Carotid baroreflex testing using the neck collar device

Cardiovascular baroreflex circuit moonlights in sleep control

Sleep disturbances are strongly associated with cardiovascular diseases. Baroreflex, a basic cardiovascular regulation mechanism, is modulated by sleep-wake states. Here, we show that neurons at key stages of baroreflex pathways also promote sleep. Using activity-dependent genetic labeling, we tagged neurons in the nucleus of the solitary tract (NST) activated by blood pressure elevation and confirmed their barosensitivity with optrode recording and calcium imaging. Chemogenetic or optogenetic activation of these neurons promoted non-REM sleep in addition to decreasing blood pressure and heart rate. GABAergic neurons in the caudal ventrolateral medulla (CVLM)-a downstream target of the NST for vasomotor baroreflex-also promote non-REM sleep, partly by inhibiting the sympathoexcitatory and wake-promoting adrenergic neurons in the rostral ventrolateral medulla (RVLM). Cholinergic neurons in the nucleus ambiguous-a target of the NST for cardiac baroreflex-promoted non-REM sleep as well. Thus, key components of the cardiovascular baroreflex circuit are also integral to sleep-wake brain-state regulation.

Stress and the baroreflex

The stress response to emotions elicits the release of glucocorticoids from the adrenal cortex, epinephrine from the adrenal medulla, and norepinephrine from the sympathetic nerves. The baroreflex adapts to buffer these responses to ensure that perfusion to the organs meets the demands while maintaining blood pressure within a within a narrow range. While stressor-evoked autonomic cardiovascular responses may be adaptive for the short-term, the recurrent exaggerated cardiovascular stress reactions can be maladaptive in the long-term. Prolonged stress or loss of the baroreflex's buffering capacity can predispose episodes of heightened sympathetic activity during stress leading to hypertension, tachycardia, and ventricular wall motion abnormalities. This review discusses 1) how the baroreflex responds to acute and chronic stressors, 2) how lesions in the neuronal pathways of the baroreflex alter the ability to respond or counteract the stress response, and 3) the techniques to assess baroreflex sensitivity and stress responses. Evidence suggests that loss of baroreflex sensitivity may predispose heightened autonomic responses to stress and at least in part explain the association between stress, mortality and cardiovascular diseases.

Neck Chamber Technique Revisited: Low-Noise Device Delivering Negative and Positive Pressure and Enabling Concomitant Carotid Artery Imaging With Ultrasonography

Background and Objectives: Recently, novel noiseless device for the assessment of baroreceptor function with the neck suction (NS) has been presented. In this study, we present another in-house approach to the variable-pressure neck chamber method. Our device offers further critical improvements. First, it enables delivery of negative (NS) as well as positive pressure (neck pressurizing, NP) in a noiseless manner. Second, we used small, 3D-printed cups positioned over the carotid sinuses instead of cumbersome neck collar to improve subject comfort and to test feasibility of tracking the pressure-induced changes in carotid artery with ultrasonography. Methods: Five healthy, non-smoking, normal-weight subjects aged 29 ± 3 years (mean ± SD) volunteered for the study. Heart rate (HR, bpm) and mean arterial pressure (MAP, mmHg) responses to short, 7-s long episodes of NS and NP were recorded. Each trial consisted of 12 episodes of variable-pressure: six episodes of NS (suction ranging between -10 and -80 mmHg) and six episodes of NP (pressure ranging between + 10 and + 80 mmHg). Carotid artery sonography was performed during the NS and NP in four subjects, on another occasion. Results: The variable-pressure episodes resulted consistently in the expected pattern of hemodynamic alterations: HR and MAP increases or decreases following the NP and NS, respectively, as evidenced by the coefficient of determination (R²) of ≥0.78 for the carotid-HR response curve (for all five participants) and the carotid-MAP response curve (for four out of five participants; the curve cannot be calculated for one subject). We found a linear, dose-dependent relation between the applied pressure and the systolic-diastolic difference in carotid artery diameter. Conclusion: The novel device enables noiseless stimulation and unloading of the carotid baroreceptors with the negative and positive pressure, respectively, applied on the subject's neck via small, asymmetric and one-side flattened, 3D-printed cups. The unique design of the cups enables concomitant visualizing of the carotid artery during the NS or NP administration, and thereby direct monitoring of the intensity of mechanical stimulus targeting the carotid baroreceptors.

