Wearable‐Based Monitoring of Autonomic and Gastrointestinal Function in Disorders of Gut‐Brain Interaction: A Systematic Review and Meta‐Analyses

Disorders of gut-brain interaction (DGBI) are common in pediatrics. Though the name clearly implies a neural contribution, the role of the autonomic nervous system remains unclear. Heart rate in healthy subjects (HC) follows a circadian pattern with dipping during the night with increased high frequency (hf) heart rate variability (HRV) and root mean square of successive differences (RMSSD). Our hypothesis was that reduced vagal modulation in adolescents with DGBI is associated with blunted rise in nocturnal vagal modulation. An institutional review board (IRB) approved this study, which included children aged 12-18years with a DGBI and carefully screened HC. All subjects underwent 24h HRV recording. The following questionnaires/tools were included: Pain Catastrophizing Scale for Children (PCS-C), Pain Catastrophizing Scale for Parents (PCS-P), Revised Child Anxiety and Depression Scale (RCADS), Trauma Symptom Checklist for Children (TSCC), and Functional Disability Inventory (FDI). In total, 12 HC and 15 participants with DGBI participated (female individuals with DGBI versus HC: 93% versus 58%, p = 0.08). There was no age difference (median [range] HC 16.6 years [13.4, 18.2]; DGBI 16.8 years [13.8, 18.7], p = 0.92). The 24h, daytime and nighttime HRV demonstrated that RMSSD, low-frequency (lf) HRV, and high-frequency (hf) HRV were lower in the DGBI group (p < 0.001). A nocturnal rise in RMSSD was present in the DGBI group, but less so in the HC group (p = 0.021). Higher catastrophizing correlated with lower nocturnal RMSSD (PCS-C correlation coefficient [CC]: -0.46), depression (RCADS depression CC: -0.51), and post-traumatic stress disorder (PTSD; TSCC PTSD CC: -0.58). HRV is reduced in patients with DGBI with a blunted rise in nocturnal RMSSD at night. As expected, vagal modulation is inversely correlated with catastrophizing, depression, PTDS, and FDI, with nocturnal HRV showing generally better correlations than daytime HRV.

Multiple sclerosis (MS) is a neurodegenerative disease with diverse symptoms, including autonomic nervous system impairment. While autonomic dysfunction is well-documented in MS, debate continues over its extent and whether it primarily affects the sympathetic or parasympathetic divisions. This review aimed to investigate heart rate variability (HRV) as an indicator of cardiac autonomic function in people with MS (PwMS)." PubMed/MEDLINE, Web of Science, Embase, and Scopus were systematically searched up to August 2024 to identify observational studies that assessed HRV in PwMS and healthy controls (HCs). The standard mean difference (SMD) of HRV parameters, along with its 95% confidence interval (CI), between PwMS and HCs was estimated utilizing R version 4.4.0 within the random-effects model. This study combined data from 43 studies on 1518 PwMS and 1062 HCs. The meta-analysis revealed significant autonomic dysfunction in PwMS compared to HCs. Specifically, the time-domain measures such as root mean square of successive differences (RMSSD) (SMD = -0.81, p < 0.01) and pNN50 (SMD = -0.63, p < 0.001) showed significant reductions in PwMS, compared to HCs. Furthermore, among frequency domains, high frequency (HF) power (SMD = -0.51, p < 0.01) and low frequency (LF) power were significantly lower in PwMS (SMD = -0.44, p < 0.05) than HCs. This review revealed that PwMS exhibited different HRV than healthy individuals. Additionally, HRV findings demonstrated greater sympathetic dominance in PwMS compared to healthy individuals. Future studies are needed to explore HRV in PwMS across varying severity levels, subtypes, and treatments to understand autonomic dysfunction in this population better.

