Comparison of the prognostic values of invasive and noninvasive assessments of baroreflex sensitivity in heart failure

The autonomic nervous system in atrial fibrillation-pathophysiology and non-invasive assessment

The autonomic nervous system plays a crucial role in atrial fibrillation pathophysiology. Parasympathetic hyperactivity result in a shortening of the action potential duration, a reduction of the conduction wavelength, and as such facilitates reentry in the presence of triggers. Further, autonomic remodeling of atrial myocytes in AF includes progressive sympathetic hyperinnervation by increased atrial sympathetic nerve density and sympathetic atrial nerve sprouting. Knowledge on the pathophysiological process in AF, including the contribution of the autonomic nervous system, may in the near future guide personalized AF management. This review focuses on the role of the autonomic nervous system in atrial fibrillation pathophysiology and non-invasive assessment of the autonomic nervous system.

Baroreflex sensitivity but not microvolt T-wave alternans can predict major adverse cardiac events in ischemic heart failure

BACKGROUND

Major adverse cardiovascular events (MACE) constitutes the main cause of morbidity and mortality in ischemic heart failure (HF) patients. The prognostic value of the autonomic nervous system parameters and microvolt T-wave alternans (MTWA) in this issue has not been identified to date. The aim herein, was to assess the usefulness of the abovementioned parameters in the prediction of MACE in HF patients with left ventricular systolic dysfunction of ischemic origin.

METHODS

Baroreflex sensitivity (BRS), heart rate variability (HRV), MTWA and other well-known clinical parameters were analyzed in 188 ischemic HF outpatients with left ventricular ejection fraction (LVEF) ≤ 50%. During 34 (14-71) months of follow-up, 56 (30%) endpoints were noted.

RESULTS

Univariate Cox analyses revealed BRS (but not HRV), MTWA, age, New York Heart Association functional class III, LVEF, implantable cardioverter-defibrillator presence, use of diuretics and antiarrhythmic drugs, diabetes, and kidney insufficiency were defined as significant predictors of MACE. Pre-specified cut-off values for MACE occurrence for the aforementioned continuous parameters (age, LVEF, and BRS) were: ≥ 72 years, ≤ 33%, and ≤ 3 ms/mmHg, respectively. In a multivariate Cox analysis only BRS (HR 2.97, 95% CI 1.35-6.36, p < 0.006), and LVEF (HR 1.98, 95% CI 0.61-4.52, p < 0.038) maintained statistical significance in the prediction of MACE.

CONCLUSIONS

Baroreflex sensitivity and LVEF are independent of other well-known clinical parameters in the prediction of MACE in patients with HF of ischemic origin and LVEF up to 50%. BRS ≤ 3 ms/mmHg and LVEF ≤ 33% identified individuals with the highest probability of MACE during the follow-up period.

Effect of Algorithmic Music Listening on Cardiac Autonomic Nervous System Activity: An Exploratory, Randomized Crossover Study

It is proven that music listening can have a therapeutic impact in many clinical fields. However, to assume a curative value, musical stimuli should have a therapeutic logic. This study aimed at assessing short-term effects of algorithmic music on cardiac autonomic nervous system activity. Twenty-two healthy subjects underwent a crossover study including random listening to relaxing and activating algorithmic music. Electrocardiogram (ECG) and non-invasive arterial blood pressure were continuously recorded and were later analyzed to measure Heart Rate (HR) mean, HR variability and baroreflex sensitivity (BRS). Statistical analysis was performed using a general linear model, testing for carryover, period and treatment effects. Relaxing tracks decreased HR and increased root mean square of successive squared differences of normal-to-normal (NN) intervals, proportion of interval differences of successive NN intervals greater than 50 ms, low-frequency (LF) and high-frequency (HF) power and BRS. Activating tracks caused almost no change or an opposite effect in the same variables. The difference between the effects of the two stimuli was statistically significant in all these variables. No difference was found in the standard deviation of normal-to-normal RR intervals, LF_power in normalized units and LF_power/HF_power variables. The study suggests that algorithmic relaxing music increases cardiac vagal modulation and tone. These results open interesting perspectives in various clinical areas.

