Examining the Influence of Cognitive Load and Environmental Conditions on Autonomic Nervous System Response in Military Aircrew: A Hypoxia-Normoxia Study.

Executing flight operations demand that military personnel continuously perform tasks that utilize low- and high-order cognitive functions. The autonomic nervous system (ANS) is crucial for regulating the supply of oxygen (O2) to the brain, but it is unclear how sustained cognitive loads of different complexities may affect this regulation. Therefore, in the current study, ANS responses to low and high cognitive loads in hypoxic and normoxic conditions were evaluated. The present analysis used data from a previously conducted, two-factor experimental design. Healthy subjects (n = 24) aged 19 to 45 years and located near Fort Novosel, AL, participated in the parent study. Over two, 2-h trials, subjects were exposed to hypoxic (14.0% O2) and normoxic (21.0% O2) air while simultaneously performing one, 15-min and one, 10-min simulation incorporating low- and high-cognitive aviation-related tasks, respectively. The tests were alternated across five, 27-min epochs; however, only epochs 2 through 4 were used in the analyses. Heart rate (HR), HR variability (HRV), and arterial O2 saturation were continuously measured using the Warfighter MonitorTM (Tiger Tech Solutions, Inc., Miami, FL, USA), a previously validated armband device equipped with electrocardiographic and pulse oximetry capabilities. Analysis of variance (ANOVA) regression models were performed to compare ANS responses between the low- and high-cognitive-load assessments under hypoxic and normoxic conditions. Pairwise comparisons corrected for familywise error were performed using Tukey's test within and between high and low cognitive loads under each environmental condition. Across epochs 2 through 4, in both the hypoxic condition and the normoxic condition, the high-cognitive-load assessment (MATB-II) elicited heightened ANS activity, reflected by increased HR (+2.4 ± 6.9 bpm) and decreased HRV (-rMSSD: -0.4 ± 2.7 ms and SDNN: -13.6 ± 14.6 ms). Conversely, low cognitive load (ADVT) induced an improvement in ANS activity, with reduced HR (-2.6 ± 6.3 bpm) and increased HRV (rMSSD: +1.8 ± 6.0 ms and SDNN: vs. +0.7 ± 6.3 ms). Similar observations were found for the normoxic condition, albeit to a lower degree. These within-group ANS responses were significantly different between high and low cognitive loads (HR: +5.0 bpm, 95% CI: 2.1, 7.9, p < 0.0001; rMSSD: -2.2 ms, 95% CI: -4.2, -0.2, p = 0.03; SDNN: -14.3 ms, 95% CI: -18.4, -10.1, p < 0.0001) under the hypoxic condition. For normoxia, significant differences in ANS response were only observed for HR (+4.3 bpm, 95% CI: 1.2, 7.4, p = 0.002). Lastly, only high cognitive loads elicited significant differences between hypoxic and normoxic conditions but just for SDNN (-13.3 ms, 95% CI, -17.5, -8.9, p < 0.0001). Our study observations suggest that compared to low cognitive loads, performing high-cognitive-load tasks significantly alters ANS activity, especially under hypoxic conditions. Accounting for this response is critical, as military personnel during flight operations sustain exposure to high cognitive loads of unpredictable duration and frequency. Additionally, this is likely compounded by the increased ANS activity consequent to pre-flight activities and anticipation of combat-related outcomes.

Tracking Cancer: Exploring Heart Rate Variability Patterns by Cancer Location and Progression.

