Orthostatic intolerance: different abnormalities in the neural sympathetic response to a gravitational stimulus

Vasovagal syncope: An overview of pathophysiological mechanisms

Syncope is a short-term transient loss of consciousness, characterized by rapid onset and complete spontaneous recovery. According to the 2018 European Society of Cardiology guidelines, three different types of syncope have been identified. However, all forms of syncope share a common final pathophysiological event, global cerebral hypoperfusion, which results from the inability of the circulatory system to maintain blood pressure at the level required to efficiently supply blood to the brain. The vasovagal syncope (VVS) is the most common form of syncope. Although, VVS is generally harmless, its frequent occurrence can negatively affect quality of life and increase the risk of adverse events. The pathophysiological mechanisms underlying VVS remain obscure. The multifaceted nature of VVS presents a veritable challenge to understanding this condition and developing preventative strategies. Thus, the aim of this review was to discuss the factors contributing to the pathogenesis of VVS and provide guidance for future research.

Orthostatic hypotension after cervicomedullary junction surgery: illustrative case

BACKGROUND

Surgery at the cervicomedullary junction carries a risk of damaging vital brainstem functions. Because the nucleus of the solitary tract (NS) is involved in the baroreceptor reflex, damage to its integrity may lead to orthostatic hypotension.

OBSERVATIONS

A 56-year-old man with a medical history of hypertension, von Hippel-Lindau disease, and previous bilateral adrenalectomy due to pheochromocytoma was referred with symptoms of dysphagia and paralysis of the left vocal cord. Paralysis of the left vagus nerve was suspected. Magnetic resonance imaging revealed a contrast-enhancing cystic process in the cervicomedullary junction. Twenty-three years earlier, the patient had undergone surgical treatment for a hemangioblastoma in the same region. After repeated surgery, the patient temporarily developed orthostatic hypotension. At discharge, the patient no longer needed antihypertensive medication.

LESSONS

Surgery near the cervicomedullary junction can affect the NS, leading to disruption of the baroreceptor response that regulates blood pressure.

Orthostatic Hypertension: Critical Appraisal of an Overlooked Condition

Orthostatic hypertension, which appears to be mediated through excess neurohumoral activation while standing, is a common blood pressure trait among patients with and without arterial hypertension. However, lack of consensus regarding the definition of orthostatic hypertension makes it difficult to assess the true prevalence of this condition. Orthostatic hypertension appears to predict the risk for progression to arterial hypertension in younger and risk of cardiovascular morbidity and mortality in older persons. Yet, the risk may differ between populations. Whether orthostatic hypertension indicates a generally increased risk of death, constitutes an intermediate variable in the causal pathway of cardiovascular risk factors, a simple measure of disease severity, or an independently acting mechanism is not known. Since both orthostatic hypotension and orthostatic hypertension herald increased risk of cardiovascular disease, it appears reasonable to screen the patients for abnormal orthostatic blood pressure responses using simple orthostatic testing. However, how presence of orthostatic hypertension may affect clinical management decisions such as the choice of antihypertensive drugs is currently difficult to ascertain. Clearly, this issue deserves more attention.

Cardiac and Vascular Sympathetic Baroreflex Control during Orthostatic Pre-Syncope

We hypothesized that sympathetic baroreflex mediated uncoupling between neural sympathetic discharge pattern and arterial pressure (AP) fluctuations at 0.1 Hz during baroreceptor unloading might promote orthostatic pre-syncope. Ten volunteers (32 ± 6 years) underwent electrocardiogram, beat-to-beat AP, respiratory activity and muscle sympathetic nerve activity (MSNA) recordings while supine (REST) and during 80° head-up tilt (HUT) followed by -10 mmHg stepwise increase of lower body negative pressure until pre-syncope. Cardiac and sympathetic baroreflex sensitivity were quantified. Spectrum analysis of systolic and diastolic AP (SAP and DAP) and calibrated MSNA (cMSNA) variability assessed the low frequency fluctuations (LF, ~0.1 Hz) of SAP, DAP and cMSNA variability. The squared coherence function (K²) quantified the coupling between cMSNA and DAP in the LF band. Analyses were performed while supine, during asymptomatic HUT (T₁) and at pre-syncope onset (T₂). During T₂ we found that: (1) sympathetic baroreceptor modulation was virtually abolished compared to T₁; (2) a progressive decrease in AP was accompanied by a persistent but chaotic sympathetic firing; (3) coupling between cMSNA and AP series at 0.1 Hz was reduced compared to T₁. A negligible sympathetic baroreceptor modulation during pre-syncope might disrupt sympathetic discharge pattern impairing the capability of vessels to constrict and promote pre-syncope.

