Respiratory modulation of human autonomic rhythms

Acute cardiac autonomic and hemodynamic responses to resistive breathing: Effect of loading type and intensity

OBJECTIVES

This study aimed to assess the acute impact of distinct loading breathing types and intensities on cardiac autonomic function and hemodynamic responses in healthy young adults.

METHODS

A randomized, crossover trial involved 28 participants who underwent inspiratory resistive breathing, expiratory resistive breathing (ERB) and combined resistive breathing, each at 30% and 60% of maximal respiratory pressures. Data on heart rate variability (HRV) and hemodynamic parameters were collected during each trial.

RESULTS

The study revealed significant main and interaction effects for both the performed task and the intensity across all measured variables (all p < 0.001). ERB at 60% load demonstrated significantly higher HRV values in the standard deviation of normal-to-normal RR intervals, the square root of the mean squared difference of successive normal-to-normal RR intervals and high-frequency power, as well as significantly lower values in heart rate, stroke volume, stroke volume index, cardiac output, cardiac index, end-diastolic volume and end-diastolic volume index, compared to other loaded protocols (all p < 0.001).

CONCLUSION

These findings highlight the acute effect of type-specific and load-dependent resistive breathing on cardiac autonomic and hemodynamic functions, where ERB at 60% intensity showed the most significant cardiovagal modulation while causing the least hemodynamic alterations.

Guideline for the application of heart rate and heart rate variability in occupational medicine and occupational health science

This updated guideline replaces the "Guideline for the application of heart rate and heart rate variability in occupational medicine and occupational health science" first published in 2014. Based on the older version of the guideline, the authors have reviewed and evaluated the findings on the use of heart rate (HR) and heart rate variability (HRV) that have been published in the meantime and incorporated them into a new version of this guideline.This guideline was developed for application in clinical practice and research purposes in the fields of occupational medicine and occupational science to complement evaluation procedures with respect to exposure and risk assessment at the workplace by the use of objective physiological workload indicators. In addition, HRV is also suitable for assessing the state of health and for monitoring the progress of illnesses and preventive medical measures. It gives an overview of factors influencing the regulation of the HR and HRV at rest and during work. It further illustrates methods for measuring and analyzing these parameters under standardized laboratory and real workload conditions, areas of application as well as the quality control procedures to be followed during the recording and evaluation of HR and HRV.

Combined effect of transcutaneous auricular vagus nerve stimulation and 0.1 Hz slow-paced breathing on working memory

Background

Previous research has found that transcutaneous auricular vagus nerve stimulation (taVNS) can improve working memory (WM) performance. It has also been shown that 0.1 Hz slow-paced breathing (SPB, i.e., breathing at a rate of approximately 6 breaths/min) can significantly influence physical state and cognitive function via changes in autonomic afferent activity. In the present study, we investigated the synergistic effects of taVNS and SPB on WM performance.

Methods

A total of 96 healthy people participated in this within-subjects experiment involving four conditions, namely taVNS, SPB, combined taVNS with SPB (taVNS + SPB), and sham. Each participant underwent each intervention for 30 min and WM was compared pre- and post-intervention using the spatial and digit n-back tasks in a random order four times. Permutation-based analysis of variance was used to assess the interaction between time and intervention.

Results

For the spatial 3-back task, a significant interaction between time and intervention was found for the accuracy rate of matching trials (mACC, p = 0.03). Post hoc analysis suggested that both taVNS and taVNS + SPB improved WM performance, however, no significant difference was found in the SPB or sham groups.

Conclusion

This study has replicated the effects of taVNS on WM performance reported in previous studies. However, the synergistic effects of combined taVNS and SPB warrant further research.

Validity of Two Consumer Multisport Activity Tracker and One Accelerometer against Polysomnography for Measuring Sleep Parameters and Vital Data in a Laboratory Setting in Sleep Patients

Two commercial multisport activity trackers (Garmin Forerunner 945 and Polar Ignite) and the accelerometer ActiGraph GT9X were evaluated in measuring vital data, sleep stages and sleep/wake patterns against polysomnography (PSG). Forty-nine adult patients with suspected sleep disorders (30 males/19 females) completed a one-night PSG sleep examination followed by a multiple sleep latency test (MSLT). Sleep parameters, time in bed (TIB), total sleep time (TST), wake after sleep onset (WASO), sleep onset latency (SOL), awake time (WASO + SOL), sleep stages (light, deep, REM sleep) and the number of sleep cycles were compared. Both commercial trackers showed high accuracy in measuring vital data (HR, HRV, SpO2, respiratory rate), r > 0.92. For TIB and TST, all three trackers showed medium to high correlation, r > 0.42. Garmin had significant overestimation of TST, with MAE of 84.63 min and MAPE of 25.32%. Polar also had an overestimation of TST, with MAE of 45.08 min and MAPE of 13.80%. ActiGraph GT9X results were inconspicuous. The trackers significantly underestimated awake times (WASO + SOL) with weak correlation, r = 0.11−0.57. The highest MAE was 50.35 min and the highest MAPE was 83.02% for WASO for Garmin and ActiGraph GT9X; Polar had the highest MAE of 21.17 min and the highest MAPE of 141.61% for SOL. Garmin showed significant deviations for sleep stages (p < 0.045), while Polar only showed significant deviations for sleep cycle (p = 0.000), r < 0.50. Garmin and Polar overestimated light sleep and underestimated deep sleep, Garmin significantly, with MAE up to 64.94 min and MAPE up to 116.50%. Both commercial trackers Garmin and Polar did not detect any daytime sleep at all during the MSLT test. The use of the multisport activity trackers for sleep analysis can only be recommended for general daily use and for research purposes. If precise data on sleep stages and parameters are required, their use is limited. The accuracy of the vital data measurement was adequate. Further studies are needed to evaluate their use for medical purposes, inside and outside of the sleep laboratory. The accelerometer ActiGraph GT9X showed overall suitable accuracy in detecting sleep/wake patterns.

Beyond the Baroreflex: A New Measure of Autonomic Regulation Based on the Time-Frequency Assessment of Variability, Phase Coherence and Couplings

For decades the role of autonomic regulation and the baroreflex in the generation of the respiratory sinus arrhythmia (RSA) - modulation of heart rate by the frequency of breathing - has been under dispute. We hypothesized that by using autonomic blockers we can reveal which oscillations and their interactions are suppressed, elucidating their involvement in RSA as well as in cardiovascular regulation more generally. R-R intervals, end tidal CO2, finger arterial pressure, and muscle sympathetic nerve activity (MSNA) were measured simultaneously in 7 subjects during saline, atropine and propranolol infusion. The measurements were repeated during spontaneous and fixed-frequency breathing, and apnea. The power spectra, phase coherence and couplings were calculated to characterise the variability and interactions within the cardiovascular system. Atropine reduced R-R interval variability (p < 0.05) in all three breathing conditions, reduced MSNA power during apnea and removed much of the significant coherence and couplings. Propranolol had smaller effect on the power of oscillations and did not change the number of significant interactions. Most notably, atropine reduced R-R interval power in the 0.145-0.6 Hz interval during apnea, which supports the hypothesis that the RSA is modulated by a mechanism other than the baroreflex. Atropine also reduced or made negative the phase shift between the systolic and diastolic pressure, indicating the cessation of baroreflex-dependent blood pressure variability. This result suggests that coherent respiratory oscillations in the blood pressure can be used for the non-invasive assessment of autonomic regulation.