Reduced heart rate variability (HRV) is an autonomic nervous system (ANS) response that may indicate dysfunction in the human body. Consistent evidence shows cancer patients elicit lower HRV; however, only select cancer locations were previously evaluated. Thus, the aim of the current study was to explore HRV patterns in patients diagnosed with and in varying stages of the most prevalent cancers. At a single tertiary academic medical center, 798 patients were recruited. HRV was measured via an armband monitor (Warfighter MonitorTM, Tiger Tech Solutions, Inc., Miami, FL, USA) equipped with electrocardiographic capabilities and was recorded for 5 to 7 min with patients seated in an upright position. Three time-domain metrics were calculated: SDNN (standard deviation of the NN interval), rMSSD (the root mean square of successive differences of NN intervals), and the percentage of time in which the change in successive NN intervals exceeds 50ms within a measurement (pNN50). Of the 798 patients, 399 were diagnosed with cancer. Cancer diagnoses were obtained via medical records one week following the measurement. Analysis of variance models were performed comparing the HRV patterns between different cancers, cancer stages (I-IV), and demographic strata. A total of 85% of the cancer patients had breast, gastrointestinal, genitourinary, or respiratory cancer. The cancer patients were compared to a control non-cancer patient population with similar patient size and distributions for sex, age, body mass index, and co-morbidities. For all HRV metrics, non-cancer patients exhibited significantly higher rMSSDs (11.1 to 13.9 ms, p < 0.0001), SDNNs (22.8 to 27.7 ms, p < 0.0001), and pNN50s (6.2 to 8.1%, p < 0.0001) compared to stage I or II cancer patients. This significant trend was consistently observed across each cancer location. Similarly, compared to patients with stage III or IV cancer, non-cancer patients possessed lower HRs (-11.8 to -14.0 bpm, p < 0.0001) and higher rMSSDs (+31.7 to +32.8 ms, p < 0.0001), SDNNs (+45.2 to +45.8 ms), p < 0.0001, and pNN50s (19.2 to 21.6%, p < 0.0001). The HR and HRV patterns observed did not significantly differ between cancer locations (p = 0.96 to 1.00). The depressed HRVs observed uniformly across the most prevalent cancer locations and stages appeared to occur independent of patients' co-morbidities. This finding highlights the potentially effective use of HRV as a non-invasive tool for determining common cancer locations and their respective stages. More studies are needed to delineate the HRV patterns across different ages, between sexes and race/ethnic groups.

Continuous Physiological Monitoring of the Combined Exposure to Hypoxia and High Cognitive Load in Military Personnel.

Military aviators endure high cognitive loads and hypoxic environments during flight operations, impacting the autonomic nervous system (ANS). The synergistic effects of these exposures on the ANS, however, are less clear. This study investigated the simultaneous effects of mild hypoxia and high cognitive load on the ANS in military personnel. This study employed a two-factor experimental design. Twenty-four healthy participants aged between 19 and 45 years were exposed to mild hypoxia (14.0% O2), normoxia (21.0% O2), and hyperoxia (33.0% O2). During each epoch (n = 5), participants continuously performed one 15 min and one 10 min series of simulated, in-flight tasks separated by 1 min of rest. Exposure sequences (hypoxia-normoxia and normoxia-hyperoxia) were separated by a 60 min break. Heart rate (HR), heart rate variability (HRV), and O2 saturation (SpO2) were continuously measured via an armband monitor (Warfighter MonitorTM, Tiger Tech Solutions, Inc., Miami, FL, USA). Paired and independent t-tests were used to evaluate differences in HR, HRV, and SpO2 within and between exposure sequences. Survival analyses were performed to assess the timing and magnitude of the ANS responses. Sympathetic nervous system (SNS) activity during hypoxia was highest in epoch 1 (HR: +6.9 bpm, p = 0.002; rMSSD: -9.7 ms, p = 0.003; SDNN: -11.3 ms, p = 0.003; SpO2: -8.4%, p < 0.0000) and appeared to slightly decline with non-significant increases in HRV. During normoxia, SNS activity was heightened, albeit non-significantly, in epoch 1, with higher HR (68.5 bpm vs. 73.0 bpm, p = 0.06), lower HRV (rMSSD: 45.1 ms vs. 38.7 ms, p = 0.09 and SDNN: 52.5 ms vs. 45.1 ms, p = 0.08), and lower SpO2 (-0.7% p = 0.05). In epochs 2-4, HR, HRV, and SpO2 trended towards baseline values. Significant between-group differences in HR, HRV, and O2 saturation were observed. Hypoxia elicited significantly greater HRs (+5.0, p = 0.03), lower rMSSD (-7.1, p = 0.03), lower SDNN (-8.2, p = 0.03), and lower SpO2 (-1.4%, p = 0.002) compared to normoxia. Hyperoxia appeared to augment the parasympathetic reactivation reflected by significantly lower HR, in addition to higher HRV and O2 relative to normoxia. Hypoxia induced a greater ANS response in military personnel during the simultaneous exposure to high cognitive load. The significant and differential ANS responses to varying O2 levels and high cognitive load observed highlight the importance of continuously monitoring multiple physiological parameters during flight operations.