Effects of Prolonged Head-Down Bed Rest on Cardiac and Vascular Baroreceptor Modulation and Orthostatic Tolerance in Healthy Individuals

Orthostatic intolerance commonly occurs after prolonged bed rest, thus increasing the risk of syncope and falls. Baroreflex-mediated adjustments of heart rate and sympathetic vasomotor activity (muscle sympathetic nerve activity – MSNA) are crucial for orthostatic tolerance. We hypothesized that prolonged bed rest deconditioning alters overall baroreceptor functioning, thereby reducing orthostatic tolerance in healthy volunteers. As part of the European Space Agency Medium-term Bed Rest protocol, 10 volunteers were studied before and after 21 days of −6° head down bed rest (HDBR). In both conditions, subjects underwent ECG, beat-by-beat blood pressure, respiratory activity, and MSNA recordings while supine (REST) and during a 15-min 80° head-up tilt (TILT) followed by a 3-min −10 mmHg stepwise increase of lower body negative pressure to pre-syncope. Cardiac baroreflex sensitivity (cBRS) was obtained in the time (sequence method) and frequency domain (spectrum and cross-spectrum analyses of RR interval and systolic arterial pressure – SAP, variability). Baroreceptor modulation of sympathetic discharge activity to the vessels (sBRS) was estimated by the slope of the regression line between the percentage of MSNA burst occurrence and diastolic arterial pressure. Orthostatic tolerance significantly decreased after HDBR (12 ± 0.6 min) compared to before (21 ± 0.6 min). While supine, heart rate, SAP, and cBRS were unchanged before and after HDBR, sBRS gain was slightly depressed after than before HDBR (sBRS: −6.0 ± 1.1 versus −2.9 ± 1.5 burst% × mmHg⁻¹, respectively). During TILT, HR was higher after than before HDBR (116 ± 4 b/min versus 100 ± 4 b/min, respectively), SAP was unmodified in both conditions, and cBRS indexes were lower after HDBR (α index: 3.4 ± 0.7 ms/mmHg; BRS_SEQ 4.0 ± 1.0) than before (α index: 6.4 ± 1.0 ms/mmHg; BRS_SEQ 6.8 ± 1.2). sBRS gain was significantly more depressed after HDBR than before (sBRS: −2.3 ± 0.7 versus −4.4 ± 0.4 burst% × mmHg⁻¹, respectively). Our findings suggest that baroreflex-mediated adjustments in heart rate and MSNA are impaired after prolonged bed rest. The mechanism likely contributes to the decrease in orthostatic tolerance.

Cerebral hemodynamics in orthostatic intolerance with normal head-up tilt test

OBJECTIVES

Orthostatic hypotension (OH) and postural orthostatic tachycardia syndrome (POTS) are well-known causes of orthostatic intolerance (OI). In addition, there are OI patients who are characterized by the symptoms of OI and lack of abnormal findings in head-up tilt (HUT) test. The aim of this study was to determine the cerebral hemodynamic changes in HUT test of OI patients with normal HUT (OINH).

MATERIALS AND METHODS

Two hundred and sixty-one OI patients and 50 healthy controls were enrolled in this study. All subjects underwent transcranial Doppler test while performing the HUT test. Forty-five patients had OH, 33 patients had POTS, and 183 patients had OINH. Blood pressures, heart rate, cerebral blood flow velocities (CBFVs), end-tidal carbon dioxide (ET-PCO2 ), cerebral critical closing pressure (CCP), cerebral perfusion pressure (CPP), and cerebral vascular resistance (CVR) were measured during HUT test. We compared the hemodynamic parameters of OINH with those of OH, POTS, and healthy controls.

RESULTS

Reduced CBFVs, CPP, and ET-PCO2 and elevated CCP were observed in the HUT test of all four groups. CVR was reduced in three OI patients. The drops in systolic CBFV, CPP, and CVR of OINH patients were greater than those of healthy controls. The changes in parameters in the HUT test of OINH group were not different from those of OH and POTS groups except prominent decrements of CPP and CVR in OH group.

CONCLUSION

Our findings suggest that OINH is true OI sharing the common pathomechanism of OH and POTS.

