Comparison of various techniques used to estimate spontaneous baroreflex sensitivity (the EuroBaVar study)

Vasopressin and cardiovascular autonomic adjustment in chronic hypertensive pregnancy

Chronic hypertensive pregnancy (CHP) is a growing health issue with unknown etiology. Vasopressin (VP), a nonapeptide synthesized in paraventricular (PVN) and supraoptic nucleus (SON), is a well-known neuroendocrine and autonomic modulator of the cardiovascular system, related to hypertension development. We quantified gene expression of VP and its receptors, V1aR and V1bR, within the PVN and SON in CHP and normal pregnancy, and assessed levels of secreted plasma VP. Also, we evaluated autonomic cardiovascular adaptations to CHP using spectral indices of blood pressure (BPV) and heart rate (HRV) short-term variability, and spontaneous baroreflex sensitivity (BRS). Experiments were performed in female spontaneously hypertensive rats (SHRs) and in normotensive Wistar rats (WRs). Animals were equipped with a radiotelemetry probe for continuous hemodynamic recordings before and during pregnancy. BPV, HRV and BRS were assessed using spectral analysis and the sequence method, respectively. Plasma VP was determined by ELISA whilst VP, V1aR, and V1bR gene expression was analyzed by real-time-quantitative PCR (RT-qPCR). The results show that non-pregnant SHRs exhibit greater VP, V1aR, and V1bR gene expression in both PVN and SON respectively, compared to Wistar dams. Pregnancy decreased VP gene expression in the SON of SHRs but increased it in the PVN and SON of WRs. Pregnant SHRs exhibited a marked drop in plasma VP concentration associated with BP normalization. This triggered marked tachycardia, heart rate variability increase, and BRS increase in pregnant SHRs. It follows that regardless of BP normalization in late pregnancy, SHRs exhibit cardiovascular vulnerability and compensate by recruiting vagal mechanisms. Pregnant SHR dams have reduced expression of VP in SON associated with increased V1bR expression, lower plasma VP, normal BP during late pregnancy and marked signs of enhanced sympathetic cardiac stimulation (increased HR and LFHR variability) and recruitment of vagal mechanisms (enhancement of BRS and HFHR variability).

Non-invasive parameters of autonomic function using beat-to-beat cardiovascular variations and arterial stiffness in hypertensive individuals: a systematic review

PURPOSE

Non-invasive, beat-to-beat variations in physiological indices provide an opportunity for more accessible assessment of autonomic dysfunction. The potential association between the changes in these parameters and arterial stiffness in hypertension remains poorly understood. This systematic review aims to investigate the association between non-invasive indicators of autonomic function based on beat-to-beat cardiovascular signals with arterial stiffness in individuals with hypertension.

METHODS

Four electronic databases were searched from inception to June 2022. Studies that investigated non-invasive parameters of arterial stiffness and autonomic function using beat-to-beat cardiovascular signals over a period of > 5min were included. Study quality was assessed using the STROBE criteria. Two authors screened the titles, abstracts, and full texts independently.

RESULTS

Nineteen studies met the inclusion criteria. A comprehensive overview of experimental design for assessing autonomic function in terms of baroreflex sensitivity and beat-to-beat cardiovascular variabilities, as well as arterial stiffness, was presented. Alterations in non-invasive indicators of autonomic function, which included baroreflex sensitivity, beat-to-beat cardiovascular variabilities and hemodynamic changes in response to autonomic challenges, as well as arterial stiffness, were identified in individuals with hypertension. A mixed result was found in terms of the association between non-invasive quantitative autonomic indices and arterial stiffness in hypertensive individuals. Nine out of 12 studies which quantified baroreflex sensitivity revealed a significant association with arterial stiffness parameters. Three studies estimated beat-to-beat heart rate variability and only one study reported a significant relationship with arterial stiffness indices. Three out of five studies which studied beat-to-beat blood pressure variability showed a significant association with arterial structural changes. One study revealed that hemodynamic changes in response to autonomic challenges were significantly correlated with arterial stiffness parameters.

CONCLUSIONS

The current review demonstrated alteration in autonomic function, which encompasses both the sympathetic and parasympathetic modulation of sinus node function and vasomotor tone (derived from beat-to-beat cardiovascular signals) in hypertension, and a significant association between some of these parameters with arterial stiffness. By employing non-invasive measurements to monitor changes in autonomic function and arterial remodeling in individuals with hypertension, we would be able to enhance our ability to identify individuals at high risk of cardiovascular disease. Understanding the intricate relationships among these cardiovascular variability measures and arterial stiffness could contribute toward better individualized treatment for hypertension in the future.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO ID: CRD42022336703. Date of registration: 12/06/2022.

Regional associations of white matter integrity and neurological, post-traumatic stress disorder and autonomic symptoms in Veterans with and without history of loss of consciousness in mild TBI

Background

Posttraumatic stress disorder (PTSD) and mild traumatic brain injury (mTBI) share overlapping symptom presentations and are highly comorbid conditions among Veteran populations. Despite elevated presentations of PTSD after mTBI, mechanisms linking the two are unclear, although both have been associated with alterations in white matter and disruptions in autonomic regulation. The present study aimed to determine if there is regional variability in white matter correlates of symptom severity and autonomic functioning in a mixed sample of Veterans with and without PTSD and/or mTBI (N = 77).

Methods

Diffusion-weighted images were processed to extract fractional anisotropy (FA) values for major white matter structures. The PTSD Checklist-Military version (PCL-M) and Neurobehavioral Symptom Inventory (NSI) were used to determine symptom domains within PTSD and mTBI. Autonomic function was assessed using continuous blood pressure and respiratory sinus arrythmia during a static, standing angle positional test. Mixed-effect models were used to assess the regional specificity of associations between symptom severity and white matter, with FA, global symptom severity (score), and white matter tract (tract) as predictors. Additional interaction terms of symptom domain (i.e., NSI and PCL-M subscales) and loss of consciousness (LoC) were added to evaluate potential moderating effects. A parallel analysis was conducted to explore concordance with autonomic functioning.

Results

Results from the two-way Score × Tract interaction suggested that global symptom severity was associated with FA in the cingulum angular bundle (positive) and uncinate fasciculus (negative) only, without variability by symptom domain. We also found regional specificity in the relationship between FA and autonomic function, such that FA was positively associated with autonomic function in all tracts except the cingulum angular bundle. History of LoC moderated the association for both global symptom severity and autonomic function.

Conclusions

Our findings are consistent with previous literature suggesting that there is significant overlap in the symptom presentation in TBI and PTSD, and white matter variability associated with LoC in mTBI may be associated with increased PTSD-spectra symptoms. Further research on treatment response in patients with both mTBI history and PTSD incorporating imaging and autonomic assessment may be valuable in understanding the role of brain injury in treatment outcomes and inform treatment design.

BaroWavelet: An R-based tool for dynamic baroreflex evaluation through wavelet analysis techniques

BACKGROUND AND OBJECTIVE

Baroreflex sensitivity constitutes an indicator of the function of the baroreceptor control mechanism of blood pressure levels. It can be computed after estimating heart rate and blood pressure variability. We propose a novel tool for the evaluation of baroreflex sensitivity using wavelet analysis methods. This tool, known as BaroWavelet, incorporates an algorithm proposal based on the analysis methodology of the RHRV software package, as well as other conventional techniques. Our objectives are to develop and evaluate the tool, by testing its ability to detect changes in baroreflex sensitivity in humans.

METHODS

The code for this tool was designed in the R programming environment and was organized into two analysis routines and a graphical interface. Simulated recordings of blood pressure and inter-beat intervals were employed for an initial evaluation of the tool in a controlled environment. Finally, similar recordings obtained during supine and orthostatic postural evaluations, from patients that belonged to the open-access EUROBAVAR data set, were analyzed.

RESULTS

BaroWavelet identified the scripted changes of the baroreflex sensitivity in the simulated data. The algorithm proposal was also able to better retain additional information regarding the dynamics of the baroreflex. In the EUROBAVAR subjects, baroreflex sensitivity components were significantly smaller during orthostatism when compared with the supine position.

CONCLUSIONS

BaroWavelet managed to characterize baroreflex dynamics from the recordings, which were consistent with the findings reported in the literature. This demonstrates its effectiveness to perform these analyses. We suggest that this tool may be of use in research and for the evaluation of baroreflex sensitivity with clinical and therapeutic purposes. The new tool is available at the official GitHub repository of the Autonomic Nervous System Unit of the University of Málaga (https://github.com/CIMES-USNA-UMA/BaroWavelet).

