Clinical relevance of heart rate variability

Heart Rate Variability in Subjects with Severe Allergic Background Undergoing COVID-19 Vaccination

Anti-Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) vaccination is the world’s most important strategy for stopping the pandemic. Vaccination challenges the body’s immune response and can be complicated by hypersensitivity reactions. The autonomic nervous system can modulate the inflammatory immune response, therefore constituting a potential marker to characterize individuals at high risk of hypersensitivity reactions. Autonomic nervous system functionality was assessed through measurement of the heart rate variability (HRV) in subjects with a history of severe allergic reactions and 12 control subjects. HRV parameters included the mean electrocardiograph RR interval and the standard deviation of all normal R–R intervals (SDNN). All measurements were performed immediately before the anti-SARS-CoV-2 vaccination. The median RR variability was lower in the study than in the control group: 687 ms (645–759) vs. 821 ms (759–902); p = 0.02. The SDNN was lower in the study group than in the control group: 32 ms (23–36) vs. 50 ms (43–55); p < 0.01. No correlation was found between age and the SDNN. Autonomic nervous system activity is unbalanced in people with a severe allergy background.

Minimal Influence of Hypobaria on Heart Rate Variability in Hypoxia and Normoxia

Introduction

The present study evaluated the putative effect of hypobaria on resting HRV in normoxia and hypoxia.

Methods

Fifteen young pilot trainees were exposed to five different conditions in a randomized order: normobaric normoxia (NN, P_B = 726 ± 5 mmHg, F_IO₂ = 20.9%), hypobaric normoxia (HN, P_B = 380 ± 6 mmHg, F_IO₂≅40%), normobaric hypoxia (NH, P_B = 725 ± 4 mmHg, F_IO₂≅11%); and hypobaric hypoxia (HH at 3000 and 5500 m, HH3000 and HH5500, P_B = 525 ± 6 and 380 ± 8 mmHg, respectively, F_IO₂ = 20.9%). HRV and pulse arterial oxygen saturation (SpO₂) were measured at rest seated during a 6 min period in each condition. HRV parameters were analyzed (Kubios HVR Standard, V 3.0) for time (RMSSD) and frequency (LF, HF, LF/HF ratio, and total power). Gas exchanges were collected at rest for 10 min following HRV recording.

Results

SpO₂ decreased in HH3000 (95 ± 3) and HH5500 (81 ± 5), when compared to NN (99 ± 0). SpO₂ was higher in NH (86 ± 4) than HH5500 but similar between HN (98 ± 2) and NN. Participants showed lower RMSSD and total power values in NH and HH5500 when compared to NN. In hypoxia, LF/HF ratio was greater in HH5500 than NH, whereas in normoxia, LF/HF ratio was lower in HN than NN. Minute ventilation was higher in HH5500 than in all other conditions.

Discussion

The present study reports a slight hypobaric effect either in normoxia or in hypoxia on some HRV parameters. In hypoxia, with a more prominent sympathetic activation, the hypobaric effect is likely due to the greater ventilation stimulus and larger desaturation. In normoxia, the HRV differences may come from the hyperoxic breathing and slight breathing pattern change due to hypobaria in HN.

Cardiac autonomic function evaluation in pediatric and adult patients with congenital myasthenic syndromes

Cardiac autonomic dysfunction has been examined in myasthenia gravis but not in congenital myasthenic syndromes (CMS). We aimed to evaluate cardiac autonomic functions in genetically defined CMS. Patients diagnosed with and under treatment for CMS were reviewed for 24-hour cardiac rhythm monitoring. Heart rate variability (HRV) measures were defined as: SDNN, mean of the standard deviations for all R-R intervals; SDNNi, standard deviation of all R-R intervals in successive five-minute epochs; RMSSD, square root of the mean of squared differences between successive R-R intervals. Ten patients with mutations in the epsilon subunit of the acetylcholine receptor (AChRε) and five patients with mutations in the collagen-like tail of asymmetric acetylcholinesterase (ColQ) were included. Median age at evaluation was 17 (2.5-46) years. In the AChRε group, RMSSD values; and in the ColQ group, SDNN, SDNNi and RMSSD values were significantly lower than those of healthy subjects. This first extensive report examining HRV in CMS showed alterations in patients with ColQ mutations and, to a lesser extent, in the group with AChRε mutations. This might indicate an increased risk of cardiac arrhythmias. We suggest cardiological follow-up in CMS, and consideration of any potential cardiovascular effects of therapeutic agents used in management.