Cardiovascular parameters and neural sympathetic discharge variability before orthostatic syncope: role of sympathetic baroreflex control to the vessels

We tested the hypothesis that altered sympathetic baroreceptor control to the vessels (svBRS) and disrupted coupling between blood pressure (BP) fluctuations and muscle sympathetic activity (MSNA) discharge pattern in the low frequency band (LF, around 0.1 Hz) precede vasovagal syncope. Seven healthy males underwent ECG, BP, respiratory, and MSNA recordings at baseline (REST) and during a 15 min 80° head-up tilt, followed by a -10 mmHg step wise increase of lower body negative pressure up to presyncope. Spectral and coherence analyses of systolic arterial pressure (SAP) and MSNA variability provided the indexes of vascular sympathetic modulation, LFSAP, and of the linear coupling between MSNA and SAP in the low frequency band (around 0.1 Hz), K(2)MSNA-SAP(LF). svBRS was assessed as the slope of the regression line between MSNA and diastolic arterial pressure (DAP). Data were analyzed at REST, during asymptomatic and presyncope periods of tilt. svBRS declined during presyncope period compared to REST and asymptomatic tilt. The presyncope period was characterized by a decrease of RR interval, LFMSNA, LFSAP, and K(2)MSNA-SAP(LF) values compared to the asymptomatic one, whereas MSNA burst rate was unchanged. The reduction of svBRS producing an altered coupling between MSNA and SAP variability at 0.1 Hz, may provoke circulatory changes leading to presyncope.

Utility of corrected QT interval in orthostatic intolerance

We performed this study to determine whether electrocardiographic corrected QT (QTc) interval predicts alterations in sympathovagal balance during orthostatic intolerance (OI). We reviewed 1,368 patients presenting with symptoms suggestive of OI who underwent electrocardiography and composite autonomic function tests (AFTs). Patients with a positive response to the head-up tilt test were classified into orthostatic hypotension (OH), neurocardiogenic syncope (NCS), or postural orthostatic tachycardia syndrome (POTS) groups. A total of 275 patients (159 OH, 54 NCS, and 62 POTS) were included in the final analysis. Between-group comparisons of OI symptom grade, QTc interval, QTc dispersion, and each AFT measure were performed. QTc interval and dispersion were correlated with AFT measures. OH Patients had the most severe OI symptom grade and NCS patients the mildest. Patients with OH showed the longest QTc interval (448.8±33.6 msec), QTc dispersion (59.5±30.3 msec) and the lowest values in heart rate response to deep breathing (HRDB) (10.3±6.0 beats/min) and Valsalva ratio (1.3±0.2). Patients with POTS showed the shortest QTc interval (421.7±28.6 msec), the highest HRDB values (24.5±9.2 beats/min), Valsalva ratio (1.8±0.3), and proximal and distal leg sweat volumes in the quantitative sudomotor axon reflex test. QTc interval correlated negatively with HRDB (r = −0.443, p<0.001) and Valsalva ratio (r = −0.425, p<0.001). We found negative correlations between QTc interval and AFT values representing cardiovagal function in patients with OI. Our findings suggest that prolonged QTc interval may be considered to be a biomarker for detecting alterations in sympathovagal balance, especially cardiovagal dysfunction in OH.

Leg vasoconstriction during head-up tilt in patients with autonomic failure is not abolished

Maintaining blood pressure during orthostatic challenges is primarily achieved by baroreceptor-mediated activation of the sympathetic nervous system, which can be divided into preganglionic and postganglionic parts. Despite their preganglionic autonomic failure, spinal cord-injured individuals demonstrate a preserved peripheral vasoconstriction during orthostatic challenges. Whether this also applies to patients with postganglionic autonomic failure is unknown. Therefore, we assessed leg vasoconstriction during 60° head-up tilt in five patients with pure autonomic failure (PAF) and two patients with autonomic failure due to dopamine-β-hydroxylase (DBH) deficiency. Ten healthy subjects served as controls. Leg blood flow was measured using duplex ultrasound in the right superficial femoral artery. Leg vascular resistance was calculated as the arterial-venous pressure gradient divided by blood flow. DBH-deficient patients were tested off and on the norepinephrine pro-drug l-threo-dihydroxyphenylserine (l-DOPS). During 60° head-up tilt, leg vascular resistance increased significantly in PAF patients [0.40 ± 0.38 (+30%) mmHg·ml−1·min−1]. The increase in leg vascular resistance was not significantly different from controls [0.88 ± 1.04 (+72%) mmHg·ml−1·min−1]. In DBH-deficient patients, leg vascular resistance increased by 0.49 ± 0.01 (+153%) and 1.52 ± 1.47 (+234%) mmHg·ml−1·min−1 off and on l-DOPS, respectively. Despite the increase in leg vascular resistance, orthostatic hypotension was present in PAF and DBH-deficient patients. Our results demonstrate that leg vasoconstriction during orthostatic challenges in patients with PAF or DBH deficiency is not abolished. This indicates that the sympathetic nervous system is not the sole or pivotal mechanism inducing leg vasoconstriction during orthostatic challenges. Additional vasoconstrictor mechanisms may compensate for the loss in sympathetic nervous system control.