Influence of Spontaneous and Mechanical Ventilation on Frequency-Based Measures of Heart Rate Variability

Frequency-based measures of heart rate variability have been shown to be a useful physiological marker in both clinical and research settings providing insight into the functioning of the autonomic nervous system. Ongoing interactions between the autonomic nervous system control of the heart and lung occurs during each ventilation cycle because of their anatomical position within the closed thoracic cavity. Mechanical ventilation and subsequent removal change the normal ventilator mechanics producing alterations in the tidal volume, intrathoracic pressure, and oxygen delivery. A noninvasive method called heart rate variability (HRV) can be used to evaluate this interaction during ventilation and can be quantified by applying frequency-based measures of the variability between heartbeats. Although HRV is a reliable method to measure alteration of the autonomic nervous system (ANS) function and cardiopulmonary interaction, there have been limited reports concerning the changes in the frequency-based measure of HRV during both spontaneous and mechanical ventilation. The purpose of this methodological study is therefore to describe the physiological influence of both spontaneous and mechanical ventilation on frequency-based measures of HRV.

Successful continuous positive airway pressure treatment reduces skin sympathetic nerve activity in patients with obstructive sleep apnea

BACKGROUND

Obstructive sleep apnea (OSA) is associated with cardiovascular diseases and increased sympathetic tone. We previously demonstrated that patients with OSA have increased skin sympathetic nerve activity (SKNA).

OBJECTIVE

The purpose of this study was to test the hypothesis that continuous positive airway pressure (CPAP) treatment reduces SKNA.

METHODS

The electrocardiogram, SKNA, and polysomnographic recording were recorded simultaneously in 9 patients with OSA. After baseline recording, CPAP titration was performed and the pressure was adjusted gradually for the optimal treatment, defined by reducing the apnea-hypopnea index (AHI) to ≤5/h. Otherwise the treatment was considered suboptimal (AHI > 5/h). Fast Fourier transform analyses were performed to investigate the frequency spectrum of SKNA.

RESULTS

There were very low frequency (VLF), low frequency (LF), and high frequency (HF) oscillations in SKNA. The HF oscillation matched the frequency of respiration. OSA episodes were more frequently associated with the VLF and LF than with the HF oscillations of SKNA. Compared with baseline, CPAP significantly decreased the arousal index and AHI and increased the minimal and mean oxyhemoglobin levels. Optimal treatment significantly increased the dominant frequency and reduced the heart rate, average SKNA (aSKNA), SKNA burst duration, and total burst area. The dominant frequency negatively correlated with aSKNA.

CONCLUSION

VLF, LF, and HF oscillations are observed in human SKNA recordings. Among them, VLF and LF oscillations are associated with OSA while HF oscillations are associated with normal breathing. CPAP therapy reduces aSKNA and shifts the frequency of SKNA oscillation from VLF or LF to HF.

Rostral ventrolateral medulla, retropontine region and autonomic regulations

The rostral half of the ventrolateral medulla (RVLM) and adjacent ventrolateral retropontine region (henceforth RVLMRP) have been divided into various sectors by neuroscientists interested in breathing or autonomic regulations. The RVLMRP regulates respiration, glycemia, vigilance and inflammation, in addition to blood pressure. It contains interoceptors that respond to acidification, hypoxia and intracranial pressure and its rostral end contains the retrotrapezoid nucleus (RTN) which is the main central respiratory chemoreceptor. Acid detection by the RTN is an intrinsic property of the principal neurons that is enhanced by paracrine influences from surrounding astrocytes and CO₂-dependent vascular constriction. RTN mediates the hypercapnic ventilatory response via complex projections to the respiratory pattern generator (CPG). The RVLM contributes to autonomic response patterns via differential recruitment of several subtypes of adrenergic (C1) and non-adrenergic neurons that directly innervate sympathetic and parasympathetic preganglionic neurons. The RVLM also innervates many brainstem and hypothalamic nuclei that contribute, albeit less directly, to autonomic responses. All lower brainstem noradrenergic clusters including the locus coeruleus are among these targets. Sympathetic tone to the circulatory system is regulated by subsets of presympathetic RVLM neurons whose activity is continuously restrained by the baroreceptors and modulated by the respiratory CPG. The inhibitory input from baroreceptors and the excitatory input from the respiratory CPG originate from neurons located in or close to the rhythm generating region of the respiratory CPG (preBötzinger complex).

Interictal autonomic dysfunction

PURPOSE OF REVIEW

Epilepsy is associated with autonomic dysfunction. Here, we provide an up-to-date review on measures of interictal autonomic function, focusing on heart rate variability (HRV), baroreflex sensitivity (BRS) and electrodermal activity (EDA).

RECENT FINDINGS

Resting HRV, BRS and EDA are altered in patients with epilepsy compared with healthy controls. A larger body of work is available for HRV compared with BRS and EDA, and points to interictal HRV derangements across a wide range of epilepsies, including focal, generalized, and combined generalized and focal epilepsies. HRV alterations are most pronounced in temporal lobe epilepsy, Dravet syndrome and drug-resistant and chronic epilepsies. There are conflicting data on the effect of antiseizure medications on measures of interictal autonomic function. However, carbamazepine has been associated with decreased HRV. Epilepsy surgery and vagus nerve stimulation do not appear to have substantial impact on measures of interictal autonomic function but well designed studies are lacking.

SUMMARY

Patients with epilepsy, particularly those with longstanding uncontrolled seizures, have measurable alterations of resting autonomic function. These alterations may be relevant to the increased risk of premature mortality in epilepsy, including sudden unexpected death in epilepsy, which warrants investigation in future research.

Impact of Altered Breathing Patterns on Interaction of EEG and Heart Rate Variability

Background

Altered pattern of respiration has been shown to affect both the cardiac as well as cortical activity, which is the basis of central-autonomic dual interaction concept. On the other hand, effect of this association between altered breathing with slow cortical activity, that is, electroencephalography (EEG) theta waves (associated with learning and relaxed alertness) on the cardiac autonomic balance is largely unclear.

Objective

The study aims to understand this interaction in response to altered respiratory patterns, for example, voluntary apnea, bradypnea, and tachypnea in terms of EEG and heart rate variability (HRV) correlates in normal healthy subjects.

Methods

This study was conducted on 32 adult male subjects. EEG from F3, F4, P3, P4, O1 and O2 cortical areas and Lead II electrocardiography for HRV analysis was continuously recorded during aforesaid respiratory interventions. Power spectral analysis of EEG for theta waves and HRV measures, that is, RMSSD, pNN50, HF, LF, and LF/HF was calculated as % change taking resting value as 100%.

Results

Apnea caused decrease in theta power, whereas an increase in LF/HF was observed in HRV. Bradypnea on the other hand, did not elicit any significant change in power of theta waves. However, decreased RMSSD and pNN50 were observed in HRV. Tachypnea led to increase in theta power with HRV depicting significantly decreased RMSSD and pNN50. Besides, significant correlation between EEG and HRV measures was found during tachypnea, which shifted toward posterior cortical sites as compared to resting condition.

Conclusion

Various altered respiratory patterns caused either depressed parasympathetic or increased sympathetic output, whereas increased theta power along with posterior shift of correlation between theta power and HRV measures observed during post tachypnea might be due to involvement of global brain areas due to respiration-coupled neuronal activity. Thus, a definite link between cortical activity and autonomic output in relation to altered respiratory patterns may be suggested.

