Mechanical and neural contributions to hysteresis in the cardiac vagal limb of the arterial baroreflex

Unraveling autonomic cardiovascular control complexity during orthostatic stress: Insights from a mathematical model

Understanding cardiovascular control mediated by the autonomic system remains challenging due to its inherent complexity. Consequently, syndromes such as orthostatic intolerance continue to evoke debates regarding the underlying pathophysiological mechanisms. This study develops a comprehensive mathematical model simulating the control of the sympathetic branch of the cardiovascular system in individuals with normal and abnormal responses to the head-up-tilt test. We recruited four young women: one control, one with vasovagal syncope, one with orthostatic hypertension, and one with orthostatic hypotension, exposing them to an orthostatic head-up tilt test (HUTT) employing non-invasive methods to measure electrocardiography and continuous blood pressure. Our work encompasses a compartmental model formulated using a system of ordinary differential equations. Using heart rate as input, we predict blood pressure, flow, and volume in compartments representing the veins, arteries, heart, and the sympathetic branch of the baroreflex control system. The latter is modulated by high- and low-pressure baroreceptor afferents activated by changes in blood pressure induced by the HUTT. Sensitivity analysis, parameter subset selection, and optimization are employed to estimate patient-specific parameters associated with autonomic performance. The model has seven sensitive and identifiable parameters with significant physiological relevance that can serve as biomarkers for patient classification. Results show that the model can reproduce a spectrum of blood pressure responses successfully, fitting the trajectory displayed by the experimental data. The controller exhibits behavior that emulates the operation of the sympathetic system. These encouraging findings underscore the potential of computational methods in evaluating pathologies associated with autonomic nervous system control, warranting further exploration and novel approaches.

Short communication: Baroreflex function in embryonic emus (Dromiceius novaehollandiae)

The baroreflex involves cardiovascular homeostatic mechanisms that buffer the system against acute deviations in arterial blood pressure. It is comprised of the cardiac limb which involves adjustments in heart rate and the peripheral limb which involves adjustments in vascular resistance. This negative feedback loop mechanism has been investigated in numerous species of adult vertebrates, however our understanding of the maturation and functional importance of the reflex in developing animals remains poorly understood. In egglaying species, our knowledge of this mechanism is limited to the domestic chicken embryo and the embryonic alligator. While each of these species possess a cardiac baroreflex prior to hatching, they differ in the timing when it becomes functional, with the embryonic chicken possessing the reflex at 90% of incubation, while the alligator possesses the reflex at 70% of incubation. In an effort to determine if bird species might share similar patterns of active baroreflex function, we studied embryonic emus (Dromiceius novaehollandiae). However, we hypothesized that emus would possess a pattern of baroreflex function similar to that of the American alligator given the emu embryo possesses functional vagal tone at 70% of incubation, possibly indicating a more mature collection of cardiovascular control mechanism than those found in embryonic chickens. Our findings illustrate that emu embryos possess a hypotensive baroreflex at 90% of incubation. Therefore, our data fail to support our original hypothesis. While only two species of birds have been studied in this context, it could indicate that baroreflex function is not essential for cardiovascular homeostasis in birds for the majority of in ovo development.

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.

Greater post-exercise hypotension in healthy young untrained men after exercising in a hot compared to a temperate environment

This research examined the effects of exercising in a hot compared to a temperate environment on post-exercise hemodynamics in untrained men. We hypothesized exercise in a hot compared to a temperate environment would elicit greater post-exercise hypotension, and this would be attributable to higher cutaneous vascular conductance and sweat loss, and lower heart rate variability (HRV) and cardiac baroreflex sensitivity (cBRS). In a randomized counterbalanced order, 12 untrained healthy men completed two trials involving 40-min leg-cycling exercise at either 23 °C (CON) or 35 °C (HOT). Post-exercise participants rested supine for 60 min at 23 °C whilst hemodynamic and thermoregulatory measurements were assessed. Post-exercise hypotension was greater after exercising in a hot than a temperate environment as indicated by a lower mean arterial pressure at 60 min recovery (CON 83 ± 5 mmHg, HOT 78 ± 5 mmHg, Mean difference [95% confidence interval], -5 [-8, -3] mmHg). Throughout recovery, cutaneous vascular conductance was higher, and cBRS and HRV were lower after exercising in a hot than in a temperate environment (P < 0.05). Sweat loss was greater on HOT than on CON (P < 0.001). Post-exercise hypotension after exercising in the hot environment was associated with sweat loss (r = 0.66, P = 0.02), and changes in cutaneous vascular conductance (r = 0.64, P = 0.03), and HRV (Root mean square of the successive difference in R-R interval [RMSSD]) r=0.75, P = 0.01 and and log high frequency [HF] r=0.66, P = 0.02), but not cBRS (all, r ≤ 0.2, P > 0.05). Post-exercise hypotension was greater after exercise in a hot compared to a temperate environment and may be partially explained by greater sweat loss and cutaneous vascular conductance, and lower HRV.

Age-associated reductions in cardiovagal baroreflex sensitivity are exaggerated in middle-aged and older men with low testosterone

Aging is associated with reductions in cardiovagal baroreflex sensitivity (cBRS), which increases cardiovascular disease risk. Preclinical data indicate that low testosterone reduces cBRS. We determined whether low testosterone is associated with reduced cBRS in healthy men.

METHODS

Twenty-six men categorized as young (N=6; age=31±4 years; testosterone=535±60 ng/dL), middle-aged/older with normal (N=10; aged 56±3 years; testosterone=493±85 ng/dL), or low (N=10; age=57±6 years; testosterone=262±31 ng/dL) testosterone underwent recordings of beat-by-beat blood pressure and R-R interval during rest and two Valsalva maneuvers, and measures of carotid artery compliance. IL-6, CRP, oxidized LDL cholesterol and TAS were measured.

RESULTS

Middle-aged/older men had lower cBRS compared to young men (17.0±6.5 ms/mmHg; p=0.028); middle-age/older men with low testosterone had lower cBRS (5.5±3.2 ms/mmHg; p=0.039) compared to age-matched men with normal testosterone (10.7±4.0 ms/mmHg). No differences existed among groups during Phase II of the Valsalva maneuver; middle-aged/older men with low testosterone had reduced cBRS (4.7±2.6 ms/mmHg) compared to both young (12.8±2.8ms/mmHg; p<0.001) and middle-aged/older men with normal testosterone (8.6±4.4ms/mmHg; p=0.046) during Phase IV of the Valsalva maneuver. There were no differences in oxidized LDL, (p=0.882) or TAS across groups (p=0.633). IL-6 was significantly higher in middle-aged/older men with low testosterone compared to the other groups (p<0.05 for all) and inversely correlated with cBRS (r=-0.594, p=0.007). Middle-aged/older men had reduced carotid artery compliance compared to young, regardless of testosterone status (p<0.001).

CONCLUSIONS

These observations indicate that low testosterone in middle-aged/older men may contribute to a reduction in cBRS; increased inflammation may also contribute to a reduction in cBRS.

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.

Baroreflex Curve Fitting Using a WYSIWYG Boltzmann Sigmoidal Equation

Arterial baroreflex assessment using vasoactive substances enables investigators to collect data pairs over a wide range of blood pressures and reflex reactions. These data pairs relate intervals between heartbeats or sympathetic neural activity to blood pressure values. In an X-Y plot the data points scatter around a sigmoidal curve. After fitting the parameters of a sigmoidal function to the data, the graph’s characteristics represent a rather comprehensive quantitative reflex description. Variants of the 4-parameter Boltzmann sigmoidal equation are widely used for curve fitting. Unfortunately, their ‘slope parameters’ do not correspond to the graph’s actual slope which complicates the analysis and bears the risk of misreporting. We propose a modified Boltzmann sigmoidal function with preserved goodness of fit whose parameters are one-to-one equivalent to the sigmoidal curve’s characteristics.

