Association between Cold Face Test‐induced vagal inhibition and cortisol response to acute stress

Low vagal function is related to several disorders. One possible underlying mechanism linking the vagus nerve and disorders is the HPA axis. Thirty‐three healthy male subjects participated in a stress task, while heart rate (HR), respiratory sinus arrhythmia (RSA), salivary cortisol, and mood were assessed. Vagal function was determined using baseline, stress‐induced inhibition, and Cold Face Test (CFT)‐induced stimulation. The stress task induced a significant increase in cortisol and HR, a decrease in RSA, and a worsening of mood. A linear regression model with the time from CFT onset until maximum bradycardia as the independent variable explained 17.9% of the total variance in cortisol in response to the stressor (mood: 36.5%). The results indicate that a faster CFT response is associated with reduced cortisol increase and enhanced mood after acute stress. Our data support an inverse relationship between vagal function and the HPA axis.     

Differential Association of Adiposity Measures with Heart Rate Variability Measures in Koreans

Purpose

Although obesity has been associated with imbalances in cardiac autonomic nervous system, it is unclear whether there are differential relationships between adiposity measures and heart rate variability (HRV) measures. We aimed to examine differences in the relationship between adiposity measures and HRV indices in a healthy Korean population.

Materials and Methods

In all, 1409 non-smokers (811 males, 598 females) without known histories of cardiovascular (CV), endocrine, or neurological diseases underwent adiposity measurements [(body mass index (BMI), percentage of body fat mass (PBF), and waist-to-hip ratio (WHR)], the HRV assessment (SDNN, RMSSD, LF, HF, LF/HF, and pNN50), and examination for CV risk factors (fasting glucose, LDL-cholesterol, HDL-cholesterol, triglycerides, hs-CRP, and blood pressure).

Results

Compared with BMI and PBF, WHR was more strongly correlated with each HRV index and more likely to predict decreased HRV (<15 percentile vs. ≥15 percentile of each HRV index) in ROC curves analysis. In linear regression analysis, all adiposity measures were inversely associated with each HRV measure before adjusting for age, gender, and CV risk factors (p<0.05). After adjusting for the covariates, WHR was inversely related to RMSSD, LF, and pNN50; PBF with RMSSD, HF, and pNN50; BMI with RMSSD (p<0.05). The inversed association between HRV indices and the gender-specific WHR tertile was significant for subjects with BMI ≥25 kg/m², but not for those with BMI <25 kg/m².

Conclusion

WHR and PBF appear to be better indicators for low HRV than BMI, and the association between abdominal adiposity and HRV may be stronger in overweight subjects.     

Impact of chronic obstructive pulmonary disease on linear and nonlinear dynamics of heart rate variability in patients with heart failure

The objective of this study was to investigate the impact of chronic obstructive pulmonary disease (COPD)-heart failure (HF) coexistence on linear and nonlinear dynamics of heart rate variability (HRV). Forty-one patients (14 with COPD-HF and 27 HF) were enrolled and underwent pulmonary function and echocardiography evaluation to confirm the clinical diagnosis. Heart rate (HR) and R-R intervals (iRR) were collected during active postural maneuver (APM) [supine (10 min) to orthostasis (10 min)], respiratory sinus arrhythmia maneuver (RSA-M) (4 min), and analysis of frequency domain, time domain, and nonlinear HRV. We found expected autonomic response during orthostatic changes with reduction of mean iRR, root mean square of successive differences between heart beats (RMSSD), RR tri index, and high-frequency [HF (nu)] and an increased mean HR, low-frequency [LF (nu)], and LF/HF (nu) compared with supine only in HF patients (P<0.05). Patients with COPD-HF coexistence did not respond to postural change. In addition, in the orthostatic position, higher HF nu and lower LF nu and LF/HF (nu) were observed in COPD-HF compared with HF patients. HF patients showed an opposite response during RSA-M, with increased sympathetic modulation (LF nu) and reduced parasympathetic modulation (HF nu) (P<0.05) compared with COPD-HF patients. COPD-HF directly influenced cardiac autonomic modulation during active postural change and controlled breathing, demonstrating an autonomic imbalance during sympathetic and parasympathetic maneuvers compared with isolated HF.     