How to Use Heart Rate Variability: Quantification of Vagal Activity in Toddlers and Adults in Long-Term ECG

Recent developments in noninvasive electrocardiogram (ECG) monitoring with small, wearable sensors open the opportunity to record high-quality ECG over many hours in an easy and non-burdening way. However, while their recording has been tremendously simplified, the interpretation of heart rate variability (HRV) data is a more delicate matter. The aim of this paper is to supply detailed methodological discussion and new data material in order to provide a helpful notice of HRV monitoring issues depending on recording conditions and study populations. Special consideration is given to the monitoring over long periods, across periods with different levels of activity, and in adults versus children. Specifically, the paper aims at making users aware of neglected methodological limitations and at providing substantiated recommendations for the selection of appropriate HRV variables and their interpretation. To this end, 30-h HRV data of 48 healthy adults (18–40 years) and 47 healthy toddlers (16–37 months) were analyzed in detail. Time-domain, frequency-domain, and nonlinear HRV variables were calculated after strict signal preprocessing, using six different high-frequency band definitions including frequency bands dynamically adjusted for the individual respiration rate. The major conclusion of the in-depth analyses is that for most applications that implicate long-term monitoring across varying circumstances and activity levels in healthy individuals, the time-domain variables are adequate to gain an impression of an individual’s HRV and, thus, the dynamic adaptation of an organism’s behavior in response to the ever-changing demands of daily life. The sound selection and interpretation of frequency-domain variables requires considerably more consideration of physiological and mathematical principles. For those who prefer using frequency-domain variables, the paper provides detailed guidance and recommendations for the definition of appropriate frequency bands in compliance with their specific recording conditions and study populations.

Effect of Hyperventilation on Periodic Repolarization Dynamics

Heart and lung functions are closely connected, and the interaction is mediated by the autonomic nervous system. Hyperventilation has been demonstrated to especially activate its sympathetic branch. However, there is still a lack of methods to assess autonomic activity within this cardiorespiratory coupling. Periodic repolarization dynamics (PRD) is an ECG-based biomarker mirroring the effect of efferent cardiac sympathetic activity on the ventricular myocardium. Its calculation is based on beat-to-beat variations of the T wave vector (dT°). In the present study, we investigated the effects of a standardized hyperventilation maneuver on changes of PRD and its underlying dT° signal in 11 healthy subjects. In response to hyperventilation, dT° revealed a characteristic pattern and normalized dT° values increased significantly compared to baseline [0.063 (IQR 0.032) vs. 0.376 (IQR 0.093), p < 0.001] and recovery [0.082 (IQR 0.029) vs. 0.376 (IQR 0.093), p < 0.001]. During recovery, dT° remained on a higher level compared to baseline (p = 0.019). When calculating PRD, we found significantly increased PRD values after hyperventilation compared to baseline [3.30 (IQR 2.29) deg² vs. 2.76 (IQR 1.43) deg², p = 0.018]. Linear regression analysis revealed that the increase in PRD level was independent of heart rate (p = 0.63). Our pilot data provide further insights in the effect of hyperventilation on sympathetic activity associated repolarization instability.

Cardiorespiratory and autonomic function in epileptic seizures: A video-EEG monitoring study

PURPOSE

Seizure-induced cardiorespiratory and autonomic dysfunction has long been recognized, and growing evidence points to its implication in sudden unexpected death in epilepsy (SUDEP). However, a comprehensive understanding of cardiorespiratory function in the preictal, ictal, and postictal periods are lacking.

METHODS

We examined continuous cardiorespiratory and autonomic function in 157 seizures (18 convulsive and 139 nonconvulsive) from 70 consecutive patients who had a seizure captured on concurrent video-encephalogram (EEG) monitoring and polysomnography between February 1, 2012 and May 31, 2017. Heart and respiratory rates, heart rate variability (HRV), and oxygen saturation were assessed across four distinct periods: baseline (120 s), preictal (60 s), ictal, and postictal (300 s). Heart and respiratory rates were further followed for up to 60 min after seizure termination to assess return to baseline.

RESULTS

Ictal tachycardia occurred during both convulsive and nonconvulsive seizures, but the maximum rate was higher for convulsive seizures (mean: 138.8 beats/min, 95% confidence interval (CI): 125.3-152.4) compared with nonconvulsive seizures (mean: 105.4 beats/min, 95% CI: 101.2-109.6; p < 0.001). Convulsive seizures were associated with a lower ictal minimum respiratory rate (mean: 0 breaths/min, 95% CI: 0-0) compared with nonconvulsive seizures (mean: 11.0 breaths/min, 95% CI: 9.5-12.6; p < 0.001). Ictal obstructive apnea was associated with convulsive compared with nonconvulsive seizures. The low-frequency (LF) power band of ictal HRV was higher among convulsive seizures than nonconvulsive seizures (ratio of means (ROM): 2.97, 95% CI: 1.34-6.60; p = 0.008). Postictal tachycardia was substantially prolonged, characterized by a longer return to baseline for convulsive seizures (median: 60.0 min, interquartile range (IQR): 46.5-60.0) than nonconvulsive seizures (median: 0.26 min, IQR: 0.008-0.9; p < 0.001). For postictal hyperventilation, the return to baseline was longer in convulsive seizures (median: 25.3 min, IQR: 8.1-60) than nonconvulsive seizures (median: 1.0 min, IQR: 0.07-3.2; p < 0.001). The LF power band of postictal HRV was lower in convulsive seizures than nonconvulsive seizures (ROM: 0.33, 95% CI: 0.11-0.96; p = 0.043). Convulsive seizures with postictal generalized EEG suppression (PGES; n = 12) were associated with lower postictal heart and respiratory rate, and increased HRV, compared with those without (n = 6).

CONCLUSIONS

Profound cardiorespiratory and autonomic dysfunction associated with convulsive seizures may explain why these seizures carry the greatest risk of SUDEP.

Influence of sympathetic activation on myocardial contractility measured with ballistocardiography and seismocardiography during sustained end-expiratory apnea

Ballistocardiography (BCG) and seismocardiography (SCG) assess vibrations produced by cardiac contraction and blood flow, respectively, through micro-accelerometers and micro-gyroscopes. BCG and SCG kinetic energies (KE) and their temporal integrals (iK) during a single heartbeat are computed in linear and rotational dimensions. Our aim was to test the hypothesis that iK from BCG and SCG are related to sympathetic activation during maximal voluntary end-expiratory apnea. Multiunit muscle sympathetic nerve traffic [burst frequency (BF), total muscular sympathetic nerve activity (tMSNA)] was measured by microneurography during normal breathing and apnea (n = 28, healthy men). iK of BCG and SCG were simultaneously recorded in the linear and rotational dimension, along with oxygen saturation ([Formula: see text]) and systolic blood pressure (SBP). The mean duration of apneas was 25.4 ± 9.4 s. SBP, BF, and tMSNA increased during the apnea compared with baseline (P = 0.01, P = 0.002,and P = 0.001, respectively), whereas [Formula: see text] decreased (P = 0.02). At the end of the apnea compared with normal breathing, changes in iK computed from BCG were related to changes of tMSNA and BF only in the linear dimension (r = 0.85, P < 0.0001; and r = 0.72, P = 0.002, respectively), whereas changes in linear iK of SCG were related only to changes of tMSNA (r = 0.62, P = 0.01). We conclude that maximal end expiratory apnea increases cardiac kinetic energy computed from BCG and SCG, along with sympathetic activity. The novelty of the present investigation is that linear iK of BCG is directly and more strongly related to the rise in sympathetic activity than the SCG, mainly at the end of a sustained apnea, likely because the BCG is more affected by the sympathetic and hemodynamic effects of breathing cessation. BCG and SCG may prove useful to assess sympathetic nerve changes in patients with sleep disturbances.NEW & NOTEWORTHY Ballistocardiography (BCG) and seismocardiography (SCG) assess vibrations produced by cardiac contraction and blood flow, respectively, through micro-accelerometers and micro-gyroscopes. Kinetic energies (KE) and their temporal integrals (iK) during a single heartbeat are computed from the BCG and SCG waveforms in a linear and a rotational dimension. When compared with normal breathing, during an end-expiratory voluntary apnea, iK increased and was positively related to sympathetic nerve traffic rise assessed by microneurography. Further studies are needed to determine whether BCG and SCG can probe sympathetic nerve changes in patients with sleep disturbances.