The degree of engagement of cardiac and sympathetic arms of the baroreflex does not depend on the absolute value and sign of arterial pressure variations

Objective.The percentages of cardiac and sympathetic baroreflex patterns detected via baroreflex sequence (SEQ) technique from spontaneous variability of heart period (HP) and systolic arterial pressure (SAP) and of muscle nerve sympathetic activity (MSNA) burst rate and diastolic arterial pressure (DAP) are utilized to assess the level of the baroreflex engagement. The cardiac baroreflex patterns can be distinguished in those featuring both HP and SAP increases (cSEQ++) and decreases (cSEQ--), while the sympathetic baroreflex patterns in those featuring a MSNA burst rate decrease and a DAP increase (sSEQ+-) and vice versa (sSEQ-+). The present study aims to assess the modifications of the involvement of the cardiac and sympathetic arms of the baroreflex with age and postural stimulus intensity.Approach.We monitored the percentages of cSEQ++ (%cSEQ++) and cSEQ-- (%cSEQ--) in 100 healthy subjects (age: 21-70 years, 54 males, 46 females), divided into five sex-balanced groups consisting of 20 subjects in each decade at rest in supine position and during active standing (STAND). We evaluated %cSEQ++, %cSEQ--, and the percentages of sSEQ+- (%sSEQ+-) and sSEQ-+ (%sSEQ-+) in 12 young healthy subjects (age 23 ± 2 years, 3 females, 9 males) undergoing incremental head-up tilt.Main results.We found that: (i) %cSEQ++ and %cSEQ-- decreased with age and increased with STAND and postural stimulus intensity; (ii) %sSEQ+- and %sSEQ-+ augmented with postural challenge magnitude; (iii) the level of cardiac and sympathetic baroreflex engagement did not depend on either the absolute value of arterial pressure or the direction of its changes.Significance.This study stresses the limited ability of the cardiac and sympathetic arms of the baroreflex in controlling absolute arterial pressure values and the equivalent ability of both positive and negative arterial pressure changes in soliciting them.

CardioRVAR: A New R Package and Shiny Application for the Evaluation of Closed-Loop Cardiovascular Interactions

CardioRVAR is a new R package designed for the complete evaluation of closed-loop cardiovascular interactions and baroreflex sensitivity estimated from continuous non-invasive heart rate and blood pressure recordings. In this work, we highlight the importance of this software tool in the context of human cardiovascular and autonomic neurophysiology. A summary of the main algorithms that CardioRVAR uses are reviewed, and the workflow of this package is also discussed. We present the results obtained from this tool after its application in three clinical settings. These results support the potential clinical and scientific applications of this tool. The open-source tool can be downloaded from a public GitHub repository, as well as its specific Shiny application, CardioRVARapp. The open-source nature of the tool may benefit the future continuation of this work.

Correlation among Poincare plot and traditional heart rate variability indices in adults with different risk levels of metabolic syndrome: a cross-sectional approach from Southern India

OBJECTIVES

Heart rate variability (HRV) is an important marker of cardiac autonomic modulation. Metabolic syndrome (MetS) can alter cardiac autonomic modulation, raising the risk of cardiovascular disease (CVD). Poincaré plot analysis (PPA) is a robust scatter plot-based depiction of HRV and carries similar information to the traditional HRV measures. However, no prior studies have examined the relationship between PPA and traditional HRV measures among different risk levels of MetS. We evaluated the association between the Poincare plot and traditional heart rate variability indices among adults with different risk levels of MetS.

METHODS

We measured anthropometric data and collected fasting blood samples to diagnose MetS. The MetS risk was assessed in 223 participants based on the number of MetS components and was classified as control (n=64), pre-MetS (n=49), MetS (n=56), and severe MetS (n=54). We calculated the Poincaré plot (PP) and traditional HRV measures from a 5 min HRV recording.

RESULTS

Besides the traditional HRV measures, we found that various HRV indices of PPA showed significant differences among the groups. The severe MetS group had significantly lower S (total HRV), SD1 (short-term HRV), SD2 (long-term HRV), and higher SD2/SD1. The values of S, SD1, SD2, and SD2/SD1 were significantly correlated with most traditional HRV measures.

CONCLUSIONS

We found gradual changes in HRV patterns as lower parasympathetic and higher sympathetic activity alongside the rising number of MetS components. The HRV indices of PPA integrating the benefits of traditional HRV indices distinguish successfully between different risk levels of MetS and control subjects.

Spectral decomposition of cerebrovascular and cardiovascular interactions in patients prone to postural syncope and healthy controls

We present a framework for the linear parametric analysis of pairwise interactions in bivariate time series in the time and frequency domains, which allows the evaluation of total, causal and instantaneous interactions and connects time- and frequency-domain measures. The framework is applied to physiological time series to investigate the cerebrovascular regulation from the variability of mean cerebral blood flow velocity (CBFV) and mean arterial pressure (MAP), and the cardiovascular regulation from the variability of heart period (HP) and systolic arterial pressure (SAP). We analyze time series acquired at rest and during the early and late phase of head-up tilt in subjects developing orthostatic syncope in response to prolonged postural stress, and in healthy controls. The spectral measures of total, causal and instantaneous coupling between HP and SAP, and between MAP and CBFV, are averaged in the low-frequency band of the spectrum to focus on specific rhythms, and over all frequencies to get time-domain measures. The analysis of cardiovascular interactions indicates that postural stress induces baroreflex involvement, and its prolongation induces baroreflex dysregulation in syncope subjects. The analysis of cerebrovascular interactions indicates that the postural stress enhances the total coupling between MAP and CBFV, and challenges cerebral autoregulation in syncope subjects, while the strong sympathetic activation elicited by prolonged postural stress in healthy controls may determine an increased coupling from CBFV to MAP during late tilt. These results document that the combination of time-domain and spectral measures allows us to obtain an integrated view of cardiovascular and cerebrovascular regulation in healthy and diseased subjects.

Advanced Cross-Correlation Function Application to Identify Arterial Baroreflex Sensitivity Variations From Healthy to Diabetes Mellitus

Diabetes mellitus (DM) is a chronic disease characterized by elevated blood glucose levels, which leads over time to serious damage to the heart, blood vessels, eyes, kidneys, and nerves. DM is of two types–types 1 or 2. In type 1, there is a problem with insulin secretion, and in type 2–insulin resistance. About 463 million people worldwide have diabetes, and 80% of the majority live in low- and middle-income countries, and 1.5 million deaths are directly attributed to diabetes each year. Autonomic neuropathy (AN) is one of the common diabetic complications, leading to failure in blood pressure (BP) control and causing cardiovascular disease. Therefore, early detection of AN becomes crucial to optimize treatment. We propose an advanced cross-correlation function (ACCF) between BP and heart rate with suitable threshold parameters to analyze and detect early changes in baroreflex sensitivity (BRS) in DM with AN (DM+). We studied heart rate (HR) and systolic BP responses during tilt in 16 patients with diabetes mellitus only (DM−), 19 diabetes mellitus with autonomic dysfunction (DM+), and 10 healthy subjects. The ACCF analysis revealed that the healthy and DM groups had different filtered percentages of significant maximum cross-correlation function (CCF) value (p < 0.05), and the maximum CCF value after thresholds was significantly reduced during tilt in the DM+ group (p < 0.05). The maximum CCF index, a parameter for the phase between HR and BP, separated the healthy group from the DM groups (p < 0.05). Due to the maximum CCF index in DM groups being located in the positive range and significantly different from healthy ones, it could be speculated that BRS dysfunction in DM and AN could cause a phase change from lead to lag. ACCF could detect and separate DM+ from DM groups. This fact could represent an advantage of the ACCF algorithm. A common cross-correlation analysis was not easy to distinguish between DM− and DM+. This pilot study demonstrates that ACCF analysis with suitable threshold parameters could explore hidden changes in baroreflex control in DM+ and DM−. Furthermore, the superiority of this ACCF algorithm is useful in distinguishing whether AN is present or not in DM.

A Novel Method for Baroreflex Sensitivity Estimation Using Modulated Gaussian Filter

The evaluation of baroreflex sensitivity (BRS) has proven to be critical for medical applications. The use of α indices by spectral methods has been the most popular approach to BRS estimation. Recently, an algorithm termed Gaussian average filtering decomposition (GAFD) has been proposed to serve the same purpose. GAFD adopts a three-layer tree structure similar to wavelet decomposition but is only constructed by Gaussian windows in different cutoff frequency. Its computation is more efficient than that of conventional spectral methods, and there is no need to specify any parameter. This research presents a novel approach, referred to as modulated Gaussian filter (modGauss) for BRS estimation. It has a more simplified structure than GAFD using only two bandpass filters of dedicated passbands, so that the three-level structure in GAFD is avoided. This strategy makes modGauss more efficient than GAFD in computation, while the advantages of GAFD are preserved. Both GAFD and modGauss are conducted extensively in the time domain, yet can achieve similar results to conventional spectral methods. In computational simulations, the EuroBavar dataset was used to assess the performance of the novel algorithm. The BRS values were calculated by four other methods (three spectral approaches and GAFD) for performance comparison. From a comparison using the Wilcoxon rank sum test, it was found that there was no statistically significant dissimilarity; instead, very good agreement using the intraclass correlation coefficient (ICC) was observed. The modGauss algorithm was also found to be the fastest in computation time and suitable for the long-term estimation of BRS. The novel algorithm, as described in this report, can be applied in medical equipment for real-time estimation of BRS in clinical settings.