ABSTRACTBackground:Heart rate variability (HRV) is a crucial indicator of autonomic nervous system function and cardiovascular health. This study aimed to establish normative HRV data for healthy populations residing at high altitudes (approximately 3500 m) in the Keylong district of Himachal Pradesh, India, and to investigate the influence of age and gender on HRV parameters.Methodology:A total of 145 healthy subjects (101 females and 44 males) without chronic diseases were recruited for HRV measurement using an Equivital wireless device and LabChart software. Time domain, frequency domain, and non-linear HRV parameters were analyzed. Shapiro–Wilk test, multivariate analysis of variance (MANOVA), aligned rank test, and post-hoc tests were applied for statistical analysis.Result:Among time domain parameters, females exhibited significantly higher values of average Mean time duration between two successive R waves in ECG during the period of measurement (RR), root mean square of successive differences (RMSSD), and percentage of successive RR intervals that differ by more than 50 milliseconds (pRR50) compared to males. Age significantly influenced pRR50, with the youngest group (18–30 years) showing higher values than the oldest group (51–70 years). The frequency domain and non-linear parameters showed no significant influence of age or gender. Regression analysis revealed that RMSSD, pRR50, and LF/HF were significantly related to age in females, suggesting a decrease in parasympathetic tone and an increase in sympathetic tone with age. The normative HRV values for high-altitude populations differed from those at sea level.Conclusion:These findings provide valuable reference data for assessing cardiovascular health and monitoring disease prognosis in high-altitude populations.

Redux: Do Little Bellyachers Grow up to Become Big Bellyachers?

Functional gastrointestinal disorders (FGID) and gastroesophageal reflux (GERD) disease affect a large global population and incur substantial health care costs. Impairment in gut-brain communication is one of the main causes of these disorders. The central nervous system (CNS) provides its inputs to the enteric nervous system (ENS) by modulating the autonomic nervous system (ANS) to control the gastrointestinal functions. Therefore, GERD and FGID’s might be associated with autonomic dysfunction, which can be identified via heart rate variability (HRV). FGIDs may be treated by restoring the autonomic dysfunction via neuromodulation. This article reviews the roles of HRV in the assessment of autonomic function and dysfunction in (i) gastroesophageal reflux (GERD), and the following FGIDs: (ii) functional dyspepsia (FD) and gastroparesis, (iii) irritable bowel syndrome (IBS) and (iv) constipation. The roles of HRV in the assessment of autonomic responses to various interventions were also reviewed. We used PUBMED, Web of Science, Elsevier/Science direct and Scopus to search the eligible studies for each disorder, which also included the keyword ‘heart rate variability’. The retrieved studies were screened and filtered to identify the most suitable studies using HRV parameters to associate the autonomic function with any of the above disorders. Studies involving both human and animal models were included. Based on analyses of HRV, GERD as well as the FGIDs were found to be associated with decreased parasympathetic activity and increased sympathetic nervous system activity with the autonomic balance shifted towards the sympathetic nervous system. In addition, the HRV methods were also reported to be able to assess the autonomic responses to various interventions (mostly neuromodulation), typically the enhancement of parasympathetic activity. In summary, GERD and FGIDs are associated with impaired autonomic dysfunction, mainly due to suppressed vagal and overactive sympathetic tone, which can be assessed noninvasively using HRV.

Deep breathing exercises are the second most used complementary health approach in the United States. Two heart rate variability (HRV) parameters, the root mean square of successive differences (RMSSD) and the respiratory sinus arrhythmia (RSA), are used to assess parasympathetic reactivity to deep breathing, but they are often not in agreement. Our purpose was to determine the cause of the disagreement. We investigated HRV parameters in 38 subjects during baseline, deep breathing, and recovery. Here we show that RMSSD as a measure of parasympathetic reactivity is unreliable; it does not reflect the increase in HRV during deep breathing as determined by RSA. We observed a decrease in RMSSD values despite a marked increase in HRV as determined by RSA and the standard deviation of normal heartbeat interval (SDNN) in healthy subjects and patients with functional bowel disorders. We show that RSA captures all aspects of HRV, whereas successive differences in heart rate intervals are only a small part of HRV, with decreasing variability during deep breathing in most subjects. We present a new measure of calculating RSA during deep breathing that may become an essential tool for researchers and clinicians. We also provide a unique visualization of the increased heart rate variability during deep breathing. Hence, RMSSD cannot be used to assess parasympathetic reactivity during deep breathing; using RSA is recommended. The use of RMSSD in previous influential studies may have led to erroneous conclusions about parasympathetic reactivity during deep breathing. Its continued use may undervalue the effects of the autonomic nervous system in slow deep breathing.