Effects of Respiratory Muscle Training on Baroreflex Sensitivity, Respiratory Function, and Serum Oxidative Stress in Acute Cervical Spinal Cord Injury

Background: respiratory complications are a leading cause of morbidity and mortality in individuals with spinal cord injury (SCI). We examined the effects of respiratory muscle training (RMT) in patients with acute cervical SCI. Methods: this prospective trial enrolled 44 adults with acute cervical SCI, of which twenty received RMT and twenty-four did not receive RMT. Respiratory function, cardiovascular autonomic function, and reactive oxidative species (ROS) were compared. The experimental group received 40-min high-intensity home-based RMT 7 days per week for 10 weeks. The control group received a sham intervention for a similar period. The primary outcomes were the effects of RMT on pulmonary and cardiovascular autonomic function, and ROS production in individuals with acute cervical SCI. Results: significant differences between the two groups in cardiovascular autonomic function and the heart rate response to deep breathing (p = 0.017) were found at the 6-month follow-up. After RMT, the maximal inspiratory pressure (p = 0.042) and thiobarbituric acid-reactive substances (TBARS) (p = 0.006) improved significantly, while there was no significant difference in the maximal expiratory pressure. Significant differences between the two groups in tidal volume (p = 0.005) and the rapid shallow breathing index (p = 0.031) were found at 6 months. Notably, the SF-36 (both the physical (PCS) and mental (MCS) component summaries) in the RMT group had decreased significantly at the 6-month follow-up, whereas the clinical scores did not differ significantly (p = 0.333) after RMT therapy. Conclusions: High-intensity home-based RMT can improve pulmonary function and endurance and reduce breathing difficulties in patients with respiratory muscle weakness after injury. It is recommended for rehabilitation after spinal cord injury.

Supine Parasympathetic Withdrawal and Upright Sympathetic Activation Underly Abnormalities of the Baroreflex in Postural Tachycardia Syndrome: Effects of Pyridostigmine and Digoxin

[Figure: see text].

Baroreflex function in Parkinson's disease: insights from the modified-Oxford technique

Nonmotor symptoms are common in Parkinson's disease (PD) and they include dysregulation of cardiovascular system, which adversely affects quality of life. Recent studies provide indirect evidence that baroreflex dysfunction may be one of the mechanisms of cardiovascular dysregulation in PD. Herein, we tested the hypothesis that the baroreflex gain, assessed across an extensive range of the reflex arc by eliciting rapid changes in blood pressure (BP) induced by sequential boluses of vasoactive drugs (modified-Oxford technique) would be attenuated in middle-aged patients with PD. Beat-to-beat heart rate (electrocardiography) and BP (finger photoplethysmography) were obtained during 10 min of supine rest preceding the modified-Oxford (bolus of nitroprusside followed by phenylephrine 1 min afterward) in 11 patients with PD (51 ± 6 yr) and 7 age-matched controls (47 ± 6 yr). The resulting systolic BP and R-R interval responses were plotted and fitted with segmental linear regression and symmetric sigmoid model. Spontaneous indices obtained via sequence technique were also used to estimate baroreflex gain. Compared with controls, the estimated gains measured by segmental linear regression (patients: 3.83 ± 2.6 ms/mmHg versus controls: 7.78 ± 1.7 ms/mmHg; P = 0.003) and symmetric sigmoid model (patients: 12.36 ± 6.9 ms/mmHg versus controls: 32.02 ± 19.0 ms/mmHg; P = 0.009) were lower in patients with PD. The operating range of BP was larger in patients with PD compared with controls (13 ± 7 mmHg versus controls: 7 ± 3 mmHg; P = 0.032). Of note, the gain obtained from spontaneous indices was similar between groups. These data indicate that baroreflex gain was reduced by >50% in PD, thereby providing clear and direct evidence that cardiovagal baroreflex dysfunction occurs in PD.NEW & NOTEWORTHY Attenuated baroreflex gain may contribute to adverse cardiovascular outcomes, including orthostatic intolerance symptoms typically observed in patients with Parkinson's disease. We found that the baroreflex gain (assessed by the modified-Oxford technique) is attenuated and accompanied by an increased operating range in patients with Parkinson's disease. These findings highlight that cardiovascular perturbations are required to detect baroreflex impairments and that spontaneous indices do not reveal cardiovagal-baroreflex dysfunction in a middle-aged group of patients with Parkinson's disease.

Effects of inspiratory muscle-training intensity on cardiovascular control in amateur cyclists

Chronic effects of inspiratory muscle training (IMT) on autonomic function and baroreflex regulation are poorly studied. This study aims at evaluating chronic effects of different IMT intensities on cardiovascular control in amateur cyclists. A longitudinal, randomized, controlled blind study was performed on 30 recreational male cyclists undergoing IMT for 11 wk. Participants were randomly allocated into sham-trained group (SHAM, n = 9), trained group at 60% of the maximal inspiratory pressure (MIP60, n = 10), and trained group at critical inspiratory pressure (CIP, n = 11). Electrocardiogram, finger arterial pressure, and respiratory movements were recorded before (PRE) and after (POST) training at rest in supine position (REST) and during active standing (STAND). From the beat-to-beat series of heart period (HP) and systolic arterial pressure (SAP), we computed time domain markers, frequency domain indexes in the low frequency (0.04-0.15 Hz) and high frequency (HF, 0.15-0.4 Hz) bands, an entropy-based complexity index (CI), and baroreflex markers estimated from spontaneous HP-SAP sequences. Compared with SHAM, the positive effect of MIP60 over the HP series led to the HF power increase during REST (PRE: 521.2 ± 447.5 ms²; POST: 1,161 ± 878.9 ms²) and the CI rise during STAND (PRE: 0.82 ± 0.18; POST: 0.97 ± 0.13). Conversely, the negative effect of CIP took the form of the decreased HP mean during STAND (PRE: 791 ± 71 ms; POST: 737 ± 95 ms). No effect of IMT was visible over SAP and baroreflex markers. These findings suggest that moderate-intensity IMT might be beneficial when the goal is to limit cardiac sympathetic hyperactivity at REST and/or in response to STAND.

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.

Characterization of the Asymmetry of the Cardiac and Sympathetic Arms of the Baroreflex From Spontaneous Variability During Incremental Head-Up Tilt

Hysteresis of the baroreflex (BR) is the result of the different BR sensitivity (BRS) when arterial pressure (AP) rises or falls. This phenomenon has been poorly studied and almost exclusively examined by applying pharmacological challenges and static approaches disregarding causal relations. This study inspects the asymmetry of the cardiac BR (cBR) and vascular sympathetic BR (sBR) in physiological closed loop conditions from spontaneous fluctuations of physiological variables, namely heart period (HP) and systolic AP (SAP) leading to the estimation of cardiac BRS (cBRS) and muscle sympathetic nerve activity (MSNA) and diastolic AP (DAP) leading to the estimation of vascular sympathetic BRS (sBRS). The assessment was carried out in 12 young healthy subjects undergoing incremental head-up tilt with table inclination gradually increased from 0 to 60°. Two analytical methods were exploited and compared, namely the sequence (SEQ) and phase-rectified signal averaging (PRSA) methods. SEQ analysis is based on the detection of joint causal schemes representing the HP and MSNA burst rate delayed responses to spontaneous SAP and DAP modifications, respectively. PRSA analysis averages HP and MSNA burst rate patterns after aligning them according to the direction of SAP and DAP changes, respectively. Since cBRSs were similar when SAP went up or down, hysteresis of cBR was not detected. Conversely, hysteresis of sBR was evident with sBRS more negative when DAP was falling than rising. sBR hysteresis was no longer visible during sympathetic activation induced by the orthostatic challenge. These results were obtained via the SEQ method, while the PRSA technique appeared to be less powerful in describing the BR asymmetry due to the strong association between BRS estimates computed over positive and negative AP variations. This study suggests that cBR and sBR provide different information about the BR control, sBR exhibits more relevant non-linear features that are evident even during physiological changes of AP, and the SEQ method can be fruitfully exploited to characterize the BR hysteresis with promising applications to BR branches different from cBR and sBR.

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.

Hypoxic tachycardia is not a result of increased respiratory activity in healthy subjects

NEW FINDINGS

What is the central question of this research? Does increased ventilation contribute to the increase in heart rate during transient exposure to hypoxia in humans? What is the main finding and its importance? Voluntary suppression of the ventilatory response to transient hypoxia does not affect the magnitude of the heart rate response to the stimulus. This indicates that hypoxic tachycardia is not secondary to hyperpnoea in humans. Better understanding of the physiology underlying the cardiovascular response to hypoxia might help in identification of new markers of elevated chemoreceptor activity, which has been proposed as a target in treatment of sympathetically mediated diseases.