Effects of the number of sit-stand maneuver repetitions on baroreflex sensitivity and cardiovascular risk assessments

Sit-stand maneuvers (SSM) have increasingly been used for baroreflex sensitivity (BRS) measurement in physiological research, but it remains unknown as to how many SSM need to be performed to measure BRS and assess its relation with cardiovascular disease (CVD) risk. Therefore, this study aimed to determine 1) the effect of the number of SSM repetitions on BRS and 2) the association between BRS and CVD risk factors. Data were collected from 174 individuals during 5 minutes of spontaneous rest and 5 minutes of repeated SSM at 0.05 Hz (i.e., 15 cycles of 10-second sit and 10-second stand). During SSM, BRS was calculated from the incremental cycles of 3, 6, 9, 12, and 15 SSM using transfer function analysis of heart rate (HR) and systolic blood pressure (SBP). General CVD risk factors, carotid arterial stiffness, and cardiorespiratory fitness were measured. In result, HR and SBP increased during SSM (p<0.05). The BRS remained at a similar level during the resting and SSM conditions, while the coherence function reached its peak after 3 cycles of SSM. BRS with ≥6 cycles of SSM was strongly correlated with age (r=-0.721 to -0.740), carotid distensibility (r=0.625 to 0.629), and cardiorespiratory fitness (r=0.333 to 0.351) (all p<0.001). Multiple regression analysis demonstrated that BRS with ≥6 cycles of SSM explained >60% of the variance in CVD risk factors. Therefore, our findings suggest that repeated SSM significantly strengthens the association between BRS and CVD risk factors. Particularly, BRS with ≥6 cycles of SSM is strongly associated with CVD risk.

Valsalva-derived Measures and Phenylephrine Test in Patients With Heart Failure With Reduced Ejection Fraction Receiving Comprehensive Neurohormonal Blockade Drug Therapy: A 5-year Event-free Survival Analysis

BACKGROUND

To assess the relationships between Valsalva- and phenylephrine test-derived measures and outcome in patients with heart failure with reduced ejection fraction (HFrEF) receiving comprehensive neurohormonal blockade pharmacotherapy.

METHODS AND RESULTS

Data from 56 patients with HFrEF (mean left ventricle ejection fraction of 32 ± 6%) subjected to Valsalva and phenylephrine tests were analyzed retrospectively. Baroreflex-related (Valsalva-ratio and blood pressure-RR interval slope from phase IV) and non-baroreflex-related measures (systolic blood pressure rise in phase IV [ΔSBP_{PHASE_IV}], and pulse amplitude ratio [PAR]) were calculated from Valsalva. Short-term outcomes (HF-related hospitalization, implantable cardioverter-defibrillator shock or all-cause death within 24 months from examination) and long-term outcomes (implantable cardioverter-defibrillator shock or all-cause death within 60 months) were analyzed. The end point occurred in 16 and 18 patients, for the short- and long-term outcomes, respectively. A low ΔSBP_{PHASE_IV} identified patients at risk in the long term, as evidenced by a low vs high ΔSBP_{PHASE_IV} comparison (square-wave response patients assigned to low ΔSBP_{PHASE_IV} group, P = .002), and Cox model (hazard ratio 0.91, 95% confidence interval 0.86-0.96, P < .001), and tended to identify patients at risk in the short term outcome (hazard ratio 0.95, 95% confidence interval 0.91-1.00, P = .055). There was a tendency toward a higher event-free survival in the low PAR group (low vs high PAR; hazard ratio 0.44, 95% CI 0.17-1.18, P = .104).

CONCLUSIONS

Non-baroreflex-related measures obtained from Valsalva-namely, ΔSBP_{PHASE_IV} and PAR-might carry prognostic value in patients with HFrEF receiving neurohormonal blockade pharmacotherapy.

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.

Could autonomic nervous system parameters be still helpful in identifying patients with left ventricular systolic dysfunction at the highest risk of all-cause mortality?

BACKGROUND

Autonomic imbalance is associated with poor prognosis of patients with systolic dysfunction. Most of the previous data were written several years ago and constituted to cardiovascular or arrhythmic mortality. The current treatment of these patients has improved substantially over the last decades, and thus, the population at risk of death may have altered as well. Consequently, data on high-risk patients with systolic dysfunction in the modern era are sparse and those from previous trials may no longer be applicable. The aim herein, was to verify whether well-known autonomic indices - baroreflex sensitivity (BRS) and heart rate variability (HRV) - remain accurate predictors of mortality in patients with systolic dysfunction.

METHODS

Non-invasively obtained BRS and HRV were analyzed in 205 clinically stable patients with left ventricular ejection fraction (LVEF) ≤ 40%. 28 patients died within 28 ± 9 month follow-up.

RESULTS

Baroreflex sensitivity, low-frequency (LF) in normalized units, LF to high-frequency ratio and standard deviation of average R-R intervals were significantly associated with mortality; cut-off values of the highest discriminatory power for abovementioned parameters were ≤ 3.0 ms/mmHg, ≤ 41, ≤ 0.7 and ≤ 25 ms, respectively. In bivariate Cox analyses (adjusted for LVEF, New York Heart Association [NYHA] or absence of implantable cardioverter-defibrillator [ICD]) autonomic indices remain significant predictors of death.