Evaluation of autonomic functions of patients with multiple system atrophy and Parkinson's disease by head-up tilt test

The aim of this study was to evaluate the autonomic neural function in Parkinson's disease (PD) and multiple system atrophy (MSA) with head-up tilt test and spectral analysis of cardiovascular parameters. This study included 15 patients with MSA, 15 patients with PD, and 29 healthy control (HC) subjects. High frequency power of the RR interval (RR-HF), the ratio of low frequency power of RR interval to RR-HF (RR-LF/HF) and LF power of systolic BP were used to evaluate parasympathetic, cardiac sympathetic and vasomotor sympathetic functions, respectively. Both patients with PD and MSA showed orthostatic hypotension and lower parasympathetic function (RR-HF) at tilt position as compared to HC subjects. Cardiac sympathetic function (RR-LF/HF) was significantly high in patients with PD than MSA at supine position. RR-LF/HF tended to increase in MSA and HC, but decreased in PD by tilting. Consequently, the change of the ratio due to tilting (ΔRR-LF/HF) was significantly lower in patients with PD than in HC subjects. Further analysis showed that compared to mild stage of PD, RR-LF/HF at the supine position was significantly higher in advanced stage. By tilting, it was increased in mild stage and decreased in the advanced stage of PD, causing ΔRR-LF/HF to decrease significantly in the advanced stage. Thus, we demonstrated that spectral analysis of cardiovascular parameters is useful to identify sympathetic and parasympathetic disorders in MSA and PD. High cardiac sympathetic function at the supine position, and its reduction by tilting might be a characteristic feature of PD, especially in the advanced stage.

Myocardial (123)I-MIBG Uptake and Cardiovascular Autonomic Function in Parkinson's Disease

Introduction. Patients with Parkinson's disease (PD) showed reduced myocardial ¹²³I-MIBG uptake, which may affect autonomic regulation. We investigated correlation between MIBC accumulation and cardiovascular autonomic function in PD. Methods. We performed myocardial MIBG scintigraphy, heart rate variability (HRV) analysis, and the head-up tilt test (HUT) in 50 PD patients (66.4 ± 7.8 years; duration 5.5 ± 5.9 years). Autonomic function tests were also performed in 50 healthy controls (66.5 ± 8.9 years). As HRV parameters, a high-frequency power (HF, 0.15–0.4 Hz), a low-frequency power (LF, 0.04–0.15 Hz), and LF/HF ratio were used. Results. Our PD patients had a significant reduction in LF and HF compared with the controls (P = 0.005 and P = 0.01). In HUT, systolic and diastolic blood pressure falls in the PD group were significantly greater than those in the controls (P = 0.02 and P = 0.02). The washout rate of MIBG was negatively correlated with blood pressure changes during HUT. Conclusion. Our PD patients showed reduced HRV, blood pressure dysregulation, and reduced MIBG accumulation, which was correlated with blood pressure dysregulation. Orthostatic hypotension in PD may be mainly caused by sympathetic postganglionic degeneration.

Complex nonlinear autonomic nervous system modulation link cardiac autonomic neuropathy and peripheral vascular disease

BACKGROUND

Physiological interactions are abundant within, and between, body systems. These interactions may evolve into discrete states during pathophysiological processes resulting from common mechanisms. An association between arterial stenosis, identified by low ankle-brachial pressure index (ABPI) and cardiovascular disease (CVD) as been reported. Whether an association between vascular calcification-characterized by high ABPI and a different pathophysiology-is similarly associated with CVD, has not been established. The current study aims to investigate the association between ABPI, and cardiac rhythm, as an indicator of cardiovascular health and functionality, utilizing heart rate variability (HRV).