Arterial Baroreceptor Physiology: Differences Between Normal Subjects and Pediatric Patients with Postural Tachycardia and Neurocardiogenic Syncope

The arterial baroreceptor reflex in children and adolescents has not been well studied in the current literature with a lack of agreed upon normal values, particularly in postural orthostatic tachycardia syndrome (POTS) or neurocardiogenic syncope (NCS). We used the sequence method and head-up tilt test (HUTT) to evaluate baroreceptor function in 3 phases: baseline supine position for 10 min, head-up position at 70° for 30 min or until syncope, and post-tilt supine reposition for 10 min. We measured the number of baroreceptor events, baroreceptor effectiveness index (BEI), and the magnitude of sensitivity of the events at each phase of HUTT. We studied 198 individuals (49 normal subjects, 67 POTS, 82 NCS) with age ranges from 8 to 21 years. The data show a statistically significant decrease in slope and BEI in patients with POTS and NCS during the head-up phase, with an increase in activity in the lag 1 and 2 portions of all phases in patients with POTS. This study provides terminology to describe baroreceptor function and identifies the slope and BEI portions of the baroreceptor reflex as the most useful objective measures to differentiate pediatric patients with POTS and NCS from normal subjects.

The Feasibility of Baroreflex Sensitivity Measurements in Heart Failure Subjects: The Role of Slow-patterned Breathing

INTRODUCTION

Increased muscle sympathetic nerve activity (MSNA) indicates an adverse outcome in heart failure. Decreased baroreflex modulation of MSNA is well known feature of the disease. The determinability of cardiovagal baroreflex sensitivity (BRS) in heart failure is low, however, the determinability of sympathetic BRS is not known.

METHODS

We have assessed the spontaneous, MSNA burst incidence-based baroreflex index (BRSsymp) in 33 stable heart failure patients and in 10 healthy controls using the traditional r≥0.5 cut-off for acceptable individual diastolic pressure-burst incidence slopes, and also a more stringent r≥0.7 cut-off. We have also assessed the influence of 6/min breathing.

RESULTS

The determinability of BRSsymp in heart failure patients was 64% during spontaneous breathing with r≥0.5 cut-off, and 39% using the r≥0.7 cut-off. The determinability of these indices further decreased during 6/min breathing, dropping to 29% with the r≥0.7 cut-off. In contrast, the determinability of the cardiovagal BRS indices increased significantly with 6/min breathing (from 24% to 66%; p<0.001). Patients who still had determinable BRSsymp at the r≥0.7 cut-off had a significantly lower baseline burst incidence than those with an undeterminable index (70±14 vs.89±10 burst/100 cycles; p<0.002). Neither the 6/min breathing, nor the r≥0.7 cut-off limit influenced the high availability of BRSsymp in healthy subjects.

CONCLUSION

The determinability of BRSsymp in heart failure patients is limited, especially with the 0.7 limit for correlation. Undeterminable BRSsymp in patients is associated with higher sympathetic activity. 6/min breathing improves the determinability of cardiovagal BRS indices, but not that of BRSsymp. This article is protected by copyright. All rights reserved.

Transcutaneous auricular vagus nerve stimulation augments postprandial inhibition of ghrelin

Vagus nerve stimulation (VNS) facilitates weight loss in animals and patients treated with VNS for depression or epilepsy. Likewise, chronic transcutaneous auricular VNS (taVNS) reduces weight gain and improves glucose tolerance in Zucker diabetic fatty rats. If these metabolic effects of taVNS observed in rats translate to humans is unknown. Therefore, the hypothesis of this study was that acute application of taVNS affects glucotropic and orexigenic hormones which could potentially facilitate weight loss and improve glucose tolerance if taVNS were applied chronically. In two single-blinded randomized cross-over protocols, blood glucose levels, plasma concentrations of insulin, C-peptide, glucagon, leptin, and ghrelin, together with heart rate variability and baroreceptor-heart rate reflex sensitivity were determined before and after taVNS (left ear, 10 Hz, 300 µs, 2.0-2.5 mA, 30 min) or sham-taVNS (electrode attached to ear with the stimulator turned off). In a first protocol, subjects (n = 16) were fasted throughout the protocol and in a second protocol, subjects (n = 10) received a high-calorie beverage (220 kCal) after the first blood sample, just before initiation of taVNS or sham-taVNS. No significant effects of taVNS on heart rate variability and baroreceptor-heart rate reflex sensitivity and only minor effects on glucotropic hormones were observed. However, in the second protocol taVNS significantly lowered postprandial plasma ghrelin levels (taVNS: -115.5 ± 28.3 pg/ml vs. sham-taVNS: -51.2 ± 30.6 pg/ml, p < 0.05). This finding provides a rationale for follow-up studies testing the hypothesis that chronic application of taVNS may reduce food intake through inhibition of ghrelin and, therefore, may indirectly improve glucose tolerance through weight loss.

Synchronization of the Processes of Autonomic Control of Blood Circulation in Humans Is Different in the Awake State and in Sleep Stages

The influence of higher nervous activity on the processes of autonomic control of the cardiovascular system and baroreflex regulation is of considerable interest, both for understanding the fundamental laws of the functioning of the human body and for developing methods for diagnostics and treatment of pathologies. The complexity of the analyzed systems limits the possibilities of research in this area and requires the development of new tools. Earlier we propose a method for studying the collective dynamics of the processes of autonomic control of blood circulation in the awake state and in different stages of sleep. The method is based on estimating a quantitative measure representing the total percentage of phase synchronization between the low-frequency oscillations in heart rate and blood pressure. Analysis of electrocardiogram and invasive blood pressure signals in apnea patients in the awake state and in different sleep stages showed a high sensitivity of the proposed measure. It is shown that in slow-wave sleep the degree of synchronization of the studied rhythms is higher than in the awake state and lower than in sleep with rapid eye movement. The results reflect the modulation of the processes of autonomic control of blood circulation by higher nervous activity and can be used for the quantitative assessment of this modulation.

Cardiovagal Baroreflex Hysteresis Using Ellipses in Response to Postural Changes

The cardiovagal branch of the baroreflex is of high clinical relevance when detecting disturbances of the autonomic nervous system. The hysteresis of the baroreflex is assessed using provoked and spontaneous changes in blood pressure. We propose a novel ellipse analysis to characterize hysteresis of the spontaneous respiration-related cardiovagal baroreflex for orthostatic test. Up and down sequences of pressure changes as well as the working point of baroreflex are considered. The EuroBaVar data set for supine and standing was employed to extract heartbeat intervals and blood pressure values. The latter values formed polygons into which a bivariate normal distribution was fitted with its properties determining proposed ellipses of baroreflex. More than 80% of ellipses are formed out of nonoverlapping and delayed up and down sequences highlighting baroreflex hysteresis. In the supine position, the ellipses are more elongated (by about 46%) and steeper (by about 4.3° as median) than standing, indicating larger heart interval variability (70.7 versus 47.9 ms) and smaller blood pressure variability (5.8 versus 8.9 mmHg) in supine. The ellipses show a higher baroreflex sensitivity for supine (15.7 ms/mmHg as median) than standing (7 ms/mmHg). The center of the ellipse moves from supine to standing, which describes the overall sigmoid shape of the baroreflex with the moving working point. In contrast to regression analysis, the proposed method considers gain and set-point changes during respiration, offers instructive insights into the resulting hysteresis of the spontaneous cardiovagal baroreflex with respiration as stimuli, and provides a new tool for its future analysis.

Perfusion of Brain Preautonomic Areas in Hypertension: Compensatory Absence of Capillary Rarefaction and Protective Effects of Exercise Training

Remodeling of capillary rarefaction and deleterious arteries are characteristic hallmarks of hypertension that are partially corrected by exercise training. In addition, experimental evidence showed capillary rarefaction within the brain cortex and reduced cerebral blood flow. There is no information on hypertension- and exercise-induced effects on capillary profile and function within preautonomic nuclei. We sought now to evaluate the effects of hypertension and exercise training (T) on the capillary network within hypothalamic paraventricular (PVN) and solitary tract (NTS) nuclei, and on the remodeling of brain arteries. Age-matched spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY), submitted to moderate T or kept sedentary (S) for three months, were chronically cannulated for hemodynamic recordings at rest. Rats were anesthetized for i.v. administration of fluorescein isothiocyanate (FITC)-dextran (capillary volume/density measurements) or 4% paraformaldehyde perfusion (basilar, middle, and posterior arteries' morphometry) followed by brain harvesting and processing. Other groups of conscious rats had carotid blood flow (CBF, ultrasound flowmeter) acquired simultaneously with hemodynamic recordings at rest and exercise. SHR-S exhibited elevated pressure and heart rate, reduced CBF, increased wall/lumen ratio of arteries, but no capillary rarefaction within the PVN and NTS. T improved performance gain and caused resting bradycardia in both groups; reduction of pressure and sympathetic vasomotor activity and normalization of the wall/lumen ratio were only observed in SHR-T. T groups responded with marked PVN and NTS capillary angiogenesis and augmented CBF during exercise; to avoid overperfusion at rest, reduced basal CBF was observed only in WKY-T. Data indicated that the absence of SHR-S capillary rarefaction and the intense SHR-T angiogenesis within autonomic areas associated with correction of deleterious arteries' remodeling are essential adjustments to hypertension and exercise training, respectively. These adaptive responses maintain adequate baseline perfusion in SHR-S and SHR-T preautonomic nuclei, augmenting it in exercised rats when a well-coordinated autonomic control is required.