The Effect of Breath Pacing on Task Switching and Working Memory

The cortical and subcortical circuit regulating both cognition and cardiac autonomic interactions are already well established. This circuit has mainly been analyzed from cortex to heart. Thus, the heart rate variability (HRV) is usually considered a reflection of cortical activity. In this paper, we investigate whether HRV changes affect cortical activity. Short-term local autonomic changes were induced by three breathing strategies: spontaneous (Control), normal (NB) and slow paced breathing (SB). We measured the performance in two cognition domains: executive functions and processing speed. Breathing maneuvres produced three clearly differentiated autonomic states, which preconditioned the cognitive tasks. We found that the SB significantly increased the HRV low frequency (LF) power and lowered the power spectral density (PSD) peak to 0.1[Formula: see text]Hz. Meanwhile, executive function was assessed by the working memory test, whose accuracy significantly improved after SB, with no significant changes in the response times. Processing speed was assessed by a multitasking test. Consistently, the proportion of correct answers (success rate) was the only dependent variable affected by short-term and long-term breath pacing. These findings suggest that accuracy, and not timing of these two cognitive domains would benefit from short-term SB in this study population.

Cardiac interaction between mother and infant: enhancement of heart rate variability

The vagal activity of infants is represented by heart rate variability (HRV) and associated with both growth and socioemotional development. The enhancement of an infant's vagal tone activity might be beneficial for development. This study explored whether HRV in infants aged 3-8 months can be enhanced by influencing HRV in mothers (40 dyads). The power of the low frequency (LF) component of maternal HRV was facilitated using slow-paced breathing. We investigated whether the change in maternal HRV affected the LF component in infants held by their mothers. In older infants (N = 14, 6-8 months) the LF power showed an increase during maternal paced breathing, whereas a delayed increase occurred after termination of maternal paced breathing in younger infants (N = 16, 3-5 months). These results show that the effects of maternal cardiac activity on the infant's HRV are age-dependent. This age-dependent reactivity of the infant's HRV could be due to the development of the inner model in infants which regulates physiological functions, including cardiac activity. This finding might help develop efficient methods for enhancing vagal nerve activity in infants.

Circadian Rhythms and Measures of CNS/Autonomic Interaction

The physiological role and relevance of the mechanisms sustaining circadian rhythms have been acknowledged. Abnormalities of the circadian and/or sleep-wakefulness cycles can result in major metabolic disorders or behavioral/professional inadequacies and stand as independent risk factors for metabolic, psychiatric, and cerebrovascular disorders and early markers of disease. Neuroimaging and clinical evidence have documented functional interactions between autonomic (ANS) and CNS structures that are described by a concept model (Central Autonomic Network) based on the brain-heart two-way interplay. The circadian rhythms of autonomic function, ANS-mediated processes, and ANS/CNS interaction appear to be sources of variability adding to a variety of environmental factors, and may become crucial when considering the ANS major role in internal environment constancy and adaptation that are fundamental to homeostasis. The CNS/ANS interaction has not yet obtained full attention and systematic investigation remains overdue.

Multiscale Information Decomposition Dissects Control Mechanisms of Heart Rate Variability at Rest and During Physiological Stress

Heart rate variability (HRV; variability of the RR interval of the electrocardiogram) results from the activity of several coexisting control mechanisms, which involve the influence of respiration (RESP) and systolic blood pressure (SBP) oscillations operating across multiple temporal scales and changing in different physiological states. In this study, multiscale information decomposition is used to dissect the physiological mechanisms related to the genesis of HRV in 78 young volunteers monitored at rest and during postural and mental stress evoked by head-up tilt (HUT) and mental arithmetics (MA). After representing RR, RESP and SBP at different time scales through a recently proposed method based on multivariate state space models, the joint information transfer TRESP,SBP→RR is decomposed into unique, redundant and synergistic components, describing the strength of baroreflex modulation independent of respiration (USBP→RR), nonbaroreflex (URESP→RR) and baroreflex-mediated (RRESP,SBP→RR) respiratory influences, and simultaneous presence of baroreflex and nonbaroreflex respiratory influences (SRESP,SBP→RR), respectively. We find that fast (short time scale) HRV oscillations—respiratory sinus arrhythmia—originate from the coexistence of baroreflex and nonbaroreflex (central) mechanisms at rest, with a stronger baroreflex involvement during HUT. Focusing on slower HRV oscillations, the baroreflex origin is dominant and MA leads to its higher involvement. Respiration influences independent on baroreflex are present at long time scales, and are enhanced during HUT.

A Heartbeat Away From Consciousness: Heart Rate Variability Entropy Can Discriminate Disorders of Consciousness and Is Correlated With Resting-State fMRI Brain Connectivity of the Central Autonomic Network

Background: Disorders of consciousness are challenging to diagnose, with inconsistent behavioral responses, motor and cognitive disabilities, leading to approximately 40% misdiagnoses. Heart rate variability (HRV) reflects the complexity of the heart-brain two-way dynamic interactions. HRV entropy analysis quantifies the unpredictability and complexity of the heart rate beats intervals. We here investigate the complexity index (CI), a score of HRV complexity by aggregating the non-linear multi-scale entropies over a range of time scales, and its discriminative power in chronic patients with unresponsive wakefulness syndrome (UWS) and minimally conscious state (MCS), and its relation to brain functional connectivity. Methods: We investigated the CI in short (CIs) and long (CIl) time scales in 14 UWS and 16 MCS sedated. CI for MCS and UWS groups were compared using a Mann-Whitney exact test. Spearman's correlation tests were conducted between the Coma Recovery Scale-revised (CRS-R) and both CI. Discriminative power of both CI was assessed with One-R machine learning model. Correlation between CI and brain connectivity (detected with functional magnetic resonance imagery using seed-based and hypothesis-free intrinsic connectivity) was investigated using a linear regression in a subgroup of 10 UWS and 11 MCS patients with sufficient image quality. Results: Higher CIs and CIl values were observed in MCS compared to UWS. Positive correlations were found between CRS-R and both CI. The One-R classifier selected CIl as the best discriminator between UWS and MCS with 90% accuracy, 7% false positive and 13% false negative rates after a 10-fold cross-validation test. Positive correlations were observed between both CI and the recovery of functional connectivity of brain areas belonging to the central autonomic networks (CAN). Conclusion: CI of MCS compared to UWS patients has high discriminative power and low false negative rate at one third of the estimated human assessors' misdiagnosis, providing an easy, inexpensive and non-invasive diagnostic tool. CI reflects functional connectivity changes in the CAN, suggesting that CI can provide an indirect way to screen and monitor connectivity changes in this neural system. Future studies should assess the extent of CI's predictive power in a larger cohort of patients and prognostic power in acute patients.

Gastric cooling and menthol cause an increase in cardiac parasympathetic efferent activity in healthy adult human volunteers

NEW FINDINGS

What is the central question of this study? How do gastric stretch and gastric cooling stimuli affect cardiac autonomic control? What is the main finding and its importance? Gastric stretch causes an increase in cardiac sympathetic activity. Stretch combined with cold stimulation result in an elimination of the sympathetic response to stretch and an increase in cardiac parasympathetic activity, in turn resulting in a reduction in heart rate. Gastric cold stimulation causes a shift in sympathovagal balance towards parasympathetic dominance. The cold-induced bradycardia has the potential to decrease cardiac workload, which might be significant in individuals with cardiovascular pathologies.