METHODS AND RESULTS

Two hundred and thirty six patients underwent ABPI assessment. Standard time and frequency domain, and non-linear HRV measures were determined from 5-min electrocardiogram. ABPI data were divided into normal (n = 101), low (n = 67) and high (n = 66) and compared to HRV measures.(DFAα1 and SampEn were significantly different between the low ABPI, high ABPI and control groups (p < 0.05).

CONCLUSION

A possible coupling between arterial stenosis and vascular calcification with decreased and increased HRV respectively was observed. Our results suggest a model for interpreting the relationship between vascular pathophysiology and cardiac rhythm. The cardiovascular system may be viewed as a complex system comprising a number of interacting subsystems. These cardiac and vascular subsystems/networks may be coupled and undergo transitions in response to internal or external perturbations. From a clinical perspective, the significantly increased sample entropy compared to the normal ABPI group and the decreased and increased complex correlation properties measured by DFA for the low and high ABPI groups respectively, may be useful indicators that a more holistic treatment approach in line with this more complex clinical picture is required.

Heart rate variability is severely impaired among type 2 diabetic patients with hypertension

INTRODUCTION

The aim of our study was to evaluate the relative effect of diabetes and hypertension on heart rate variability.

RESEARCH DESIGN AND METHODS

Four age-matched groups including type 2 diabetic patients with and without hypertension, non-diabetic patients with essential hypertension and healthy control subjects were studied. Autonomic function was evaluated by the standard cardiovascular reflex tests and 24-hour heart rate variability measurement. Heart rate variability was characterized by the triangular index value and by the spectral components of the frequency domain analysis.

RESULTS

According to the two-way analysis of variance on ranks, all parameters were influenced negatively by diabetes (heart rate variability triangular index: p < 0.001; low-frequency component: p < 0.0001; high-frequency component: p < 0.001; and total power: p < 0.0001), whereas hypertension had a negative effect only on the low-frequency component (p < 0.05). The interaction between hypertension and diabetes was not significant, indicating that their effects on the heart rate variability parameters are additive. Beat-to-beat variation upon deep breathing, the most sensitive cardiovascular reflex test was also negatively influenced by both diabetes (p < 0.001) and hypertension, (p < 0.05), and their effects were additive.

CONCLUSIONS

Diabetes appears to have a greater effect on autonomic dysfunction compared with hypertension. Patients suffering from both diabetes and hypertension are at the highest risk of reduced heart rate variability. Early assessment of the autonomic nerve function is suggested in diabetic patients with hypertension.

Titanium-treated surroundings attenuate psychological stress associated with autonomic nerve regulation in office workers with daily emotional stress

Housing mice in the presence of small particles of titanium has been shown to reduce stress-responsive behavior via the autonomic nervous system. Here, we examined the effects of nighttime titanium exposure on stress parameters and autonomic nerve activity in office workers with emotional stress. A randomized double-blind, placebo controlled trial was performed in 24 male subjects with desk jobs, who were randomly allocated to spend 5 nights in rooms with or without titanium. The serum concentrations of stress-responsive hormones (cortisol, adrenocorticotropin, and catecholamine) were measured, and profiles of emotional stress were collected to subjectively assess relaxation. Autonomic nerve activity was examined by power spectra analysis of heart rate variability. In psychological tests, factors related to tension (-14.5%, 95% CI=-15.7--2.1), anger (-11.3%, 95% CI=-13.9--0.7), and emotional stress (-28.5%, 95% CI=-30.0--5.3) were significantly decreased by exposure to titanium. The serum level of adrenocorticotropin was gradually elevated throughout the experimental period in the placebo group (day 4, 80.5%, 95% CI=7.1-37.5 vs. before trial) but not the titanium group. Power spectral analysis of R-R interval data showed a significant elevation in the high-frequency power spectral ratio in subjects housed in titanium rooms (days 1-2, 14.3%, 95% CI=4.7-21.9; days 3-4, 26.8%, 95% CI=4.9-38.7; and days 5-6, 24.1%, 95% CI=5.8-34.0 vs. before trial). These results suggest that sleeping in a room containing titanium lowers physiological and psychological stress.