Optimizing phase variability threshold for automated synchrogram analysis of cardiorespiratory interactions in amateur cyclists

We propose a procedure suitable for automated synchrogram analysis for setting the threshold below which phase variability between two marker event series is of such a negligible amount that the null hypothesis of phase desynchronization can be rejected. The procedure exploits the principle of maximizing the likelihood of detecting phase synchronization epochs and it is grounded on a surrogate data approach testing the null hypothesis of phase uncoupling. The approach was applied to assess cardiorespiratory phase interactions between heartbeat and inspiratory onset in amateur cyclists before and after 11-week inspiratory muscle training (IMT) at different intensities and compared to a more traditional approach to set phase variability threshold. The proposed procedure was able to detect the decrease in cardiorespiratory phase locking strength during vagal withdrawal induced by the modification of posture from supine to standing. IMT had very limited effects on cardiorespiratory phase synchronization strength and this result held regardless of the training intensity. In amateur athletes training, the inspiratory muscles did not limit the decrease in cardiorespiratory phase synchronization observed in the upright position as a likely consequence of the modest impact of this respiratory exercise, regardless of its intensity, on cardiac vagal control. This article is part of the theme issue 'Advanced computation in cardiovascular physiology: new challenges and opportunities'.

Stand Up to Excite the Spine: Neuromuscular, Autonomic, and Cardiometabolic Responses During Motor Imagery in Standing vs. Sitting Posture

Motor imagery (MI) for health and performance strategies has gained interest in recent decades. Nevertheless, there are still no studies that have comprehensively investigated the physiological responses during MI, and no one questions the influence of low-level contraction on these responses. Thus, the aim of the present study was to investigate the neuromuscular, autonomic nervous system (ANS), and cardiometabolic changes associated with an acute bout of MI practice in sitting and standing condition. Twelve young healthy males (26.3 ± 4.4 years) participated in two experimental sessions (control vs. MI) consisting of two postural conditions (sitting vs. standing). ANS, hemodynamic and respiratory parameters, body sway parameters, and electromyography activity were continuously recorded, while neuromuscular parameters were recorded on the right triceps surae muscles before and after performing the postural conditions. While MI showed no effect on ANS, the standing posture increased the indices of sympathetic system activity and decreased those of the parasympathetic system (p < 0.05). Moreover, MI during standing induced greater spinal excitability compared to sitting posture (p < 0.05), which was accompanied with greater oxygen consumption, energy expenditure, ventilation, and lower cardiac output (p < 0.05). Asking individuals to perform MI of an isometric contraction while standing allows them to mentally focus on the motor command, not challenge balance, and produce specific cardiometabolic responses. Therefore, these results provide further evidence of posture and MI-related modulation of spinal excitability with additional autonomic and cardiometabolic responses in healthy young men.

Autonomic nervous system activity changes in patients with hypertension and overweight: role and therapeutic implications

The incidence and prevalence of hypertension is increasing worldwide, with approximately 1.13 billion of people currently affected by the disease, often in association with other diseases such as diabetes mellitus, chronic kidney disease, dyslipidemia/hypercholesterolemia, and obesity. The autonomic nervous system has been implicated in the pathophysiology of hypertension, and treatments targeting the sympathetic nervous system (SNS), a key component of the autonomic nervous system, have been developed; however, current recommendations provide little guidance on their use. This review discusses the etiology of hypertension, and more specifically the role of the SNS in the pathophysiology of hypertension and its associated disorders. In addition, the effects of current antihypertensive management strategies, including pharmacotherapies, on the SNS are examined, with a focus on imidazoline receptor agonists.

Persons with type 1 diabetes have low blood oxygen levels in the supine and standing body positions

INTRODUCTION

Blood oxygen saturation is low compared with healthy controls (CONs) in the supine body position in individuals with type 1 diabetes (T1D) and has been associated with complications. Since most of daily life occurs in the upright position, it is of interest if this also applies in the standing body position. In addition, tissue oxygenation in other anatomical sites could show different patterns in T1D. Therefore, we investigated blood, arm and forehead oxygen levels in the supine and standing body positions in individuals with T1D (n=129) and CONs (n=55).

RESEARCH DESIGN AND METHODS

Blood oxygen saturation was measured with pulse oximetry. Arm and forehead mixed tissue oxygen levels were measured with near-infrared spectroscopy sensors applied on the skin.

RESULTS

Data are presented as least squares means±SEM and differences (95% CIs). Overall blood oxygen saturation was lower in T1D (CON: 97.6%±0.2%; T1D: 97.0%±0.1%; difference: -0.5% (95% CI -0.9% to -0.0%); p=0.034). In all participants, blood oxygen saturation increased after standing up (supine: 97.1%±0.1%; standing: 97.6%±0.2%; difference: +0.6% (95% CI 0.4% to 0.8%); p<0.001). However, the increase was smaller in T1D compared with CON (CON supine: 97.3%±0.2%; CON standing: 98.0%±0.2%; T1D supine: 96.9%±0.2%; T1D standing: 97.2%±0.1%; difference between groups in the change: -0.4% (95% CI -0.6% to -0.2%); p<0.001). Arm oxygen saturation decreased in both groups after standing and more in those with T1D. Forehead oxygen saturation decreased in both groups after standing and there were no differences between the changes when comparing the groups.

CONCLUSION

Compared with CON, individuals with T1D exhibit possible detrimental patterns of tissue oxygen adaptation to standing, with preserved adaptation of forehead oxygenation. Further studies are needed to explore the consequences of these differences.

Closed-Loop Dynamic Modeling of the Heart-Rate Reflex to Concurrent Spontaneous Changes of Arterial Blood Pressure and CO2 Tension: Quantification of the Effects of Mild Cognitive Impairment

OBJECTIVE

To extend closed-loop modeling of the heart-rate reflex (HRR) by including the dynamic effects of concurrent changes in blood CO2 tension. This extended dynamic model can be used to generate physio-markers of "baroreflex gain" (BRG) and "chemoreflex gain" (CRG) that allow quantitative assessment of the possible impact of pathologies upon HRR. Mild Cognitive Impairment (MCI) is used as an example.

METHODS

The proposed data-based closed-loop modeling methodology estimates the forward and reverse dynamic components of the model via Laguerre kernel expansions of two open-loop models using spontaneous time-series data collected in 45 MCI patients and 15 controls. The BRG and CRG physio-markers are subsequently computed for each subject via simulation of the obtained closed-loop model for unit-step change of arterial pressure or blood CO2 tension, respectively.

RESULTS

Both open-loop and closed-loop HRR modeling revealed that MCI patients exhibit significantly smaller CRG relative to controls (p<0.001), but not significantly different BRG. Furthermore, the closed-loop model captured the dynamic effect of sympathetic activity as resonant peak around 0.1 Hz (Mayer wave) in the chemoreflex and baroreflex transfer functions (not captured via open-loop modeling). This may prove valuable in advancing our understanding of how sympathetic activity impacts HRR in various pathologies.

CONCLUSION

The extended HRR model, incorporating the dynamic effects of concurrent changes of blood CO2 tension, revealed significantly reduced chemoreflex gain (but not baroreflex gain) in MCI patients. Furthermore, the closed-loop model captured the sympathetic influence around 0.1 Hz.

SIGNIFICANCE

Multivariate closed-loop dynamic modeling is valuable for understanding physiological autoregulation.

PyBioS: A freeware computer software for analysis of cardiovascular signals

BACKGROUND AND OBJECTIVE

Several software applications have been proposed in the past years as computational tools for assessing biomedical signals. Many of them are focused on heart rate variability series only, with their strengths and limitations depending on the necessity of the user and the scope of the application. Here, we introduce new software, named PyBioS, intended for the analysis of cardiovascular signals, even though any type of biomedical signal can be used. PyBioS has some functionalities that differentiate it from the other software.

METHODS

PyBioS was developed in Python language with an intuitive, user-friendly graphical user interface. The basic steps for using PyBioS comprise the opening or creation (simulation) of signals, their visualization, preprocessing and analysis. Currently, PyBioS has 8 preprocessing tools and 15 analysis methods, the later providing more than 50 metrics for analysis of the signals' dynamics.

RESULTS

The possibility to create simulated signals and save the preprocessed signals is a strength of PyBioS. Besides, the software allows batch processing of files, making the analysis of a large amount of data easy and fast. Finally, PyBioS has plenty of analysis methods implemented, with the focus on nonlinear and complexity analysis of signals and time series.

CONCLUSIONS

Although PyBioS is not intended to overcome all the necessities from users, it has useful functionalities that may be helpful in many situations. Moreover, PyBioS is continuously under improvement and several simulated signals, tools and analysis methods are still to be implemented. Also, a new module is being implemented on it to provide machine learning algorithms for classification and regression of data extracted from the biomedical signals.

Myths and methodologies: Reliability of non-invasive estimates of cardiac autonomic modulation during whole-body passive heating

Observed individual variability in cardiac baroreflex sensitivity (cBRS) and heart rate variability (HRV) is extensive, especially during exposure to stressors such as heat. A large part of the observed variation may be related to the reliability (consistency) of the measurement. We therefore examined the test-retest reliability of cBRS and HRV measurements on three separate occasions in 14 young men (age: 24 (SD 5) years), at rest and during whole-body heating (water-perfused suit) to raise and clamp oesophageal temperature 0.6°C, 1.2°C and 1.8°C above baseline. Beat-to-beat measurements of RR interval and systolic blood pressure (BP) were obtained for deriving HRV (from RR), and cBRS calculated via (i) the spontaneous method, α coefficients and transfer function analysis at each level of heat strain, and (ii) during forced oscillations via squat-stand manoeuvres (0.1 Hz) before and after heating. Absolute values and changes in all cBRS estimates were variable but generally consistent with reductions in parasympathetic activity. cBRS estimates demonstrated poor absolute reliability (coefficient of variation ≥25%), but relative reliability (intraclass correlation coefficient; ICC) of some frequency estimates was acceptable (ICC ≥0.70) during low-heat strain (ICC: 0.56-0.74). After heating, forced oscillations in BP demonstrated more favourable responses than spontaneous oscillations (better reliability, lower minimum detectable change). Absolute reliability of HRV estimates were poor, but relative reliability estimates were often acceptable (≥0.70). Our findings illustrate how measurement consistency of cardiac autonomic modulation estimates are altered during heat stress, and we demonstrate the possible implications on research design and data interpretation.