COVID-19-Induced Changes in Photoplethysmography.

INTRODUCTION: Photoplethysmography (PPG) is the science behind many commonly used medical devices such as the pulse oximeter. PPG changes, herein as "PPG dropouts," have been described in existing in vitro studies following artificially induced clot activation. Because COVID-19 causes increased arterial, venous, and microvascular clot formation, our hypothesis is that PPG dropouts identified in vitro can also be found in vivo in patients with COVID-19. The aim of this study is to evaluate PPG recordings and D-dimer levels for patients hospitalized with COVID-19 and compare them with the PPG tracings from non-COVID controls. MATERIALS AND METHODS: PPG recordings were obtained for 197 ICU patients with COVID-19 and 300 non-COVID controls. PPG tracings were obtained using a TigerTech CovidPlus monitor, which received U.S. FDA emergency use authorization in March 2020 for monitoring the biometrics of patients with COVID-19 and featured unfiltered red and infrared spectrum PPG monitoring. D-dimer lab results were also recorded whenever available. RESULTS: The results demonstrated significant differences in the prevalence rate of PPG dropout among patients with COVID-19 vs. non-COVID controls. The median PPG dropout rate was 0.58 for COVID-19 patients (median 0.58, IQR 0.42-0.72, P < .05) as opposed to a median 0.0 for non-COVID patients (median 0.0, IQR 0.0-0.0, P < .05). Furthermore, at least one incidence of PPG dropout was detected in 100% of COVID-19 patients, as opposed to 2.3% of non-COVID controls (P < .05). PPG dropout also correlated closely with the normalized serum D-dimer levels taken on the same day. The change in the normalized D-dimer levels was plotted against the change in PPG dropout, and a line of best fit was created. Linear regression resulted in R2 = 0.743 (P < .05), indicating that changes in the PPG dropout rate correlate with hemorheological changes in COVID-19 patients. CONCLUSIONS: PPG dropout, like D-dimer, may not be specific for COVID-19. However, the inflammatory nature of the disease and the prevalence of prolonged ICU created a large sample size and allowed the authors to observe PPG changes in vivo in a statistically meaningful way. Further confirmatory studies are needed to confirm the potential application of PPG dropout as a measure of inflammation in other disease processes.

Exposures to Elevated Core Temperatures during Football Training: The Impact on Autonomic Nervous System Recovery and Function.

Exercising with elevated core temperatures may negatively affect autonomic nervous system (ANS) function. Additionally, longer training duration under higher core temperatures may augment these negative effects. This study evaluated the relationship between exercise training duration and 24 h ANS recovery and function at ≥37 °C, ≥38 °C and ≥39 °C core temperature thresholds in a sample of male Division I (D1) collegiate American football athletes. Fifty athletes were followed over their 25-week season. Using armband monitors (Warfighter MonitorTM, Tiger Tech Solutions, Inc., Miami, FL, USA), core temperature (°C) and 24 h post-exercise baseline heart rate (HR), HR recovery and heart rate variability (HRV) were measured. For HRV, two time-domain indices were measured: the root mean square of the standard deviation of the NN interval (rMSSD) and the standard deviation of the NN interval (SDNN). Linear regression models were performed to evaluate the associations between exercise training duration and ANS recovery (baseline HR and HRV) and function (HR recovery) at ≥37 °C, ≥38 °C and ≥39 °C core temperature thresholds. On average, the athletes were 21.3 (± 1.4) years old, weighed 103.0 (±20.2) kg and had a body fat percentage of 15.4% (±7.8%, 3.0% to 36.0%). The duration of training sessions was, on average, 161.1 (±40.6) min and they ranged from 90.1 to 339.6 min. Statistically significant associations between training duration and 24 h ANS recovery and function were observed at both the ≥38.0 °C (baseline HR: ß = 0.10 ± 0.02, R2 = 0.26, p < 0.0000; HR recovery: ß = -0.06 ± 0.02, R2 = 0.21, p = 0.0002; rMSSD: ß = -0.11 ± 0.02, R2 = 0.24, p < 0.0000; and SDNN: ß = -0.16 ± 0.04, R2 = 0.22, p < 0.0000) and ≥39.0 °C thresholds (ß = 0.39 ± 0.05, R2 = 0.62, p < 0.0000; HR recovery: ß = -0.26 ± 0.04, R2 = 0.52, p < 0.0000; rMSSD: ß = -0.37 ± 0.05, R2 = 0.58, p < 0.0000; and SDNN: ß = -0.67 ± 0.09, R2 = 0.59, p < 0.0000). With increasing core temperatures, increases in slope steepness and strengths of the associations were observed, indicating accelerated ANS deterioration. These findings demonstrate that exercise training under elevated core temperatures (≥38 °C) may negatively influence ANS recovery and function 24 h post exercise and progressively worsen.