ABSTRACT

Animal data suggest that hypoxic tachycardia is secondary to hyperpnoea, and for years this observation has been extrapolated to humans, despite a lack of experimental evidence. We addressed this issue in 17 volunteers aged 29 ± 7 (SD) years. A transient hypoxia test, comprising several nitrogen-breathing episodes, was performed twice in each subject. In the first test, the subject breathed spontaneously (spontaneous breathing). In the second test, the subject was repeatedly asked to adjust his or her depth and rate of breathing according to visual (real-time inspiratory flow) and auditory (metronome sound) cues, respectively (controlled breathing), to maintain respiration at the resting level during nitrogen-breathing episodes. Hypoxic responsiveness, including minute ventilation [Hyp-VI; in liters per minute per percentage of blood oxygen saturation ( S p O 2 )], tidal volume [Hyp-VT; in litres per S p O 2 ], heart rate [Hyp-HR; in beats per minute per S p O 2 ], systolic [Hyp-SBP; in millimetres of mercury per S p O 2 ] and mean blood pressure [Hyp-MAP; in millimetres of mercury per S p O 2 ] and systemic vascular resistance [Hyp-SVR; in dynes seconds (centimetres)^-5 per S p O 2 ] was calculated as the slope of the regression line relating the variable to S p O 2 , including pre- and post-hypoxic values. The Hyp-VI and Hyp-VT were reduced by 69 ± 25 and 75 ± 10%, respectively, in controlled versus spontaneous breathing (Hyp-VI, -0.30 ± 0.15 versus -0.11 ± 0.09; Hyp-VT, -0.030 ± 0.024 versus -0.007 ± 0.004; both P < 0.001). However, the cardiovascular responses did not differ between spontaneous and controlled breathing (Hyp-HR, -0.62 ± 0.24 versus -0.71 ± 0.33; Hyp-MAP, -0.43 ± 0.19 versus -0.47 ± 0.21; Hyp-SVR, 9.15 ± 5.22 versus 9.53 ± 5.57; all P ≥ 0.22), indicating that hypoxic tachycardia is not secondary to hyperpnoea. Hyp-HR was correlated with Hyp-SVR (r = -074 and -0.80 for spontaneous and controlled breathing, respectively; both P < 0.05) and resting barosensitivity assessed with the sequence technique (r = -0.60 for spontaneous breathing; P < 0.05). This might suggest that the baroreflex mechanism is involved.

Acute hydrocortisone administration reduces cardiovagal baroreflex sensitivity and heart rate variability in young men

KEY POINTS

A surge in cortisol during acute physiological and pathophysiological stress may precipitate ventricular arrhythmia and myocardial infarction. Reduced cardiovagal baroreflex sensitivity and heart rate variability are observed during acute stress and are associated with an increased risk of acute cardiac events. In the present study, healthy young men received either a single iv bolus of saline (placebo) or hydrocortisone, 1 week apart, in accordance with a randomized, placebo-controlled, cross-over study design. Hydrocortisone acutely increased heart rate and blood pressure and reduced cardiovagal baroreflex sensitivity and heart rate variability in young men. These findings suggest that, by reducing cardiovagal baroreflex sensitivity and heart rate variability, acute surges in cortisol facilitate a pro-arrhythmic milieu and provide an important mechanistic link between stress and acute cardiac events ABSTRACT: Surges in cortisol concentration during acute stress may increase cardiovascular risk. To better understand the interactions between cortisol and the autonomic nervous system, we determined the acute effects of hydrocortisone administration on cardiovagal baroreflex sensitivity (BRS), heart rate variability (HRV) and cardiovascular reactivity. In a randomized, placebo-controlled, single-blinded cross-over study, 10 healthy males received either a single iv bolus of saline (placebo) or 200 mg of hydrocortisone, 1 week apart. Heart rate (HR), blood pressure (BP) and limb blood flow were monitored 3 h later, at rest and during the sequential infusion of sodium nitroprusside and phenylephrine (modified Oxford Technique), a cold pressor test and a mental arithmetic stress task. HRV was assessed using the square root of the mean of the sum of the squares of differences between successive R-R intervals (rMSSD). Hydrocortisone markedly increased serum cortisol 3 h following infusion and also compared to placebo. In addition, hydrocortisone elevated resting HR (+7 ± 4 beats min^-1 ; P < 0.001) and systolic BP (+5 ± 5 mmHg; P = 0.008); lowered cardiovagal BRS [geometric mean (95% confidence interval) 15.6 (11.1-22.1) ms/mmHg vs. 26.2 (17.4--39.5) ms/mmHg, P = 0.011] and HRV (rMSSD 59 ± 29 ms vs. 84 ± 38 ms, P = 0.004) and increased leg vasoconstrictor responses to cold pressor test (Δ leg vascular conductance -45 ± 20% vs. -23 ± 26%; P = 0.023). In young men, an acute cortisol surge is accompanied by increases in HR and BP, as well as reductions in cardiovagal BRS and HRV, potentially providing a pro-arrhythmic milieu that may precipitate ventricular arrhythmia or myocardial infarction and increase cardiovascular risk.

Asymmetries in reciprocal baroreflex mechanisms and chronic pain severity: Focusing on irritable bowel syndrome

BACKGROUND

Objective measures of pain severity remain ill defined, although its accurate measurement is critical. Reciprocal baroreflex mechanisms of blood pressure (BP) control were found to impact differently on pain regulation, and thus their asymmetry was hypothesized to also connect to chronic pain duration and severity.

METHODS

Seventy-eight female patients with irritable bowel syndrome (IBS) and 27 healthy women were assessed for IBS severity and chronicity, negative affect, and various measures of resting autonomic function including BP, heart rate and its variability (HRV), baroreceptor-sensitivity to activations and inhibitions, gains of brady- and tachy-cardiac baro-responses, gains of BP falls/rises, and BP start points for these spontaneous baroreflexes.

KEY RESULTS

IBS directly and indirectly (through increased negative affect) was associated with asymmetry between baroreceptor activations/inhibitions compared to symmetrical baroreflex reciprocity in the healthy women. In the IBS group, independently of specific IBS symptoms, pain chronicity was associated with (i) decreased BP falls coupled with either (a) decreased tachycardia associated with lower disease severity (earlier "pain resilience" mechanism), or (b) decreased bradycardia associated with higher disease severity (later "pain decompensation" mechanism), or (ii) increased BP start point for baroreceptor activations coupled with either (a) BP increase (delayed "pain adaptation" mechanism) or (b) affect-related HRV decrease (delayed "pain aggravation" mechanism).

CONCLUSION AND INFERENCES

We anticipate the findings to be a starting point for validating these autonomic metrics of pain suffering and pain coping mechanisms in other chronic pain syndromes to suggest them as biomarkers of its severity and duration for profiling and correct management of chronic pain patients.

Effects of Respiratory Training on Heart Rate Variability and Baroreflex Sensitivity in Individuals With Chronic Spinal Cord Injury

OBJECTIVE

To evaluate the effects of pressure threshold respiratory training (RT) on heart rate variability and baroreflex sensitivity in persons with chronic spinal cord injury (SCI).

DESIGN

Before-after intervention case-controlled clinical study.

SETTING

SCI research center and outpatient rehabilitation unit.

PARTICIPANTS

Participants (N=44) consisted of persons with chronic SCI ranging from C2 to T11 who participated in RT (n=24), and untrained control subjects with chronic SCI ranging from C2 to T9 (n=20).

INTERVENTIONS

A total of 21±2 RT sessions performed 5 days a week during a 4-week period using a combination of pressure threshold inspiratory and expiratory devices.

MAIN OUTCOME MEASURES

Forced vital capacity (FVC), forced expiratory volume in 1 second (FEV₁), and beat-to-beat arterial blood pressure and heart rate changes during the 5-second-long maximum expiratory pressure maneuver (5s MEP) and the sit-up orthostatic stress test, acquired before and after the RT program.

RESULTS

In contrast to the untrained controls, individuals in the RT group experienced significantly increased FVC and FEV₁ (both P<.01) in association with improved quality of sleep, cough, and speech. Sympathetically (phase II) and parasympathetically (phase IV) mediated baroreflex sensitivity both significantly (P<.05) increased during the 5s MEP. During the orthostatic stress test, improved autonomic control over heart rate was associated with significantly increased sympathetic and parasympathetic modulation (low- and high-frequency change: P<.01 and P<.05, respectively).

CONCLUSIONS

Inspiratory-expiratory pressure threshold RT is a promising technique to positively affect both respiratory and cardiovascular dysregulation observed in persons with chronic SCI.