Temperament, health-related behaviors, and autonomic cardiac regulation: the cardiovascular risk in young Finns study

Temperament, as indicated by Cloninger's psychobiological model predicts coronary heart disease risk, but its association with autonomic cardiac regulation, a potential mediating mechanism, is unclear. We examined the associations between temperament traits and autonomic cardiac regulation in a resting situation in 798 women and 580 men derived from a population-based sample. After adjustment for age and sex, harm avoidance was associated with lower level of high-frequency (HF) variation, root mean square successive differences (RMSSDs), the percentage of successive R–R intervals >50 ms (pNN50) and higher heart rate (HR) (all p ≤ 0.005), suggesting that harm avoidance is related to low parasympathetic activity. Additional adjustments for behavioral factors attenuated these associations more than the adjustment for biological risk factors. Novelty seeking was associated with higher RMSSD (p = 0.007) and pNN50 (p = 0.012) and lower heart rate (p < 0.001). With adjustment for behavioral risk factors, the associations with RMSSD (p = 0.136) and pNN50 (p = 0.236) attenuated to the null, but adjustment for biological risk factors had little effect. Reward dependence and persistence were unrelated to indices of cardiac regulation.     

Consistency of heart rate and sympathovagal reactivity across different autonomic contexts

Theories that psychophysiological reactivity constitutes a risk factor for coronary artery disease assume that reactivity is a consistent individual characteristic. We tested this assumption by measuring reactivity to three psychologically challenging tasks performed by 22 healthy subjects across different autonomic contexts produced by positional change. Dependent variables included heart rate (HR), low-frequency (LF; 0.04–0.15 Hz) and high-frequency (HF; 0.15–0.50Hz) heart period variability, and the LF/HF ratio. HR (r= .44, p < .05) and LF/HF ratio (r= .48, p= .03) reactivity were modestly correlated across the different autonomic contexts, but HF and LF power reactivity were not. These findings suggest that HR reactivity to psychological challenge is a modestly consistent characteristic of individuals, despite differences in autonomic context. Although the same is true of cardiac sympathovagal balance, reactivity of HF and LF power were less consistent.     

急性饮酒对青年心率变异的影响及性别差异研究

目的通过检测轻度急性饮酒前后心率变异性指标,评价酒精对于自主神经活动影响的性别差异。方法20例健康志愿者．其中男性10例．年龄（20．1±0．6）岁：女性10例．年龄（19．6±0．8）岁。根据受试者体质量计算乙醇溶液饮用量（0．27g／kg）。分别在饮酒前和饮酒后15min、45min共3次记录心电信号．提取RR间期时间序列,计算心率变异性（HRV）评价指标,包括时域参数RR间期的平均值（RRI）、全部RR间期标准差（SDRR）、全程相邻RR间期之差的均方根（RMSSD）、相邻RR间期之差大于50ms的心搏动数占总心搏动数的比例（pNN50）,频域参数低频频段（LF）、高频频段（HF）和LF／HF,Poincar6图参数宽度（短轴,SDl）、长度（长轴,SD2）和SDl／SD2。结果时域参数、频域参数和P0inca诺参数存急性饮酒后降低。对于RMSSD和SDRR参数．仅在女性受试者中观测到饮酒45min后降低．差异均具有统计学意义（P〈0．05）。对于LF和SDl参数在饮酒后15min即观测到在男性受试者中降低,差异均具有统计学意义（P〈0．05）。结论少量的急性饮酒降低了心率变异程度．酒精抑制了副交感神经的活动度．女性比男性对酒精的敏感程度更高．男性自主种经活动对酒精影响的响应时间更短。     

Genetic influences on heart rate variability at rest and during stress

We tested whether the heritability of heart rate variability (HRV) under stress is different from rest and its dependency on ethnicity or gender. HRV indexed by root mean square of successive differences (RMSSD) and high-frequency (HF) power was measured at rest and during 3 stressors in 427 European and 308 African American twins. No ethnic or gender differences were found for any measures. There was a nonsignificant increase in heritability of RMSSD (from 0.48 to 0.58) and HF (from 0.50 to 0.58) under stress. Up to 81% and 60% of the heritabilities of RMSSD and HF under stress could be attributed to genes influencing rest levels. The heritabilities due to genes expressed under stress were 0.11 for RMSSD and 0.23 for HF. The findings suggest that, independent of ethnicity and gender, HRV regulation at rest and under stress is largely influenced by the same genes with a small but significant contribution of stress-specific genetic effects.     