Overweight and reduced heart rate variability in British children: an exploratory study

OBJECTIVE

To investigate the associations between heart rate variability (HRV) and weight status in children.

METHOD

Following ethics approval, 182 children (87 males, 95 females) aged between 6 and 11 years old (Mean (SD) age 9.1 (1.4) years) were assessed for height and body mass from which body mass index (BMI, kg/m(2)) was calculated. Data were collected in Central England, March-June 2011. Resting HRV was assessed in the supine position. Time domain (RMSSD and pNN50) and frequency domain (HF and LF power (ms(2)), LF/HF ratio) measures of HRV were calculated. Pearson correlations were used to assess the associations between BMI and HRV parameters. Differences in HRV parameters according to gender or weight status were analysed using t-tests.

RESULTS

BMI was significantly and inversely related to RMSSD (r=-0.23, p=0.001) and pNN50 (r=0.20, p=0.008) but not frequency domain measures of HRV (p>0.05). Overweight/obese children demonstrated lower HF power compared to normal weight children (p=0.02). RMSSD and pNN50 were significantly lower in overweight/obese children compared to normal weight children (p<0.05). Age was associated with changes in Mean R-R and RMSSD (p<0.05).

CONCLUSION

This exploratory study suggests that HRV is reduced in overweight/obese children.

The deep breathing test: median-based expiration-inspiration difference is the measure of choice

Heart rate variability (HRV) has become an important parameter for the assessment of autonomic function in many areas of medicine. In particular, respiratory sinus arrhythmia measured during the deep breathing test (DBT) is often used. Results are usually expressed in common time-domain parameters. A "most preferred measure" has not yet been identified. We investigated the sensitivity of the DBT to the following anomalies: in-test variance; shifts of mean heart rate; premature ventricular contractions; and breathing rate deviations. Frequency and magnitude of the anomalies were determined in a set of real DBTs (n=514) and transferred to computer simulations to mimic realistic conditions. The sensitivity of standard deviation, mean circular resultant (MCR), root mean square of successive differences (RMSSD), and four types of expiration-inspiration (E-I) difference were quantified statistically. Median-based E-I differences, E-I ratio, and MCR were most resistant to the anomalies. E-I difference derived by median values should be used preferentially, providing the highest precision and independence from heart rate.

Current clinical applications of heart rate variability

Heart rate variability (HRV) has become a popular method for the studies of physiologic mechanisms responsible for the control of heart rate fluctuations, in which the autonomic nervous system appears to play a primary role. Depression of HRV has been observed in many clinical scenarios, including autonomic neuropathy, heart transplantation, congestive heart failure, myocardial infarction (MI), and other cardiac and noncardiac diseases. However, it is important to realize that clinical implication of HRV analysis has been clearly recognized in only two clinical conditions: (1) as a predictor of risk of arrhythmic events or sudden cardiac death after acute MI, and (2) as a clinical marker of evolving diabetic neuropathy. Recently, its role in evaluation and management of heart failure has also been recognized. It is pertinent to recognize the limitations of HRV as far as its clinical utility at present is concerned. The methodology of HRV had remained poorly standardized until the recent publication of the Special Report of the Task Force of ESC/NASPE, and thus has been presenting difficulty in comparing earlier existing data. Also, determination of the exact sensitivity, specificity, and predictive value of HRV, as well as the normal values of standard measures in the general population, still require further investigation before better standards can be set for existing and future clinical applications. This article reviews the major concepts of HRV measurements, their clinical relevance, and the recent advances in this field.

Osteopathic manipulative treatment and its relationship to autonomic nervous system activity as demonstrated by heart rate variability: a repeated measures study

Background

The relationship between osteopathic manipulative treatment (OMT) and the autonomic nervous system has long been acknowledged, but is poorly understood. In an effort to define this relationship, cervical myofascial release was used as the OMT technique with heart rate variability (HRV) as a surrogate for autonomic activity. This study quantifies that relationship and demonstrates a cause and effect.