Baroreflex sensitivity with different lags and random forests for staging cardiovascular autonomic neuropathy in subjects with diabetes

Impaired baroreflex sensitivity (BRS) may indicate cardiovascular autonomic neuropathy (CAN), which often remains undiagnosed during the initial course of diabetes mellitus. The baroreflex mechanism can be considered negative feedback because of baroreflex delay, the time delay between a change in blood pressure and the counteracting heart rate response. This work sought to analyze BRS considering lags from 1 to 10 RR intervals. We hypothesized that diabetic patients with subclinical CAN (SCAN) have a detectable delay in autonomic nervous system activity and that this would differ from patients without CAN (NCAN) and with established CAN (ECAN). In the first stage, 30 patients were included in an exploratory analysis using the Principal Component Analysis. Six indexes related to the BRS delay were proposed and considered significant for staging diabetic patients. Three indexes allowed for the differentiating of patients with and without CAN, and three indexes distinguished subjects with SCAN from subjects with NCAN or ECAN. Then, in the second stage, a random forest model was developed with 72 subjects, using the variables selected in the first stage. It was possible to detect SCAN, and to point out those subjects with the potential to change the CAN stage, allowing for the tracking of CAN progression. The model achieved a sensitivity of 96% and specificity of 100% to detect SCAN. Thus, the BRS analysis considering delayed reaction in the dynamics of heart rate variability may contribute to an accurate screening tool to staging CAN, in addition to indicating patients with most insidious disease progress.

An Empirical Mode Decomposition Approach to Assess the Strength of Heart Period-Systolic Arterial Pressure Variability Interactions

This work proposes an empirical mode decomposition (EMD) method to assess the strength of the interactions between heart period (HP) and systolic arterial pressure (SAP) variability. EMD was exploited to decompose the original series (OR) into its first, and fastest, intrinsic mode function (IMF1) and the residual (RES) computed by subtracting the IMF1 from OR. EMD procedure was applied to both HP and SAP variability series. Then, the cross correlation function (CCF) was computed over OR, IMF1 and RES series derived from HP and SAP variability in 13 healthy subjects (age 27±8 yrs, 5 males) at rest in supine position (REST) and during head-up tilt (TILT). The first CCF maximum at negative time lags and the first CCF minimum at positive time lags were taken respectively as indexes of the coupling along the feedback baroreflex and feedforward mechanical arms of HP-SAP closed-loop control. Results showed that the coupling strength is increased during TILT on both arms and this result holds on REST. At REST the coupling along both arms was stronger when computed over IMF1 because interactions were mainly governed by respiration. This result was not observed during TILT possibly due to the reduced importance of respiration compared to other mechanisms acting at slower time scales. The proposed method allowed the investigation of the strength of HP-SAP variability interactions according to the time scales of the physiological mechanisms.The proposed EMD-based method allows the quantification of the strength of the HP-SAP closed-loop variability interactions according to the different time scales of respiration and slower baroreflex-mediated reflexes.

Mathematical modeling of the cardiovascular autonomic control in healthy subjects during a passive head-up tilt test

A mathematical model is proposed for the autonomic control of cardiovascular system, which takes into account two separated self-exciting sympathetic control loops of heart rate and peripheral vascular tone. The control loops are represented by self-exciting time-delay systems and their tone depends on activity of the aortic, carotid, and lower-body baroreceptors. The model is used to study the dynamics of the adaptive processes that manifest in a healthy cardiovascular system during the passive head-up tilt test. Computer simulation provides continuous observation of the dynamics of the indexes and variables that cannot be measured in the direct experiment, including the noradrenaline concentration in vessel wall and heart muscle, tone of the sympathetic and parasympathetic control, peripheral vascular resistance, and blood pressure. In the supine and upright positions, we estimated the spectral characteristics of the model variables, especially in the low-frequency band, and the original index of total percent of phase synchronization between the low-frequency oscillations in heart rate and blood pressure signals. The model demonstrates good quantitative agreement with the dynamics of the experimentally observed indexes of cardiovascular system that were averaged for 50 healthy subjects.

Social defeat: Vagal reduction and vulnerability to ventricular arrhythmias

Previously, a sub-population of defeated anesthetized rats (Dlow) was characterized by persistent low blood levels of brain-derived neurotrophic factor (BDNF) at day 29 and autonomic alteration at day 30 after social challenge, while the other population (Dhigh) was similar to non-defeated (ND) animals. The aims of this study were to determine the time-course of autonomic dysfunction in awake animals, and whether Dhigh and/or Dlow were vulnerable to cardiac events. Defeated animals were exposed to four daily episodes of social defeats from day 1 to day 4. At day 30, anesthetized Dlow displayed decreased experimental and spontaneous reflex responses reflecting lower parasympathetic efficiency. In addition, Dlow but not Dhigh were characterized by left ventricular hypertrophy at day 30. Telemetric recordings revealed that Dlow had increased low frequency-to-high frequency ratio (LF/HF) and diastolic (DBP) and systolic (SBP) blood pressure, associated with decreased HF and spontaneous baroreflex responses (BRS) from day 3 to day 29. LF/HF, DBP and SBP recovered at day 5, and HF and BRS recovered at day 15 in Dhigh. Ventricular premature beats (VPBs) occurred in Dlow and Dhigh animals from day 5. Time course of VBP fluctuations in Dhigh mirrored that of HF and BRS, but not that of LF/HF, DBP and SBP. These results suggest that a psychosocial stress associated to low serum BDNF levels can lead to vulnerability to persistent autonomic dysfunction, cardiac hypertrophy and ventricular ectopic beats. The parasympathetic recovery seen in Dhigh may provide protection against cardiac events in this population.

Reliability of short-term measurements of heart rate variability: Findings from a longitudinal study

Research on heart rate variability (HRV) received increasing attention. This study analysed the reliability of the most common HRV parameters for baseline measurements. 103 healthy students (83 women, M = 21.72 ± 3.31 years) participated in five short-term HRV sessions, each including supine, sitting, and standing positions, respectively, spanning a time interval of eleven months. Relative reliability was evaluated by intraclass correlation coefficients, and absolute reliability by standard errors of measurement, smallest real differences, and 95 % limits of random variation. No systematic mean differences between measurements emerged. Intraclass correlation coefficients were quite low (supine: .49-.64, sitting: .40-.57, standing: .35-.56). Absolute reliability indicators revealed pronounced variations between test and retest. Influences of posture and time between measurements on reliability were small and unsystematic. We conclude that such high levels of within-subjects variability in HRV measurements (a) hamper the detection of changes over time, and (b) should be considered carefully in future analyses.

Automatic detection algorithm for establishing standard to identify "surge blood pressure"

Blood pressure (BP) variability is one of the important risk factors of cardiovascular disease (CVD). “Surge BP,” which represents short-term BP variability, is defined as pathological exaggerated BP increase capable of triggering cardiovascular events. Surge BP is effectively evaluated by our new BP monitoring device. To the best of our knowledge, we are the first to develop an algorithm for the automatic detection of surge BP from continuous “beat-by-beat” (BbB) BP measurements. It enables clinicians to save significant time identifying surge BP in big data from their patients’ continuous BbB BP measurements. A total of 94 subjects (74 males and 20 females) participated in our study to develop the surge BP detection algorithm, resulting in a total of 3272 surges collected from the study subjects. The surge BP detection algorithm is a simple classification model based on supervised learning which formulates shape of surge BP as detection rules. Surge BP identified with our algorithm was evaluated against surge BP manually labeled by experts with 5-fold cross validation. The recall and precision of the algorithm were 0.90 and 0.64, respectively. Processing time on each subject was 11.0 ± 4.7 s. Our algorithm is adequate for use in clinical practice and will be helpful in efforts to better understand this unique aspect of the onset of CVD.

Graphical abstract

Zipf's Law of Vasovagal Heart Rate Variability Sequences

Cardiovascular self-organized criticality (SOC) has recently been demonstrated by studying vasovagal sequences. These sequences combine bradycardia and a decrease in blood pressure. Observing enough of these sparse events is a barrier that prevents a better understanding of cardiovascular SOC. Our primary aim was to verify whether SOC could be studied by solely observing bradycardias and by showing their distribution according to Zipf's law. We studied patients with vasovagal syncope. Twenty-four of them had a positive outcome to the head-up tilt table test, while matched patients had a negative outcome. Bradycardias were distributed according to Zipf's law in all of the patients. The slope of the distribution of vasovagal sequences and bradycardia are slightly but significantly correlated, but only in cases of bradycardias shorter than five beats, highlighting the link between the two methods (r = 0.32; p < 0.05). These two slopes did not differ in patients with positive and negative outcomes, whereas the distribution slopes of bradycardias longer than five beats were different between these two groups (-0.187 ± 0.004 and -0.213 ± 0.006, respectively; p < 0.01). Bradycardias are distributed according to Zipf's law, providing clear insight into cardiovascular SOC. Bradycardia distribution could provide an interesting diagnosis tool for some cardiovascular diseases.