Increased sympathetic nerve activity and reduced cardiac baroreflex sensitivity in rheumatoid arthritis

KEY POINTS

Rheumatoid arthritis (RA) is a chronic inflammatory condition associated with an increased risk of cardiovascular mortality. Increased sympathetic nerve activity and reduced cardiac baroreflex sensitivity heighten cardiovascular risk, althogh whether such autonomic dysfunction is present in RA is not known. In the present study, we observed an increased sympathetic nerve activity and reduced cardiac baroreflex sensitivity in patients with RA compared to matched controls. Pain was positively correlated with sympathetic nerve activity and negatively correlated with cardiac baroreflex sensitivity. The pattern of autonomic dysfunction that we describe may help to explain the increased cardiovascular risk in RA, and raises the possibility that optimizing pain management may resolve autonomic dysfunction in RA.

ABSTRACT

Rheumatoid arthritis (RA) is a chronic inflammatory condition associated with increased cardiovascular morbidity/mortality and an incompletely understood pathophysiology. In animal studies, central and blood borne inflammatory cytokines that can be elevated in RA evoke pathogenic increases in sympathetic activity and reductions in baroreflex sensitivity (BRS). We hypothesized that muscle sympathetic nerve activity (MSNA) was increased and BRS decreased in RA. MSNA, blood pressure and heart rate (HR) were recorded in age- and sex-matched RA-normotensive (n = 13), RA-hypertensive patients (RA-HTN; n = 17), normotensive (NC; n = 17) and hypertensive controls (HTN; n = 16). BRS was determined using the modified Oxford technique. Inflammation and pain were determined using serum high sensitivity C-reactive protein (hs-CRP) and a visual analogue scale (VAS), respectively. MSNA was elevated similarly in RA, RA-HTN and HTN patients (32 ± 9, 35 ± 14, 37 ± 8 bursts min^-1 ) compared to NC (22 ± 9 bursts min^-1 ; P = 0.004). Sympathetic BRS was similar between groups (P = 0.927), whereas cardiac BRS (cBRS) was reduced in RA, RA-HTN and HTN patients [5(3-8), 4 (2-7), 6 (4-9) ms mmHg^-1 ] compared to NC [11 (8-15) ms mmHg^-1 ; P = 0.002]. HR was independently associated with hs-CRP. Increased MSNA and reduced cBRS were associated with hs-CRP although confounded in multivariable analysis. VAS was independently associated with MSNA burst frequency, cBRS and HR. We provide the first evidence for heightened sympathetic outflow and reduced cBRS in RA that can be independent of hypertension. In RA patients, reported pain was positively correlated with MSNA and negatively correlated with cBRS. Future studies should assess whether therapies to ameliorate pain and inflammation in RA restores autonomic balance and reduces cardiovascular events.

Arterial baroreflex control of sympathetic nerve activity and heart rate in patients with type 2 diabetes

Despite greater blood pressure reactivity to acute cardiovascular stressors and a higher prevalence of hypertension in type 2 diabetes (T2D) patients, limited information is available regarding arterial baroreflex (ABR) control in T2D. We hypothesized that ABR control of muscle sympathetic nerve activity (MSNA) and heart rate (HR) are attenuated in T2D patients. Seventeen T2D patients (50 ± 2 yr; 31 ± 1 kg/m²), 9 weight-matched controls (WM-CON, 46 ± 2 yr; 32 ± 2 kg/m²) and 10 lean controls (Lean-CON, 49 ± 3 yr; 23 ± 1 kg/m²), underwent bolus infusions of sodium nitroprusside (100 μg) followed 60 s later by phenylephrine (150 μg) and weighted linear regression performed. No group differences in overall sympathetic baroreflex gain were observed (T2D: -2.5 ± 0.3 vs. WM-CON: -2.6 ± 0.2 vs. Lean-CON: -2.7 ± 0.4 arbitrary units·beat·mmHg^-1, P > 0.05) or in sympathetic baroreflex gain when derived separately during blood pressure (BP) falls (nitroprusside) and BP rises (phenylephrine). In contrast, overall cardiac baroreflex gain was reduced in T2D patients compared with Lean-CON (T2D: 8.2 ± 1.5 vs. Lean-CON: 15.6 ± 2.9 ms·mmHg^-1, P < 0.05) and also tended to be reduced in WM-CON (9.3 ± 1.9 ms·mmHg^-1) compared with Lean-CON (P = 0.059). Likewise, during BP rises, cardiac baroreflex gain was reduced in T2D patients and weight-matched controls compared with lean controls (P < 0.05), whereas no group differences were found during BP falls (P > 0.05). Sympathetic and cardiac ABR gains were comparable between normotensive and hypertensive T2D patients (P > 0.05). These findings suggest preserved ABR control of MSNA in T2D patients compared with both obese and lean age-matched counterparts, with a selective impairment in ABR HR control in T2D that may be related to obesity.

Reductions in carotid chemoreceptor activity with low-dose dopamine improves baroreflex control of heart rate during hypoxia in humans

The purpose of the present investigation was to examine the contribution of the carotid body chemoreceptors to changes in baroreflex control of heart rate with exposure to hypoxia. We hypothesized spontaneous cardiac baroreflex sensitivity (scBRS) would be reduced with hypoxia and this effect would be blunted when carotid chemoreceptor activity was reduced with low-dose dopamine. Fifteen healthy adults (11 M/4 F) completed two visits randomized to intravenous dopamine or placebo (saline). On each visit, subjects were exposed to 5-min normoxia (~99% SpO2), followed by 5-min hypoxia (~84% SpO2). Blood pressure (intra-arterial catheter) and heart rate (ECG) were measured continuously and scBRS was assessed by spectrum and sequence methodologies. scBRS was reduced with hypoxia (P < 0.01). Using the spectrum analysis approach, the fall in scBRS with hypoxia was attenuated with infusion of low-dose dopamine (P < 0.01). The decrease in baroreflex sensitivity to rising pressures (scBRS "up-up") was also attenuated with low-dose dopamine (P < 0.05). However, dopamine did not attenuate the decrease in baroreflex sensitivity to falling pressures (scBRS "down-down"; P > 0.05). Present findings are consistent with a reduction in scBRS with systemic hypoxia. Furthermore, we show this effect is partially mediated by the carotid body chemoreceptors, given the fall in scBRS is attenuated when activity of the chemoreceptors is reduced with low-dose dopamine. However, the improvement in scBRS with dopamine appears to be specific to rising blood pressures. These results may have important implications for impairments in baroreflex function common in disease states of acute and/or chronic hypoxemia, as well as the experimental use of dopamine to assess such changes.

Impact of Restless Legs Syndrome on Cardiovascular Autonomic Control

STUDY OBJECTIVES

To examine whether patients with restless legs syndrome demonstrate specific alterations in cardiovascular autonomic control.

METHODS

Patients with moderate-severe restless legs syndrome (n = 20, 80% female) and controls (n = 20) matched for age, sex, body mass index, and free of hypertension and cardiovascular disease were enrolled. We assessed cardiovagal baroreflex gain via the modified Oxford technique, sympathetically mediated vascular responses to isometric exercise to fatigue, bradycardiac response to Valsalva maneuver, and respiratory sinus arrhythmia during paced breathing. Standard electrocardiography, beat-by-beat arterial pressure, respiration, and popliteal blood flow velocity were recorded continuously.

RESULTS

Resting blood pressure and heart rate were similar between groups. However, baroreflex gain averaged 14.3 ± 1.4 msec/mm Hg in restless legs syndrome and was lower than in controls (22.6 ± 3.5 msec/mm Hg, P = 0.04). Hemodynamic responses to isometric exercise were similar between groups, though participants with restless legs syndrome had lower leg blood flow (P < 0.001), with greater leg vascular resistance (P < 0.0001), before and during isometric exercise. Respiratory sinus arrhythmia and Valsalva ratios were similar between groups. Neither baroreflex gain nor vascular resistance was correlated with sleep duration, sleep quality, or symptom duration.

CONCLUSION

Patients with restless legs syndrome demonstrate compromised cardiovagal control, specific to the arterial baroreflex, with greater peripheral vascular resistance, potentially due to heightened sympathetic outflow. These autonomic alterations may directly relate to the higher prevalence of cardiovascular disease in restless legs syndrome.