CONCLUSIONS

Baroreflex sensitivity and HRV - may still be helpful in identifying patients with left ventricular systolic dysfunction at the highest risk of all-cause mortality, independently of LVEF, NYHA class, and ICD implantation.

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.

Diet associated with exercise improves baroreflex control of sympathetic nerve activity in metabolic syndrome and sleep apnea patients

PURPOSE

We tested the hypothesis that (i) diet associated with exercise would improve arterial baroreflex (ABR) control in metabolic syndrome (MetS) patients with and without obstructive sleep apnea (OSA) and (ii) the effects of this intervention would be more pronounced in patients with OSA.

METHODS

Forty-six MetS patients without (noOSA) and with OSA (apnea-hypopnea index, AHI > 15 events/h) were allocated to no treatment (control, C) or hypocaloric diet (- 500 kcal/day) associated with exercise (40 min, bicycle exercise, 3 times/week) for 4 months (treatment, T), resulting in four groups: noOSA-C (n = 10), OSA-C (n = 12), noOSA-T (n = 13), and OSA-T (n = 11). Muscle sympathetic nerve activity (MSNA), beat-to-beat BP, and spontaneous arterial baroreflex function of MSNA (ABR_MSNA, gain and time delay) were assessed at study entry and end.

RESULTS

No significant changes occurred in C groups. In contrast, treatment in both patients with and without OSA led to a significant decrease in weight (P < 0.05) and the number of MetS factors (P = 0.03). AHI declined only in the OSA-T group (31 ± 5 to 17 ± 4 events/h, P < 0.05). Systolic BP decreased in both treatment groups, and diastolic BP decreased significantly only in the noOSA-T group. Treatment decreased MSNA in both groups. Compared with baseline, ABR_MSNA gain increased in both OSA-T (13 ± 1 vs. 24 ± 2 a.u./mmHg, P = 0.01) and noOSA-T (27 ± 3 vs. 37 ± 3 a.u./mmHg, P = 0.03) groups. The time delay of ABR_MSNA was reduced only in the OSA-T group (4.1 ± 0.2 s vs. 2.8 ± 0.3 s, P = 0.04).

CONCLUSIONS

Diet associated with exercise improves baroreflex control of sympathetic nerve activity and MetS components in patients with MetS regardless of OSA.

Microvolt T-wave alternans and autonomic nervous system parameters can be helpful in the identification of low-arrhythmic risk patients with ischemic left ventricular systolic dysfunction

INTRODUCTION

The role of implantable cardioverter-defibrillator (ICD) placement in the primary prevention of sudden cardiac death (SCD) in all consecutive patients with left ventricular ejection fraction (LVEF) ≤ 35% is still a matter of hot debate due to the fact that the population of these patients is highly heterogeneous in terms of the SCD risk. Nevertheless, reduced LVEF is still the only established criterion during qualification of patients for ICD implantation in the primary prevention of SCD, therefore identification of persons with particularly high risk among patients with LVEF ≤35% is currently of lesser importance. More important seems to be the selection of individuals with relatively low risk of SCD in whom ICD implantation can be safely postponed. The aim of the study was to determine whether well-known, non-invasive parameters, such as microvolt T-wave alternans (MTWA), baroreflex sensitivity (BRS) and short-term heart rate variability (HRV), can be helpful in the identification of low-arrhythmic risk patients with ischemic left ventricular systolic dysfunction.

METHODS

In 141 patients with coronary artery disease and LVEF ≤ 35%, MTWA testing, as well as BRS and short-term HRV parameters, were analysed. During 34 ± 13 months of follow-up 37 patients had arrhythmic episode (EVENT): SCD, non-fatal sustained ventricular arrhythmia (ventricular tachycardia [VT] or ventricular fibrillation [VF]), or adequate high-voltage ICD intervention (shock) due to a rapid ventricular arrhythmia ≥200/min. LVEF, non-negative MTWA (MTWA_non-neg), BRS and low frequency power in normalized units (LFnu) turned out to be associated with the incidence of EVENT in univariate Cox analysis. The cut-off values for BRS and LFnu that most accurately distinguished between patients with and without EVENT were 3 ms/mmHg and 23, respectively. The only variable that provided 100% negative predictive value (NPV) for EVENT was negative MTWA result (MTWA_neg), but solely for initial 12 months of the follow-up; the NPVs for other potential predictors of the EVENT were lower. The cut-off values for BRS and LFnu that provide 100% NPV for EVENT during 12 and 24 months were higher: 6.0 ms/mmHg and 73 respectively, but the gain in the NPV occurred at an expense of the number of identified patients. However, the number of identified non-risk patients turned out to be higher when the predictive model included MTWA_neg and the lower cut-off values for ANS parameters: 100% NPV for 12 and 24 months of follow-up was obtained for combination MTWA_neg and BRS ≥ 3 ms/mmHg, for combination MTWA_neg and LFnu ≥ 23 100% NPV was obtained for 12 months.