Methods

Seventeen healthy subjects, nine males and eight females aged 19–50 years from the faculty, staff, and students at Oklahoma State University Center for Health Sciences College of Osteopathic Medicine, acted as their own controls and received interventions, administered in separate sessions at least 24 hours apart, of cervical myofascial OMT, touch-only sham OMT, and no-touch control while at a 50-degree head-up tilt. Each group was dichotomized into extremes of autonomic activity using a tilt table. Comparisons were made between measurements taken at tilt and those taken at pre- and post-intervention in the horizontal.

The variance of the spectral components of HRV, expressed as frequencies, measured the response to change in position of the subjects. Normalized low frequency (LF) and high frequency (HF) values, including LF/HF ratio, were calculated and used to determine the effect of position change on HRV.

Results

Predominantly parasympathetic responses were observed with subjects in the horizontal position, while a 50-degree tilt provided a significantly different measure of maximum sympathetic tone (p < 0.001). Heart rate changed in all subjects with change in position; respirations remained constant. When OMT was performed in a sympathetic environment (tilt), a vagal response was produced that was strong enough to overcome the sympathetic tone. There was no HRV difference between sham and control in either the horizontal or tilt positions.

Conclusion

The vagal response produced by the myofascial release procedure in the maximally stimulated sympathetic environment could only have come from the application of the OMT. This demonstrates the association between OMT and the autonomic nervous system. The lack of significance between control and sham in all positions indicates that HRV may be a useful method of developing sham controls in future studies of OMT.

Trial registration

clinicaltrials.gov NCT00516984.

QT & RR variability spots the earliest autonomic deregulation in diabetes. Fading of vagal sino-atrial drive but not of sympathetic ventricular responsiveness to life challenges

27 consecutive insulin-dependent diabetic patients (pts), under 50 years, with blood glucose controlled within normal limits and no significant or multiple cardiovascular/neurological complications in the lights of clinical tests, went through a protocol as follows: laiddown at relaxed rest for 10 min, then stood-up quietly for 7 min, and finally experienced a stress-interview for 10 min while supine. A thoracic ECG lead was digitized at I ms (Codas, Dataq Instr.), RR and QT intervals were software-detected, resampled at 500 ms, and Fourier-transformed over 3 min epochs to get auto-or cross-spectra. RR-by-QT mean square coherence detached the RR-independent fraction of QT low fequency (LF) spectral power, called idioventricular QT-LF. We detected autonomic impairment of three types (discriminant score = 92.31%), presumably differentiated upon the locus of lesion, using RR's basal variance and mean RR shortening when standing as follows: (I) RR shortening > 200 ms in 10 pts; (II) normal RR shortening but no RR variance in 4 pts; (III) stiff RR around 600 ms and no RR variance in 2 pts. The above pts have been excluded from further analysis. The remaining 11 pts with no such impairments (5M and 6F, 36.4 y +/- 4.4 SD, history of 6.0 y +/- 5.2) have been compared with 11 normal subjects in an age and gender-paired control group in two steps. Step 1: Preliminary MANOVA/ANOVA showed significant effects on the ensemble of spectral variables of every single factor (status: normal or patient group; intervention; gender) with no significant factor interactions. Significant effects of intervention or status on main RR spectral variables and on a few QT spectral variables were also documented. Step 2: Non-parametric tests showed that diabetics had (mildly to moderately) shorter mean RR, while their RR-LF was always significantly lower than those found in normals--a difference propagated to QT-LF but not to idioventricular QT-LF. In the intra-group there were similar responses to interventions except stress with respect to mean RR. Consistent reduction in RR-LF under moderate or no change in mean RR suggests vagal down- regulation that, judging by idioventricular QT-LF showing, goes perhaps before a similar process with sympathetic control of ventricles. This phase delay may introduce an early arrhythmic risk worth dealing with in secondary prevention.