A recent history of preeclampsia is associated with elevated central pulse wave velocity and muscle sympathetic outflow

We demonstrate that resting muscle sympathetic nerve activity is elevated in women with a recent history of preeclampsia relative to women who have recently had uncomplicated pregnancies and without a history of preeclampsia. Structural changes in the central arteries are associated with arterial stiffness following preeclampsia, independent of changes in the sympathetic nervous system. The structural changes are observed in these relatively young previously preeclamptic women, indicating elevated cardiovascular risk. Our data suggest that with aging (and the gradual loss of vascular protection for women, as established by others), this risk will become exaggerated compared with women who have had normal pregnancies.

Similar cardiovascular and autonomic responses in trained type 1 diabetes mellitus and healthy participants in response to half marathon

AIMS

This field experiment examined whether trained people with type 1 diabetes mellitus (T1D) have similar cardiovascular and baroreflex alterations after a 21-km running race when compared to healthy people.

METHODS

Nine T1D (39.0 ± 11.1 yr; 175.0 ± 10.2 cm; 70.8 ± 8.7 kg) were matched with 9 healthy participants (42.4 ± 5.8 yr; 175.7 ± 6.7 cm; 72.1 ± 8.5 kg) who ran an official half-marathon. Before and 1-hour after the race, cardiovascular variables, sympathetic activity (catecholamines), parasympathetic (heart rate variability analysis) modulation and cardiac baroreflex function (transfer function analysis) were assessed during supine rest and a squat stand test (forced blood pressure change).

RESULTS

Performance time and weight loss [104.0 ± 13.2 and 111.0 ± 18.7 min; -2.57 ± 1.05 kg (-1.88 ± 0.88%) and -2.29 ± 1.15 kg (-1.59 ± 0.59%)] for healthy and T1D participants, respectively) were similar. Before running, no significant differences in any cardiovascular or autonomic variables were noted between the groups. After 1 h of recovery, both groups exhibited post-exercise hypotension, accompanied by increased sympathetic activity, decreased parasympathetic modulation, and reduced cardiac baroreflex sensitivity.

CONCLUSIONS

Our results showed that the pattern of change in cardiovascular and autonomic nervous activity to strenuous exercise are well maintained in T1D participants with a training history of at least 5 years.

Does glyceryl trinitrate cause central sympatholytic effects? Insights from a case of baroreflex failure

Whether part of the blood pressure lowering effects of glyceryl trinitrate (GTN) is the result of centrally mediated reduction in sympathetic activity is debated. In humans, baroreflex activity potentially obscures the central sympatholytic effects of GTN. We examined this in a routine clinical tilt test in a patient with baroreflex failure secondary to previous neck radiotherapy. With reduced baroreflex function we observed an exaggerated fall in blood pressure and reduced sympathetic activity with GTN, supporting a peripheral vasodilation and central sympatholytic effect.

AltitudeOmics: Spontaneous Baroreflex Sensitivity During Acclimatization to 5,260 m: A Comparison of Methods

Introduction

Baroreflex sensitivity (BRS) is essential to ensure rapid adjustment to variations in blood pressure (BP). Spontaneous baroreflex function can be assessed using continuous recordings of blood pressure. The goal of this study was to compare four methods for BRS quantification [the sequence, Bernardi's (BER), frequency and transfer function methods] to identify the most consistent method across an extreme range of conditions: rest and exercise, in normoxia, hypoxia, hypocapnia, and hypercapnia.

Methods

Using intra-radial artery BP in young healthy participants, BRS was calculated and compared using the four methods in normoxia, acute and chronic hypoxia (terrestrial altitude of 5,260 m) in hypocapnia (hyperventilation), hypercapnia (rebreathing) and during ramp exercise to exhaustion.

Results

The sequence and BER methods for BRS estimation showed good agreement during the resting and exercise protocols, whilst the ultra- and very-low frequency bands of the frequency and transfer function methods were more discrepant. Removing respiratory frequency from the blood pressure traces affected primarily the sequence and BER methods and occasionally the frequency and transfer function methods.

Discussion/Conclusion

The sequence and BER methods contained more respiratory related information than the frequency and transfer function methods, indicating that the former two methods predominantly rely on respiratory effects of BRS. BER method is recommended because it is the easiest to compute and even though it tends to overestimate BRS compared to the sequence method, it is consistent with the other methods, whilst its interquartile range is the smallest.

Baroreflex Sensitivity Predicts Response to Metoprolol in Children With Vasovagal Syncope: A Pilot Study

Objective: To explore the role of baroreflex sensitivity (BRS) in the head-up tilt test (HUTT) in predicting the therapeutic response of vasovagal syncope (VVS) patients to metoprolol. Materials and Methods: Vasovagal syncope patients treated with metoprolol were enrolled in this study and were classified as responders or non-responders according to changes in their symptom scores before and after metoprolol treatment. Values of BRS in the supine position and at positive response occurrence in the HUTT were obtained, and BRS changes from supine to positive response occurrence were calculated. Differences between responders and non-responders were analyzed. Receiver operating characteristic curve analysis was performed to assess the value of BRS for predicting the therapeutic efficacy of metoprolol in pediatric patients with VVS. Results: Forty patients (14 boys; 11.8 ± 2.5 years) diagnosed with VVS were recruited in the study, 28 of whom were verified to be responders to metoprolol and 12 of whom were verified as non-responders. They did not show any differences in baseline characteristics and hemodynamics in the HUTT (p > 0.05). However, the responders had an obviously increased supine BRS value compared to the non-responders (16.9 ± 7.7 ms/mmHg vs. 7.6 ± 3.8 ms/mmHg; p < 0.01). No difference in BRS at positive response occurrence was observed between the two groups (8.9 ± 8.5 ms/mmHg vs. 10.6 ± 9.8 ms/mmHg; p > 0.05). Accordingly, the changes in the BRS of responders were more obvious than in non-responders (8.0 ± 7.8 ms/mmHg vs. -3.0 ± 10.4 ms/mmHg; p < 0.01). The area under the receiver operating characteristic curve for the predictive value of supine BRS was 0.887 (95% CI, 0.779-0.995; p < 0.01). A cut-off value of 10 ms/mmHg yielded a sensitivity and specificity of 82 and 83%, respectively, in predicting the therapeutic efficacy of metoprolol in pediatric VVS patients. The area under the receiver operating characteristic curve for the predictive value of BRS changes was 0.827 (95% CI, 0.693-0.962; p < 0.01). A cut-off value of 4 ms/mmHg yielded a sensitivity and specificity of 71 and 83%, respectively. Conclusion: Baroreflex sensitivity may predict the response of children with VVS to metoprolol.

Autonomic cardiovascular adaptations to acute head-out water immersion, head-down tilt and supine position

PURPOSE

Thermoneutral head-out water immersion (WI) and 6° head-down tilt (HDT) have been considered as suitable models to increase central blood volume and simulate autonomic cardiovascular adaptations to microgravity, swimming or scuba diving. However, any differences in autonomic cardiovascular adaptations are still unclear. In this study, we hypothesized that WI induces a higher activation of arterial baroreceptors and the parasympathetic system.

METHODS

Ten healthy men underwent 30 min of WI, HDT, and a supine position (SP). RR intervals (RRI) and blood pressure (BP) were continuously monitored. High frequency power (HF), low frequency power (LF) and LF/HF ratio were calculated to study sympathetic and parasympathetic activities, and a spontaneous baroreflex method was used to study arterial baroreflex sensitivity (aBRS). Lung transfer of nitric oxide and carbon monoxide (TLNO/TLCO), vital capacity and alveolar volume (Vc/VA) were measured to study central blood redistribution.

RESULTS

We observed (1) a similar increase in RRI and decrease in BP; (2) a similar increase in HF power during all experimental conditions, whereas LF increased after; (3) a similar rise in aBRS; (4) a similar increase in Vc/VA and decrease in TLNO/TLCO in all experimental conditions.

CONCLUSIONS

These results showed a cardiac parasympathetic dominance to the same extent, underpinned by a similar arterial baroreflex activation during WI and HDT as well as control SP. Future studies may address their association with cold or hyperoxia to assess their ability to replicate autonomic cardiovascular adaptations to microgravity, swimming or scuba diving.