High Intensity Aerobic Exercise Training Improves Deficits of Cardiovascular Autonomic Function in a Rat Model of Type 1 Diabetes Mellitus with Moderate Hyperglycemia

Indices of cardiovascular autonomic neuropathy (CAN) in experimental models of Type 1 diabetes mellitus (T1DM) are often contrary to clinical data. Here, we investigated whether a relatable insulin-treated model of T1DM would induce deficits in cardiovascular (CV) autonomic function more reflective of clinical results and if exercise training could prevent those deficits. Sixty-four rats were divided into four groups: sedentary control (C), sedentary T1DM (D), control exercise (CX), or T1DM exercise (DX). Diabetes was induced via multiple low-dose injections of streptozotocin and blood glucose was maintained at moderate hyperglycemia (9-17 mM) through insulin supplementation. Exercise training consisted of daily treadmill running for 10 weeks. Compared to C, D had blunted baroreflex sensitivity, increased vascular sympathetic tone, increased serum neuropeptide Y (NPY), and decreased intrinsic heart rate. In contrast, DX differed from D in all measures of CAN (except NPY), including heart rate variability. These findings demonstrate that this T1DM model elicits deficits and exercise-mediated improvements to CV autonomic function which are reflective of clinical T1DM.

The repeated sit-to-stand maneuver is a superior method for cardiac baroreflex assessment: a comparison with the modified Oxford method and Valsalva maneuver

Baroreflex assessment has diagnostic and prognostic utility in the clinical and research environments, and there is a need for a reliable, simple, noninvasive method of assessment. The repeated sit-to-stand method induces oscillatory changes in blood pressure (BP) at a desired frequency and is suitable for assessing dynamic baroreflex sensitivity (BRS). However, little is known about the reliability of this method and its ability to discern fundamental properties of the baroreflex. In this study we sought to: 1) evaluate the reliability of the sit-to-stand method for assessing BRS and compare its performance against two established methods (Oxford method and Valsalva maneuver), and 2) examine whether the frequency of the sit-to-stand method influences hysteresis. Sixteen healthy participants underwent three trials of each method. For the sit-to-stand method, which was performed at 0.1 and 0.05 Hz, BRS was quantified as an integrated response (BRSINT) and in response to falling and rising BP (BRSDOWN and BRSUP, respectively). Test retest reliability was assessed using the intraclass correlation coefficient (ICC). Irrespective of frequency, the ICC for BRSINT during the sit-to-stand method was ≥0.88. The ICC for a rising BP evoked by phenylephrine (PEGAIN) in the Oxford method was 0.78 and ≤0.5 for the remaining measures. During the sit-to-stand method, hysteresis was apparent in all participants at 0.1 Hz but was absent at 0.05 Hz. These findings indicate the sit-to-stand method is a statistically reliable BRS assessment tool and suitable for the examination of baroreflex hysteresis. Using this approach we showed that baroreflex hysteresis is a frequency-dependent phenomenon.

Early intrinsic hyperexcitability does not contribute to motoneuron degeneration in amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS) the large motoneurons that innervate the fast-contracting muscle fibers (F-type motoneurons) are vulnerable and degenerate in adulthood. In contrast, the small motoneurons that innervate the slow-contracting fibers (S-type motoneurons) are resistant and do not degenerate. Intrinsic hyperexcitability of F-type motoneurons during early postnatal development has long been hypothesized to contribute to neural degeneration in the adult. Here, we performed a critical test of this hypothesis by recording from identified F- and S-type motoneurons in the superoxide dismutase-1 mutant G93A (mSOD1), a mouse model of ALS at a neonatal age when early pathophysiological changes are observed. Contrary to the standard hypothesis, excitability of F-type motoneurons was unchanged in the mutant mice. Surprisingly, the S-type motoneurons of mSDO1 mice did display intrinsic hyperexcitability (lower rheobase, hyperpolarized spiking threshold). As S-type motoneurons are resistant in ALS, we conclude that early intrinsic hyperexcitability does not contribute to motoneuron degeneration.

DOI:http://dx.doi.org/10.7554/eLife.04046.001

Amyotrophic lateral sclerosis (ALS), which is also known as Lou Gherig's disease or motoneuron disease, is a neurodegenerative disorder in which muscles throughout the body gradually waste away due to the death of the neurons that control their activity. The disease often begins with weakness of the arms or legs, but progresses to include difficulties with movements such as swallowing and breathing. Around half of those affected die within 3 or 4 years of diagnosis.

Although the causes of the disease are unclear, one leading theory is that the neurons that control muscle activity—motoneurons—are hyperexcitable during early development, and therefore fire too frequently. This causes too much calcium to enter the neurons and, because calcium is toxic to cells in high quantities, leads ultimately to the death of the neurons. But despite the popularity of this idea, and the fact that many therapeutic assays for ALS are based on attempts to reverse this process, there is no direct evidence that early hyperexcitability of motoneurons causes their death in ALS.

Leroy et al. have now tested this theory directly by taking advantage of the fact that not all motoneurons are affected by ALS. The large ‘F-type’ motoneurons that control fast-contracting muscle fibres degenerate in ALS, whereas the small ‘S-type’ motoneurons that control slow-contracting muscle fibres do not. A comparison of F-type and S-type motoneurons in a mouse model of ALS revealed that, surprisingly, S-type motoneurons are hyperexcitable in young ALS mice, whereas F-type motoneurons are not.

Given that S-type motoneurons are resistant to the effects of ALS, this indicates that early hyperexcitability cannot be the cause of motoneuron degeneration. Previous studies have tended to pool different types of motoneurons together, which might explain why this difference has not been seen before. Further experiments are now required to determine whether the hyperexcitability of S-type motoneurons persists into adulthood, and whether it might even contribute to their survival in ALS.

DOI:http://dx.doi.org/10.7554/eLife.04046.002

Assessment of human baroreflex function using carotid ultrasonography: what have we learnt?

The arterial baroreflex is critical to both short- and long-term regulation of blood pressure. However, human baroreflex research has been largely limited to the association between blood pressure and cardiac period (or heart rate) or indices of vascular sympathetic function. Over the past decade, emerging techniques based on carotid ultrasound imaging have allowed new means of understanding and measuring the baroreflex. In this review, we describe the assessment of the mechanical and neural components of the baroreflex through the use of carotid ultrasound imaging. The mechanical component refers to the change in carotid artery diameter in response to changes in arterial pressure, and the neural component refers to the change in R-R interval (cardiac baroreflex) or muscle sympathetic nerve activity (sympathetic baroreflex) in response to this barosensory vessel stretch. The key analytical concepts and techniques are discussed, with a focus on the assessment of baroreflex sensitivity via the modified Oxford method. We illustrate how the application of carotid ultrasound imaging has contributed to a greater understanding of baroreflex physiology in humans, covering topics such as ageing and diurnal variation, and physiological challenges including exercise, postural changes and mental stress.

Increased central arterial stiffness explains baroreflex dysfunction in spinal cord injury

After cervical spinal cord injury (SCI), orthostatic hypotension and intolerance commonly ensue. The cardiovagal baroreflex plays an important role in the acute regulation of blood pressure (BP) and is associated with the onset of presyncope. The cardiovagal baroreflex is dysfunctional after SCI; however, this may be influenced by either increased stiffening of the arteries containing the stretch-receptors (which has been shown in SCI) or a more downstream neural mechanism (i.e., solitary nucleus, sinoatrial node). Identifying where along this pathway baroreflex dysfunction occurs may highlight a potential therapeutic target. This study examined the relationship between spontaneous cardiovagal baroreflex sensitivity (BRS) and common carotid artery (CCA) stiffness in those with high level SCI before and after midodrine (alpha1-agonist) administration, as well as in able-bodied controls, to evaluate: (1) the role arterial stiffening plays mediating baroreflex function after SCI and (2) the effect of normalizing BP on these parameters. Three to five min recordings of beat-by-beat BP and heart rate, as well as 30 sec duration recordings of CCA diameter were used for analysis. All participants were tested supine and during upright-tilt. Arterial stiffness (β-stiffness index) was elevated in those with SCI when upright (+12%; p<0.05). Further, β-stiffness index was negatively related to reduced BRS in those with SCI when upright (R2=0.55; p<0.05), but not in able-bodied persons. Normalizing BP did not improve BRS or CCA stiffness. This study clearly shows that reduced BRS is closely related to increased arterial stiffness in the population with SCI.