ABSTRACT

Gastric distension increases blood pressure and heart rate in young, healthy humans, but little is known about the effect of gastric stretch combined with cooling. We used a randomized crossover study to assess the cardiovascular responses to drinking 300 ml of ispaghula husk solution at either 6 or 37°C in nine healthy humans (age 24.08 ± 9.36 years) to establish the effect of gastric stretch with and without cooling. The effect of consuming peppermint oil capsules to activate cold thermoreceptors was also investigated. The ECG, respiratory movements and continuous blood pressure were recorded during a 5 min baseline period, followed by a 115 min post-drink period, during which 5 min epochs of data were recorded. Cardiac autonomic activity was assessed using time and frequency domain analyses of respiratory sinus arrhythmia to quantify parasympathetic autonomic activity, and corrected QT (QTc) interval analysis to quantify sympathetic autonomic activity. Gastric stretch only caused a significant reduction in QTc interval lasting up to 15 min, with a concomitant but non-significant increase in heart rate, indicating an increased sympathetic cardiac tone. The additional effect of gastric cold stimulation was significantly to reduce heart rate for up to 15 min, elevate indicators of cardiac parasympathetic tone and eliminate the reduction in QTc interval seen with gastric stretch only. Stimulation of gastric cold thermoreceptors with menthol also caused a significant reduction in heart rate and concomitant increase in the root mean square of successive differences. These findings indicate that gastric cold stimulation causes a shift in the sympathovagal balance of cardiac control towards a more parasympathetic dominant pattern.

THE IMPACT OF VALVULAR PATHOLOGIES ON HEART RATE, THE SECOND HEART SOUND SPLIT, AND SYSTOLIC PULMONARY ARTERIAL PRESSURE

The heart rate increases during inspiration and decreases during expiration; the study of this variation and the change of the second heart sound split (a change related to inspiration and expiration) can determine at what time in a cardiac cycle is the inspiration and the expiration. It would also be interesting to study the variation in systolic pulmonary artery pressure (SPAP) estimated over several cardiac cycles and to understand its evolution as its variation is related to the pulmonary valve, on the one hand, and inspiration and expiration, on the other hand. The algorithm developed based on the Hilbert transform and the energy of Shannon gives the second heart sound split. The SPAP will be estimated from spectral parameters of the second heart S2. The results show an excellent performance of the algorithm proposed to extract different information on the variation of heart rate. The results of the change in pressure and split are encouraging and promising for the use of the proposed method in a clinical context of hypertension in non-invasive pulmonary pathways, for example.

The physiological effects of slow breathing in the healthy human

Slow breathing practices have been adopted in the modern world across the globe due to their claimed health benefits. This has piqued the interest of researchers and clinicians who have initiated investigations into the physiological (and psychological) effects of slow breathing techniques and attempted to uncover the underlying mechanisms. The aim of this article is to provide a comprehensive overview of normal respiratory physiology and the documented physiological effects of slow breathing techniques according to research in healthy humans. The review focuses on the physiological implications to the respiratory, cardiovascular, cardiorespiratory and autonomic nervous systems, with particular focus on diaphragm activity, ventilation efficiency, haemodynamics, heart rate variability, cardiorespiratory coupling, respiratory sinus arrhythmia and sympathovagal balance. The review ends with a brief discussion of the potential clinical implications of slow breathing techniques. This is a topic that warrants further research, understanding and discussion.

Slow breathing techniques have been used in asthma but are there effects in healthy individuals?http://ow.ly/gCPO30eQOPZ

Left Recumbent Position Decreases Heart Rate without Alterations in Cardiac Autonomic Nervous System Activity in Healthy Young Adults

Some studies have reported that recumbent position may have advantages in patients with heart disease and in pregnancy. However, it remains controversial whether recumbent position affects autonomic nervous system activity and hemodynamics in healthy adults. The aim of this study was to evaluate alterations in heart rate variability (HRV) and hemodynamics in the supine, left recumbent and right recumbent positions in healthy young adults. A total of 80 participants aged 22.8 ± 3.1 years were enrolled in this observational study. Fifty-eight volunteers (29 men and 29 women) maintained the supine position followed by the left and right recumbent positions, while electrocardiographic data were recorded for spectral analysis of HRV to assess cardiac vagal nerve and sympathetic nerve activities. The heart rate (HR) was significantly lower in the left recumbent position than in the other positions. There were no statistically significant differences in HRV among the three positions. Considering the possibility that the echographic procedure affects autonomic nervous system (ANS) activity, the other 22 participants (11 men and 11 women) underwent an echographic evaluation of hemodynamics in the heart and inferior vena cava (IVC) across the three positions. Although a low HR was also observed, there were no statistically significant differences in the IVC or the heart blood volume between the supine and the left recumbent positions. A postural change to the left recumbent position does not affect the cardiac blood circulation or ANS activity, though it does decrease HR in healthy young adults. This finding indicates that the lower HR in the left recumbent position is not attributable to the ANS activity.

Effects of different stresses on cardiac autonomic control and cardiovascular coupling

The objective of this study was to investigate the impacts of different stresses on time-varying autonomic reactivity and cardiovascular coupling. In total, 25 male subjects were recruited. RR intervals (RRI), systolic and diastolic blood pressure (SBP, DBP), stroke volume (SV), cardiac output (CO), and systemic vascular resistance (SVR) values were collected during rest, mental arithmetic task (MAT), and cold pressor test (CPT). Baroreflex sensitivity (BRS) was derived using the transfer function method. Continuous wavelet transformation of RRI was used to describe the time-variant patterns of autonomic neural activities. Wavelet cross correlation and phase synchronization were used to estimate the amplitude and phase coupling between RRI and SBP. MAT was characterized by increased heart rate (HR), SBP, DBP, and CO with decreased BRS attributable to prolonged parasympathetic withdrawal. Moreover, cardiovascular coupling was disrupted in MAT. These results indicated that baroreflex was depressed, and the top-down system started to take action under mental stress. In CPT, SBP, DBP, and SVR increased significantly, whereas HR and BRS remained unchanged. The increase of sympathetic activity was transient, and cardiovascular coupling did not change in CPT. Intriguingly, the frequency of the maximum cross-correlation coefficient in the low-frequency band (0.04-0.15 Hz) was significantly decreased in CPT, which may be due to the change of resonance frequency of the baroreflex loop.NEW & NOTEWORTHY The study is the first to compare the time-variant pattern of autonomic nervous activities and cardiovascular coupling between the mental arithmetic task (MAT) and the cold pressor test (CPT). Our results demonstrated that MAT and CPT elicited different time-varying patterns of autonomic neural activities and cardiovascular synchronization. Both the amplitude and phase consistency of blood pressure and heart rate decreased in MAT. CPT may affect the harmonic frequency of the baroreflex loop.

A Single Session of Neuromuscular Electrical Stimulation Enhances Vascular Endothelial Function and Peripheral Blood Circulation in Patients With Acute Myocardial Infarction

This study aimed to investigate whether a single session of neuromuscular electrical stimulation (NMES) can enhance vascular endothelial function and peripheral blood circulation in patients with acute myocardial infarction (AMI). Thirty-four male patients with AMI were alternately assigned to 2 groups, and received NMES with muscle contraction (NMES group, n = 17) or without muscle contraction (control group, n = 17) after admission. NMES was performed for quadriceps and gastrocnemius muscles of both legs for 30 minutes. We measured systolic blood pressure as a parameter of cardiovascular responses and the low-frequency component of blood pressure variability as an index of sympathetic activity. Reactive hyperemia peripheral arterial tonometry (RH-PAT) index and transcutaneous oxygen pressure in foot (Foot-tcPO₂) were also measured as parameters of vascular endothelial function and peripheral blood circulation, respectively. All patients completed the study without severe adverse events. Systolic blood pressure and the low-frequency component increased significantly during the NMES session in both groups (P < 0.01 and P < 0.05, respectively). However, elevation from systolic blood pressure at rest was < 10 mmHg in both groups. In the NMES group, the RH-PAT index and Foot-tcPO₂ increased significantly after NMES (P < 0.05 and P < 0.001, respectively). No significant changes were observed in these parameters throughout the session in the control group. In conclusion, a single session of NMES with muscle contraction enhanced vascular endothelial function, leading to improvement in peripheral blood circulation without inducing excessive cardiovascular and autonomic responses in patients with AMI (UMIN000014196).

