Heart rate variability in novice pilots during and after a multi-leg cross-country flight.

BACKGROUND: A pilot's workload induces autonomic nervous system modulations which could be related to a decrease of vigilance that could impair safety. Kinetics during flight and recovery are not well known. OBJECTIVE: The aim of this study was to assess linear and nonlinear heart rate variability (HRV) modulations and vigilance during a high mental workload induced by a complex flight and subsequent recovery. METHODS: There were 10 novice pilots (37.8 +/- 4.4 yr, 115.8 +/- 15.7 h flight experience) who performed a 3 h 30 min (09:30-13:00) multi-leg cross-country flight (Piper Pa28 airplane: 160 hp). We recorded electrocardiogram (ECG) during the flight and performed tests during the 24 h before and after the flight (13:30, 16:00, 18:30, 21:00, and 06:45). Tests included a stand test (10 min supine, 10 min standing), a Mackworth 'clock' vigilance test, and a Karolinska Sleepiness Scale questionnaire. We assessed HRV components by time and frequency domains in parallel with the Poincaré plot analysis. RESULTS: The flight induced a progressive decrease of RR intervals, standard deviation between normal-to-normal intervals (SDNN), Poincaré SD1 and SD2 indices, and an increase of the low-frequency to high-frequency ratio (LF/HF). During recovery, vigilance remained depressed for 2 h 30 min after the flight. The decreased RR intervals and SD1 persisted for 5 h postflight both in supine and standing positions. LF/HF stayed elevated for 2 h 30 min after the flight. CONCLUSION: A multi-leg cross-country flight involves a vagal withdrawal and an increase of sympathetic activity lasting 5 h after landing. This delay could be recommended as a safety period.

Heart rate and autonomic balance during stand tests before and after fighter combat missions.

BACKGROUND: High workload during combat missions is a critical factor in the use of modern aircraft. Our objective was to evaluate the impact of piloting in war zones on the kinetics of the sympathovagal balance during recovery. METHODS: There were 40 military pilots who were monitored during operational flights in Afghanistan. Electrocardiographic activity was recorded during stand tests performed 1 h before takeoff (T-1), immediately after landing (L+0), 2 h after (L+2), and 4 h after (L+4) the flight. Missions were divided in two groups according to flight duration. RESULTS: The mean length of long flights was 4:31 +/- 0:53 h and of short flights 1:27 +/- 0:09 h. For long flights, at L+0, all indices related to parasympathetic modulation rose significantly in comparison to T-1, L+2, and L+4 (total power L+0: 2083 +/- 414 ms2 x Hz(-1),T-1: 1269 +/- 158 ms2 x Hz(-1), L+2: 1095 +/- 148 ms2 x Hz(-1), and L+4: 1238 +/- 124 ms2 x Hz(-1); high-frequency normalized units (HFnu) L+0: 16 +/- 2%, T-1: 11 +/- 1%, L+2: 10 +/- 1%, and L+4: 11 +/- 1%). At the same time the sympathetic frequency components significantly decreased (low-frequency normalized units (LFnu) L+0: 83 +/- 2%, T-1: 88 +/- 1%, L+2: 90 +/- 1%, and L+4: 89 +/- 1%; LF/HF L+0: 7 +/- 1, T-1: 11 +/- 1, L+2: 13 +/- 2, and L+4: 16 +/- 3). For short flights, the sympathetic components were higher at L+0 (LFnu: 77 +/- 2%; LF/HF: 14 +/- 3) than at T-1 (LFnu: 66 +/- 5%; LF/HF: 6 +/- 1). A concomitant reduction of vagal components was observed. CONCLUSIONS: Modulations of autonomic balance differed with the type of mission. A postflight sympathetic increase represents an autonomic adaptation due to stress and flight. A raise of parasympathetic modulation after flight may be related to the decrease of alertness.

Operational impact of health problems observed during a four-month military deployment in Ivory Coast.