Relation between emotional distress and heart rate variability in patients with an implantable cardioverter‐defibrillator

We investigated the relationship between Type D personality, depression, and anxiety, and heart rate variability (HRV) in 64 patients with an implantable cardioverter‐defibrillator (ICD). HRV was obtained via 24‐h Holter monitoring, and 24‐h, 30‐min daytime rest and 30‐min nighttime sleep HRV were analyzed. In adjusted analyses, significant associations (standard deviation of normal‐to‐normal [NN] intervals [SDNN]: p = .043; standard deviation of NN intervals over 5‐min periods [SDANN]: p = .010) and a trend (HRV triangular index: p = .09) were found for Type D personality, and trends were found for depression (lower RMSSD: p = .10; lower pNN50: p = .09). During daytime rest, similar results were found for anxiety and depression. During sleep, only noteworthy adjusted associations were found for depression (lower root mean square of successive differences in NN intervals [RMSSD]: p = .06; lower pNN50: p = .043). A Benjamini‐Hochberg correction for multiple testing led to reduction of the number of significant relationships, but there was still support for lower autonomic control patients with Type D personality and depression. Future research with larger sample sizes is warranted.     

Linear and nonlinear analysis of heart rate variability in healthy subjects and after acute myocardial infarction in patients

The objectives of this study were to evaluate and compare the use of linear and nonlinear methods for analysis of heart rate variability (HRV) in healthy subjects and in patients after acute myocardial infarction (AMI). Heart rate (HR) was recorded for 15 min in the supine position in 10 patients with AMI taking β-blockers (aged 57 ± 9 years) and in 11 healthy subjects (aged 53 ± 4 years). HRV was analyzed in the time domain (RMSSD and RMSM), the frequency domain using low- and high-frequency bands in normalized units (nu; LFnu and HFnu) and the LF/HF ratio and approximate entropy (ApEn) were determined. There was a correlation (P < 0.05) of RMSSD, RMSM, LFnu, HFnu, and the LF/HF ratio index with the ApEn of the AMI group on the 2nd (r = 0.87, 0.65, 0.72, 0.72, and 0.64) and 7th day (r = 0.88, 0.70, 0.69, 0.69, and 0.87) and of the healthy group (r = 0.63, 0.71, 0.63, 0.63, and 0.74), respectively. The median HRV indexes of the AMI group on the 2nd and 7th day differed from the healthy group (P < 0.05): RMSSD = 10.37, 19.95, 24.81; RMSM = 23.47, 31.96, 43.79; LFnu = 0.79, 0.79, 0.62; HFnu = 0.20, 0.20, 0.37; LF/HF ratio = 3.87, 3.94, 1.65; ApEn = 1.01, 1.24, 1.31, respectively. There was agreement between the methods, suggesting that these have the same power to evaluate autonomic modulation of HR in both AMI patients and healthy subjects. AMI contributed to a reduction in cardiac signal irregularity, higher sympathetic modulation and lower vagal modulation.     

电刺激大脑岛叶对大鼠心率变异的影响

目的 研究电刺激大脑岛叶对大鼠心率变异(Heart Rate Variability,HRV)的影响.方法 检测麻醉状态下大鼠的心电图(ECG),将动物分成未处理组、假手术组、500以及1000μA点刺激组,对各组大鼠进行ECG数据的采集,对各组大鼠心率变异的频域指标进行分析.结果 HRV各参数值发生变化,表现为心动周...     

Analysis of cardiac autonomic modulation in obese and eutrophic children

INTRODUCTION:

Obesity causes alterations in cardiac autonomic function. However, there are scarce and conflicting data on this function with regard to heart rate variability in obese children.

OBJECTIVE:

To compare the autonomic function of obese and eutrophic children by analyzing heart rate variability.

METHODS:

One hundred twenty-one children (57 male and 64 female) aged 8 to 12 years were distributed into two groups based on nutritional status [obese (n  =  56) and eutrophic (ideal weight range; n  =  65) according to the body mass index reference for gender and age]. For the analysis of heart rate variability, heart rates were recorded beat by beat as the children rested in the dorsal (prone) position for 20 minutes. Heart rate variability analysis was carried out using linear approaches in the domains of frequency and time. Either Student''s t-test or the Mann-Whitney U-test was applied to compare variables between groups. Statistical significance was set at 5%.