Respiratory modulation of human autonomic function on Earth

KEY POINTS

We studied healthy supine astronauts on Earth with electrocardiogram, non-invasive arterial pressure, respiratory carbon dioxide concentrations, breathing depth and sympathetic nerve recordings. The null hypotheses were that heart beat interval fluctuations at usual breathing frequencies are baroreflex mediated, that they persist during apnoea, and that autonomic responses to apnoea result from changes of chemoreceptor, baroreceptor or lung stretch receptor inputs. R-R interval fluctuations at usual breathing frequencies are unlikely to be baroreflex mediated, and disappear during apnoea. The subjects' responses to apnoea could not be attributed to changes of central chemoreceptor activity (hypocapnia prevailed); altered arterial baroreceptor input (vagal baroreflex gain declined and muscle sympathetic nerve burst areas, frequencies and probabilities increased, even as arterial pressure climbed to new levels); or altered pulmonary stretch receptor activity (major breathing frequency and tidal volume changes did not alter vagal tone or sympathetic activity). Apnoea responses of healthy subjects may result from changes of central respiratory motoneurone activity.

ABSTRACT

We studied eight healthy, supine astronauts on Earth, who followed a simple protocol: they breathed at fixed or random frequencies, hyperventilated and then stopped breathing, as a means to modulate and expose to view important, but obscure central neurophysiological mechanisms. Our recordings included the electrocardiogram, finger photoplethysmographic arterial pressure, tidal volume, respiratory carbon dioxide concentrations and peroneal nerve muscle sympathetic activity. Arterial pressure, vagal tone and muscle sympathetic outflow were comparable during spontaneous and controlled-frequency breathing. Compared with spontaneous, 0.1 and 0.05 Hz breathing, however, breathing at usual frequencies (∼0.25 Hz) lowered arterial baroreflex gain, and provoked smaller arterial pressure and R-R interval fluctuations, which were separated by intervals that were likely to be too short and variable to be attributed to baroreflex physiology. R-R interval fluctuations at usual breathing frequencies disappear during apnoea, and thus cannot provide evidence for the existence of a central respiratory oscillation. Apnoea sets in motion a continuous and ever changing reorganization of the relations among stimulatory and inhibitory inputs and autonomic outputs, which, in our study, could not be attributed to altered chemoreceptor, baroreceptor, or pulmonary stretch receptor activity. We suggest that responses of healthy subjects to apnoea are driven importantly, and possibly prepotently, by changes of central respiratory motoneurone activity. The companion article extends these observations and asks the question, Might terrestrial responses to our 20 min breathing protocol find expression as long-term neuroplasticity in serial measurements made over 20 days during and following space travel?

Cardiorespiratory interactions: Noncontact assessment using laser Doppler vibrometry

The application of a noncontact physiological recording technique, based on the method of laser Doppler vibrometry (LDV), is described. The effectiveness of the LDV method as a physiological recording modality lies in the ability to detect very small movements of the skin, associated with internal mechanophysiological activities. The method is validated for a range of cardiovascular variables, extracted from the contour of the carotid pulse waveform as a function of phase of the respiration cycle. Data were obtained from 32 young healthy participants, while resting and breathing spontaneously. Individual beats were assigned to four segments, corresponding with inspiration and expiration peaks and transitional periods. Measures relating to cardiac and vascular dynamics are shown to agree with the pattern of effects seen in the substantial body of literature based on human and animal experiments, and with selected signals recorded simultaneously with conventional sensors. These effects include changes in heart rate, systolic time intervals, and stroke volume. There was also some evidence for vascular adjustments over the respiration cycle. The effectiveness of custom algorithmic approaches for extracting the key signal features was confirmed. The advantages of the LDV method are discussed in terms of the metrological properties and utility in psychophysiological research. Although used here within a suite of conventional sensors and electrodes, the LDV method can be used on a stand-alone, noncontact basis, with no requirement for skin preparation, and can be used in harsh environments including the MR scanner.

Novel application of multi dynamic trend analysis as a sensitive tool for detecting the effects of aging and congestive heart failure on heart rate variability

The complex fluctuations in heart rate variability (HRV) reflect cardiac autonomic modulation and are an indicator of congestive heart failure (CHF). This paper proposes a novel nonlinear approach to HRV investigation, the multi dynamic trend analysis (MDTA) method, based on the empirical mode decomposition algorithm of the Hilbert-Huang transform combined with a variable-sized sliding-window method. Electrocardiographic signal data obtained from the PhysioNet database were used. These data were from subjects with CHF (mean age = 59.4 ± 8.4), an age-matched elderly healthy control group (59.3 ± 10.6), and a healthy young group (30.3 ± 4.8); the HRVs of these subjects were processed using the MDTA method, time domain analysis, and frequency domain analysis. Among all HRV parameters, the MDTA absolute value slope (MDTS) and MDTA deviation (MDTD) exhibited the greatest area under the curve (AUC) of the receiver operating characteristics in distinguishing between the CHF group and the healthy controls (AUC = 1.000) and between the healthy elderly subject group and the young subject group (AUC = 0.834 ± 0.067 for MDTS; 0.837 ± 0.066 for MDTD). The CHF subjects presented with lower MDTA indices than those of the healthy elderly subject group. Furthermore, the healthy elderly subjects exhibited lower MDTA indices than those of the young controls. The MDTA method can adaptively and automatically identify the intrinsic fluctuation on variable temporal and spatial scales when investigating complex fluctuations in the cardiac autonomic regulation effects of aging and CHF.

Dynamic response of cardiac autonomic nervous system activity to habitual exercise during gradual variation of breathing frequency

The purpose of this study is to measure cardiac autonomic nervous system activity during breathing control with gradual alteration of the frequency between habitual exercise and sedentary young male subjects. In this study, to evaluate CANS activity, Tone-Entropy analysis, which is based on statistical property of acceleration between consecutive R-R intervals, was used. Sixteen healthy young male subjects (21.6+/-1.4yrs) were participated in these experiments and their R-R interval sequences were recorded. The controlled breathing trials let the subjects synchronize their breathing frequency ranging 3 to 30 breathing per minute. After that, breathing frequency was gradually and reversely decreased from 30 to 3 breathing per minute. Before and after the breathing controlled trials, 5 minute voluntary breathing trials were performed. Our results showed that total CANS activities of HE group were activated more than those of SE group in the entire sections and also that, as compared with HE group, maximum of average HR in SE group was appeared at 30 breathing per minute and it is recognized that the statistically significant difference between HE and SE group was shown. In conclusion, our results suggest that efficiency of cardiac function on habitual exercise in breathing control may be quantitatively and graphically evaluated with HR and Tone-Entropy analysis without any physical stimulation.

Effects of Effortful Swallow on Cardiac Autonomic Regulation

Swallowing-induced changes in heart rate have been recently reported. However, it is not apparent the responses of heart rate variability (HRV) elicited by effortful swallow maneuver. We investigated the acute effects of effortful swallowing maneuver on HRV. This study was performed on 34 healthy women between 18 and 35 years old. We assessed heart rate variability in the time (SDNN, RMSSD, and pNN50) and frequency (HF, LF, and LF/HF ratio) domains and, visual analysis through the Poincaré plot. The subjects remained at rest for 5 min during spontaneous swallowing and then performed effortful swallowing for 5 min. HRV was analyzed during spontaneous and effortful swallowing. We found no significant differences for SDNN, pNN50, RMSSD, HF in absolute units (ms(2)). There is a trend for increase of LF in absolute (p = 0.05) and normalized (p = 0.08) units during effortful swallowing. HF in normalized units reduced (p = 0.02) during effortful swallowing and LF/HF ratio (p = 0.03) increased during effortful swallowing. In conclusion effortful swallow maneuver in healthy women increased sympathetic cardiac modulation, indicating a cardiac overload.