CONCLUSION

Well-known, non-invasive parameters, such as MTWA, BRS and short-term HRV indices may be helpful in the identification of individuals with a relatively low risk of malignant ventricular arrhythmias among patients with ischemic left ventricular systolic dysfunction; in such persons, implantation of ICD could be safely postponed.

Different estimation methods of spontaneous baroreflex sensitivity have different predictive value in heart failure patients

OBJECTIVE

Several methods have been developed so far to estimate cardiac baroreflex sensitivity (BRS) through the analysis of spontaneous fluctuations of systolic arterial pressure (SAP) and R-R interval. Their relative performance in predicting cardiac mortality in heart failure patients is not known. The aim of this study was to compare the prognostic value of a set of representative indexes of spontaneous BRS in these patients.

METHODS

We studied 228 stable, moderate-to-severe heart failure patients with reduced ejection fraction, in sinus rhythm, who underwent an 8-min supine recording of ECG, arterial blood pressure and respiration during paced breathing (15 breaths/min). BRS was estimated according to the following methods: sequence (BRSSeq); nonparametric transfer function in the low-frequency band (BRSTF_NP-LF); parametric spectral computed in the low-frequency and high-frequency bands (BRSPS-LF and BRSPS-HF); parametric transfer function computed in the low-frequency and high-frequency bands (BRSTF_P-LF and BRSTF_P-HF); model-based closed loop (BRSCL); and bivariate phase-rectified signal averaging (BRSPRSA).

RESULTS

During a median follow-up of 36 months, 45 patients experienced a cardiac event. Only BRSTF_NP-LF, BRSPS-LF, BRSTF_P-LF and BRSPRSA were significantly associated with the outcome (P < 0.01), and statistical significance remained (P ≤ 0.03) after adjusting for clinical covariates. BRSTF_NP-LF and BRSPRSA also significantly improved the risk classification.

CONCLUSION

This study shows that different spontaneous BRS indexes have different predictive value in patients with heart failure. It also shows that the prognostic information of BRS estimates is linked to SAP and RR oscillations in the low-frequency band.

Autonomic control of blood circulation in patients undergoing elective carotid endarterectomy

BACKGROUND

Internal carotid artery stenosis (ICAS) is associated with significantly higher risk of stroke. Autonomic function can be impaired in the presence of atheroma in the carotid sinus region. Two parameters of autonomic nervous system (ANS) function e.g. heart rate variability (HRV) and baroreflex sensitivity (BRS) are respected predictors of cardiovascular prognosis. We assessed the effect of elective unilateral carotid endarterectomy (CEA) on cardiovascular autonomic functions as a major prognostic factor for cardiovascular health.

METHODS

Nineteen patients indicated for CEA underwent formal autonomic assessment in the laboratory. Hemodynamic profiles, HRV and BRS were evaluated with the dedicated high-tech device Task Force Monitor before surgery (day-1) and postoperatively (day 3±1). Data were obtained during 5 min orthostatic challenge and subsequent 5 min in a supine position.

RESULTS

There were no significant early postoperative changes in evaluated parameters after CEA. There was a mild decrease of blood pressure and therefore only a slight increase in BRS. It was also possible to observe a rise in the value of total power and high frequency power.

CONCLUSION

In the early postoperative period, healing processes are occurring and the sympatho-vagal interaction is probably still unbalanced. Given the considerable clinical potential of BRS and HRV measurement, further short-term and, more importantly, long-term investigations are needed.