RESULTS:

The SDNN, RMSSD, pNN50, SD1, SD2, LF and HF indices in milliseconds squared were lower among the obese children when compared to the eutrophic group. There were no alterations in the SD1/SD2 ratio, LF/HF ratio, LF index or HF index in normalized units. There was a significant difference between groups in the RR interval (R-to-R EKG interval).

CONCLUSION:

The obese children exhibited modifications in heart rate variability, characterized by a reduction in both sympathetic and parasympathetic activity. These findings stress the need for the early holistic care of obese children to avoid future complications.     

Responses of calves to acute stress: individual consistency and relations between behavioral and physiological measures

The present study examined the consistency over time of individual differences in behavioral and physiological responsiveness of calves to intuitively alarming test situations as well as the relationships between behavioral and physiological measures. Twenty Holstein Friesian heifer calves were individually subjected to the same series of two behavioral and two hypothalamo-pituitary-adrenocortical (HPA) axis reactivity tests at 3, 13 and 26 weeks of age. Novel environment (open field, OF) and novel object (NO) tests involved measurement of behavioral, plasma cortisol and heart rate responses. Plasma ACTH and/or cortisol response profiles were determined after administration of exogenous CRH and ACTH, respectively, in the HPA axis reactivity tests. Principal component analysis (PCA) was used to condense correlated measures within ages into principal components reflecting independent dimensions underlying the calves' reactivity. Cortisol responses to the OF and NO tests were positively associated with the latency to contact and negatively related to the time spent in contact with the NO. Individual differences in scores of a principal component summarizing this pattern of inter-correlations, as well as differences in separate measures of adrenocortical and behavioral reactivity in the OF and NO tests proved highly consistent over time. The cardiac response to confinement in a start box prior to the OF test was positively associated with the cortisol responses to the OF and NO tests at 26 weeks of age. HPA axis reactivity to ACTH or CRH was unrelated to adrenocortical and behavioral responses to novelty. These findings strongly suggest that the responsiveness of calves was mediated by stable individual characteristics. Correlated adrenocortical and behavioral responses to novelty may reflect underlying fearfulness, defining the individual's susceptibility to the elicitation of fear. Other independent characteristics mediating reactivity may include activity or coping style (related to locomotion) and underlying sociality (associated with vocalization).     

Equine behaviour and heart rate in temperament tests with or without rider or handler

The aim of the present study was to compare horses' heart rate (HR), heart rate variability (RMSSD, pNN50) and behaviour in the same temperament test when being ridden, led, and released free. Behavioural measurements included scores and linear measurements for reactivity (R), activity (A), time to calm down (T) and emotionality (E), recorded during the approach (1) and/or during confrontation with the stimulus (2). Sixty-five horses were each confronted 3 times (1 ridden, 1 led, 1 free running in balanced order) with 3 novel and/or sudden stimuli. Mixed model analysis indicated that leading resulted in the lowest (P < 0.05 throughout) reactions as measured by A1, A2, E1, E2, R2, and pNN50 while riding produced the strongest (A1, T2, HR, RMSSD, pNN50) or medium (E1, E2, R2) reactions. Free running resulted either in the strongest (A2, E1, E2, R2) or in the lowest (A1, T2, HR, RMSSD, pNN50) reactions. The repeatability across tests for HR (0.57), but not for RMSSD (0.23) or pNN50 (0.25) was higher than for any behavioural measurement: the latter ranged from values below 0.10 (A1, A2, T2) to values between 0.30 and 0.45 (E1, E2, R2). Overall, the results show that a rider or handler influences, but not completely masks, the horses' intrinsic behaviour in a temperament test, and this influence appeared to be stronger on behavioural variables and heart rate variability than on the horses' heart rates. Taking both practical considerations and repeatabilities into account, reactivity appears to be the most valuable parameter. Emotionality and heart rate can also yield valid results reflecting additional dimensions of temperament although their practical relevance may be less obvious. If a combination of observed variables is chosen with care, a valid assessment of a horse's temperament may be possible in all types of tests. However, in practice, tests that resemble the practical circumstances most closely, i.e. testing riding horses under a rider, should be chosen.     