Comparison of Causal and Non-causal Strategies for the Assessment of Baroreflex Sensitivity in Predicting Acute Kidney Dysfunction After Coronary Artery Bypass Grafting

Coronary artery bypass graft (CABG) surgery may lead to postoperative complications such as the acute kidney dysfunction (AKD), identified as any post-intervention increase of serum creatinine level. Cardiovascular control reflexes like the baroreflex can play a role in the AKD development. The aim of this study is to test whether baroreflex sensitivity (BRS) estimates derived from non-causal and causal approaches applied to spontaneous systolic arterial pressure (SAP) and heart period (HP) fluctuations can help in identifying subjects at risk of developing AKD after CABG and which BRS estimates provide the best performance. Electrocardiogram and invasive arterial pressure were acquired from 129 subjects (67 ± 10 years, 112 males) before (PRE) and after (POST) general anesthesia induction with propofol and remifentanil. Subjects were divided into AKDs (n = 29) or no AKDs (noAKDs, n = 100) according to the AKD development after CABG. The non-causal approach assesses the transfer function from the HP-SAP cross-spectrum in the low frequency (LF, 0.04–0.15 Hz) band. BRS was estimated according to three strategies: (i) sampling of the transfer function gain at the maximum of the HP-SAP squared coherence in the LF band; (ii) averaging of the transfer function gain in the LF band; (iii) sampling of the transfer function gain at the weighted central frequency of the spectral components of the SAP series dropping in the LF band. The causal approach separated the two arms of cardiovascular control (i.e., from SAP to HP and vice versa) and accounted for the confounding influences of respiration via system identification and modeling techniques. The causal approach provided a direct estimate of the gain from SAP to HP by observing the HP response to a simulated SAP rise from the identified model structure. Results show that BRS was significantly lower in AKDs than noAKDs during POST regardless of the strategy adopted for its computation. Moreover, all the BRS estimates during POST remained associated with AKD even after correction for demographic and clinical factors. Non-causal and causal BRS estimates exhibited similar performances. Baroreflex impairment is associated with post-CABG AKD and both non-causal and causal methods can be exploited to improve risk stratification of AKD after CABG.

The blood pressure variability and baroreflex sensitivity in healthy participants are not determined by sex or cardiorespiratory fitness

BACKGROUND

Heart rate (HR) and blood pressure (BP) autonomic modulation and baroreflex sensitivity (BRS) are important indexes of cardiovascular homeostasis. However, methodological errors are often observed, such as joint analysis of men and women. Another important aspect is that we still do not know whether cardiorespiratory fitness influences these autonomic parameters in healthy individuals.

OBJECTIVES

This study aimed to investigate whether sex can affect BRS, autonomic modulation of HR and BP variabilities (HRV and BPV, respectively), as well as the influence of cardiorespiratory fitness on these autonomic parameters.

METHODS

Healthy men and women (N=120) were assigned to groups according to the peak oxygen consumption (VO2 peak) obtained in the cardiorespiratory test: low cardiorespiratory fitness (VO2 peak: 22-38 ml/kg/min), moderate cardiorespiratory fitness (VO2 peak: 38-48 ml/kg/min), and high cardiorespiratory fitness (VO2 peak>48 ml/kg/min). HRV and BPV evaluations were performed for all groups in the frequency domain by spectral analysis. Spontaneous BRS was assessed using the sequence method.

RESULTS

Women presented lower BP values compared with men. HR did not differ between sexes, but showed an inverse relationship with cardiorespiratory performance. The HRV analysis showed greater sympathetic modulation for men and greater vagal modulation for women. Men and women presented similar results for systolic BPV and BRS, and cardiorespiratory performance did not influence any of the autonomic parameters evaluated.

CONCLUSION

Cardiorespiratory fitness does not interfere with HRV and BPV autonomic modulation or BRS. However, the cardiac modulatory balance differs between sexes, with a greater influence of the autonomic vagal component in women and the sympathetic component in men.

Autonomic Abnormalities in Patients With Primary Sjogren's Syndrome - Preliminary Results

Primary Sjögren's syndrome (pSS) is an autoimmune disease affecting exocrine glands and extra-glandular organs. There are conflicting reports on the presence of autonomic dysfunction in pSS and no data are available on the functional status of sympathetic outflow to the vessels and baroreceptor [baroreflex sensitivity (BRS)] control mechanisms. We investigated the cardiac (cBRS) and sympathetic (sBRS) baroreceptor modulation in both time and frequency domains and the cardiovascular autonomic profile in pSS patients compared to healthy controls. Autonomic symptoms were quantified by the Composite Autonomic Symptom Scale (COMPASS31) three-item questionnaire. The EULAR Sjogren's syndrome patient reported index (ESSPRI) questionnaire evaluated the magnitude of pSS clinical symptoms, i.e., fatigue, pain, and sicca symptoms. Electrocardiogram, beat-by-beat arterial pressure (AP) and respiratory activity were continuously recorded in 17 pSS patients and 16 healthy controls, while supine and during 75° head-up tilt. In seven patients and seven controls, muscle sympathetic nerve activity (MSNA) was measured. Spectrum analysis of RR variability provided markers of cardiac vagal modulation (HF_RR nu) and sympatho-vagal balance [low frequency (LF)/high frequency (HF)]. The power of LF (0.1 Hz) oscillations of systolic arterial pressure (SAP) variability (LF_SAP) evaluated the vasomotor response to sympathetic stimulation. Compared to controls, pSS patients scored higher in total COMPASS31 (p < 0.0001) and all ESSPRI subdomains (fatigue, p = 0.005; pain, p = 0.0057; dryness, p < 0.0001). Abnormal scialometry (<1.5 ml/15 min) and Schirmer tests (<5 mm/5 min) were found in pSS patients and salivary flow rate was negatively associated with ESSPRI dryness (p = 0.0014). While supine, pSS patients had lower SEQ_cBRS index of cardiac baroreceptor sensitivity, higher HF_RRnu (p = 0.021), lower LF/HF (p = 0.007), and greater MSNA (p = 0.038) than controls. No differences were observed in LF_SAP between groups. During orthostatic challenge, although LF_SAP increased similarly in both groups, MSNA was greater in pSS patients (p = 0.003). At rest pSS patients showed lower cBR control and greater parasympathetic modulation. Furthermore, greater sympathetic nerve activity was observed in pSS patients while supine and in response to gravitational challenge. We hypothesized that such enhanced sympathetic vasoconstrictor activity might reflect an attempt to maintain blood pressure in a setting of likely reduced vascular responsiveness.

Cardiac baroreflex hysteresis is one of the determinants of the heart period variability asymmetry

In heart period (HP) variability (HPV) recordings the percentage of negative HP variations tends to be greater than that of positive ones and this pattern is referred to as HPV asymmetry (HPVA). HPVA has been studied in several experimental conditions in healthy and pathological populations, but its origin is unclear. The baroreflex (BR) exhibits an asymmetric behavior as well given that it reacts more importantly to positive than negative arterial pressure (AP) variations. We tested the hypothesis that the BR asymmetry (BRA) is a HPVA determinant over spontaneous fluctuations of HP and systolic AP (SAP). We studied 100 healthy subjects (age from 21 to 70 yr, 54 men) comprising 20 subjects in each age decade. Electrocardiogram and noninvasive AP were recorded for 15 min at rest in supine position (REST) and during active standing (STAND). The HPVA was evaluated via Porta's index and Guzik's index, while the BRA was assessed as the difference, and normalized difference, between BR sensitivities computed over positive and negative SAP variations via the sequence method applied to HP and SAP variability. HPVA significantly increased during STAND and decreased progressively with age. BRA was not significantly detected both at REST and during STAND. However, we found a significant positive association between BRA and HPVA markers during STAND persisting even within the age groups. This study supports the use of HPVA indexes as descriptors of BRA and identified a challenge soliciting the BR response like STAND to maximize the association between HPVA and BRA markers.

Closed-Loop Cardiovascular Interactions and the Baroreflex Cardiac Arm: Modulations Over the 24 h and the Effect of Hypertension

Closed-loop models of the interactions between blood pressure (BP) and heart rate variations allow for estimation of baroreflex sensitivity (feedback effects of BP changes on heart rate) while also considering the feedforward effects of heart rate on BP. Our study is aimed at comparing modulations of feedback and feedforward couplings over 24 h in normotensive and hypertensive subjects, by assessing closed-loop baroreflex models in ambulatory conditions. Continuous intra-arterial BP recordings were performed for 24 h in eight normotensive and eight hypertensive subjects. Systolic BP (SBP) and pulse interval (PI) beat-by-beat series were analyzed by an autoregressive moving average model over consecutive 6-min running windows, estimating closed-loop feedback and feedforward gains in each window. The open-loop feedback gain was estimated for comparison. Normotensive and hypertensive patients were compared during wake (18:00–22:00) and sleep (23:00–5:00) periods by a mixed-effect linear model at p < 0.05. In both groups feedback (feedforward) gain averaged values were higher (lower) in sleep than in wake. Moreover, the closed-loop feedback gain was higher in normotensive subjects both in wake and sleep, whereas the closed-loop feedforward gain was higher in hypertensive subjects during sleep. By contrast, no significant differences were found between the normotensive and hypertensive groups for the open-loop feedback gain. Therefore, the closed-loop SBP-PI model can detect circadian alterations in the feedforward gain of PI on SBP and derangements of spontaneous baroreflex sensitivity in hypertension not detectable with the open-loop approach. These findings may help to obtain a more comprehensive assessment of the autonomic dysfunction underlying hypertension and for the in-depth evaluation of the benefits of rehabilitation procedures on autonomic cardiovascular modulation.

Baroreceptor activity and sensitivity: normal values in children and young adults using the head up tilt test

BACKGROUND

Baroreceptor function in children and adolescents has not been well studied in the current literature, with a lack of agreed upon normal values. The authors hypothesize that there is a significant increase in the baroreceptor activity with a decrease in the sensitivity during the upright phase in comparison to the supine phases of the head upright tilt test (HUTT) protocol.

