Continuous low-level vagus nerve stimulation reduces stellate ganglion nerve activity and paroxysmal atrial tachyarrhythmias in ambulatory canines

Vagus nerve stimulation: state of the art of stimulation and recording strategies to address autonomic function neuromodulation

OBJECTIVE

Neural signals along the vagus nerve (VN) drive many somatic and autonomic functions. The clinical interest of VN stimulation (VNS) is thus potentially huge and has already been demonstrated in epilepsy. However, side effects are often elicited, in addition to the targeted neuromodulation.

APPROACH

This review examines the state of the art of VNS applied to two emerging modulations of autonomic function: heart failure and obesity, especially morbid obesity.

MAIN RESULTS

We report that VNS may benefit from improved stimulation delivery using very advanced technologies. However, most of the results from fundamental animal studies still need to be demonstrated in humans.

Effect of vagotomy on the activity of cardiac autonomic ganglia: Insight from left atrial high density frequency mapping

BACKGROUND

Both extrinsic and intrinsic cardiac autonomic nervous systems are important for initiation and maintenance of atrial fibrillation (AF). We aimed to evaluate the effect of vagotomy on the activity of cardiac autonomic ganglionated plexi (GP) and the change of dominant frequency (DF) distribution in the left atrium (LA) during AF.

METHODS

A mid-sternal thoracotomy was performed in 6 dogs. High frequency stimulation was applied to locate the GPs. There were four major LA GPs, which were located near the pulmonary vein ostia, and a superior vena cava-aorta (SVC-Ao) GP that was located near the superior vena cava-right atrial junction. Acetylcholine patch was applied on GPs to induce intrinsic vagal response. Vagal denervation was performed thereafter. An Ensite Array was deployed in the LA to record atrial signals before and after vagotomy during induced AF.

RESULTS

The LA mean DF values (8.2±0.1 vs. 7.6±0.1Hz, p=0.002) were higher during GP activation before than after vagotomy. The maximal DF distribution was located at the primary GPs and the nearby secondary GPs during LA GPs activation and at the LA septum and right superior pulmonary vein during SVC-Ao GP activation before vagotomy. After vagotomy, the maximal DF distribution shifted to non-GP LA sites during activation of the GPs.

CONCLUSIONS

The findings suggest the important role of the extrinsic neural input in the activation and interaction of the intrinsic cardiac autonomic activity during cholinergic AF, whereas the non-GP LA sites were responsible for the AF induced without the physiological extrinsic neural input.

Acute effects of unilateral temporary stellate ganglion block on human atrial electrophysiological properties and atrial fibrillation inducibility

BACKGROUND

In experimental models, stellate ganglion block (SGB) reduces the induction of atrial fibrillation (AF), while data in humans are limited.

OBJECTIVE

The aim of this study was to assess the effect of unilateral SGB on atrial electrophysiological properties and AF induction in patients with paroxysmal AF.

METHODS

Thirty-six patients with paroxysmal AF were randomized in a 2:1 order to temporary, transcutaneous, pharmaceutical SGB with lidocaine or placebo before pulmonary vein isolation. Lidocaine was 1:1 randomly infused to the right or left ganglion. Before and after randomization, atrial effective refractory period (ERP) of each atrium, difference between right and left atrial ERP, intra- and interatrial conduction time, AF inducibility, and AF duration were assessed.

RESULTS

After SGB, right atrial ERP was prolonged from a median (1st-3rd quartile) of 240 (220-268) ms to 260 (240-300) ms (P < .01) and left atrial ERP from 235 (220-260) ms to 245 (240-280) ms (P < .01). AF was induced by atrial pacing in all 24 patients before SGB, but only in 13 patients (54%) after the intervention (P < .01). AF duration was shorter after SGB: 1.5 (0.0-5.8) minutes from 5.5 (3.0-12.0) minutes (P < .01). Intra- and interatrial conduction time was not significantly prolonged. No significant differences were observed between right and left SGB. No changes were observed in the placebo group.

CONCLUSION

Unilateral temporary SGB prolonged atrial ERP, reduced AF inducibility, and decreased AF duration. An equivalent effect of right and left SGB on both atria was observed. These findings may have a clinical implication in the prevention of drug refractory and postsurgery AF and deserve further clinical investigation.