The possible role of the vagal nervous system in the recovery of the blood pressure control after cardiac arrest: a porcine model study

Previous studies have proved that the baroreceptor reflex (baroreflex) control of heart rate can be used for stratification of post-infarction population and, in general, cardiovascular disease populations. Many methods have been proposed to estimate the so-called baroreflex sensitivity (BRS) expressed as ms mmhg^-1. Most of the studies that exploit BRS focus mainly on acute myocardial infarction (AMI) and there are no important works that investigate the role of BRS immediately after cardiac arrest (CA). The present work is a continuation of the published work of Ristagno et al (2014 Shock 41 72-8). In particular, the main objectives are: (1) to study the evolution of BRS after CA and following cardiopulmonary resuscitation (CPR); (2) to verify if the recovery of cardiovascular stability and arterial blood pressure is accompanied by a recovery of BR in a porcine model; (3) to investigate the possible causes of the BRS variations in response to CA and following cardiopulmonary resuscitation. All the BRS estimators adopted in this study show a significant decrease after CA. However, partial recovery is obtained in the last hours of post resuscitation. Analysis of impulse response showed a decrease in peak delay after CA and was significantly shorter 4 hours after CPR. This finding hints at a compensation mechanism: a faster response when baroreflex gain is not fully restored. The increase in the speed of baroreflex response is in line with the hypothesis of a key role of the parasympathetic nervous system, which is known to act at a higher firing rate.

Central Chemoreceptor Sensitivity Is Not Enhanced in Contemporary Patients With Chronic Systolic Heart Failure Receiving Optimal Treatment

BACKGROUND

Clinical and prognostic consequences of enhanced central chemosensitivity in the contemporary optimally treated patients with chronic heart failure (CHF) are unknown.

METHODS AND RESULTS

We studied central chemosensitivity (defined as hypercapnic ventilatory response [HCVR; L/min/mmHg]) in 161 CHF patients (mean left ventricular ejection fraction [LVEF] 31 ± 6%, all receiving a combination of angiotensin-converting enzyme inhibitor/angiotensin receptor blocker and beta-blocker) and 55 sex- and age-matched healthy controls. HCVR did not differ between CHF patients and controls (median 0.63 vs 0.57 L/min^-1/mmHg^-1, P = .76). When the CHF patients were divided into tertiles according to their HCVR values, there were no significant differences in clinical characteristics (except for ischemic etiology, which was more frequent in those with the highest HCVR), results of the cardiopulmonary exercise testing, and indices of heart rate variability. During the follow-up (median 28 months, range 1-48 months, ≥15 months in all survivors), 21 patients died. HCVR was not related to survival in the Cox proportional hazards analysis.

CONCLUSIONS

Central chemosensitivity is not enhanced in contemporary, optimally treated CHF patients and its assessment does not provide significant clinical or prognostic information.

Exercise training-induced modification in autonomic nervous system: An update for cardiac patients

Patients with cardiovascular disease show autonomic dysfunction, including sympathetic activation and vagal withdrawal, which leads to fatal events. This review aims to place sympathovagal balance as an essential element to be considered in management for cardiovascular disease patients who benefit from a cardiac rehabilitation program. Many studies showed that exercise training, as non-pharmacologic treatment, plays an important role in enhancing sympathovagal balance and could normalize levels of markers of sympathetic flow measured by microneurography, heart rate variability or plasma catecholamine levels. This alteration positively affects prognosis with cardiovascular disease. In general, cardiac rehabilitation programs include moderate-intensity and continuous aerobic exercise. Other forms of activities such as high-intensity interval training, breathing exercises, relaxation and transcutaneous electrical stimulation can improve sympathovagal balance and should be implemented in cardiac rehabilitation programs. Currently, the exercise training programs in cardiac rehabilitation are individualized to optimize health outcomes. The sports science concept of the heart rate variability (HRV)-vagal index used to manage exercise sessions (for a goal of performance) could be implemented in cardiac rehabilitation to improve cardiovascular fitness and autonomic nervous system function.

Spontaneous baroreceptor reflex sensitivity for risk stratification of heart failure patients: optimal cut-off and age effects

This study provides statistical support for 3 ms/mmHg as a cut-off value that is the identifier of a homogeneous group of patients with depressed BRS, and highlights the robustness of this threshold in risk stratification due to its age-independent prognostic value.

Assessment of baroreflex sensitivity from spontaneous oscillations of blood pressure and heart rate: proven clinical value?