Diseases always have a significant impact during military deployments. We evaluated the operational impact of health problems observed in a French infantry battalion (n = 690) during a 4-month assignment in Ivory Coast. In all, 55.7% of soldiers consulted at least once and sought care for 608 health problems. A total operational incapacity was observed in 22.2% of cases (7.6/1,000 person-days). The 5 diseases causing the greatest operational incapacity were diarrhea (2.1 days lost/1,000 person-days), musculoskeletal diseases and injuries (53.7 days), malaria (29 days), dental diseases (30.9 days), and fevers of undetermined origin (7 days). The incidence of diarrhea and skin infections was higher in rank-and-file troops than among noncommissioned officers. It was also higher during the mission's first month, when individual susceptibility to infections is suspected to be highest. Some diseases that are not serious nonetheless have a significant operational impact and should be better studied to determine preventive measures.

Short half-life hypnotics preserve physical fitness and altitude tolerance during military mountainous training.

OBJECTIVE: We study the effect of short half-life hypnotics (zaleplon or zolpidem against placebo) on altitude tolerance in 12 nonacclimated male soldiers (age, 22.1 +/- 0.8 years; height, 177.8 +/- 1.7 cm; weight, 69.8 +/- 1.7 kg). METHODS: Soldiers were trained to practice mountaineering at high altitude (2,533-4,810 meters) during 3 periods (one per medication tested) of 4 days (D1-D4). In each period the nights were spent in a hut (3,613 m). Administration of hypnotics or placebo was then implemented at 9:45 p.m. Nocturnal arterial oxygen saturation (SaO2) and heart rate variability (HRV) were monitored. At 5:00 a.m. and 9:00 p.m. physical fitness was assessed using acute mountain sickness (AMS) score. At 5:00 p.m., a posteffort stand test was carried out to evaluate the orthoparasympathetic imbalance with fatigue. RESULTS: Nocturnal SaO2 correlated negatively with morning AMS scores (R = -0.820, p < 0.02) and HRV analysis favored the sympathetic modulation. Posteffort stand tests revealed that sympathetic modulation attenuated from D2 to D3 in treated groups. CONCLUSION: The present study provides evidence that zolpidem or zaleplon improves sleep and subsequent physical fitness at altitude.

The vastus lateralis neuromuscular activity during all-out cycling exercise.

OBJECTIVE: The objective of this work was to study modifications in motor control through surface electromyographic (sEMG) activity during a very short all-out cycling exercise. METHODS: Twelve male cyclists (age 23+/-4 years) participated in this study. After a warm-up period, each subject performed three all-out cycling exercises of 6s separated by 2 min of complete rest. This protocol was repeated three times with a minimum of 2 days between each session. The braking torque imposed on cycling motion was 19 Nm. The sEMG of the vastus lateralis was recorded during the first seven contractions of the sprint. Time-frequency analysis of sEMG was performed using continuous wavelet transform. The mean power frequency (MPF, qualitative modifications in the recruitment of motor units) and signal energy (a quantitative indicator of modifications in the motor units recruitment) were computed for the frequency range 10-500 Hz. RESULTS: sEMG energy increased (P0.05) between contraction number 1 and 2, decreased (P < or =0.05) between contraction number 2 and 3 then stabilized between contraction number 3 and 7 during the all-out test. MPF increased (P < or =0.05) during the all-out test. This increase was more marked during the first two contractions. CONCLUSIONS: The decrease in energy and the increase in the sEMG MPF suggest a large spatial recruitment of motor units (MUs) at the beginning of the sprint followed by a preferential recruitment of faster MUs at the end of the sprint, respectively.

A dynamic network involving M1-S1, SII-insular, medial insular, and cingulate cortices controls muscular activity during an isometric contraction reaction time task.

