≥ 0.09). On nine occasions (8 in HYPER), the subjects reached our safety threshold (oxygen saturation 65%) and were asked to abort their apneas, with the preponderance of these incidents (6 of 9) occurring during the third repetition. Across the sequential attempts, longer apneas were recorded in HYPER [median(range), 220(123–324) s vs. 185(78–296) s, P ≤ 0.001], with involuntary breathing movements occurring later [134(65–234) s vs. 97(42–200) s, P ≤ 0.001] and end-apneic partial end-tidal pressures of oxygen (PETO2) being lower (P ≤ 0.02). During the final repetition, partial end-tidal pressure of carbon dioxide [(PETCO2), 6.53 ± 0.46 kPa vs. 6.01 ± 0.45 kPa, P = 0.005] was lower in HYPER. Over the serial attempts, preapneic tidal volume was gradually elevated [from apnea 1 to 3, by 0.26 ± 0.24 L (HYPER) and 0.28 ± 0.30 L (NORM), P ≤ 0.025], with a correlation noted with preapneic PETCO2 (r = −0.57, P < 0.001) and PETO2 (r = 0.76, P < 0.001), respectively. In a fasted state, preapnea hyperventilation compared with normal breathing leads to longer apneas but may increase the susceptibility to a hypoxic blackout.

During coordinated flight and centrifugation, pilots show interindividual variability in perceived roll tilt. The study explored how this variability is related to perceptual and cognitive functions. Twelve pilots underwent three 6-min centrifugations on two occasions (G levels: 1.1G, 1.8G, and 2.5G; gondola tilts: 25°, 56°, and 66°). The subjective visual horizontal (SVH) was measured with an adjustable luminous line and the pilots gave estimates of experienced G level. Afterward, they were interrogated regarding the relationship between G level and roll tilt and adjusted the line to numerically mentioned angles. Generally, the roll tilt during centrifugation was underestimated, and there was a large interindividual variability. Both knowledge on the relationship between G level and bank angle, and ability to adjust the line according to given angles contributed to the prediction of SVH in a multiple regression model. However, in most cases, SVH was substantial smaller than predictions based on specific abilities.

Without visual references, nonpilots exposed to coordinated flight turns underestimate the bank angle, because of discordant information of the roll-angular displacement from the otoliths, consistently signaling vertical position, versus the semicircular canals, enabling detection of the displacement. Pilots may also use their ability to perceive the G load and knowledge of the relation between load and angle to assess the bank angle. Our aim was to investigate whether the perception of bank angle can be improved by spatial orientation training in a centrifuge. Sixteen pilots/pilot students assessed their roll tilt, in complete darkness, during both real coordinated flight turns and gondola centrifugation, at roll tilts of 30◦ and 60◦. The experiments were repeated after a 3-wk period, during which eight of the subjects performed nine training sessions in the centrifuge, comprising feedback on roll angle vs. G load, and on indicating requested angles. Before training, the subjects perceived in the aircraft and centrifuge, respectively: 37 (17)◦, 38 (14)◦ during 60◦ turns and 19 (12)◦, 20 (10)◦ during 30◦ turns. Training improved the perception of angle during the 60◦ [to 60 (7)◦, 55 (10)◦; P ≤ 0.04] but not the 30◦ [21 (10)◦, 15 (9)◦; P ≥ 0.30] turns; the improvement disappeared within 2 yr after training. Angle assessments did not change in the untrained group. The results suggest that it is possible to, in a centrifuge, train a pilot’s ability to perceive large but not discrete-to-moderate roll-angular displacements. The transient training effect is attributable to improved capacity to perceive and translate G load into roll angle and/or to increased reliance on semicircular canal signals. NEW & NOTEWORTHY Spatial disorientation is a major problem in aviation. When performing coordinated flight turns without external visual cues (e.g., flying in clouds or darkness), the pilot underestimates the aircraft bank angle because the vestibular system provides unreliable information of roll tilt. The present study demonstrates that it is possible to, in a long-arm centrifuge, train a pilot’s ability to perceive large but not discrete-to-moderate roll-angular displacements.