METHODS

Baroreceptor evaluation was performed using the sequence method in 49 subjects ages 8-21 years, during the 3 phases of HUTT: supine, head up, and post-tilt supine positions. Baroreceptor Effectiveness Index (BEI) and the number and slope of baroreceptor events were recorded.

RESULTS

On study of the events and slope values, there was a significant difference between the three phases of the HUTT with regard to the baroreceptor activity and sensitivity. No significant difference was found in BEI.

CONCLUSIONS

Our study presents normal values of baroreceptor activity, sensitivity, and BEI in pediatric patients under controlled supine and head up position. We also present new data regarding delayed heart rate responses to pressure changes in the Lag 1 and Lag 2 parameters, which may be beneficial in diagnosis and management of pediatric patients with orthostatic intolerance.

Baroreflex sensitivity assessed with the sequence method is associated with ventricular arrhythmias in patients implanted with a defibrillator for the primary prevention of sudden cardiac death

BACKGROUND

Left ventricular ejection fraction lacks accuracy in predicting sudden cardiac death, resulting in unnecessary implantation of cardioverter defibrillators for the primary prevention of sudden cardiac death. Baroreflex sensitivity could help to stratify patients at risk of ventricular arrhythmia.

AIM

To assess the association between cardiac baroreflex sensitivity and ventricular arrhythmias in patients implanted with an implantable cardioverter defibrillator for the primary prevention of sudden cardiac death after myocardial infarction.

METHODS

This case-control single-centre study took place between 2015 and 2016. Cases (n=10) had experienced ventricular arrhythmias treated by the implantable cardioverter defibrillator in the previous 3 years; controls (n=22) had no arrhythmia during the same period. Baroreflex sensitivity was assessed using the temporal sequence method (mean slope) and cross-spectral analysis (low-frequency gain and high-frequency gain).

RESULTS

The mean age was 65 years; 94% of the patients were men. 24-hour Holter electrocardiogram autonomous nervous system variables, left ventricular ejection fraction and N-terminal prohormone of B-type natriuretic peptide (NT-proBNP) concentration did not differ between cases and controls. The mean slope was lower in cases than in controls (8 vs. 15ms/mmHg [P=0.009] in the supine position; 7 vs. 12ms/mmHg [P=0.038] in the standing position). The mean slope in the supine position was still significantly different between groups after adjustment for age, left ventricular ejection fraction and NT-proBNP (P=0.03). By comparison, low-frequency gain and high-frequency gain did not differ between groups in either the supine or the standing position.

CONCLUSION

Patients with ventricular arrhythmias had a lower mean slope compared with those who were free of arrhythmia. A prospective study is needed to confirm this association.

Assessment of arterial baroreflex sensitivity by different computational analyses of pressure wave signals alone

BACKGROUND AND OBJECTIVE

Baroreflex sensitivity (BRS) is an important indicator of the functionality of the arterial baroreceptors, and its assessment may have major research and clinical implications. An important requirement for its quantification is the continuous recording of electrocardiography (ECG) signal, so as to extract the RR interval, in parallel with continuous beat-to-beat blood pressure recording. We aimed to accurately calculate the RR Interval from pressure wave recordings per se, namely, the Pulse Interval (PI) using various arterial pulse wave analysis algorithms and to evaluate the precision and accuracy of BRS values calculated with the PI compared to BRS values calculated with the RR Interval.

METHODS

We analyzed the open access data of the Eurobavar study, which contains a set of ECG and arterial blood pressure (BP) wave signals recorded at 11 European centers. Pressure waveforms were continuously recorded by the Finapres apparatus which uses a finger cuff. The cuff pressure around the finger is dynamically adjusted by a servo-system to equal intra-arterial pressure, thus allowing the continuous recording of beat-to-beat BP waves. RR Interval was calculated from the ECG, whereas, PI was extracted from the arterial pulse waveforms, using 4 different methods (minimum, maximum, maximum 1st derivative and intersecting tangents method). BRS values were estimated by time domain and frequency domain methods. In order to compare agreement, accuracy, precision, variability, and the association between the reference BRS using the RR Interval and the BRS values using PI, standard statistical methods (i.e. intraclass correlation coefficients, RMSE, regression analysis) and Bland-Altman methods were performed.

RESULTS

We found that analysis of pressure waves alone by frequency-based (i.e. spectral) methods, provides the most accurate results of BRS estimation compared to time-domain methods (ICC > 0.9, R > 0.9, RMSE > 0.8 ms/mmHg). Concerning the spectral method, any algorithm for PI calculation is sufficient, as all show excellent agreement with the respective RR-intervals determined by ECG time series. Only the intersecting tangents and the maximum 1st derivative methods for PI calculation produce the most accurate results in time domain BRS estimation.

CONCLUSION

BRS estimation by proper analysis of pressure wave signals alone is feasible and accurate. Further studies are needed to investigate the clinical validity and relevance of the different BRS estimations in diagnostic, prognostic and therapeutic levels.

Assessment of Baroreflex Sensitivity Using Time-Frequency Analysis during Postural Change and Hypercapnia

Baroreflex is a mechanism of short-term neural control responsible for maintaining stable levels of arterial blood pressure (ABP) in an ABP-heart rate negative feedback loop. Its function is assessed by baroreflex sensitivity (BRS)—a parameter which quantifies the relationship between changes in ABP and corresponding changes in heart rate (HR). The effect of postural change as well as the effect of changes in blood O₂ and CO₂ have been the focus of multiple previous studies on BRS. However, little is known about the influence of the combination of these two factors on dynamic baroreflex response. Furthermore, classical methods used for BRS assessment are based on the assumption of stationarity that may lead to unreliable results in the case of mostly nonstationary cardiovascular signals. Therefore, we aimed to investigate BRS during repeated transitions between squatting and standing in normal end-tidal CO₂ (EtCO₂) conditions (normocapnia) and conditions of progressively increasing EtCO₂ with a decreasing level of O₂ (hypercapnia with hypoxia) using joint time and frequency domain (TF) approach to BRS estimation that overcomes the limitation of classical methods. Noninvasive continuous measurements of ABP and EtCO₂ were conducted in a group of 40 healthy young volunteers. The time course of BRS was estimated from TF representations of pulse interval variability and systolic pressure variability, their coherence, and phase spectra. The relationship between time-variant BRS and indices of ABP and HR was analyzed during postural change in normocapnia and hypercapnia with hypoxia. In normocapnia, observed trends in all measures were in accordance with previous studies, supporting the validity of presented TF method. Similar but slightly attenuated response to postural change was observed in hypercapnia with hypoxia. Our results show the merits of the nonstationary methods as a tool to study the cardiovascular system during short-term hemodynamic changes.

The additional impact of type 2 diabetes on baroreflex sensitivity of coronary artery disease patients might be undetectable in presence of deterioration of mechanical vascular properties

Both deterioration of the mechanical vascular properties of barosensitive vessels and autonomic derangement lead to modification of baroreflex sensitivity (BRS) in coronary artery disease (CAD) individuals. Type 2 diabetes (T2D) reduces BRS as well even in absence of cardiac autonomic neuropathy. The aim of the study is to clarify whether, assigned the degree of mechanical vascular impairment and without cardiac autonomic neuropathy, the additional autonomic dysfunction imposed in CAD patients by T2D (CAD-T2D) decreases BRS further. We considered CAD (n = 18) and CAD-T2D (n = 19) males featuring similar increases of average carotid intima media thickness (ACIMT) and we compared them to age- and gender-matched healthy (H, n = 19) subjects. BRS was computed from spontaneous beat-to-beat variability of heart period (HP) and systolic arterial pressure (SAP) at supine resting (REST) and during active standing (STAND). BRS was estimated via methods including time domain, spectral, cross-spectral, and model-based techniques. We found that (i) at REST BRS was lower in CAD and CAD-T2D groups than in H subjects but no difference was detected between CAD and CAD-T2D individuals; (ii) STAND induced an additional decrease of BRS visible in all the groups but again BRS estimates of CAD and CAD-T2D patients were alike; (iii) even though with different statistical power, BRS markers reached similar conclusions with the notable exception of the BRS computed via model-based approach that did not detect the BRS decrease during STAND. In presence of a mechanical vascular impairment, indexes estimating BRS from spontaneous HP and SAP fluctuations might be useless to detect the additional derangement of the autonomic control in CAD-T2D without cardiac autonomic neuropathy compared to CAD, thus limiting the applications of cardiovascular variability analysis to typify CAD-T2D individuals. Graphical abstract Graphical representation of the baroreflex sensitivity (BRS) estimated from spontaneous fluctuations of heart period and systolic arterial pressure via transfer function (TF) in low frequency (LF) band (from 0.04 to 0.15 Hz). BRS was reported as a function of the group (i.e., healthy (H), coronary artery disease (CAD) and CAD with type 2 diabetes (CAD-T2D) groups) at REST (black bars) and during STAND (white bars). Values are shown as mean plus standard deviation. The symbol "*" indicates a significant difference between conditions within the same group (i.e., H, CAD, or CAD-T2D) and the symbol "§" indicates a significant difference between groups within the same experimental condition (i.e., REST or STAND). BRS cannot distinguish CAD and CAD-T2D groups both at REST and during STAND, while it is useful to distinguish experimental conditions and separate pathological groups from H subjects.