Magnetoencephalographic, electromyographic (EMG), work, and reaction time (RT) were recorded from nine subjects during visually triggered intermittent isometric contractions of the middle finger under two conditions: unloaded and loaded (30% of maximal voluntary contraction). The effect of muscle fatigue was studied over three consecutive periods under both conditions. In the loaded condition, the motor evoked field triggered by the EMG onset decreased with fatigue, whereas movement-evoked fields (MEFs) increased (P < 0.01). Fatigue was demonstrated in the loaded condition, since (i) RT increased due to an increase in the electromechanical delay (P < 0.002); (ii) work decreased from Periods 1 to 3 (P < 0.005), while (iii) the myoelectric RMS amplitude of both flexor digitorum superficialis and extensor muscles increased (P < 0.003) and (iv) during Period 3, the spectral deflection of the EMG median frequency of the FDS muscle decreased (P < 0.001). In the unloaded condition and at the beginning of the loaded condition, a parallel network including M1-S1, posterior SII-insular, and posterior cingulate cortices accounted for the MEF activities. However, under the effect of fatigue, medial insular and posterior cingulate cortices drove this network. Moreover, changes in the location of insular and M1-S1 activations were significantly correlated with muscle fatigue (increase of RMS-EMG; P < 0.03 and P < 0.01, respectively). These results demonstrate that a plastic network controls the strength of the motor command as fatigue occurs: sensory information, pain, and exhaustion act through activation of the medial insular and posterior cingulate cortices to decrease the motor command in order to preserve muscle efficiency and integrity.

Stress modulation of the memory retrograde-enhancing effects of the awakening drug modafinil in mice.

PURPOSE: This study investigated the dose-effect relationship of modafinil administration on contextual memory processes, in parallel with the measurements of plasma corticosterone levels in acutely stressed mice. MATERIALS AND METHODS: Memory was first evaluated in normal (nonstressed) mice either in contextual (CSD) or spatial (SSD) tasks. Thus, C57 Bl/6 Jico mice learned two consecutive discriminations (D1 and D2) in a four-hole board. The discriminations occurred on either distinct (CSD) or identical (SSD) floors (internal contextual cues). All mice received a vehicle intraperitoneal injection before learning and were injected 24 h later (20 min before the test session) either with vehicle or modafinil. RESULTS: Results showed that modafinil-treated mice behaved similarly as vehicles in the spatial SSD task, whereas in contrast, memory of the first-learned discrimination (D1) in the CSD task was enhanced by a 32- but not a 16-mg/kg modafinil dose. Hence, we studied the effect of a pretest acute stress (electric footshocks) specifically on D1 performance in modafinil-treated subjects. Immediately after behavioral testing, blood was sampled to measure plasma corticosterone levels. CONCLUSIONS: Results showed that: (1) stress significantly improved performance in vehicles, (2) stress decreased the efficiency threshold of modafinil, as performance was enhanced at the low dose (16 mg/kg), whereas this enhancement was obtained for the high dose (32 mg/kg) under nonstress conditions, (3) the performance was impaired at the high (32 mg/kg) dose, and (4) modafinil significantly reduced the magnitude of the stress-induced corticosterone secretion, mainly at the dose of 32 mg/kg.

Zaleplon and zolpidem objectively alleviate sleep disturbances in mountaineers at a 3,613 meter altitude.

STUDY OBJECTIVES: To assess the effects of zolpidem and zaleplon on nocturnal sleep and breathing patterns at altitude, as well as on daytime attention, fatigue, and sleepiness. DESIGN: Double-blind, randomized, placebo-controlled, cross-over trial. SETTING: 3 day and night alpine expedition at 3,613 m altitude. PARTICIPANTS: 12 healthy male trekkers. PROCEDURE: One week spent at 1,000 m altitude (baseline control), followed by 3 periods of 3 consecutive treatment nights (N1-3) at altitude, to test 10 mg zolpidem, 10 mg zaleplon, and placebo given at 21:45. MEASURES: Sleep from EEG, actigraphy and sleep logs; overnight arterial saturation in oxygen (SpO2) from infrared oximetry; daytime attention, fatigue and sleepiness from a Digit Symbol Substitution Test, questionnaires, and sleep logs; acute mountain sickness (AMS) from the Lake Louise questionnaire. RESULTS: Compared to baseline control, sleep at altitude was significantly impaired in placebo subjects as shown by an increase in the amount of Wakefulness After Sleep Onset (WASO) from 17 +/- 8 to 36 +/- 13 min (P<0.05) and in arousals from 5 +/- 3 to 20 +/- 8 (P<0.01). Slow wave sleep (SWS) and stage 4 respectively decreased from 26.7% +/- 5.8% to 20.6% +/- 5.8% of total sleep time (TST) and from 18.2% +/- 5.2% to 12.4% +/- 3.1% TST (P<0.05 and P<0.001, respectively). Subjects also complained from a feeling of poor sleep quality combined with numerous 02 desaturation episodes. Subjective fatigue and AMS score were increased. Compared to placebo control, WASO decreased by approximately 6 min (P<0.05) and the sleep efficiency index increased by 2% (P<0.01) under zaleplon and zolpidem, while SWS and stage 4 respectively increased to 22.5% +/- 5.4% TST (P<0.05) and to 15.0% +/- 3.4% TST (P<0.0001) with zolpidem only; both drugs further improved sleep quality. No adverse effect on nighttime SpO2, daytime attention level, alertness, or mood was observed under either hypnotic. AMS was also found to be reduced under both medications. CONCLUSIONS: Both zolpidem and zaleplon have positive effects on sleep at altitude without adversely affecting respiration, attention, alertness, or mood. Hence, they may be safely used by climbers.