Translational examination of changes in baroreflex function after renal denervation in hypertensive rats and humans

Renal denervation has shown promise in the treatment of resistant hypertension, although the mechanisms underlying the blood pressure (BP) reduction remain unclear. In a translational study of spontaneously hypertensive rats (n=7, surgical denervation) and resistant hypertensive human patients (n=8; 5 men, 33-71 years), we examined the relationship among changes in BP, sympathetic nerve activity, and cardiac and sympathetic baroreflex function after renal denervation. In humans, mean systolic BP (SBP; sphygmomanometry) and muscle sympathetic nerve activity (microneurography) were unchanged at 1 and 6 months after renal denervation (P<0.05). Interestingly, 4 of 8 patients showed a 10% decrease in SBP at 6 months, but sympathetic activity did not necessarily change in parallel with SBP. In contrast, all rats showed significant and immediate decreases in telemetric SBP and lumbar sympathetic activity (P<0.05), 7 days after denervation. Despite no change in SBP, human cardiac and sympathetic baroreflex function (sequence and threshold techniques) showed improvements at 1 and 6 months after denervation, particularly through increased sympathetic baroreflex sensitivity to falling BP. This was mirrored in spontaneously hypertensive rats; cardiac and sympathetic baroreflex sensitivity (spontaneous sequence and the Oxford technique) improved 7 days after denervation. The more consistent results in rats may be because of a more complete (>90% reduction in renal norepinephrine content) denervation. We conclude that (1) renal denervation improves BP in some patients, but sympathetic activity does not always change in parallel, and (2) baroreflex sensitivity is consistently improved in animals and humans, even when SBP has not decreased. Determining procedural success will be crucial in advancing this treatment modality.

Baroreflex sensitivity analysis: spontaneous methodology vs. Valsalva's maneuver

INTRODUCTION

Spontaneous analysis techniques and Valsalva's maneuver (VM) are often used as a non-pharamcological approach to assess both sympathetic (sBRS) and cardiovagal (cBRS) baroreflex sensitivity. Despite their wide utilization, no studies have assessed the intra-individual reliability between these analysis techniques. Accordingly, we hypothesized that spontaneous BRS would be positively correlated to VM BRS.

METHODS

Heart rate (HR), blood pressure (BP), and muscle sympathetic nerve activity (MSNA) were measured in 26 healthy subjects (age, 22 ± 1 year; 14 men and 12 women) during 10 min supine rest and 3 VM at 40 mmHg expiratory pressure (15 s, 1 min recoveries). For spontaneous BRS, relations between diastolic BP and MSNA were used to determine sBRS, while relations between systolic BP and R-R interval were used to determine cBRS. During VM, sBRS was the ratio of MSNA and the maximum diastolic BP reduction during early phase II, and cBRS was derived from linear relations between systolic BP and R-R interval during early phase II (i.e., hypotensive stimulus) and phase IV (i.e., hypertensive stimulus).

RESULTS

Spontaneous sBRS was significantly correlated to VM sBRS (r = 0.516, p = 0.036). In contrast, spontaneous cBRS from up-up sequence was not correlated to VM phase IV cBRS (r = 0.274, p = 0.109). Similarly, spontaneous cBRS from down-down sequence was not correlated to VM phase II cBRS (r = 0.199, p = 0.207).

CONCLUSION

In conclusion, our findings demonstrate positive association between spontaneous sBRS and VM sBRS, but there is no correlation between spontaneous and VM cBRS.

Postural influences on the mechanical and neural components of the cardiovagal baroreflex

AIM

The ability to maintain arterial blood pressure when faced with a postural challenge has implications for the occurrence of syncope and falls. It has been suggested that posture-induced declines in the mechanical component of the baroreflex response drive reductions in cardiovagal baroreflex sensitivity associated with postural stress. However, these conclusions are largely based upon spontaneous methods of baroreflex assessment, the accuracy of which has been questioned. Therefore, the aim was to engage a partially open-loop approach to explore the influence of posture on the mechanical and neural components of the baroreflex.

METHODS

In nine healthy participants, we measured continuous blood pressure, heart rate, RR interval and carotid artery diameter during supine and standing postures. The modified Oxford method was used to quantify baroreflex sensitivity.

RESULTS

In response to falling pressures, baroreflex sensitivity was similar between postures (P = 0.798). In response to rising pressures, there was an attenuated (P = 0.042) baroreflex sensitivity (mean ± SE) in the standing position (-0.70 ± 0.11 beats min(-1) mmHg(-1)) compared with supine (-0.83 ± 0.06 beats min(-1) mmHg(-1)). This was explained by a diminished (P = 0.016) neural component whilst standing (-30.17 ± 4.16 beats min(-1) mm(-1)) compared with supine (-38.23 ± 3.31 beats min(-1) mm(-1)). These effects were consistent when baroreflex sensitivity was determined using RR interval.

CONCLUSION

Cardiovagal baroreflex sensitivity in response to rising pressures is reduced in young individuals during postural stress. Our data suggest that the mechanical component is unaffected by standing, and the reduction in baroreflex sensitivity is driven by the neural component.

Spontaneous fluctuation indices of the cardiovagal baroreflex accurately measure the baroreflex sensitivity at the operating point during upright tilt

Spontaneous fluctuation indices of cardiovagal baroreflex have been suggested to be inaccurate measures of baroreflex function during orthostatic stress compared with alternate open-loop methods (e.g. neck pressure/suction, modified Oxford method). We therefore tested the hypothesis that spontaneous fluctuation measurements accurately reflect local baroreflex gain (slope) at the operating point measured by the modified Oxford method, and that apparent differences between these two techniques during orthostasis can be explained by a resetting of the baroreflex function curve. We computed the sigmoidal baroreflex function curves supine and during 70° tilt in 12 young, healthy individuals. With the use of the modified Oxford method, slopes (gains) of supine and upright curves were computed at their maxima (Gmax) and operating points. These were compared with measurements of spontaneous indices in both positions. Supine spontaneous analyses of operating point slope were similar to calculated Gmax of the modified Oxford curve. In contrast, upright operating point was distant from the centering point of the reset curve and fell on the nonlinear portion of the curve. Whereas spontaneous fluctuation measurements were commensurate with the calculated slope of the upright modified Oxford curve at the operating point, they were significantly lower than Gmax. In conclusion, spontaneous measurements of cardiovagal baroreflex function accurately estimate the slope near operating points in both supine and upright position.

The arterial baroreflex resets with orthostasis

The arterial baroreflexes, located in the carotid sinus and along the arch of the aorta, are essential for the rapid short term autonomic regulation of blood pressure. In the past, they were believed to be inactivated during exercise because blood pressure, heart rate, and sympathetic activity were radically changed from their resting functional relationships with blood pressure. However, it was discovered that all relationships between carotid sinus pressure and either HR or sympathetic vasoconstriction maintained their curvilinear sigmoidal shape but were reset or shifted so as to best defend BP during exercise. To determine whether resetting also occurs during orthostasis, we examined the arterial baroreflexes measured supine and upright tilt. We studied the relationships between systolic BP and HR (the cardiovagal baroreflex), mean BP, and ventilation (the ventilatory baroreflex) and diastolic BP and sympathetic nerve activity (the sympathetic baroreflex). We accomplished these measurements by using the modified Oxford method in which BP was rapidly varied with bolus injections of sodium nitroprusside followed 1 min later by bolus injections of phenylephrine. Both the cardiovagal and ventilatory baroreflexes were “reset” with no change in gain or response range. In contrast, the sympathetic baroreflex was augmented as well as shifted causing an increase in peripheral resistance that improved the subjects’ defense against hypotension. This contrasts with findings during exercise in which peripheral resistance in active skeletal muscle is not increased. This difference is likely selective for exercising muscle and may represent the actions of functional sympatholysis by which exercise metabolites interfere with adrenergic vasoconstriction.

Can baroreflex measurements with spontaneous sequence analysis be improved by also measuring breathing and by standardization of filtering strategies?

Baroreflex sensitivity (BRS) is known to be attenuated by inspiration and all the original BRS methodologies took this into account by measuring only in expiration. Spontaneous sequence analysis (SSA) is a non-invasive clinical tool widely used to estimate BRS in Man but does not take breathing into account. We have therefore modified it to test whether it too can detect inspiratory attenuation. Traditional SSA is also entangled with issues of distinguishing causal from random relationships between blood pressure and heart period and of the optimum choice of data filter settings. We have also tested whether the sequences our modified SSA rejects do behave as random relationships and show the limitations of the absence of filter standardization. SSA was performed on eupneic data from 1 h periods in 20 healthy subjects. Applying SSA traditionally produced a mean BRS of 23 ± 3 ms mmHg(-1). After modification to measure breathing, SSA detected significant inspiratory attenuation (11 ± 1 ms mmHg(-1)), and the mean expiratory BRS was significantly higher (26 ± 5 ms mmHg(-1)). Traditional SSA therefore underestimates BRS by an amount (3 ms mmHg(-1)) as big as the major physiological and clinical factors known to alter BRS. We show that the sequences rejected by SSA do behave like random associations between pressure and period. We also show the minimal effect of the r(2) filter and the biases that some pressure and heart period filters can introduce. We discuss whether SSA might be improved by standardization of filter settings and by also measuring breathing.