Two separable mechanisms are responsible for mental stress effects on high frequency heart rate variability: an intra-individual approach in a healthy and a diabetic sample

Central withdrawal of parasympathetic cardiac control and increased respiratory frequency represent two important determinants of reduced respiratory-related heart rate variability (HRV). However, studies are missing to disentangle their relative contribution during mental stress. Healthy subjects (n=10) and type 2 diabetic patients (n=8), the latter with evidence of cardiac autonomic neuropathy, participated in this study. Using an intra-individual approach, high-frequency (hf) HRV was assessed for spontaneous (during rest and mental stress) and paced breathing (0.15, 0.2, 0.25, 0.3, 0.35, 0.4 and 0.45 Hz; randomized sequence). Mental stress was induced by a challenging reaction time task. Effects of respiratory frequency on hf HRV were individually predicted by paced breathing data. Mental stress decreased hf HRV (p<.001), and increased respiratory frequency (p=.01). Individual prediction of hf HRV by stress respiratory frequency resulted in lower values (p=.02) than observed during rest, indicating that respiratory stress effects were sufficient to reduce hf HRV. However, observed hf HRV values during stress were even lower (p<.001). These results indicate that hf HRV reductions during stress can only partly be explained by concomitant respiratory frequency changes. This effect is detectable in healthy subjects and in patients with evidence of diabetic cardiac autonomic neuropathy.

Cardiovascular and cardiorespiratory phase synchronization in normovolemic and hypovolemic humans

We investigated whether and how cardiovascular and cardiorespiratory phase synchronization would respond to changes in hydration status and orthostatic stress. Four men and six women were tested during graded head-up tilt (HUT) in both euhydration and dehydration (DEH) conditions. Continuous R-R intervals (RRI), systolic blood pressure (SBP) and respiration were investigated in low (LF 0.04-0.15 Hz) and high (HF 0.15-0.4 Hz) frequency ranges using a phase synchronization index (λ) ranging from 0 (complete lack of interaction) to 1 (perfect interaction) and a directionality index (d), where a positive value of d reflects oscillator 1 driving oscillator 2, and a negative value reflects the opposite driving direction. Surrogate data analysis was used to exclude relationships that occurred by chance. In the LF range, respiration was not synchronized with RRI or SBP, whereas RRI and SBP were phase synchronized. In the HF range, phases among all variables were synchronized. DEH reduced λ among all variables in the HF and did not affect λ between RRI and SBP in the LF region. DEH reduced d between RRI and SBP in the LF and did not affect d among all variables in the HF region. Increasing λ and decreasing d between SBP and RRI were observed in the LF range during HUT. Decreasing λ between SBP and RRI, respiration and RRI, and decreasing d between respiration and SBP were observed in the HF range during HUT. These results show that orthostatic stress disassociated interactions among RRI, SBP and respiration, and that DEH exacerbated the disconnection.

Mapping the end-tidal CO2 response function in the resting-state BOLD fMRI signal: spatial specificity, test-retest reliability and effect of fMRI sampling rate

The blood oxygenation level dependent (BOLD) signal measures brain function indirectly through physiological processes and hence is susceptible to global physiological changes. Specifically, fluctuations in end-tidal CO2 (PETCO2), in addition to cardiac rate variation (CRV), and respiratory volume per time (RVT) variations, have been known to confound the resting-state fMRI (rs-fMRI) signal. Previous studies addressed the resting-state fMRI response function to CRV and RVT, but no attempt has been made to directly estimate the voxel-wise response function to PETCO2. Moreover, the potential interactions among PETCO2, CRV, and RVT necessitate their simultaneous inclusion in a multi-regression model to estimate the PETCO2 response. In this study, we use such a model to estimate the voxel-wise PETCO2 response functions directly from rs-fMRI data of nine healthy subjects. We also characterized the effect of sampling rate (TR=2seconds vs. 323ms) on the temporal and spatial variability of the PETCO2 response function in addition to that of CRV and RVT. In addition, we assess the test-retest reproducibility of the response functions to PETCO2, CRV and RVT. We found that despite overlaps across their spatial patterns, PETCO2 explains a unique portion of the rs-fMRI signal variance compared to RVT and CRV. We also found the shapes of the estimated responses are very similar between long- and short-TR data, although responses estimated from short-TR data have higher reproducibility.

Additive effect of simultaneously varying respiratory frequency and tidal volume on respiratory sinus arrhythmia

Our aims were to assess, in healthy young females and males, the effects of the linear joint variation of respiratory frequency (RF) and tidal volume (VT) on the logarithmic transformation of high-frequency power of RR intervals (lnHF). ECG and VT were recorded from 18 females and 20 males during three visually guided 30-s breathing maneuvers: linearly increasing RF (RFLI) at constant VT; linearly increasing VT (VTLI) followed by decreasing VT (VTLD) at fixed RF, and RFLI and VTLI-VTLD combined. VT of females was 20% smaller. Instantaneous RF and lnHF were computed from the time-frequency distributions of respiratory series and RR intervals. LnHF-RF and lnHF-VT relations were similar between genders. LnHF and RR intervals control-maneuver differences during combined maneuver were approximately equal to the sum of those of the independent maneuvers. LnHF-RFLI relation showed strong negative correlations in separated and combined conditions, with steeper slope in the latter (p < 0.001). LnHF-VTLI and lnHF-VTLD relations presented, in the independent maneuvers, three combinations of slopes of different sign, all with hysteresis, and in the combined maneuver, strong correlations with negative slope for VTLI and positive slope for VTLD, steeper (p < 0.001) and with greater hysteresis (p < 0.001) than the independent ones. LnHF responses to our fast, non-fatiguing and non-steady-state breathing maneuvers are: similar between genders; consistent attenuation due to RFLI, whether applied alone or combined; ambiguous and with hysteresis to independent VTLI-VTLD variations; systematic greater attenuation during RFLI combined with VTLI-VTLD, equal to the sum of the independent effects, indicating that there is no interference between them.

Heart rate variability biofeedback: how and why does it work?

In recent years there has been substantial support for heart rate variability biofeedback (HRVB) as a treatment for a variety of disorders and for performance enhancement (Gevirtz, 2013). Since conditions as widely varied as asthma and depression seem to respond to this form of cardiorespiratory feedback training, the issue of possible mechanisms becomes more salient. The most supported possible mechanism is the strengthening of homeostasis in the baroreceptor (Vaschillo et al., 2002; Lehrer et al., 2003). Recently, the effect on the vagal afferent pathway to the frontal cortical areas has been proposed. In this article, we review these and other possible mechanisms that might explain the positive effects of HRVB.

Cardiac autonomic testing and diagnosing heart disease. "A clinical perspective"

Background

Coronary heart disease (CHD) is a major health concern, affecting nearly half the middle-age population and responsible for nearly one-third of all deaths. Clinicians have responsibilities beyond diagnosing CHD, including risk stratification of patients for major adverse cardiac events (MACE), modifying the risks and treating the patient. In this first of a two-part review, identifying risk factors is reviewed, including more potential benefit from autonomic testing.

Methods

Traditional and non-traditional, and modifiable and non-modifiable risk factors for MACE where compared, including newer risk factors, such as inflammation, carotid intimal thickening, ankle-brachial index, CT calcium scoring, and autonomic function testing, specifically independent measurement of parasympathetic and sympathetic (P&S) activity.

Results

The Framingham Heart Study, and others, have identified traditional risk factors for the development of CHD. These factors effectively target high-risk patients, but a large number of individuals who will develop CHD and MACE are not identified. Many patients with CHD who appear to be well-managed by traditional therapies still experience MACE. In order to identify these patients, other possible risk factors have been explored. Advanced autonomic dysfunction, and its more severe form, cardiac autonomic neuropathy, have been strongly associated with an elevated risk of cardiac mortality and are diagnosable through P&S testing.