Asthma is associated with abnormal autonomic function, and heart rate variability is considered a simple, accurate, and noninvasive tool for monitoring the autonomic system. Thus, the aim of this study was to investigate the impact of asthma on heart rate variability in children and adolescents. This systematic review of observational studies and clinical trials evaluated heart rate variability in children and adolescents with asthma compared to healthy controls. The data were extracted independently by two reviewers. The quality of the selected articles was assessed using Agency for Health Care Research and Quality indicators. Random effects meta-analysis was performed for each outcome, with the effect size reported as standardized mean difference. Fifteen studies were included, of which five were classified as having high methodological quality. In the meta-analysis, long-term recording of the standard deviation of all normal-to-normal sinus R-R intervals differed significantly between groups (standardized mean difference [SMD] = -0.46 [95% confidence interval {CI}: -0.79 to -0.14], p < 0.005, I2 = 0%). Moreover, R-R intervals (long-term) were significantly shorter in asthmatic children than controls (SMD = -0.47 [95% CI: -0.68 to -0.25], p < 0.0001, I2 = 0). There were no significant differences between adjacent normal R-R intervals that exceed 50 ms (long-term) and the root mean square of successive differences between normal sinus R-R intervals (short-term). Regarding frequency-domain variables, long-term low frequency measurements differed significantly between groups (SMD = -0.34 [95% CI: -0.58 to -0.10], p < 0.005, I2 = 15%). There were no differences in high frequency measurements or in the ratio between low/high frequency powers (short- or long-term) between groups. The results confirm the impact of asthma on heart rate variability in children and adolescents, indicating lower heart rate variability and sympathetic modulation.

Heart rate variability (HRV) indicates brain-body interaction and has been associated with a variety of mental and physical health indicators. Transient reductions in HRV, independent of bodily movement (so-called additional HRV reduction [AddHRVr]), may reflect moments of psychophysiological vulnerability. This metric is quantified by regressing bodily movement on the root mean square of successive differences and identifying reductions <0.5 SD of the predicted value in real time in everyday life. We aimed to apply this measure using wearables in everyday life to trigger low-threshold, 1-minute just-in-time adaptive interventions (JITAIs) to stabilize autonomic function and relieve perceived stress and ruminative thoughts. Thus, we compared moments of AddHRVr with random points in time with respect to the effects of brief JITAIs. In 2 preregistered microrandomized trials, participants underwent a 1-day calibration period to derive individualized trigger settings and then received AddHRVr-triggered and random prompts throughout the following 3 days, asking for perceived stress and rumination. In study 1, participants (N=60) underwent a slow breathing intervention (0.1 Hz slow-paced breathing) following each prompt, and in study 2, participants (N=50) were microrandomized to both an external attention (ie, focusing attention on a nonliving object) and a mindful breathing intervention. HRV was assessed before, during, and following each intervention by means of the root mean squares of successive differences, SD of normal-to-normal beats, and high- and low-frequency HRV. Participants also reported on perceived stress and ruminative thoughts before and after the interventions. Following interventions in both studies, perceived stress and ruminative thoughts significantly declined irrespective of the kind of prompt and intervention (study 1-perceived stress: b=-0.12; P<.001 and ruminative thoughts: b=-0.11; P<.001 and study 2-perceived stress: b=-0.07; P=.01 and ruminative thoughts: b=-0.10; P=.002). AddHRVr-triggered prompts resulted in a stronger increase in HRV during the slow-paced breathing (b=0.08; P=.02) and mindful breathing interventions (b=0.10; P=.03), and elevated HRV metrics in a time frame of 10 minutes following the interventions in contrast to random prompts (study 1: b=0.12; P<.001 and study 2: b=0.10; P=.03). Both studies show, for the first time, that transient, nonmetabolic reductions in HRV (AddHRVr) can be used to trigger brief JITAIs in real time by wearables to stabilize autonomic function, thus potentially promoting cardiac health. The findings suggest that although psychological benefits emerged independent of the cardiac autonomic state, slow-paced breathing or directing attention for 1 minute to either the own body or nonliving objects seemed to boost autonomic flexibility when HRV was compromised. German Clinical Trial Register DRKS00035684; https://www.drks.de/search/de/trial/DRKS00035684 and DRKS00035685; https://www.drks.de/search/de/trial/DRKS00035685.