Evaluation of autonomic nervous system function with spectral analysis of heart rate variability in a case of tetanus

The autonomic nervous system is affected in a wide variety of neurological disorders. Its dysfunction may play an important role in the clinical course and may result in serious complications, such as cardiac arrest. We report a case of tetanus who presented with severe autonomic nervous system dysfunction which was detected by spectral analysis of heart rate variability monitored over 24 h. This is a semi-quantitative method for evaluation of the status of the autonomic nervous system. In the present case, the analysis revealed profoundly decreased activity of both sympathetic and parasympathetic nervous system modulation of cardiac rhythm. The parasympathetic nervous system activity was more severely impaired than that of the sympathetic nervous system. The relative predominance of the sympathetic nervous system in the present case may have resulted in unopposed sympathetic nervous system hyperactivity manifested in this patient by tachycardia and excessive sweating. We further infer that the documented diminished buffering capacity of the autonomic nervous system may have lead to a sudden cardiac arrest in our case. Thus, spectral analysis of heart rate variability is a non-invasive and sensitive method for evaluating the status of the autonomic nervous system of critically ill patients in the hospital setting.

Reduced autonomic activity during stepwise exposure to high altitude

Several studies have shown increased sympathetic activity during acute exposure to hypobaric hypoxia. In a recent field study we found reduced plasma catecholamines during the first days after a stepwise ascent to high altitude. In the present study 14 subjects were exposed to a simulated ascent in a hypobaric chamber to test the hypothesis of a temporary reduction in autonomic activity. The altitude was increased stepwise to 4500 m over 3 days. Heart rate variability (HRV) was assessed continuously in seven subjects. Baroreceptor reflex sensitivity (BRS) was determined in eight subjects with the 'Transfer Function' method at baseline, at 4500 m and after returning to baseline. Resting plasma catecholamines and cardiovascular- and plasma catecholamine- responses to cold pressor- (CPT) and mental stress-test (MST) were assessed daily in all and 12 subjects, respectively. Data are mean +/- SEM. Compared with baseline at 4500 m there were lower total power (TP) (35 457 +/- 26 302 vs. 15 001 +/- 11 176 ms2), low frequency (LF) power (3112 +/- 809 vs. 1741 +/- 604 ms2), high frequency (HF) power (1466 +/- 520 vs. 459 +/- 189 ms2) and HF normalized units (46 +/- 0.007 vs. 44 +/- 0.006%), P < or = 0.001. Baroreceptor reflex sensitivity decreased (15.6 +/- 2.1 vs. 9.5 +/- 2.6 ms mmHg(-1), P = 0.015). Resting noradrenaline (NA) decreased (522 +/- 98 vs. 357 +/- 60 pmol L(-1), P = 0.027). The increase in systolic blood pressure (SBP) and NA during mental stress was less pronounced (21 +/- 4 vs. 10 +/- 2% and 25 +/- 9 vs. -2 +/- 8%, respectively, P < 0.05). The increase in SBP during cold pressor test decreased (16 +/- 3 vs. 1 +/- 6%, P = 0.03). Diastolic blood pressure, HR and adrenaline displayed similar tendencies. We conclude that a transient reduction in parasympathetic and sympathetic activity was demonstrated during stepwise exposure to high altitude.

24-hour ECG frequency-domain measures in preeclamptic and healthy pregnant women during and after pregnancy

OBJECTIVE

The aim of the present study was to evaluate the autonomic balance in women with preeclampsia and in healthy women during and after pregnancy by means of a 24-h ECG Holter recording combined with power spectral analysis.

METHODS

Fifteen preeclamptic and 15 healthy women underwent 24-h Holter monitoring in the 32nd-36th week of gestation and 3-6 months postpartum. The power spectrum of the maternal electrocardiogram was analyzed with an autoregressive algorithm.

MAIN OUTCOME MEASURES

The power spectrum contains two major components: a low-frequency peak, primarily attributed to sympathetic tone, and a high-frequency peak, reflecting vagal tone.