Conclusions

Independent measures of P&S activity, provides additional information and has the potential to incrementally add to risk assessment. This additional information enables physicians to (1) specifically target more high-risk patients and (2) titrate therapies, with autonomic testing guidance, in order to minimize risk of cardiac mortality and morbidity.

Cardiac autonomic testing and treating heart disease. "A clinical perspective"

Background

Coronary heart disease (CHD) is a major health concern, affecting nearly half the middle-age population and responsible for nearly one-third of all deaths. Clinicians have several major responsibilities beyond diagnosing CHD, such as risk stratification of patients for major adverse cardiac events (MACE) and treating risks, as well as the patient. This second of a two-part review series discusses treating risk factors, including autonomic dysfunction, and expected outcomes.

Methods

Therapies for treating cardiac mortality risks including cardiovascular autonomic neuropathy (CAN), are discussed.

Results

While risk factors effectively target high-risk patients, a large number of individuals who will develop complications from heart disease are not identified by current scoring systems. Many patients with heart conditions, who appear to be well-managed by traditional therapies, experience MACE. Parasympathetic and Sympathetic (P&S) function testing provides more information and has the potential to further aid doctors in individualizing and titrating therapy to minimize risk. Advanced autonomic dysfunction (AAD) and its more severe form cardiovascular autonomic neuropathy have been strongly associated with an elevated risk of cardiac mortality and are diagnosable through autonomic testing. This additional information includes patient-specific physiologic measures, such as sympathovagal balance (SB). Studies have shown that establishing and maintaining proper SB minimizes morbidity and mortality risk.

Conclusions

P&S testing promotes primary prevention, treating subclinical disease states, as well as secondary prevention, thereby improving patient outcomes through (1) maintaining wellness, (2) preventing symptoms and disorder and (3) treating subclinical manifestations (autonomic dysfunction), as well as (4) disease and symptoms (autonomic neuropathy).

Electrophysiological properties of rostral ventrolateral medulla presympathetic neurons modulated by the respiratory network in rats

The respiratory pattern generator modulates the sympathetic outflow, the strength of which is enhanced by challenges produced by hypoxia. This coupling is due to the respiratory-modulated presympathetic neurons in the rostral ventrolateral medulla (RVLM), but the underlining electrophysiological mechanisms remain unclear. For a better understanding of the neural substrates responsible for generation of this respiratory-sympathetic coupling, we combined immunofluorescence, single cell qRT-pCR, and electrophysiological recordings of the RVLM presympathetic neurons in in situ preparations from normal rats and rats submitted to a metabolic challenge produced by chronic intermittent hypoxia (CIH). Our results show that the spinally projected cathecholaminergic C1 and non-C1 respiratory-modulated RVLM presympathetic neurons constitute a heterogeneous neuronal population regarding the intrinsic electrophysiological properties, respiratory synaptic inputs, and expression of ionic currents, albeit all neurons presented persistent sodium current-dependent intrinsic pacemaker properties after synaptic blockade. A specific subpopulation of non-C1 respiratory-modulated RVLM presympathetic neurons presented enhanced excitatory synaptic inputs from the respiratory network after CIH. This phenomenon may contribute to the increased sympathetic activity observed in CIH rats. We conclude that the different respiratory-modulated RVLM presympathetic neurons contribute to the central generation of respiratory-sympathetic coupling as part of a complex neuronal network, which in response to the challenges produced by CIH contribute to respiratory-related increase in the sympathetic activity.

Cardiorespiratory dynamic response to mental stress: a multivariate time-frequency analysis

Mental stress is a growing problem in our society. In order to deal with this, it is important to understand the underlying stress mechanisms. In this study, we aim to determine how the cardiorespiratory interactions are affected by mental arithmetic stress and attention. We conduct cross time-frequency (TF) analyses to assess the cardiorespiratory coupling. In addition, we introduce partial TF spectra to separate variations in the RR interval series that are linearly related to respiration from RR interval variations (RRV) that are not related to respiration. The performance of partial spectra is evaluated in two simulation studies. Time-varying parameters, such as instantaneous powers and frequencies, are derived from the computed spectra. Statistical analysis is carried out continuously in time to evaluate the dynamic response to mental stress and attention. The results show an increased heart and respiratory rate during stress and attention, compared to a resting condition. Also a fast reduction in vagal activity is noted. The partial TF analysis reveals a faster reduction of RRV power related to (3 s) than unrelated to (30 s) respiration, demonstrating that the autonomic response to mental stress is driven by mechanisms characterized by different temporal scales.

Effects of respiratory time ratio on heart rate variability and spontaneous baroreflex sensitivity

Paced breathing is a frequently performed technique for cardiovascular autonomic studies. The relative timing of inspiration and expiration during paced breathing, however, is not consistent. We, therefore, examined whether indexes of heart rate variability and spontaneous baroreflex sensitivity would be affected by the respiratory time ratio that is set. We studied 14 healthy young adults who controlled their breathing rates to either 0.1 or 0.25 Hz in the supine and sitting positions. Four different inspiratory-to-expiratory time ratios (I/E) (uncontrolled, 1:1, 1:2, and 1:3) were examined for each condition in a randomized order. The results showed spectral indexes of heart rate variability and spontaneous baroreflex sensitivity were not influenced by the I/E that was set during paced breathing under supine and sitting positions. Porta's and Guzik's indexes of heart rate asymmetry were also not different at various I/E during 0.1-Hz breathing, but had larger values at 1:1 during 0.25-Hz breathing, although significant change was found in the sitting position only. At the same time, Porta's and Guzik's indexes obtained during 0.1-Hz breathing were greater than during 0.25-Hz breathing in both positions. The authors suggest that setting the I/E during paced breathing is not necessary when measuring spectral indexes of heart rate variability and spontaneous baroreflex sensitivity under the conditions used in this study. The necessity of paced breathing for the measurement of heart rate asymmetry, however, requires further investigation.

High-pass filter characteristics of the baroreflex--a comparison of frequency domain and pharmacological methods

Pharmacological methods to assess baroreflex sensitivity evoke supra-physiological blood pressure changes whereas computational methods use spontaneous fluctuations of blood pressure. The relationships among the different baroreflex assessment methods are still not fully understood. Although strong advocates for each technique exist, the differences between these methods need further clarification. Understanding the differences between pharmacological and spontaneous baroreflex methods could provide important insight into the baroreflex physiology. We compared the modified Oxford baroreflex gain and the transfer function modulus between spontaneous RR interval and blood pressure fluctuations in 18 healthy subjects (age: 39±10 yrs., BMI: 26±4.9). The transfer function was calculated over the low-frequency range of the RR interval and systolic blood pressure oscillations during random-frequency paced breathing. The average modified Oxford baroreflex gain was lower than the average transfer function modulus (15.7±9.2 ms/mmHg vs. 19.4±10.5 ms/mmHg, P<0.05). The difference between the two baroreflex measures within the individual subjects comprised a systematic difference (relative mean difference: 20.7%) and a random variance (typical error: 3.9 ms/mmHg). The transfer function modulus gradually increased with the frequency within the low-frequency range (LF), on average from 10.4±7.3 ms/mmHg to 21.2±9.8 ms/mmHg across subjects. Narrowing the zone of interest within the LF band produced a decrease in both the systematic difference (relative mean difference: 0.5%) and the random variance (typical error: 2.1 ms/mmHg) between the modified Oxford gain and the transfer function modulus. Our data suggest that the frequency dependent increase in low-frequency transfer function modulus between RR interval and blood pressure fluctuations contributes to both the systematic difference (bias) and the random variance (error) between the pharmacological and transfer function baroreflex measures. This finding suggests that both methodological and physiological factors underlie the observed disagreement between the pharmacological and the transfer function method. Thus both baroreflex measures contribute complementary information and can be considered valid methods for baroreflex sensitivity assessment.