Diurnal Variation in the Mechanical and Neural Components of the Baroreflex

Diminished baroreflex sensitivity in the morning negatively influences morning coronary blood flow and blood pressure control in hypertensive patients. Our aim was to determine the contribution of the mechanical and neural components of the cardiac baroreflex to diurnal variation in blood pressure control. In 12 healthy participants, we used the modified Oxford method to quantify baroreflex sensitivity for rising (G up ) and falling (G down ) pressures in the morning (7:00 am ) and afternoon (4:00 pm ). Beat-to-beat blood pressure, R-R intervals, and carotid artery diameter measurements were recorded. Integrated sensitivity was determined by plotting R-R intervals against systolic blood pressure. The mechanical component was carotid artery diameter plotted against systolic blood pressure, and the neural component was R-R intervals plotted against carotid artery diameter. Linear mixed models were used to compare the integrated, mechanical, and neural sensitivities between morning and afternoon. We found significant diurnal variation in integrated sensitivity, with an attenuated response in the morning (G up =13.0±0.6; G down =6.3±0.4 ms/mm Hg) when compared with the afternoon (G up =15.1±0.6; G down =12.6±0.4 ms/mm Hg). For rising pressures, the diminished integrated sensitivity in the morning was caused by a reduction in mechanical sensitivity, whereas for falling pressures it was caused by a reduction in neural sensitivity. Our findings explicate the mechanisms underlying diurnal variation in baroreflex function. Pharmacological and lifestyle interventions targeted specifically at the diminished component of the cardiac baroreflex in the morning may lead to better management of hypertension.

Neuromechanical Features of the Cardiac Baroreflex After Exercise

A single bout of exercise is associated with postexercise hypotension, transient decreases in autonomic function, and changes in baroreflex sensitivity. The baroreflex is less sensitive to falling blood pressure than to rising blood pressure; we characterized the cardiac baroreflex in terms of hysteresis and its mechanical and neural components. We hypothesized that hysteresis would be exacerbated postexercise because of a greater relative decrease in falling blood pressure. In 10 healthy young humans (5 men), we used bolus injections of sodium nitroprusside and phenylephrine hydrochloride to drive transient decreases and increases in blood pressure, respectively, to quantify cardiac baroreflex sensitivity to falling and rising blood pressure. This was completed before and at 10, 30, and 60 minutes after 40 minutes of cycling at 60% estimated maximal oxygen consumption. Analyses of beat-to-beat blood pressure, R-R intervals and heart rate, and carotid artery diameter were used to determine the integrated cardiac baroreflex response; this was further quantified into a mechanical component (systolic blood pressure versus carotid diameter) and a neural component (carotid diameter versus R-R interval). There were 2 principle findings: after aerobic exercise baroreflex sensitivity is reduced and hysteresis manifests, and the reduction in sensitivity to falling blood pressure is mediated by decreased mechanical and neural gains, whereas the decreased baroreflex sensitivity to rising blood pressure is mediated by a reduced mechanical gain only. We suggest that impaired neural transduction of the cardiac baroreflex, and its influence on hysteresis, plays an important role in transient autonomic dysfunction after exercise.

Methods of assessing vagus nerve activity and reflexes

The methods used to assess cardiac parasympathetic (cardiovagal) activity and its effects on the heart in both humans and animal models are reviewed. Heart rate (HR)-based methods include measurements of the HR response to blockade of muscarinic cholinergic receptors (parasympathetic tone), beat-to-beat HR variability (HRV) (parasympathetic modulation), rate of post-exercise HR recovery (parasympathetic reactivation), and reflex-mediated changes in HR evoked by activation or inhibition of sensory (afferent) nerves. Sources of excitatory afferent input that increase cardiovagal activity and decrease HR include baroreceptors, chemoreceptors, trigeminal receptors, and subsets of cardiopulmonary receptors with vagal afferents. Sources of inhibitory afferent input include pulmonary stretch receptors with vagal afferents and subsets of visceral and somatic receptors with spinal afferents. The different methods used to assess cardiovagal control of the heart engage different mechanisms, and therefore provide unique and complementary insights into underlying physiology and pathophysiology. In addition, techniques for direct recording of cardiovagal nerve activity in animals; the use of decerebrate and in vitro preparations that avoid confounding effects of anesthesia; cardiovagal control of cardiac conduction, contractility, and refractoriness; and noncholinergic mechanisms are described. Advantages and limitations of the various methods are addressed, and future directions are proposed.

Cardiac baroreflex sensitivity is not correlated to sympathetic baroreflex sensitivity within healthy, young humans

The purpose of this study was to evaluate the relationship between the cardiac and sympathetic baroreflex sensitivities within healthy, young humans. The sensitivities of the cardiac and sympathetic baroreflexes were compared in 53 normotensive individuals (28 men and 25 women; age: 24.0 ± 0.9 years; body mass index: 24.0 ± 0.3 cm/kg², mean ± SEM). Heart rate, arterial blood pressure, and peroneal muscle sympathetic nerve activity were recorded under resting conditions (heart rate: 58 ± 1 bpm; systolic blood pressure: 126 ± 2 mm Hg; diastolic blood pressure: 72 ± 1 mm Hg; mean arterial blood pressure: 89 ± 1 mm Hg; muscle sympathetic nerve activity: 18 ± 1 bursts per min) and during rapid changes in blood pressure induced by sequential boluses of nitroprusside and phenylephrine. Cardiac and sympathetic baroreflex sensitivities were analyzed using the slopes of the linear portions of the muscle sympathetic nerve activity-diastolic blood pressure and R-R interval-systolic blood pressure relationships, respectively. When individual cardiac baroreflex sensitivity was compared with sympathetic baroreflex sensitivity, no correlation (R-R interval: r = -0.13; heart rate: r = 0.21) was observed when studied as a group. Analysis by sex unveiled a correlation in women between the cardiac and sympathetic baroreflex sensitivities (R-R interval: r = -0.54; P = 0.01; no correlation with hazard ratio: r = 0.29). No relationship was found in men (R-R interval: r = 0.17; heart rate: r = 0.12). These results indicate that, although both cardiac and sympathetic efferents function in baroreflex control of arterial pressure, there is no correlation in their sensitivities within healthy normotensive humans. However, sex-stratified data indicate that sex-based differential correlations might exist.

Age-related effects of vagotonic atropine on cardiovagal baroreflex gain

Impaired neural transduction of barosensory vessel stretch into vagal outflow is a primary determinant of reduced cardiovagal baroreflex gain with human aging. We set out to determine whether age-related reductions in this neural component of the baroreflex might be offset by enhancing the central integration/efferent responsiveness of the neural arc. Low vagotonic doses of atropine were employed to enhance central neural outflow and peripheral sinus node effects. Baroreflex gain and its neural and mechanical components were pharmacologically assessed before and after intravenous vagotonic atropine in 16 older and 14 young healthy subjects. Vagotonic atropine increased cardiovagal baroreflex gain (∼30%) and its neural component (∼20%) in older but not young individuals. Moreover, the atropine-induced increases in integrated gain and in its neural component were inversely related to baseline levels. Thus, age-related neural deficits in the baroreflex arc appear to play a determining role in reduced cardiovagal baroreflex gain with age and the compromised neural baroreflex function can be acutely improved by a single pharmacologic intervention.

Luminal administration ex vivo of a live Lactobacillus species moderates mouse jejunal motility within minutes

Gut commensals modulate host immune, endocrine, and metabolic functions. They also affect peripheral and central neural reflexes and function. We have previously shown that daily ingestion of Lactobacillus reuteri (LR) for 9 d inhibits the pseudoaffective cardiac response and spinal single-fiber discharge evoked by visceral distension, and decreases intestinal motility and myenteric AH cell slow afterhyperpolarization (sAHP) by inhibiting a Ca-activated K (IK(Ca)) channel. We tested whether luminal LR could acutely decrease motility in an ex vivo perfusion model of naive Balb/c jejunum. Live LR dose dependently decreased motor complex pressure wave amplitudes with 9- to 16-min onset latency and an IC(50) of 5 × 10(7) cells/ml Krebs. Heat-killed LR or another live commensal, Lactobacillus salivarius, were without effect. The IK(Ca) channel blocker TRAM-34, but neither the opener (DCEBIO) nor the hyperpolarization-activated cationic channel inhibitor ZD7288 (5 μM) (or TTX 1 μM), mimicked the LR effect on motility acutely ex vivo. We provide evidence for a rapid, strain-specific, dose-dependent action of a live Lactobacillus on small intestinal motility reflexes that recapitulates the long-term effects of LR ingestion. These observations may be useful as a first step to unraveling the pathways involved in bacteria to the nervous system communication.