The baroreceptor-heart rate reflex (baroreflex sensitivity, BRS) is a key mechanism contributing to the neural regulation of the cardiovascular system. Several methods have been proposed so far to assess BRS by analyzing the spontaneous beat-to-beat fluctuations of arterial blood pressure and heart rate. These methods are inherently simple, non-invasive and low-cost. This study is an attempt to address the question of whether spontaneous baroreflex methods have proven to be of value in the clinical setting. In the first part of this article, we critically review most representative clinical studies using spontaneous BRS techniques either for risk stratification or treatment evaluation, these being major issues in the clinical management of the patients. In the second part, we address two important aspects of spontaneous BRS measurements: measurability and reliability. Estimation of BRS in the studies selected for the review was performed according to the sequence, transfer function, alpha-index and phase-rectified signal averaging method. Arterial blood pressure was recorded non-invasively during supine, short-term (<30 min) laboratory recordings. The conclusion from this review is that spontaneous BRS techniques have been shown to be of great value in clinical practice but further work is needed to confirm the validity of previous findings and to widen the field of clinical applications. Measurability and reliability can be a major issue in the measurement of spontaneous BRS, particularly in some patient populations like post-myocardial infarction and heart failure patents. Main causes of poor measurability are: non-sinus rhythm, a high rate of ectopic beats and the need for recorded time series of RR interval and arterial blood pressure to satisfy the constraints of the different BRS estimation algorithms. As for reliability, within-subject variability is rather high in the measurements of spontaneous BRS and, therefore, should be carefully taken into account when BRS measurements are used to detect treatment effects in individual patients.

Exercise training prevents the deterioration in the arterial baroreflex control of sympathetic nerve activity in chronic heart failure patients

Arterial baroreflex control of muscle sympathetic nerve activity (ABRMSNA) is impaired in chronic systolic heart failure (CHF). The purpose of the study was to test the hypothesis that exercise training would improve the gain and reduce the time delay of ABRMSNA in CHF patients. Twenty-six CHF patients, New York Heart Association Functional Class II-III, EF ≤ 40%, peak V̇o2 ≤ 20 ml·kg(-1)·min(-1) were divided into two groups: untrained (UT, n = 13, 57 ± 3 years) and exercise trained (ET, n = 13, 49 ± 3 years). Muscle sympathetic nerve activity (MSNA) was directly recorded by microneurography technique. Arterial pressure was measured on a beat-to-beat basis. Time series of MSNA and systolic arterial pressure were analyzed by autoregressive spectral analysis. The gain and time delay of ABRMSNA was obtained by bivariate autoregressive analysis. Exercise training was performed on a cycle ergometer at moderate intensity, three 60-min sessions per week for 16 wk. Baseline MSNA, gain and time delay of ABRMSNA, and low frequency of MSNA (LFMSNA) to high-frequency ratio (HFMSNA) (LFMSNA/HFMSNA) were similar between groups. ET significantly decreased MSNA. MSNA was unchanged in the UT patients. The gain and time delay of ABRMSNA were unchanged in the ET patients. In contrast, the gain of ABRMSNA was significantly reduced [3.5 ± 0.7 vs. 1.8 ± 0.2, arbitrary units (au)/mmHg, P = 0.04] and the time delay of ABRMSNA was significantly increased (4.6 ± 0.8 vs. 7.9 ± 1.0 s, P = 0.05) in the UT patients. LFMSNA-to-HFMSNA ratio tended to be lower in the ET patients (P < 0.08). Exercise training prevents the deterioration of ABRMSNA in CHF patients.

Mild obstructive sleep apnea does not modulate baroreflex sensitivity in adult patients

BACKGROUND

Obstructive sleep apnea (OSA) is a chronic and progressive disease. OSA is associated with increased cardiovascular morbidity and mortality, the risk being more frequently encountered with severe degrees of OSA. Increased sympathetic activation and impaired cardiac autonomic control as reflected by depressed baroreceptor reflex sensitivity (BRS) are possible mechanisms involved in the cardiovascular complications of OSA. However, it is not known at what stage of OSA that changes in BRS appear. The aim of this study was to evaluate BRS in patients with mild OSA.

METHODS

The study population consisted of 81 overweight patients with mild OSA and 46 body weight-matched non-OSA subjects. BRS, apnea-hypopnea index, body mass index, and metabolic parameters were assessed. The phenylephrine test was used to measure BRS.

RESULTS

Patients in the OSA group were slightly but significantly older than the non-OSA population (50.3±9.3 years vs 45.7±11.1 years, P=0.02). Body mass index, percentage body fat, blood pressure, fasting glucose, insulin, and lipid levels did not differ between the OSA patients and non-OSA subjects. Absolute BRS values in patients with mild OSA and non-OSA subjects (9.97±6.70 ms/mmHg vs 10.51±7.16 ms/mmHg, P=0.67) and BRS values proportional to age-related and sex-related reference values (91.4%±22.7% vs 92.2%±21.8%, P=0.84) did not differ from each other. BRS <50% of the sex-specific reference value was found in 6% of patients with mild OSA and in 2% of non-OSA subjects (P=0.29).