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

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

Baroreflex dysfunction in chronic kidney disease

Chronic kidney disease (CKD) patients have high cardiovascular mortality and morbidity. The presence of traditional and CKD related risk factors results in exaggerated vascular calcification in these patients. Vascular calcification is associated with reduced large arterial compliance and thus impaired baroreflex sensitivity (BRS) resulting in augmented blood pressure (BP) variability and hampered BP regulation. Baroreflex plays a vital role in short term regulation of BP. This review discusses the normal baroreflex physiology, methods to assess baroreflex function, its determinants along with the prognostic significance of assessing BRS in CKD patients, available literature on BRS in CKD patients and the probable patho-physiology of baroreflex dysfunction in CKD.

Effect of parasympathetic stimulation on brain activity during appraisal of fearful expressions

Autonomic nervous system activity is an important component of human emotion. Mental processes influence bodily physiology, which in turn feeds back to influence thoughts and feelings. Afferent cardiovascular signals from arterial baroreceptors in the carotid sinuses are processed within the brain and contribute to this two-way communication with the body. These carotid baroreceptors can be stimulated non-invasively by externally applying focal negative pressure bilaterally to the neck. In an experiment combining functional neuroimaging (fMRI) with carotid stimulation in healthy participants, we tested the hypothesis that manipulating afferent cardiovascular signals alters the central processing of emotional information (fearful and neutral facial expressions). Carotid stimulation, compared with sham stimulation, broadly attenuated activity across cortical and brainstem regions. Modulation of emotional processing was apparent as a significant expression-by-stimulation interaction within left amygdala, where responses during appraisal of fearful faces were selectively reduced by carotid stimulation. Moreover, activity reductions within insula, amygdala, and hippocampus correlated with the degree of stimulation-evoked change in the explicit emotional ratings of fearful faces. Across participants, individual differences in autonomic state (heart rate variability, a proxy measure of autonomic balance toward parasympathetic activity) predicted the extent to which carotid stimulation influenced neural (amygdala) responses during appraisal and subjective rating of fearful faces. Together our results provide mechanistic insight into the visceral component of emotion by identifying the neural substrates mediating cardiovascular influences on the processing of fear signals, potentially implicating central baroreflex mechanisms for anxiolytic treatment targets.

Breath-Holding During Exhalation as a Simple Manipulation to Reduce Pain Perception

OBJECTIVE

Baroreceptor stimulation yields antinociceptive effects. In this study, baroreceptors were stimulated by a respiratory maneuver, with the effect of this manipulation on pain perception subsequently measured.

METHODS

Thirty-eight healthy participants were instructed to inhale slowly (control condition) and to hold the air in lungs after a deep inhalation (experimental condition). It was expected that breath-holding would increases blood pressure (BP) and thus stimulate the baroreceptors, which in turn would reduce pain perception. Pain was induced by pressure algometry on the nail of the left-index finger, at three different pressure intensities, and quantified by visual analogue scales. Heart rate (HR) and BP were continuously recorded.

RESULTS

Pain perception was lower when pain pressure was administered during the breath-holding phase versus the slow inhalation phase, regardless of the pressure intensity. During breath-holding, a rapid increase in BP and decrease in HR were observed, demonstrating activation of the baroreceptor reflex.

CONCLUSION

Pain perception is reduced when painful stimulation is applied during breath-holding immediately following a deep inhalation. These results suggest that a simple and easy-to-perform respiratory maneuver could be used to reduce acute pain perception.

Modeling heart beat dynamics and fMRI signals during carotid stimulation by neck suction

Central autonomic control on the cardiovascular system has been widely investigated in the last decades. More recently, with the advent of brain imaging techniques, considerable effort is being spent on defining the role of specific brain areas, and their dynamic network, acting on autonomic efferents. A way to assess autonomic modulation is offered by carotid stimulation. In this work, we propose a methodology to investigate autonomic control in carotid stimulation experiments using heartbeat series in combination with fMRI imaging. We modeled cardiovascular signals using the point process model, and processed fMRI data in order to estimate independent components of correlated information. Using cross-correlation and surrogate analysis, we assessed the responsiveness of subjects to neck suction stimuli and identified stimulus-related fMRI independent components.

The effect of baroreceptor stimulation on pain perception depends on the elicitation of the reflex cardiovascular response: evidence of the interplay between the two branches of the baroreceptor system

We examined the impact of baroreceptor stimulation on pain and cardiovascular responses in 39 healthy participants. Carotid baroreceptors were stimulated with external suction (-50 mmHg, stimulation) or pressure (+8 mmHg, control). Pain was induced by pressure to the nail of the left-index finger and quantified by a visual analog scale. Pain decreased heart rate (HR) and increased blood pressure (BP). Baroreceptor stimulation further decreased HR and reduced the BP increase. Pain experience failed to differ between baroreceptor stimulation conditions. However, significant results were obtained when trials were categorized according to the magnitude of the HR deceleration elicited by baroreceptor stimulation. In trials with strong baroreceptor-elicited HR deceleration pain intensity was lower than in trials both with inactive baroreceptor stimulation (pressure trials) or trials with small baroreceptor-elicited HR responses. Anti-nociceptive effects of baroreceptor stimulation depend on the activation of the reflex cardiovascular response. Central nervous inhibition due to baroreceptor stimulation only occurs if the peripheral cardiovascular response is engaged.