Symptomatic comparison in efficacy on patients with benign prostatic hyperplasia treated with two therapeutic approaches

Objectives

A randomised controlled trial was performed to compare the symptomatic effects on patients with benign prostatic hyperplasia (BPH) treated by two therapeutic approaches – the Western medicine (WM) and traditional Chinese medicine (TCM).

Methods

Four primary outcome measures, namely the quality of life (QOL), maximum urine flow ratio (UFR), International Prostate Symptom Score (IPSS) and prostate volumes, as well as four urethra-related and 35 non-urethra-related symptoms, were investigated to evaluate the effects on 31 BPH patients subjected to WM (Terazosin Hydrochloride Hytrin, THH) and 30 cases to TCM (herbal Saxifrage tablet, HST). The effects of both treatments are compared by the two-sample Kolmogorov–Smirnov test. The contributions of symptoms for four assessments are analysed by the logistic regression model and the Chow test.

Results

The effect of TCM is weaker than that of WM in the assessment of the IPSS score (p < 0.05), and both treatments are similar in the prostate volumes, the maximum UFR and the QOL assessments (p > 0.05), as well as in the effective number of urethra-related or non-urethra-related symptoms before and after treatment (p > 0.05). By comparing the linear regression models, different urethra-related and non-urethra-related symptom patterns associated with TCM and WM therapies are detected for four assessments, especially for the prostate volume assessment (p < 0.01).

Conclusion

TCM (HST) is a potentially effective treatment in improving the QOL, prostate volumes and maximum UFR for patients with BPH, though it is less effective in ameliorating the IPSS score when compared with WM (THH). The non-urethra-related symptoms experienced by BPH patients might be one of the parameters to further achieve the tailored diagnosis and treatment for BPH.

Influence of ageing on carotid baroreflex peak response latency in humans

The stability of a physiological control system, such as the arterial baroreflex, depends critically upon both the magnitude (i.e. gain or sensitivity) and timing (i.e. latency) of the effector response. Although studies have examined resting arterial baroreflex sensitivity in older subjects, little attention has been given to the influence of ageing on the latency of peak baroreflex responses. First, we compared the temporal pattern of heart rate (HR) and mean arterial blood pressure (BP) responses to selective carotid baroreceptor (CBR) unloading and loading in 14 young (22 +/- 1 years) and older (61 +/- 1 years) subjects, using 5 s pulses of neck pressure (NP, +35 Torr) and neck suction (NS, -80 Torr). Second, CBR latency was assessed following pharmacological blockade of cardiac parasympathetic nerve activity in eight young subjects, to better understand how known age-related reductions in parasympathetic nerve activity influence CBR response latency. In response to NP, the time to the peak increase in HR and mean BP were similar in young and older groups. In contrast, in response to NS the time to peak decrease in HR (2.1 +/- 0.2 vs. 3.8 +/- 0.2 s) and mean BP (6.7 +/- 0.4 vs. 8.3 +/- 0.2 s) were delayed in older individuals (young vs. older, P < 0.05). The time to peak HR and mean BP were delayed in young subjects following cardiac parasympathetic blockade (glycopyrrolate). Collectively, these data suggest that ageing is associated with delayed peak cardiovascular responses to acute carotid baroreceptor loading that may be, in part, due to age-related reductions in cardiac parasympathetic tone.

A comparison of pharmacologic and spontaneous baroreflex methods in aging and hypertension

BACKGROUND

Phenylephrine bolus injection is an established technique to measure baroreflex sensitivity (BRS). This study quantified the relationship between the phenylephrine method and noninvasive measures of BRS and examined the effects of aging and hypertension on BRS. We also examined whether heart rate variability (HRV) provides as much information as does BRS.

METHODS

BRS was determined by phenylephrine bolus (BRSphe), amyl nitrite inhalation (BRSamyl), Valsalva maneuver (BRSVals) and by time (BRS(+)) and spectral domain analysis (BRS(LFalpha), 004-015 Hz) of spontaneous blood pressure and R-R interval changes over the 5-min time period.

RESULTS

The phenylephrine method significantly correlated with other methods (BRS(LFalpha) R = 0.54, BRS(+) R = 0.55, BRSVals R = 0.43 and BRSamyl R = 0.39; P < or = 0.001). Each method underestimated the BRSphe by the factors 0.62, 0.64, 0.59 and 0.33, respectively; P value less than 0.001. Only BRS(LFalpha) was significantly different between normotensive and hypertensive patients in young [24.3 +/- 1.4 (n = 40) vs. 12.2 +/- 2.3 (n = 7)] and middle-aged [16.5 +/- 1.1 (n = 71) vs. 10.8 +/- 1.1 (n = 31) groups, respectively]. HRV in the high frequency band (0.15-0.40 Hz) was significantly lower in young hypertensive patients than in normal controls (26 +/- 6.0 vs. 50 +/- 2.4, P < 0.05).

CONCLUSION

Although all methods correlated with the phenylephrine technique, none of them could be used interchangeably with that technique. BRS(LFalpha) detected the baroreflex loss of hypertension most clearly, and BRSamyl did not differ among groups.

Arterial baroreflex control of cardiac vagal outflow in older individuals can be enhanced by aerobic exercise training

Maintained cardiac vagal function is critical to cardiovascular health in human aging. Aerobic exercise training has been considered an attractive intervention to increase cardiovagal baroreflex function; however, the data are equivocal. Moreover, if regular exercise does reverse the age-related decline in cardiovagal baroreflex function, it is unknown how this might be achieved. Therefore, we assessed the effects of a 6-month aerobic training program on baroreflex gain and its mechanical and neural components in older individuals (5 women and 7 men, aged 55 to 71 years). We assessed baroreflex function using pharmacological pressure changes (bolus nitroprusside followed by bolus phenylephrine) and estimated the integrated gain (Delta R-R interval/Delta systolic blood pressure) and mechanical (Delta diameter/Delta pressure) and neural (Delta R-R interval/Delta diameter) components via measurements of carotid artery diameter in previously sedentary older individuals before and after 6 months of aerobic training. There was a significant 26% increase in baroreflex gain that was directly related to the amount of exercise performed and that was derived mainly from an increase in the neural component of the arterial baroreflex (P<0.05). We did find changes in the mechanical component, but unlike integrated gain and the neural component, these were not related to the magnitude of the exercise stimulus. These results suggest that exercise training can have a powerful effect on cardiovagal baroreflex function, but a sufficient stimulus is necessary to produce the effect. Moreover, adaptations in the afferent-efferent baroreflex control of cardiac vagal outflow may be crucial for the improvement in arterial baroreflex function in older humans.

Age- and fitness-related alterations in vascular sympathetic control

In the current study we explored (1) if there were differences in sympathetic activity and baroreflex function by age, sex, or physical activity status, (2) if any aspect of baroreflex function related to differences in resting sympathetic activity, and (3) if mechanical and/or neural baroreflex components related to differences in integrated baroreflex gain. Electrocardiogram, blood pressure, carotid diameter and muscle sympathetic nerve activity were recorded continuously at rest and during sequential bolus injections of sodium nitroprusside and phenylephrine in 22 young, 21 older sedentary and 10 older trained individuals. Analyses of co-variance were used to examine age, sex and training status differences and to explore the explanatory power of integrated baroreflex gain and its mechanical and neural components. Training status and sex influenced neither resting sympathetic outflow nor sympathetic baroreflex gain components. Older subjects had a smaller mechanical component and a strong tendency towards a greater neural component of the sympathetic baroreflex during both pressure falls and pressure rises. Opposing age-related changes in mechanical and neural components resulted in a smaller integrated gain during pressure falls, but a greater integrated gain during pressure rises in older subjects. Thus, in older individuals, compromised sympathetic activation to pressure falls was owing to the stiffening of barosensory vessels, whereas the more sensitive sympathoinhibition to pressure rise was due to an increased neural control. Enhanced neural control with age, however, did not contribute the increased resting sympathetic outflow, which indicates that these two changes are probably driven by distinct neural mechanisms.