RESULTS

The power spectrum of maternal heart rate variability did not differ between preeclamptic and normal women during pregnancy. After delivery, the amplitude of all components became significantly higher than those during pregnancy, with one exception: the high-frequency component in the patients who had been preeclamptic. In a comparison of the two groups, the high-frequency component after delivery was significantly lower in women who had preeclampsia than in normal healthy women (p = 0.03).

CONCLUSIONS

During pregnancy, the power spectrum is reduced and cannot be used to distinguish between patients with preeclampsia and normal healthy women. Three to 6 months after delivery, the high-frequency component is still reduced in the preeclamptic group of women. This indicates an impaired vagal modulation even in the nonpregnant state in this group of women, unlike those who had a normotensive pregnancy.

Mechanosensitivity as an integrative system in heart: an audit

This review examines a manifold of apparently loosely linked observations and mechanisms, from membrane to man, and assembles them to support the notion that mechanoelectric transduction is an integrative regulatory system in the heart. For this, the assemblage has to satisfy, at least to some extent, criteria that apply to other integrative regulatory systems such as the endocrine and nervous systems. The integrative effectors in the endocrine system are chemical linkages, circulating hormones: in the nervous system the linkage is a network of cables, nerve conduction and neurotransmitters. Mechanical integration is would be effected through mechanical machinery, cardiac contractile and hydraulic function with attendant stress and strain transmitted via "tensegrity". This can, through the cytoskeleton, begin with membrane integrins and transmit intracellularly for example via F actins to reach the rest of the membranous integrins. Further transmission to the organ is via cell-to-cell adhesion complexes and the extracellular matrix. This tensegrity facilitates integration of force and strain changes from area to area. In consequence, and analogous to the neurendocrine system, mechanoelectric transduction should, and does (1) operate at the molecular or membrane level--this would be via mechanotransducers affecting transmembrane ionic flow; (2) operate in the cell--to influence electrophysiology; (3) have a multicellular expression--e.g. mechanical distortion of one cell can raise intracellular calcium of an adjacent cell; (4) express in the intact organ--e.g. an increase in venous return hydraulically distends the sinoatrial node, steepening its pacemaker potential, thus increasing heart rate. It should also (5) demonstrate elements of a feedback system--"mechanoelectric feedback", and (6) interact with other systems--the cytoskeleton incorporates cell signalling complexes intersecting with other signal cascades. Finally, (7) it can malfunction to produce clinical abnormality--it contributes electrophysiologically to lethal cardiac arrhythmia. This anatomical and functional behaviour of mechanoelectric transduction could sanction the prospect of viewing it as analogous to the other integrative physiological systems.

Fitness and gender-related differences in heart period variability

OBJECTIVE

The purpose of this study was to elucidate the processes of autonomic nervous system control of the heart in high fit and low fit men and women by examining indices of vagally mediated cardiac control.

METHOD

There were 40 participants (19 women and 21 men) classified as either high fit or low fit on the basis of scores from the University of Houston Non-Exercise Test (N-EX). Both time domain and frequency domain cardiovascular measures were calculated during a resting baseline period (BL), a facial cooling task (FCT) designed to elicit parasympathetic activity, a reaction time task (RT) designed to elicit primarily sympathetic activity, and a combination task (COMBO) designed to elicit a combination of both parasympathetic and sympathetic activation and recovery periods after each task.

RESULTS

Both time and frequency domain indices of heart period variability (HPV) indicated that high fit individuals exhibited greater vagal control of the heart relative to low fit individuals at baseline and across all tasks. In addition, the results suggest greater relative vagal control of the heart in women compared with men. Reactivity scores revealed significant main effects for condition for both time and frequency domain measures of HPV, but no main effects or interactions involving fitness or gender.

CONCLUSIONS

First, small differences in aerobic capacity result in meaningful differences in cardiac autonomic control, with high fit individuals clearly demonstrating greater vagally mediated cardiac control. Second, important gender-related differences in HPV indicate that women exhibit greater vagal control and men exhibit more sympathetic cardiac control even though heart period (HP) did not differ. Third, the mechanisms underlying the beneficial effects of fitness and those placing women at a reduced risk for coronary heart disease (CHD) seem to be similar.