Single limb electrocardiogram using vector mapping: Evaluation and validation of a novel medical device.

INTRODUCTION: The Tiger Tech Warfighter Monitor (WFM) is a novel single-limb device for ECG acquisition. The WFM provides true (not derived) single limb Electrocardiogram monitoring (ECG) to provide heart rate and R-R interval monitoring between QRS complexes. Herein, we evaluate the diagnostic accuracy of the WFM heart rate, R-R interval monitoring, and heart rate variability monitoring in comparison to a 2­lead chest ECG. METHODS: Data was collected under Institutional Review Board (IRB) approval. Patients available within our institution's pre-operative holding unit were randomly selected to undergo simultaneous chest and WFM ECG monitoring. 3-5-min measurements were taken depending on the patient's availability. Data was saved to two separate mobile phones and time-stamped for synchronization. A proprietary Tiger Tech extraction algorithm was used to tag proper features on both the WFM 1-Limb ECG and Chest ECG data files. A separate algorithm was then used to compare the beat-to-beat variations between the ECGs. RESULTS: Data was extracted and analyzed on 26 subjects. Linear regression of heart rate analysis revealed excellent correlations with an R2 of 0.99 (p < 0.05). Similar linear regression evaluation of R-R interval correlation demonstrated a mean R2 value of 0.95 (p < 0.05). Statistically significant correlation was achieved in all 26 included study participants. Heart rate variability also achieved excellent correlation (SDNN R2 = 0.997, RMSSD R2 = 0.995, LnRMSSD R2 = 0.992, p << 0.05). CONCLUSION: Results demonstrate that the WFM achieves excellent correlation with chest ECG for heart rate, R-R internals, and heart rate variability.

Nanometric sensing with laser feedback interferometry.

We demonstrate a nanometric sensor based on feedback interferometry in a distributed feedback (DFB) laser by using a measurement of either the optical frequency or laser voltage. We find that in an optimal range of optical feedback, the sensor operates reliably down to an extrapolated 12 nm; for the sensor demonstrated here at ∼1550 nm, this provides a minimum detectible displacement of λ/130.

Multistate intermittency on the route to chaos of a semiconductor laser subjected to optical feedback from a long external cavity.

We observe experimentally two regimes of intermittency on the route to chaos of a semiconductor laser subjected to optical feedback from a long external cavity as the feedback level is increased. The first regime encountered corresponds to multistate intermittency involving two or three states composed of several combinations of periodic, quasiperiodic, and subharmonic dynamics. The second regime is observed for larger feedback levels and involves intermittency between period-doubled and chaotic regimes. This latter type of intermittency displays statistical properties similar to those of on-off intermittency.

Low-frequency fluctuations in an external-cavity laser leading to extreme events.

We experimentally investigate the dynamical regimes of a laser diode subject to external optical feedback in light of extreme-event (EE) analysis. We observe EEs in the low-frequency fluctuations (LFFs) regime. This number decreases to negligible values when the laser transitions towards fully developed coherence collapse as the injection current is increased. Moreover, we show that EEs observed in the LFF regime are linked to high-frequency pulsing events observed after a power dropout. Finally, we prove experimentally that the observation of EEs in the LFF regimes is robust to changes in operational parameters.