CONCLUSION

Patients with mild OSA did not show evidence of disturbed BRS in comparison with weight-matched non-OSA controls.

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.

Information domain analysis of the spontaneous baroreflex during pharmacological challenges

The information carried by heart period (HP) given systolic arterial pressure (SAP) changes was assessed to characterize spontaneous baroreflex (i.e. the relation linking SAP variability to HP variability): the larger the information carried by HP given SAP changes, the greater the unpredictability of HP given SAP variations, the smaller the strength of the causal coupling from SAP series to HP series. It was typified according to two parameters: i) the information carried by HP given SAP changes within the same heart cycle (i.e. 0-step-ahead information) describing immediate effects of SAP variations on HP; ii) the rate of increase of the information carried by HP given SAP changes as a function of the temporal distance, k, between the conditioning SAP pattern and future HP value (i.e. the rate of increase of k-step-ahead information with k) describing short-term effects of SAP modifications on HP. Both parameters were found under vagal control. Indeed, i) 0-step-ahead information suggested that HP and SAP variabilities were significantly coupled from SAP to HP at baseline and after the reduction of the inhibitory effect of sympathetic control on vagal influences performed through the administration of propranolol or clonidine; and ii) during vagal blockade induced by atropine or combined vagal and sympathetic blockade induced by the administration of propranolol after atropine k-step-ahead information reached a level incompatible with coupled HP and SAP dynamics regardless of k. In addition, it was found that the 0-step-ahead information at baseline and after propranolol and the rate of increase of k-step-ahead information with k at baseline could be exclusively explained in terms of linear HP-SAP interactions. Conversely, the same parameters after clonidine suggested the raise of nonlinear mechanisms probably unveiled by the central sympathetic blockade. Comparison with more traditional parameters describing the HP-SAP variability relation such as baroreflex sensitivity and squared HP-SAP coherence confirmed the complementary value of the proposed information domain analysis.

Clinical value of baroreflex sensitivity

The arterial baroreflex is an important determinant of the neural regulation of the cardiovascular system. It has been recognised that baroreflex-mediated sympathoexcitation contributes to the development and progression of many cardiovascular disorders. Accordingly, the quantitative estimation of the arterial baroreceptor-heart rate reflex (baroreflex sensitivity, BRS), has been regarded as a synthetic index of neural regulation at the sinus atrial node. The evaluation of BRS has been shown to provide clinical and prognostic information in a variety of cardiovascular diseases, including myocardial infarction and heart failure that are reviewed in the present article.

Baroreflex sensitivity: mechanisms and measurement

This article is a brief review of baroreflex physiology, the definition and functional meaning of baroreflex sensitivity, and the methods used to measure baroreflex sensitivity. The arterial baroreflex is important for haemodynamic stability and for cardioprotection, and it has convincingly been demonstrated that baroreflex sensitivity, even when assessed with different methods, has a strong prognostic value. Development of new baroreflex assessing procedures is still ongoing, with a focus on increased reliability in difficult measuring circumstances, e.g., in patients with a weak baroreflex and in patients with frequent arrhythmias.

Baroreflex sensitivity variations in response to propofol anesthesia: comparison between normotensive and hypertensive patients

The aim of this paper is to compare baroreflex sensitivity (BRS) following anesthesia induction via propofol to pre-induction baseline values through a systematic and mathematically robust analysis. Several mathematical methods for BRS quantification were applied to pre-operative and intra-operative data collected from patients undergoing major surgery, in order to track the trend in BRS variations following anesthesia induction, as well as following the onset of mechanical ventilation. Finally, a comparison of BRS trends in chronic hypertensive patients (CH) with respect to non hypertensive (NH) patients was performed. 10 NH and 7 CH patients undergoing major surgery with American Society of Anesthesiologists classification score 2.5 ± 0.5 and 2.6 ± 0.5 respectively, were enrolled in the study. A Granger causality test was carried out to verify the causal relationship between RR interval duration and systolic blood pressure (SBP), and four different mathematical methods were used to estimate the BRS: (1) ratio between autospectra of RR and SBP, (2) transfer function, (3) sequence method and (4) bivariate closed loop model. Three different surgical epochs were considered: baseline, anesthetic procedure and post-intubation. In NH patients, propofol administration caused a decrease in arterial blood pressure (ABP), due to its vasodilatory effects, and a reduction of BRS, while heart rate (HR) remained unaltered with respect to baseline values before induction. A larger decrease in ABP was observed in CH patients when compared to NH patients, whereas HR remained unaltered and BRS was found to be lower than in the NH group at baseline, with no significant changes in the following epochs when compared to baseline. To our knowledge, this is the first study in which the autonomic response to propofol induction in CH and NH patients was compared. The analysis of BRS through a mathematically rigorous procedure in the perioperative period could result in the availability of additional information to guide therapy and anesthesia in uncontrolled hypertensive patients, which are prone to a higher rate of hypotension events occurring during general anesthesia induction.