Brief exposure to -2 Gz reduces cerebral oxygenation in response to stand test.

The aim of the present experiment was to determine whether a single 30 s of exposure to -2 Gz (foot-to-head inertial forces) as orthostatic stress results in altered brain oxygenation control in response to active standing. Cerebral oxygenation (oxy-Hb), cerebral blood volume (CBV), and mean arterial blood pressure at brain level (MAPbrain) were recorded in 12 subjects in supine and then in standing position (10 min), before and after -2 Gz centrifugation. The decrease in oxy-Hb (-5 +/- 9 vs. -9 +/- 10 microM, P < 0.001) and in CBV (-2 +/- 11 vs. -4 +/- 12 microM, P < 0.05) upon standing was more important after -2 Gz centrifugation, with unchanged MAPbrain (-6 +/- 7 vs. -6 +/- 9 mmHg). These findings suggest a downward shift in the static cerebral autoregulation curve.

Modafinil restores memory performance and neural activity impaired by sleep deprivation in mice.

The original aims of our study have been to investigate in sleep-deprived mice, the effects of modafinil administration on spatial working memory, in parallel with the evaluation of neural activity level, as compared to non-sleep-deprived animals. For this purpose, an original sleep deprivation apparatus was developed and validated with continuous electroencephalography recording. Memory performance was evaluated using spontaneous alternation in a T-maze, whereas the neural activity level was estimated by the quantification of the c-Fos protein in various cerebral zones. This study allowed altogether: First, to evidence that a diurnal 10-h sleep deprivation period induced an impairment of spatial working memory. Second, to observe a decrease in c-Fos expression after sleep deprivation followed by a behavioural test, as compared to non-sleep-deprived mice. This impairment in neural activity was evidenced in areas involved in wake-sleep cycle regulation (anterior hypothalamus and supraoptic nucleus), but also in memory (frontal cortex and hippocampus) and emotions (amygdala). Finally, to demonstrate that modafinil 64 mg/kg is able to restore on the one hand memory performance after a 10-h sleep deprivation period, and on the other hand, the neural activity level in the very same brain areas where it was previously impaired by sleep deprivation and cognitive task.

Modafinil-induced modulation of working memory and plasma corticosterone in chronically-stressed mice.

The original aims of our study were to investigate the dose-effect relationship of modafinil administration on working memory performance, in parallel with the measurement of plasma corticosterone in chronically-stressed mice, as compared to control mice. Memory performance was evaluated by spontaneous alternation in a T-maze. Vehicle or modafinil (8, 16 or 32 mg/kg) were administered after or without chronic stress (immobilization and exposure to light) for 15 min/day over a period of consecutive 14 days. Immediately after behavioral testing, blood was sampled to measure plasma corticosterone levels. Under non-stress conditions, corticosterone significantly increased with 16 and 32 mg/kg modafinil administration. Interestingly, optimal working memory performance was revealed at the 16 mg/kg dose. Moreover, no correlation was evidenced between working memory performance and plasma corticosterone level in modafinil-treated animals. Under stress conditions, corticosterone level was lowered at 8 mg/kg and remained unchanged at 16 and 32 mg/kg modafinil. An optimal working memory performance was evidenced at 8 mg/kg, which indicated a decrease in the efficiency threshold of modafinil under stress. Furthermore, an inverse correlation emerged between working memory performance and corticosterone level. Our study evidenced for the first time the interaction between stress and memory, in the emotional modulation of working memory performance, as a function of the administered dose of modafinil.