Neural autonomic control in orthostatic intolerance

Inability to maintain the upright position is manifested by a number of symptoms shared by either human pathophysiology and conditions following weightlessness or bed rest. Alterations of the neural sympathetic cardiovascular control have been suggested to be one of the potential underlying etiopathogenetic mechanisms in these conditions. We hypothesize that the study of the autonomic profile of human orthostatic intolerance syndromes may furnish a valuable insight into the complexity of the sympathetic alterations leading to a reduced gravitational tolerance. In the present paper we describe abnormalities both in the magnitude and in the pattern of the sympathetic neural firing observed in patients affected by orthostatic intolerance, attending the upright position. Also, we discuss similarity and differences in the neural sympathetic mechanisms regulating the cardiovascular system during the gravitational stimulus both in clinical syndromes and in subjects returning from space.

Analysis of sympathetic neural discharge in rats and humans

Neural signals convey information through two different modalities: intensity and discharge pattern. The intensity code is based on the number of action potentials per unit time, which is then easily translated into neurotransmitter release. This kind of information may be assessed simply by counting the number of spikes or bursts over a time unit. However, the discharge pattern is a further, efficient means of neural information transfer. Rhythmic patterns (i.e. oscillations) can support highly structured, temporal codes based on correlation and synchronization. It is therefore clear that applying frequency domain analysis to sympathetic activity recorded in animals and humans may provide additional information about the neural control of the circulation. Over the last century, data obtained by the analysis of sympathetic activity in experimental animals, and recently also in humans, have provided fundamental contributions to our understanding of the physiological mechanisms involved in the neural control of circulation, as well as how these are altered in cardiovascular and non-cardiovascular diseases. The aim of this paper is to address some aspects related to the recording, analysis and interpretation of sympathetic activity in rats and humans, with special emphasis on analysis in the frequency domain.

Syncope: facts and fiction

According to the current state-of-art on the brainstem functional anatomy and reticular formation, authors believe that nucleus tractus solitarii (NTS) is the neural structure, which meets all the conditions of the hypothetical syncope generating, reflex centre. The afferent branch of this reflex arc represents information from different visceral sources including the brain itself. The efferent branch of this reflex arc is reticular activating system (RAS). The executive mechanism of syncope is deactivation of RAS done with the active engagement of NTS through solitarioreticular pathway (SRT) and parabrachial nuclear complex (PBC). The biological purpose of syncope would be resetting of the NTS in case of an unbearable vegetative input, which is code for triggering the mechanism described.

Effects of AF-DX116 and Other Muscarinic Receptor Antagonists on Orthostatic Hypotension in Autonomic Imbalanced (SART-Stressed) Rats

SART (specific alternation of rhythm in temperature)-stressed rats are an animal model of autonomic imbalance created by exposing animals to repeated cold stress. The SART-stressed rats have been shown to easily develop orthostatic hypotension (OH). In this study, effects of AF-DX116, a selective M(2) antagonist, and other muscarinic receptor antagonists on OH were investigated in SART-stressed and unstressed rats. Each anesthetized rat was canulated into the left common carotid artery, and blood pressure (BP) and heart rate were measured. Stimulation for postural change was initiated by head-up tilting. As the indices of OH, the maximum fall of BP, % reflex (recovery from maximum fall), and the area enclosed between the baseline and the recovery curve for BP (AUC) were used. Large AUC and small % reflex in SART-stressed rats were changed, becoming similar to those of the unstressed rats by AF-DX116 and methoctoramine. Atropine and methylatropine had similar effects to AF-DX116. However, the effects of methoctoramine, atropine, and methylatropine were less than that of AF-DX116. Pirenzepine was not effective. In conclusion, it was suggested in SART-stressed rats that OH was related to hyperactivity in the parasympathetic nerve and the M(2) receptor played the major role in OH.

Acute β-Blockade Increases Muscle Sympathetic Activity and Modifies Its Frequency Distribution

Background— The possible mechanisms by which β-adrenergic antagonists may act on the neural regulation of the cardiovascular system are still elusive. Recent studies reported a marked increase of postganglionic muscle sympathetic nerve activity (MSNA) after acute β-blockade associated with unchanged values of arterial blood pressure and baroreflex sensitivity. We tested the hypothesis that acute β-blockade might also alter the oscillatory characteristics of MSNA, thus decreasing its effectiveness on peripheral vasoconstriction.

