Limitations of stroke volume estimation by non-invasive blood pressure monitoring in hypergravity.

BACKGROUND: Altitude and gravity changes during aeromedical evacuations induce exacerbated cardiovascular responses in unstable patients. Non-invasive cardiac output monitoring is difficult to perform in this environment with limited access to the patient. We evaluated the feasibility and accuracy of stroke volume estimation by finger photoplethysmography (SVp) in hypergravity. METHODS: Finger arterial blood pressure (ABP) waveforms were recorded continuously in ten healthy subjects before, during and after exposure to +Gz accelerations in a human centrifuge. The protocol consisted of a 2-min and 8-min exposure up to +4 Gz. SVp was computed from ABP using Liljestrand, systolic area, and Windkessel algorithms, and compared with reference values measured by echocardiography (SVe) before and after the centrifuge runs. RESULTS: The ABP signal could be used in 83.3% of cases. After calibration with echocardiography, SVp changes did not differ from SVe and values were linearly correlated (p<0.001). The three algorithms gave comparable SVp. Reproducibility between SVp and SVe was the best with the systolic area algorithm (limits of agreement -20.5 and +38.3 ml). CONCLUSIONS: Non-invasive ABP photoplethysmographic monitoring is an interesting technique to estimate relative stroke volume changes in moderate and sustained hypergravity. This method may aid physicians for aeronautic patient monitoring.

Single-beat versus multibeat real-time 3D echocardiography for assessing left ventricular volumes and ejection fraction: a comparison study with cardiac magnetic resonance.

BACKGROUND: Real-time 3-dimensional echocardiography (RT3DE) is superior to 2D echocardiography in assessing left ventricular (LV) volumes and ejection fraction (EF), but its feasibility is limited by multibeat acquisition, which requires an optimal breath-hold and a regular heart rhythm. We sought to evaluate the accuracy and feasibility of single- and 2-beat RT3DE for LV volume and EF assessment. METHODS AND RESULTS: Sixty-six consecutive patients referred for cardiac magnetic resonance (CMR) underwent RT3DE and CMR on the same day. Of the 50 patients (age, 59+/-18 years; 68%men; 42% coronary artery disease; LVEF=49+/-14%; limits, 14% to 76%) with an adequate RT3DE image quality, accuracy for LV volumes and EF measurements of single- and 2-beat modalities were compared with the conventional 4-beat acquisition and CMR. Correlations with CMR for LV end-diastolic volume (161+/-59 mL, r=0.93 to 0.94) and end-systolic volume (86+/-56 mL, r=0.93 to 0.96) were excellent regardless of the number of cardiac cycles used. However, because of the low temporal resolution (7+/-2 volumes per second), single-beat underestimated LVEF (bias, -5+/-8%) with greater bias than 2-beat (bias, 1+/-6%, P<0.001) and 4-beat (bias, 3+/-7%, P<0.001) modalities. Interestingly, 2-beat provided accuracy similar to 4-beat for end-diastolic volume (bias, -17+/-21 mL versus -15+/-23 mL), end-systolic volume (bias, -9+/-16 mL versus -12+/-17 mL), and LVEF (bias, 1+/-6% versus 3+/-7%) measurements, but fewer stitching artifacts were observed with 2- than 4-beat modalities (3% versus 30%). CONCLUSIONS: Compared with conventional multibeat acquisitions, 2-beat modality provides similar accuracy in LV volume and EF measurements and should be preferred due to fewer stitching artifacts. In contrast, the temporal resolution of single-beat modality appears insufficient to provide an accurate estimation of LVEF.

Impact of longitudinal myocardial deformation on the prognosis of chronic heart failure patients.

BACKGROUND: Longitudinal myocardial deformation indexes appear superior to left ventricular ejection fraction (LVEF) in assessing myocardial contractility. However, few studies have addressed the prognostic value of longitudinal motion markers (velocity, strain, and strain rate) in predicting outcome in heart failure patients. METHODS AND RESULTS: The study included 125 consecutive symptomatic heart failure patients (63+/-16 years, 77% male, LVEF=31+/-10%). All patients underwent a complete echocardiographic and clinical examination, and brain natriuretic peptide level was assessed in 93 patients. Longitudinal myocardial velocity by tissue Doppler imaging, global-epsilon, and strain rate by speckle tracking were computed from apical views (4-, 3-, and 2-chambers views) and compared with the occurrence of major adverse cardiac events. On the whole, peak longitudinal velocity, global-epsilon, and strain rate averaged 5+/-2 cm/s (range, 1 to 9), -8+/-3% (range, -3 to -18), and -0.33+/-0.16 s(-1) (range, -0.83 to -0.05), respectively. During the follow-up period (266+/-177 days), major adverse cardiac events occurred in 47 (38%) patients (15 deaths, 29 recurrent heart failure, and 4 heart transplantations). By univariable analysis using Cox model global-epsilon, strain rate, and LVEF were associated with the occurrence of major adverse cardiac events, whereas only global-epsilon remained independently predictive of outcome by multivariate analysis. CONCLUSIONS: In the heart failure population, longitudinal global strain by speckle tracking is superior to LVEF and other longitudinal markers in identifying patients with poor outcome.

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.

Energy expenditure during an ultraendurance alpine climbing race.

OBJECTIVE: Accurate reports of energy expenditure (EE) during prolonged mountaineering activity are sparse. The purpose of this study was to estimate EE during a winter ultraendurance climbing race and individual mountaineering activities in Mont Blanc, France. METHODS: Seven days before the race, resting metabolic rate (RMR) and maximal oxygen consumption (Vo2(max)) were measured in 10 experienced male climbers (30.0 +/- 0.9 years). Three days before (reference period) and during the race, heart rate (HR) was recorded for estimation of total daily EE (TDEE), and the type and duration of all activities were collected through questionnaires. Total DEE was calculated by adding DEE during sleep (DEE sleep), sedentary (DEE sedentary), and during exercise (DEE exercise). Daily energy expenditure during exercise was determined through assumption of the rectilinear relationship between heart rate (HR) and Vo2. Anthropometric measurements were performed 7 days before, just before, and immediately after the race. RESULTS: Total time of the race averaged approximately 29 hours and 29 minutes, including 11 hours and 24 minutes in the hut, plus 18 hours and 5 minutes dedicated to climbing. During the race, TDEE was 43.6 +/- 1.2 MJ x d(-1). Energy expenditures for cross-country skiing and alpine climbing were similar (57.3 +/- 2.1 kJ x min(-1) and 54.0 +/- 2.9 kJ x min(-1), respectively). An energy deficit of 33.5 +/- 2.3 MJ resulted after the race, with a mean weight loss of 1.52 +/- 0.31 kg (P < .001). CONCLUSIONS: Experienced climbers expended a high level of energy during a winter ultraendurance alpine climbing race at moderate altitude under high degrees of difficulty and risk exposure. These results provide comparative data on the energy cost of the main mountaineering activities during a race: cross-country skiing and alpine climbing.

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.

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.

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.

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.

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 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.