Aerobatic flight effects on baroreflex sensitivity and sympathovagal balance in experienced pilots.

BACKGROUND: Aerobatic flights subject pilots to accelerations and, therefore, to heavy physical workloads. OBJECTIVE: Our aim was to document changes in spontaneous baroreflex sensitivity and disturbances of sympathovagal balance after exposure to "push-pull" accelerations. METHODS: During 30-min flights, five aerobatic pilots performed five series of descending spirals: first, 30 s under negative (-3 Gz max), and then 30 s under positive (+4 Gz max) G loading, climbing between each series to regain altitude. A stand-test was performed before (T0), immediately postflight (PF), 1 h (PF1), and 2 h after (PF2) the flight. A Finapres apparatus recorded heart rate (HR) and BP during the stand-tests. RESULTS: Resting HR was higher at PF than T0 in supine (11.2 +/- 5.3%, p < 0.01) and standing (11.0 +/- 4.9%; p < 0.05) positions. Sequence analysis of spontaneous baroflex sensitivity (BRS) and spectral analysis of HR variability showed that: a) supine spontaneous BRS did not differ between preflight and postflight, while parasympathetic modulation of HR variability tended to increase; and b) supine spontaneous BRS was higher at PF1 than PF (PF: 0.011 +/- 0.0014 ms x mmHg(-1), PF1: 0.015 +/- 0.0012 ms x mmHg(-1); p < 0.05) and parasympathetic modulation of HR variability (high frequency component) was higher at PF2 than PF (PF: 0.014 +/- 0.007, PF2: 0.039 +/- 0.009; p < 0.001). CONCLUSIONS: These findings may reflect a change in the sympathovagal balance during the second hour of recovery from repeated push-pull maneuvers.

Cerebral oxygenation declines despite maintained orthostatic tolerance after brief exposure to gravitational stress.

We examined the effect of a single 120 s of exposure to +3Gz (head-to-foot inertial forces) centrifugation as orthostatic stress on cerebral oxygenation (oxy-Hb) and cerebral blood volume (CBV) changes in response to stand test, in order to relate the occurrence of altered cerebral oxygenation control to any increase in sympathetic activity. Frontal near-infrared spectroscopy and mean arterial blood pressure at brain level (MAPbrain) were recorded in 14 subjects in supine and then in standing (10 min) position, before and after +3Gz centrifugation. The decrease in oxy-Hb (-7 +/- 5 a.u. versus -27 +/- 4 a.u., P<0.001) and in CBV (-6 +/- 10 a.u. versus -15 +/- 8 a.u., P<0.05) upon standing was more important after +3Gz centrifugation, with unchanged MAPbrain (-8 +/- 8 mmHg versus -3 +/- 11 mmHg). Upon standing, the high-frequency component of heart rate was lower (1090 +/- 460 ms2 versus 827 +/- 412 ms2, P<0.05) after +3Gz centrifugation. These findings suggest a downward shift in the static cerebral autoregulatory curve. We conclude that cerebral vasoconstriction might have occurred without centrally mediated increase in the entire peripheral sympathetic activity of the body.

EEG and ECG changes during simulator operation reflect mental workload and vigilance.

BACKGROUND: Performing mission tasks in a simulator influences many neurophysiological measures. Quantitative assessments of electroencephalography (EEG) and electrocardiography (ECG) have made it possible to develop indicators of mental workload and to estimate relative physiological responses to cognitive requirements. OBJECTIVE: To evaluate the effects of mental workload without actual physical risk, we studied the cortical and cardiovascular changes that occurred during simulated flight. METHODS: There were 12 pilots (8 novices and 4 experts) who simulated a flight composed of 10 sequences that induced several different mental workload levels. EEG was recorded at 12 electrode sites during rest and flight sequences; ECG activity was also recorded. Subjective tests were used to evaluate anxiety and vigilance levels. RESULTS: Theta band activity was lower during the two simulated flight rest sequences than during visual and instrument flight sequences at central, parietal, and occipital sites (p < 0.05). On the other hand, rest sequences resulted in higher beta (at the C4 site; p < 0.05) and gamma (at the central, parietal, and occipital sites; p < 0.05) power than active segments. The mean heart rate (HR) was not significantly different during any simulated flight sequence, but HR was lower for expert subjects than for novices. The subjective tests revealed no significant anxiety and high values for vigilance levels before and during flight. CONCLUSIONS: The different flight sequences performed on the simulator resulted in electrophysiological changes that expressed variations in mental workload. These results corroborate those found during study of real flights, particularly during sequences requiring the heaviest mental workload.