Translational neurocardiology: preclinical models and cardioneural integrative aspects

Neuronal elements distributed throughout the cardiac nervous system, from the level of the insular cortex to the intrinsic cardiac nervous system, are in constant communication with one another to ensure that cardiac output matches the dynamic process of regional blood flow demand. Neural elements in their various 'levels' become differentially recruited in the transduction of sensory inputs arising from the heart, major vessels, other visceral organs and somatic structures to optimize neuronal coordination of regional cardiac function. This White Paper will review the relevant aspects of the structural and functional organization for autonomic control of the heart in normal conditions, how these systems remodel/adapt during cardiac disease, and finally how such knowledge can be leveraged in the evolving realm of autonomic regulation therapy for cardiac therapeutics.

Median nerve stimulation reduces ventricular arrhythmias induced by dorsomedial hypothalamic stimulation

BACKGROUND

This study tested the hypothesis that median nerve stimulation (MNS) prevents ventricular arrhythmias (VAs) induced by dorsomedial hypothalamus stimulation (DMHS) and investigated the electrophysiological mechanisms underlying the anti-arrhythmic effects of MNS by recording left stellate ganglion activity (LSGA).

METHODS

Eighteen rabbits were anesthetized, the median nerve was anchored by stimulating electrodes, and a bipolar electrode was implanted into the LSG to record nerve activity. The DMH was stimulated to induce arrhythmia. All animals underwent six repetitions of DMHS (30 s). The 18 rabbits were divided into the following 3 groups: a control group, which underwent only DMHS (n = 6); an MNS group, which underwent MNS during both the third and fourth DMHS repetitions (n = 6); and an LSGA-recording group, for which LSGA was recorded at baseline, immediately following DMHS and again immediately following MNS and DMHS (n = 6).

RESULTS

Repeated DMHS-induced multiple VAs, in the rabbits. Compared with the DMHS-only group, the concurrent administration of MNS during DMHS significantly reduced the incidence of VAs (7 ± 3 and 9 ± 2 beats for the third and fourth DMHS + MNS repetitions vs. 29 ± 8 and 27 ± 9 beats for the first two DMHS repetitions, p < 0.05). The total duration of the abnormal discharges of the LSG (ADLSG) following MNS and DMHS was significantly reduced compared with that of the DMHS-only group (40 ± 18 vs. 14 ± 6 s, p < 0.05).

CONCLUSION

MNS reduced VAs induced by DMHS, which is thought to be mediated through suppressing of ADLSG.

NEW AND NOTEWORTHY

Median nerve electrical stimulation prevented ventricular arrhythmias induced by DMHS through the mechanism of suppressing abnormal discharges of left stellate ganglion.

Low-Level Vagus Nerve Stimulation Reverses Cardiac Dysfunction and Subcellular Calcium Handling in Rats With Post-Myocardial Infarction Heart Failure

Vagus nerve stimulation (VNS), targeting the imbalanced autonomic nervous system, is a promising therapeutic approach for chronic heart failure (HF). Moreover, calcium cycling is an important part of cardiac excitation-contraction coupling (ECC), which also participates in the antiarrhythmic effects of VNS. We hypothesized that low-level VNS (LL-VNS) could improve cardiac function by regulation of intracellular calcium handling properties. The experimental HF model was established by ligation of the left anterior descending coronary artery (LAD). Thirty-two male Sprague-Dawley rats were divided into 3 groups as follows; control group (sham operated without coronary ligation, n = 10), HF-VNS group (HF rats with VNS, n = 12), and HF-SS group (HF rats with sham nerve stimulation, n = 10). After 8 weeks of treatment, LL-VNS significantly improved left ventricular ejection fraction (LVEF) and attenuated myocardial interstitial fibrosis in the HF-VNS group compared with the HF-SS group. Elevated plasma norepinephrine and dopamine, but not epinephrine, were partially reduced by LL-VNS. Additionally, LL-VNS restored the protein and mRNA levels of sarcoplasmic reticulum Ca(2+) ATPase (SERCA2a), Na(+)-Ca(2+) exchanger 1 (NCX1), and phospholamban (PLB) whereas the expression of ryanodine receptor 2 (RyR2) as well as mRNA level was unaffected. Thus, our study results suggest that the improvement of cardiac performance by LL-VNS is accompanied by the reversal of dysfunctional calcium handling properties including SERCA2a, NCX1, and PLB which may be a potential molecular mechanism of VNS for HF.