Inclusion of a rest period in diaphragmatic breathing increases high frequency heart rate variability: Implications for behavioral therapy

Heart rate variability (HRV) is associated with positive physiological and psychological effects. HRV is affected by breathing parameters, yet debate remains regarding the best breathing interventions for strengthening HRV. The objective of the current study was to test whether the inclusion of a postexhalation rest period was effective at increasing HRV, while controlling for breathing rate. A within‐subject crossover design was used with 40 participants who were assigned randomly to a breathing pattern including a postexhalation rest period or a breathing pattern that omitted the postexhalation rest period. Participants completed training on each breathing pattern, practiced for 6 min, and sat quietly during a 5‐min washout period between practices. Participants were given instructions for diaphragmatic breathing at a pace of six breaths/minute with or without a postexhalation rest period. Recordings of heart rate, breathing rate, HF‐HRV, RMSSD, LF‐HRV, and SDNN were collected before and during each of the breathing trials. HRV indices were derived from Lead 1 ECG recordings. Pairwise contrasts showed that inclusion of a postexhalation rest period significantly decreased heart rate (p < .001) and increased HF‐HRV (p < .05). No differences were found for breathing rates (p > .05), RMSSD (p > .05), and SDNN (p > .05). Results indicated that omission of the postexhalation rest period resulted in higher LF‐HRV (p < .05). A postexhalation rest period improves HF‐HRV, commonly associated with self‐regulatory control, yet the importance of a postexhalation rest period requires further exploration.     

The effect of biting tails and having tails bitten in pigs

Tail-biting is a behavioral abnormality which compromises the welfare of pigs. The goal of this study was to characterize the tail-biting phenotype using behavior and measures of heart-rate (HR) and its variability (HRV) in pigs. Thirty pigs were categorized as tail-biters (n=10), tail-bite victims (n=10), and control pigs (n=10) based on the frequency of tail-biting behavior that they performed or received at the farm. The animals' behavioral responses were registered at the experimental facilities for 10 min during test sessions whereas physiological responses were registered for 10 min prior to (basal) and during sessions when subjected to a novel object test (NOT) and to a novel arena test (NAT). Phenotypes differed in most behaviors during the two tests and in the NOT their physiological responses suggested different regulation of vagal tone. Biters had a reduction from baseline values to values during testing for the root mean square of successive R-R intervals (RMSSD) and the high-frequency band (HF) compared to victims, whose RMSSD and HF increased from baseline to test values. In the low-frequency band (LF), an increase was shown in biters and controls while a decrease in victims. LF was found to be strongly positively correlated with HF and RMSSD in biters. During baseline, victims tended to have lower HF and significantly higher power of the low-frequency component divided by power of the high-frequency band (LF:HF ratio) compared to biters and controls. The activity of the autonomic nervous system, especially the suppression of parasympathetic tone, indicated that both victims and biters may have a dysfunctional autonomic regulation which may indicate psychological disturbance. We provide the first documentation of phenotypic differences between pigs that have performed tail-biting, have been victimized, or have not been involved in tail biting using HRV data.     

Comparison of time and frequency domain measures of RSA in ambulatory recordings

The extent to which various measures of ambulatory respiratory sinus arrhythmia (RSA) capture the same information across conditions in different subjects remains unclear. In this study the root mean square of successive differences (RMSSD), peak valley RSA (pvRSA), and high frequency power (HF power) were assessed during ambulatory recording in 84 subjects, of which 64 were retested after about 3 years. We used covariance structure modeling to test the equality of the correlations among three RSA measures over two test days and three conditions (daytime sitting or walking and nighttime sleep) and in groups with low, medium, and high mean heart rate (HR), or low, medium, and high mean respiration rate (RR). Results showed that ambulatory RMSSD, pvRSA, and HF power are highly correlated and that their correlation is stable across time, ambulatory conditions, and a wide range of resting HR and RR values. RMSSD appears to be the most cost-efficient measure of RSA.     

Familiality of heart rate and cardiac-related autonomic activity in five-month-old twins: the Québec newborn twins study

The familiality of infant sleeping heart rate (HR) and cardiac-related autonomic activity, as indexed by spectral analysis of HR variability (HRV) and response to postural change, was investigated in 322 5-month-old twin pairs. The postural change elicited only minor changes in cardiovascular activity. As a result, analyses focused on supine cardiovascular activity. Multivariate genetic modeling indicated that individual differences in sleeping HR and high frequency (HF) HRV were determined by unique environmental and distinct additive genetic factors. These variables, along with low frequency (LF) HRV, were also affected by overlapping familial environmental influences. Familial influences in HF HRV were more pronounced for baby girls than boys. Estimates of relative LF and HF HRV were determined by common (familial) and unique environmental factors; familial influences on these estimates did not overlap with familial influences on sleeping HR.     