EEG and ECG changes during selected flight sequences.

BACKGROUND: Mental workload has become a critical factor in the design and use of modern aircraft. Because of the complexity of the human-machine system, it is necessary to determine workload, fatigue, and level of performance using noninvasive electrophysiological measures. OBJECTIVE: Our objective was to identify the electrophysiological indicators of mental workload during piloting tasks. METHODS: Electroencephalographic (EEG) and electrocardiographic (ECG) activity was recorded during actual flight, with a profile planned to produce different levels of mental workload. RESULTS: In-flight EEG and ECG recordings enabled us to document mental workload levels. During active segments, delta and theta band activity increased (p < 0.05 or greater); results showed an increase of 22.5% for theta band activity during active flight segments compared with in-flight rest periods. Inversely, alpha band activity diminished: the decrease between ground baseline and all flight sequences was 30% (p < 0.05 or greater). These variations were reversed during the in-flight rest sequences. Instrumental flight caused an increase in the theta and alpha frequency band activity in the parietal-occipital area (p < 0.05 or greater); the alpha/beta ratio also increased. Heart rate increased during the active segments and fell during the in-flight rest periods. The mean difference between active segments and in-flight rest periods was of 8.89 bpm (i.e., an increase of 11.5%; p < 0.01). Heart rate was correlated to the EEG activity of the delta and beta bands in the central area (C3, Cz, C4, p < 0.05). CONCLUSIONS: The electrophysiological measures recorded provide useful indicators of the mental workload required by different flight sequences.

Analysis of heart rate variability after a ranger training course.

We studied the effects of prolonged physical activities on resting heart rate variability (HRV) during a training session attended by 23 cadets of the French military academy. This course lasts 1 month and is concluded by a 5-day field exercise simulation with physical and psychological stress. Data collection took place before (B) and immediately at the end (E) of the course. It included HRV recordings during a stand test (5 minutes lying down and 5 minutes standing), with a Polar R-R monitor, followed by blood sampling to assay plasma testosterone. The results (B and E) showed that the testosterone level fell by approximately 28.6 +/- 7%, indicating a high level of fatigue. During the stand test, the total power (TP) of the HRV spectrum increased in a supine position. The TP of B was 5,515.7 ms2 (SE, 718.4) and of E was 13018.9 ms2 (SE, 2,539.2; p < 0.001). High-frequency (HF) normalized values increased and low-frequency (LF) normalized values fell, regardless of position (HF normalized values and LF normalized values: supine, p < 0.01, p < 0.05; standing, p < 0.05, p < 0.01, respectively). LF:HF ratio fell 66.2 (SE, 12.9%; p < 0.01) in a lying position. During the time-domain analysis of HRV, differences between adjacent normal R-R intervals more than 50 milliseconds, expressed as a percentage, and differences between the coupling intervals of adjacent normal RR intervals increased in the lying position (p < 0.001). These results as a whole suggest that parasympathetic nervous system activity increases with fatigue.

Declarative memory impairments following a military combat course: parallel neuropsychological and biochemical investigations.

The aim of this study was to investigate the impact on several forms of memory and metabolism of a 5-day combat course including heavy and continuous physical activities and sleep deprivation. Mnemonic performance and biochemical parameters of 21 male soldiers were examined before and at the end of the course. Our results showed that short-term memory (memory span, visual memory, audiovisual association) and long-term memory were significantly impaired, whereas short-term spatial memory and planning tasks were spared. Parallel biochemical analysis showed an adaptation of energy metabolism. The observed decrease in glycaemia may be partly responsible for the long-term memory impairment, whereas the decreases in plasma cholinesterases and choline may be involved in the short-term memory deterioration. However, there are also many other reasons for the observed memory changes, one of them being chronic sleep deprivation.