BackgroundPrevious literature has highlighted that excessive alcohol use (EAU) is directly linked with permanent neurological damage. Studies have also highlighted gradual improvements in heart rate variability (HRV) after cessation of alcohol use. Moreover, chronic alcohol consumption has also been correlated with reduced HRV and an increase in skin conductance (SC) among healthy adults, leading to a combined decline in cognitive performance. This study examines the persistence of HRV and SC changes among older adults with dementia and a history of alcohol use disorder (AUD).MethodParticipants were recruited from the Geriatric Transitional Unit at Ontario Shores Mental Health Sciences, Whitby, Ontario. Eligibility included a formal diagnosis of dementia and history of lifelong EAU was recorded based on informants. Behavioral and symptoms of dementia was assessed with the Neuropsychiatric Inventory Clinician rating scale. Empatica E4 wristbands was used to record physiological data. Physiological signals such as electrodermal activity (EDA) and heart rate (HR) were recorded. HRV was analyzed using KUBIOS software. T‐tests were used to determine differences in frequency and severity of BPSD, HRV and SC.Result28 participants were recruited, 17 male, average age: 73.4, 10 had EAU history (average age 67.8, 2 female). EAU group had a significantly lower root mean square of successive differences (RMSSD) (p = 0.002) and parasympathetic nervous system (PNS) activity (p = 0.021), but significantly higher sympathetic nervous system activity (SNS) (p = 0.007) than participants without EAU. Additionally, the standard deviation of normal beats (SDNN) and RMSSD scores of EAU group were below the normal range. Moreover, EDA was higher in participants without EAU (M = [1.54], SD = [3.46]) compared subjects with EAU (M = [0.43], SD = [0.94]), but the difference was not significant (p = 0.36). There was no statistically significant difference between NPI scores of the 2 groups.ConclusionOur findings indicate that alcohol abuse results in long‐term alterations of peripheral nervous system, causing an increase in sympathetic activity and a consequent decrease in parasympathetic activity. We observed that EAU subjects have a diminished ability to vagally mediate heart rate.

BackgroundMetabolic syndrome (MetS), a cluster of interrelated risk factors including central obesity, hyperglycemia, dyslipidemia and hypertension, is increasingly recognized for its impact on cardiovascular health and cognitive function. However, the degree to which MetS associates with autonomic function and modifies risk for Alzheimer's disease (AD) in older adults is unknown. We investigated the association between MetS, cognition, and autonomic function in 440 participants enrolled in the U.S. POINTER trial (mean age: 68.1±5.3 years, 61% female, 26% people of color).MethodMetS was defined according to Adult Treatment Panel III (ATP III) criteria. Continuous blood pressure and ECG recordings were measured in the standing position and used to calculate baroreflex sensitivity (BRS), blood pressure variability (BPV), and heart rate variability (HRV) assessed as root of mean square of successive differences (rMSSD) and standard deviation of beat to beat interval (SDNN). A cognitive battery was administered to assess global and domain‐specific cognitive function. Autonomic and cognitive outcomes were compared by MetS status. Linear regression was used to examine cross‐sectional associations between the outcomes of interest adjusting for age, sex, race, ethnicity, education, and site.ResultParticipants with MetS (n = 281, 63.8%) were younger (p = 0.009), likely to be male (p <0.0001), and had lower global cognition (p = 0.0018), episodic memory (p = 0.011), executive function (p = 0.005), and processing speed (p = 0.033). Autonomic function was reduced in MetS as measured by lower BRS (p = 0.034), lower SDNN (p = 0.01), higher BPV (p = 0.024), and a trend for lower rMSSD (p = 0.067). Similarly, in linear regression analyses, MetS was negatively associated with BRS (p = 0.007), rMSSD (p = 0.044), and SDNN (p = 0.0007) and positively associated with BPV (p = 0.009). Both rMSSD and SDNN were positively associated with global cognition (p = 0.039, p = 0.001) and executive function (p = 0.038, p = 0.002). SDNN was also positively associated with episodic memory (p = 0.004). Associations between autonomic and cognitive function were similar in participants with and without MetS.ConclusionIn this population of older adults at risk for AD, measures of healthy autonomic function are positively associated with cognition and negatively associated with MetS. Further studies are needed to assess if early diagnosis and treatment of MetS in this population could delay the cognitive decline.