Purpose

We evaluated the hypothesis that repetitive gravitoinertial stress would augment the arterial-pressure response to peripheral sympathetic stimulation.

Methods

Before and after a 5-weeks G-training regimen conducted in a human-use centrifuge, twenty healthy men performed a hand cold-pressor test, and nine of them also a foot cold-pressor test (4 min; 4 °C water). Arterial pressures and total peripheral resistance were monitored.

Results

The cold-induced elevation (P ≤ 0.002) in arterial pressures and total peripheral resistance did not vary between testing periods, either in the hand [mean arterial pressure: Before =  + 16% vs. After =  + 17% and total peripheral resistance: Before =  + 13% vs. After =  + 15%], or in the foot [mean arterial pressure: Before =  + 19% vs. After =  + 21% and total peripheral resistance: Before =  + 16% vs. After =  + 16%] cold-pressor tests (P > 0.05).

Conclusion

Present results demonstrate that 5 weeks of prolonged iterative exposure to hypergravity does not alter the responsiveness of sympathetically mediated circulatory reflexes.

Purpose: This study examined the magnitude of physiological strain imposed by repeated maximal static and dynamic apneas through assessing a panel of stress-related biomarkers. Methods: Eleven healthy men performed on three separate occasions (≥72-h apart): a series of five repeated maximal (i) static (STA) or (ii) dynamic apneas (DYN) or (iii) a static eupneic protocol (CTL). Venous blood samples were drawn at 30, 90, and 180-min after each protocol to determine ischaemia modified albumin (IMA), neuron-specific enolase (NSE), myoglobin, and high sensitivity cardiac troponin T (hscTnT) concentrations. Results: IMA was elevated after the apnoeic interventions (STA,+86%;DYN,+332%,p ≤ 0.047) but not CTL (p = 0.385). Myoglobin was higher than baseline (23.6 ± 3.9 ng/mL) 30-min post DYN (+70%,38.8 ± 13.3 ng/mL,p = 0.030). A greater myoglobin release was recorded in DYN compared with STA and CTL (p ≤ 0.035). No changes were observed in NSE (p = 0.207) or hscTnT (p = 0.274). Conclusions: Five repeated maximal DYN led to a greater muscle injury compared with STA but neither elicited myocardial injury or neuronal-parenchymal damage.

The study examined intra- and interlimb variations in cutaneous vessel responsiveness to acute and repeated transmural pressure elevations. In 11 healthy men, red blood cell flux was assessed via laser-Doppler flowmetry on both glabrous and nonglabrous skin regions of an arm (finger and forearm) and leg (toe and lower leg), across a wide range of stepwise increasingdistending pressures imposed in the vessels of each limb separately. The pressure-flux cutaneous responses were evaluatedbefore and after 5 wk of intermittent (40 min, 3 sessions per week) exposures to hypergravity (2.6–3.3 G; G training). Beforeand after G training, forearm and lower leg blood flux were relatively stable up to 210 and 240 mmHg distending pressures,respectively; and then they increased two- to threefold (P < 0.001). Finger blood flux dropped promptly (P < 0.001), regardlessof the G training (P = 0.64). At 120-mmHg distending pressures, toe blood flux enhanced by 40% (P  0.05); the increasewas augmented after the G training (P = 0.01). At high distending pressures, toe blood flux dropped by 70% in both trials (P <0.001). The present results demonstrate that circulatory autoregulation is more pronounced in glabrous skin than in nonglabrousskin, and in nonglabrous sites of the leg than in those of the arm. Repetitive high-sustained gravitoinertial stress does not modifythe pressure-flow relationship in the dependent skin vessels of the arm nor in the nonglabrous sites of the lower leg. Yet it maypartly inhibit the myogenic responsiveness of the toe’s glabrous skin.