Relationship between carotid artery mechanics and the spontaneous baroreflex: a noninvasive investigation in normal humans

OBJECTIVES

This study addressed the relationship between spontaneous baroreflex sensitivity and carotid mechanical properties in a clinical setting.

METHODS

In 191 normal volunteers (age range 20-60 years, mean 44 ± 13), spontaneous indices of baroreflex regulation were obtained noninvasively in the time (baroreflex slope, BRS) and frequency domains (α index and systolic arterial pressure-RR interval transfer gain) as well as using an exogenous autoregressive causal model (A.XAR); carotid mechanical properties were estimated by ultrasound. The link between mechanical measures and spontaneous baroreflex indices was explored by multivariate analysis and linear modeling.

RESULTS

Participants were divided into five groups according to age decades. With advancing age we observed a decrease in spontaneous baroreflex indices (BRS from 31.2 to 16.3; α index from 27.4 to 13.6; RR-SAP gain at high frequency from 31 ± 3.0 to 14 ± 3.0 ms/mmHg all P < 0.001) and increase in carotid intima-media thickness (IMT: from 0.53 to 0.69 mm; P < 0.001) and stiffness (local wave speed: from 4.0 to 6.9 m/s; both P < 0.001). A significant correlation was found between spontaneous indices of baroreflex sensitivity and carotid mechanical properties, particularly wave speed (r = -0.328, P < 0.001). After adjusting for age and sex, a significant correlation remained between RR-SAP gain and wave speed and between A.XAR and IMT. Factor analysis and automatic linear modeling confirmed the observation that mechanical carotid properties are strong predictors of the age-related reduction of spontaneous baroreflex.

CONCLUSION

A significant correlation between spontaneous baroreflex indices and local carotid mechanical properties supports the idea that they should be considered in the physiology of baroreflex regulation.

Spontaneous baroreflex sensitivity: prospective validation trial of a novel technique in survivors of acute myocardial infarction

BACKGROUND

Low baroreflex sensitivity (BRS) indicates poor prognosis after acute myocardial infarction. Noninvasive BRS assessment is complicated by nonstationarities and noise in electrocardiogram and pressure signals. Phase-rectified signal averaging is a novel signal processing technology overcoming these problems.

OBJECTIVE

To prospectively validate a BRS measure (baroreflex sensitivity assessed by means of phase-rectified signal averaging [BRS(PRSA)]) based on this technology.

METHODS

Nine hundred forty-one consecutive acute myocardial infarction survivors aged 80 years or younger in sinus rhythm were prospectively enrolled at 2 German university hospitals. All patients underwent 30-minute recordings of electrocardiogram and arterial blood pressures (Portapres; TNO-TPD Biomedical Instrumentation, Amsterdam, Netherlands) within the first 2 weeks after myocardial infarction. BRS(PRSA) was prospectively dichotomized at 1.58 ms/mm Hg. Primary end point was all-cause mortality at 5 years. Multivariable analyses included Global Registry of Acute Coronary Events score (dichotomized at ≥120), sex, BRS(PRSA), left ventricular ejection fraction (dichotomized at ≤35%), and diabetes mellitus. BRS(PRSA) was compared with 3 standard noninvasive BRS measures, that is, the sequence method, the transfer function method, and the correlation method.

RESULTS

During follow-up, 72 patients (7.7%) died. BRS(PRSA) stratified the study population into a high-risk group of 405 patients (≤1.58 ms/mm Hg) with an estimated 5-year mortality of 14.2% and a low-risk group of 536 patients (>1.58 ms/mm Hg) with a 5-year mortality of 2.8% (P <.0001). On multivariable analysis, BRS(PRSA) ≤ 1.58 ms/mm Hg was associated with a hazard ratio of 3.1 (confidence interval 1.7-5.6; P = .001). Predictive power of BRS(PRSA) ≤ 1.58 ms/mm Hg was particularly strong in patients with a Global Registry of Acute Coronary Events score of ≥120 or with a left ventricular ejection fraction of ≤35%.

CONCLUSION

BRS(PRSA) is a powerful and independent predictor of mortality in postinfarction patients especially when assessed in patients with a Global Registry of Acute Coronary Events score of ≥120 or a left ventricular ejection fraction of ≤35%.