自主呼吸状态下光电容积脉搏波法对 健康人群心率变异性的分析

目的 分析自主呼吸状态下光电容积脉搏波法对健康人群心率变异性。方法 选取2016年10月~2018年9月我校本科生健康志愿者64名,所有志愿者均行心电图（ECG）及光电容积脉搏波法（PPG）检测,比较3 min的ECG与PPG检测HRV时的RR间期标准差（SDNN）、RR间期差值均方根值（RMSSD）、低频功率与标准化高频功率的比值（LF/HF）,并分析ECG与PPG检测的心率变异性参数的相关性与一致性。结果 64例志愿者3 min的ECG与PPG心率变异性参数比较,差异无统计学意义（P＞0.05）。经Pearson相关性分析发现,PPG检测SDNN、RMSSD、LF/HF与ECG各指标呈正相关（SDNN:r=0.999,P＜0.001;RMSSD:r=0.998,P＜0.001;LF/HF:r=0.998,P＜0.001）;经可靠性分析发现,PPG用于检测健康人群心率变异性与ECG一致性较高（ICC=0.915,95% CI:0.878~0.943,P＜0.001）。结论 健康人群自主呼吸状态下光电容积脉搏波法可用于心率变异性的检测,临床上有较好的应用价值。     

Temporal stability of heart period variability during a resting baseline and in response to psychological challenge

Central to the psychophysiologic reactivity hypothesis of the etiology of coronary artery disease is the assumption that reactivity is an individual characteristic that is stable over time. Although heart rate (HR) and blood pressure reactivity appear to meet this criterion, temporal stability of cardiac autonomic control as measured by analysis of heart period variability (HPV) has not been assessed. In this study, we tested the stability of HPV, measured in both the time and frequency domain, during a quiet, resting baseline and in response to 5-min mental arithmetic and reaction time tasks, in 20 normal subjects measured in three testing sessions during a 9-month period. Stability, assessed by the intraclass correlation coefficient (ICC), was excellent for resting baseline measures of HR and HPV, with ICCs of 0.68–0.86. However, HR and HPV reactivity to either arithmetic or reaction time tasks generally was less stable, with ICCs of 0.17–0.73, in contrast to results of previous studies demonstrating long-term Stability of HR responses to psychological challenge. Stability of aggregated reactivity scores was only slightly improved. Whether for individual tasks or aggregated measures, reactivity of total and low-frequency measures of HPV was moderately stable but stability of high-frequency HPV reactivity was poor.     

Cardiac measures of nuclear power plant operator stress during simulated incident and accident scenarios

Maintaining optimal performance in demanding situations is challenged by stress‐induced alterations in performance. Here, we quantified the stress of nuclear power plant (NPP) operators (N = 20) during a full‐scale simulator training for incident and accident scenarios. We compared the ambulatory electrocardiography measurements of heart rate (HR) and heart rate variability (HRV), and self‐reported stress during baselines and simulated scenarios. Perceived (scale 0–10) and physiologically measured stress were low during baseline after the scenarios and normal NPP operation (means 1.8–2.2, mean HR 75–80 bpm). During a cognitively challenging scenario simulating a sensor malfunction, the operators' stress was mild to moderate (mean 3.4; HR + 12% from baseline). During simulations of severe accidents of fire and radioactive steam leakage, the experienced stress and cardiac activity were on a moderate to high level (means 4.2 and 4.6; HR + 23% and + 14% from baseline, respectively). Cardiac activity paralleled the self‐reported stress: correlation of self‐reported stress to HR was 0.61 (p < .001) and to HRV features RMSSD, HF, LF/HF, SD1, and SD1/SD2 were ?0.26, ?0.28, 0.35, ?0.40, and ?0.39 (p < .01), respectively. The low shared variance (22%) between HR and physical activity further support the interpretation that the cardiac activity was strongly linked to the experience of stress and not accountable by operators' movement within the simulator. Cardiac measurements in naturalistic settings can thus reveal relevant information on acute stress with the benefit of not interrupting the primary task.