Patients with ankylosing spondylitis (AS) have a significantly higher risk of cardiovascular morbidities. The participation of the autonomic nervous system (ANS) in AS is still unknown and inconclusive. Heart rate variability (HRV) is defined as the variability of the time interval between two consecutive heartbeats. This meta-analysis aims to detect the association of HRV and its various parameters with AS patients by comparing them to healthy controls.Research literature was searched in PubMed, Embase, and Cochrane Library databases from inception to April 2022. The Review Manager 5 (RevMan) Version 5.4 software was used to analyze the data. In addition, the protocol of systematic review is registered in the PROSPERO database with ID CRD42022336484.This study includes a total of nine case-control studies with a total of 923 patients; 409 with AS and 514 healthy controls. The root mean square of successive differences between normal heartbeats (RMSSD) [standardized mean difference (SMD); -0.47, 95% CI: -0.69 to -0.25, p < 0.0001], proportion of NN50 (pNN50) (SMD; -0.89, 95% CI: -1.74 to -0.04, p = 0.04) and HRV (SMD; -1.11, 95% CI: -1.53 to 0.69, P < 0.00001) were significantly low in AS cases compared to healthy controls. The HRV value was also significantly low in patients with high Bath ankylosing spondylitis disease activity (BASDAI) index (SMD: -1.45, 95% CI: -2.45 to -0.36, p < 0.009).HRV (parasympathetic activity) was significantly lowered in AS patients compared to healthy controls.

Aims/IntroductionThe principal aim of the present study was to investigate the cardiovascular autonomic system status of diabetes patients using approximate entropy (ApEn) extracted from 24-h heart rate variability (HRV) and its frequency components.Materials and MethodsA total of 29 healthy controls and 63 type 2 diabetes patients were included. Participants’ 24-h HRV signals were recorded, and decomposed and reconstructed into four frequency components: high, low, very low and ultra low. The total 24-h HRV and its four components were divided into 24 1-h segments. ApEn values were extracted and statistically analyzed. Four traditional HRV indices, namely standard deviation of the RR intervals, root mean square of successive differences, coefficient of variance of RR intervals and ratio of low to high power of HRV, were also calculated.ResultsThe low-frequency component contained the most abundant non-linear information, so was potentially most suitable for studying the cardiovascular system status with non-linear methods. ApEn values extracted from low- and high-frequency components of healthy controls were higher than those of diabetes patients. Except for root mean square of successive differences, standard deviation of the RR intervals, low to high power of HRV and coefficient of variance of RR intervals of healthy controls were all higher than those of diabetes patients.ConclusionsThe results showed that ApEn contained information on disorders of autonomic system function of diabetes patients as traditional HRV indices in time and frequency domains. ApEn and three traditional indices showed accordance to some degree. Non-linear information in subcomponents of HRV was shown, which is potentially more effective for distinguishing healthy individuals and diabetes patients than that extracted from the total HRV. Compared with diabetes patients, the cardiovascular system of healthy controls showed information of higher complexity, and better regulation function in response to changes of environment.

Heart rate variability (HRV) is a non-invasive measure of autonomic function. The relationship between unselected long-term traumatic injury (TI) and HRV has not been investigated. This systematic review examines the impact of non-acute TI (>7 days post-injury) on standard HRV indices in adults. Four electronic databases (CINAHL, Medline, Scopus, and Web of Science) were searched. The quality of studies, risk of bias (RoB), and quality of evidence (QoE) were assessed using Axis, RoBANS and GRADE, respectively. Using the random-effects model, mean difference (MD) for root mean square of successive differences (RMSSD) and standard deviation of NN-intervals (SDNN), and standardized mean difference (SMD) for Low-frequency (LF): High-Frequency (HF) were pooled in RevMan guided by the heterogeneity score (I2). 2152 records were screened followed by full-text retrieval of 72 studies. 31 studies were assessed on the inclusion and exclusion criteria. Only four studies met the inclusion criteria. Three studies demonstrated a high RoB (mean RoBANS score 14.5±3.31) with a low QoE. TI was associated with a significantly higher resting heart rate. Meta-analysis of three cross-sectional studies demonstrated a statistically significant reduction in RMSSD (MD -8.45ms, 95%CI-12.78, -4.12, p<0.0001) and SDNN (MD -9.93ms, 95%CI-14.82, -5.03, p<0.0001) (low QoE) in participants with TI relative to the uninjured control. The pooled analysis of four studies showed a higher LF: HF ratio among injured versus uninjured (SMD 0.20, 95%CI 0.01-0.39, p<0.04) (very low QoE). Albeit low QoE, non-acute TI is associated with attenuated HRV indicating autonomic imbalance. The findings might explain greater cardiovascular risk following TI. Trial registration PROSPERO registration number: CRD: CRD42021298530.