Methods and Results— In 11 healthy volunteers, ECG, MSNA, arterial pressure, and respiration were recorded before and after atenolol (0.05 mg/kg IV bolus) administration. The frequency distribution of RR interval, MSNA, systolic arterial pressure (SAP), and respiratory variability was assessed by spectrum and cross-spectrum analysis. Spontaneous baroreflex sensitivity (α-index) and plasma catecholamines (high-performance liquid chromatography) were measured. Atenolol induced a significant increase in RR interval (14.3±1.6%) with no changes in systolic and diastolic arterial pressure. MSNA increased (42±13% from 18±2 bursts per minute). The low-frequency (LF) component of RR and MSNA variability decreased (−44±7% and −24±5%, respectively), whereas the high-frequency (HF) component increased (163±55% and 34±11%, respectively), expressed in normalized units. Spectral coherence, an index of oscillatory coupling, decreased between LF RR and LF MSNA , whereas it increased between HF MSNA and HF Resp . SAP variability, α-index, and plasma catecholamines remained unchanged.

Conclusions— Atenolol induced a change in MSNA frequency distribution reflecting a stronger respiratory coupling. This shift toward high frequency, despite an increase in MSNA, may lead to a less efficient sympathetic vasomotor modulation.

Spectral analysis of muscle sympathetic nerve activity in heat-stressed humans

Whole body heating increases muscle sympathetic nerve activity (MSNA); however, the effect of heat stress on spectral characteristics of MSNA is unknown. Such information may provide insight into mechanisms of heat stress-induced MSNA activation. The purpose of the present study was to test the hypothesis that heat stress-induced changes in systolic blood pressure variability parallel changes in MSNA variability. In 13 healthy subjects, MSNA, electrocardiogram, arterial blood pressure (via Finapres), and respiratory activity were recorded under both normothermic and heat stress conditions. Spectral characteristics of integrated MSNA, R-R interval, systolic blood pressure, and respiratory excursions were assessed in the low (LF; 0.03-0.15 Hz) and high (HF; 0.15-0.45 Hz) frequency components. Whole body heating significantly increased skin and core body temperature, MSNA burst rate, and heart rate, but not mean arterial blood pressure. Systolic blood pressure and R-R interval variability were significantly reduced in both the LF and HF ranges. Compared with normothermic conditions, heat stress significantly increased the HF component of MSNA, while the LF component of MSNA was not altered. Thus the LF-to-HF ratio of MSNA oscillatory components was significantly reduced. These data indicate that the spectral characteristics of MSNA are altered by whole body heating; however, heat stress-induced changes in MSNA do not parallel changes in systolic blood pressure variability. Moreover, the reduction in LF component of systolic blood pressure during heat stress is unlikely related to spectral changes in MSNA.

Effects of unilateral and bilateral carotid baroreflex stimulation on cardiac and neural sympathetic discharge oscillatory patterns

BACKGROUND

Left and right carotid baroreflex afferents participate in generating the spontaneous variability of heart rate (HR), arterial pressure (AP), and muscle sympathetic nerve activity (MSNA), but the relative contribution of each side is unclear. Pathophysiological conditions unilaterally affecting carotid baroreceptor function might result in abnormal changes of HR, AP, and MSNA variability, thus markedly affecting prognosis. We tested the hypothesis that unilateral carotid baroreceptor perturbation might differentially affect HR, AP, and MSNA variability compared with stimulation of the opposite side.

METHODS AND RESULTS

In 12 healthy volunteers, 4 sinusoidal neck suction procedures (0.1 Hz, from 0 to -50 mm Hg) were applied at the right, left, and combined right and left sides of the neck, in concordance or with phase opposition. Respiration was controlled at 0.25 Hz. Power spectrum analysis assessed the changes in the 0.1-Hz oscillatory component of the R-R interval, systolic AP (SAP), and MSNA variability induced by rhythmic baroreceptor stimulation. Mean R-R interval, SAP, and MSNA were unchanged during all procedures. The increase of the 0.1-Hz component of R-R and SAP variability during right and combined right and left carotid baroreceptor stimulation was greater than the changes induced by left-sided stimulation. The increase in the 0.1-Hz oscillatory component of MSNA variability was similar during all neck suction procedures.

CONCLUSIONS

Right carotid baroreflex loading was as efficient as bilateral stimulation and more effective than left carotid suction in modulating R-R and SAP variability. There was no asymmetry in neural sympathetic discharge responses after single-sided carotid baroreceptor stimulation.