Resetting the baroreflex during snoring: role of resistive loading and intra-thoracic pressure

Baroreflex sensitivity (BRS) is reduced during snoring in humans and animal models. We utilised our rabbit model to examine the contribution of increased upper airway resistance to baroreflex resetting during snoring, by comparing BRS and baroreflex operating point (OP) values during IS to those obtained during tracheostomised breathing through an external resistive load (RL) titrated to match IS levels of peak inspiratory pleural pressure (Ppl). During both IS and RL, BRS decreased by 45% and 49%. There was a linear relationship between the change in Ppl and the decrease in BRS, which was similar for IS and RL. During both RL and IS, there was a shift in OP driven by ~16% increase in HR and no change in arterial pressure. Snoring related depression of BRS is likely mediated via a HR driven change in OP, which itself may be the outcome of negative intra-thoracic pressure mediated effects on right atrial wall stretch reflex control of heart rate.

New insights into the effects of age and sex on arterial baroreflex function at rest and during dynamic exercise in humans

The arterial baroreflex (ABR) performs an important role in regulating blood pressure (BP) both at rest and during exercise, by carefully orchestrating autonomic neural activity to the heart and blood vessels. Reduced ABR sensitivity (i.e., gain) has been associated with increased cardiovascular risk, cardiac electrical instability and orthostatic intolerance, while 'normal' ABR function during exercise is important for ensuring an appropriate cardiovascular response is elicited. Previous studies examining the influence of age and sex on resting ABR function in humans have primarily used pharmacological methods (e.g., modified Oxford technique) to change BP and alter baroreceptor input. With this approach only reflex control of heart rate and sympathetic nerve activity may be evaluated, and as such the influence of age and sex on ABR control of BP per se remains incompletely understood. Furthermore, the majority of previous studies examining ABR function during exercise have principally assessed young men. Whether these findings can be extrapolated to young women or older men and women remains unclear. Recently the potential for age and sex to modulate the integrative neural control of the cardiovascular system is becoming appreciated. This review article will provide a detailed update of such recent advances into our understanding of the effects of age and sex on ABR control of BP both at rest and during dynamic exercise in humans.

Direct stimulation of the autonomic nervous system modulates activity of the brain at rest and when engaged in a cognitive task

The effect of autonomic perturbation (AP) on the central nervous system functioning is still largely unknown. Using an automated neck suction device to stimulate the carotid mechanoreceptors in the carotid sinus (parasympathetic pathway), operated synchronously with functional magnetic resonance imaging (fMRI) acquisition, we investigated the effects of AP on the activity of the brain at rest and when engaged in a visuo-spatial attention task. ECG was always recorded to index changes in autonomic function. At rest, AP induced increased activation in the insula and in the amygdala, which have been previously associated with the autonomic control and emotion processing, as well as in the caudate nucleus and in the medial temporal cortex, both implicated in cognitive functions. Despite a preserved performance during visuo-spatial attention task, AP induced increased reaction times and a positive modulation on the activation of the right posterior parietal cortex, the occipital cortex, the periaquiductal gray, and nuclei of the brainstem. We speculate that this modulation of brain activity represents, at different anatomical levels, a compensation mechanism to maintain cognitive efficiency under parasympathetic stimulation, which is traditionally considered as the system for energy regain and storage. In conclusion, this study provides the first evidence of a dynamic interaction between AP and higher level functions in humans.

Snoring effects on the baroreflex: an animal model

Baroreflex sensitivity (BRS) is reduced in humans during snoring, however the mechanisms are unknown. We used an anaesthetised rabbit induced snoring (IS) model, to test: (1) whether IS was associated with reduced BRS; and (2) if snoring related vibration transmission to peri-carotid tissues influenced BRS levels. BRS was quantified using the spontaneous sequence technique. During IS, BRS fell by 40%, without any associated change in blood pressure (BP) but accompanied by an increase in heart rate (HR). Direct application of a snore frequency and intensity matched vibratory stimulus to the peri-carotid tissues of non-snoring tracheostomised rabbits had no effect on BRS, HR or BP. In conclusion, IS induced depression of BRS is likely mediated via a HR driven change in BRS operating point that is unrelated to snoring-related vibration transmission to carotid baroreceptors. The anaesthetised IS rabbit provides a model in which mechanistic interactions between snoring and BRS can be further explored.

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.