Heart rate variability (HRV) is a non-invasive measure of autonomic function. The relationship between unselected long-term traumatic injury (TI) and HRV has not been investigated. This systematic review examines the impact of non-acute TI (>7 days post-injury) on standard HRV indices in adults. Four electronic databases (CINAHL, Medline, Scopus, and Web of Science) were searched. The quality of studies, risk of bias (RoB), and quality of evidence (QoE) were assessed using Axis, RoBANS and GRADE, respectively. Using the random-effects model, mean difference (MD) for root mean square of successive differences (RMSSD) and standard deviation of NN-intervals (SDNN), and standardized mean difference (SMD) for Low-frequency (LF): High-Frequency (HF) were pooled in RevMan guided by the heterogeneity score (I2). 2152 records were screened followed by full-text retrieval of 72 studies. 31 studies were assessed on the inclusion and exclusion criteria. Only four studies met the inclusion criteria. Three studies demonstrated a high RoB (mean RoBANS score 14.5±3.31) with a low QoE. TI was associated with a significantly higher resting heart rate. Meta-analysis of three cross-sectional studies demonstrated a statistically significant reduction in RMSSD (MD -8.45ms, 95%CI-12.78, -4.12, p<0.0001) and SDNN (MD -9.93ms, 95%CI-14.82, -5.03, p<0.0001) (low QoE) in participants with TI relative to the uninjured control. The pooled analysis of four studies showed a higher LF: HF ratio among injured versus uninjured (SMD 0.20, 95%CI 0.01–0.39, p<0.04) (very low QoE). Albeit low QoE, non-acute TI is associated with attenuated HRV indicating autonomic imbalance. The findings might explain greater cardiovascular risk following TI.Trial registration PROSPERO registration number: CRD: CRD42021298530.

Objective To investigate the changes in heart rate deceleration capacity(DC) and heart rate va-riability(HRV) parameters in children with vasovagal syncope(VVS), to assess the basic autonomic function of children with VVS, and to explore the reference value of DC for the diagnosis of VVS in children. Methods VVS group included 62 patients diagnosed with VVS in the Cardiovascular Department of Children′s Hospital Affiliated to Capital Institute of Pediatrics from June 2015 to December 2017, 56 outpatients undergoing physical examination were selected as the healthy control group, and the changes in DC and HRV matched to age and gender were analyzed respectively.Indexes of DC and HRV of both groups of children in health control group and children with VVS were compared among different age groups or different genders. Results (1)Analysis of DC and HRV based on age or gender: whether in healthy children or in children with VVS, DC and HRV parameters varied significantly in the range of different ages, while they were the same in children with different genders.(2)Comparison of DC and HRV in school-age and puberty children between VVS group and healthy control group: in school-aged children, DC of VVS group was significantly higher than that of healthy control group [(6.8±1.0) ms vs.(6.0±0.7) ms, t=-2.412, P<0.01]. Mean square root rates of succe-ssive normal sinus RR interval differences(rMSSD) increased, and the difference was significant (P<0.05). In children at puberty, DC in children of VVS group was significantly higher than that in the healthy control group [(7.4±1.2) ms vs.(6.6±1.1) ms, t=-2.742, P<0.01], rMSSD, LF and HF value were significantly higher compared with those of the healthy control group (all P<0.05). (3)Predictive value of DC on VVS: the binary Logistic regression analysis found that only DC was associated with VVS in different age groups.By using receiver operating characteristic curve to analyze the predictive value of DC on VVS, it was found that when making DC 6.5 ms for school age and 7.0 ms at puberty as the threshold, a better prediction of VVS could be achieved with good sensitivity and specificity. Conclusions Children′s autonomic nervous function changes with age, and DC and HRV parameters change significantly during adolescence and at school age.Children with VVS may have abnormally increased vagal tone.When DC ≥6.5 ms at school age or DC≥7.0 ms at puberty, it may have a reference value for the diagnosis of VVS. Key words: Vasovagal syncope; Deceleration capacity; Heart rate variability; Autonomic nervous function