Effects on hemodynamic variables and echocardiographic parameters after a stellate ganglion block in 15 healthy volunteers

BACKGROUND

The sympathetic nervous system has an important role in generating pain. Various pathomechanisms are involved that respond well to the application of local anesthetics (LA), for example to the stellate ganglion block (SGB).

OBJECTIVES

We wanted to know more about the effects of SGB on cardiovascular parameters.

METHODS

We included 15 healthy volunteers; another 15 healthy volunteers as a control group (sham injection of LA). In order to produce a more precise SGB, we employed only a small volume of LA (3mL), a LA with a lower permeability (procaine 1%), and a modified injection technique. Systolic and diastolic blood pressure (SBP, DBP), heart rate (HR), and echocardiographic parameters were recorded before and after SGB. We also investigated whether there are side differences (left and right SBG).

RESULTS

At baseline all parameters were within the normal range. After performing right and left SGB DBP significantly increased (on the right side from 68.73±8.61 to 73.53±11.10, p=0.015; on the left side from 70.66±13.01 to 77.93±10.40, p=0.003). In the control group no increase in DBP was observed. No side-specific differences were found, except a significant reduction in the maximum velocity of myocardial contraction during the systole with left-sided SGB.

CONCLUSIONS

Even with our methods we could not prevent the simultaneous occurrence of a partial parasympatholytic effect. For this reason, the SGB has only minor hemodynamic effects, which is desirable as it enhances the safety of the SGB.

Long-term risk of atrial fibrillation after the death of a partner

Objectives

Severe psychological stress is generally associated with an increased risk of acute cardiovascular diseases, such as myocardial infarction, but it remains unknown whether it also applies to atrial fibrillation. We conducted a population-based case–control study using nationwide Danish health registers to examine the risk of atrial fibrillation after the death of a partner.

Methods

From 1995 through 2014, we identified 88 612 cases with a hospital diagnosis of atrial fibrillation and 886 120 age-matched and sex-matched controls based on risk-set sampling. The conditional logistic regression model was used to calculate adjusted ORs of atrial fibrillation with 95% CIs.

Results

Partner bereavement was experienced by 17 478 cases and 168 940 controls and was associated with a transiently higher risk of atrial fibrillation; the risk was highest 8–14 days after the loss (1.90; 95% CI 1.34 to 2.69), after which it gradually declined. One year after the loss, the risk was almost the same as in the non-bereaved population. Overall, the OR of atrial fibrillation within 30 days after bereavement was 1.41 (95% CI 1.17 to 1.70), but it tended to be higher in persons below the age of 60 years (2.34; 95% CI 1.02 to 5.40) and in persons whose partner had a low predicted mortality 1 month before the death, that is, ≤5 points on the age-adjusted Charlson Comorbidity Index (1.57; 95% CI 1.13 to 2.17).

Conclusions

The severely stressful life event of losing a partner was followed by a transiently increased risk of atrial fibrillation lasting for 1 year, especially for the least predicted losses.

Low-level transcutaneous electrical stimulation of the auricular branch of vagus nerve ameliorates left ventricular remodeling and dysfunction by downregulation of matrix metalloproteinase 9 and transforming growth factor β1

Vagus nerve stimulation improves left ventricular (LV) remodeling by downregulation of matrix metalloproteinase 9 (MMP-9) and transforming growth factor β1 (TGF-β1). Our previous study found that low-level transcutaneous electrical stimulation of the auricular branch of the vagus nerve (LL-TS) could be substituted for vagus nerve stimulation to reverse cardiac remodeling. So, we hypothesize that LL-TS could ameliorate LV remodeling by regulation of MMP-9 and TGF-β1 after myocardial infarction (MI). Twenty-two beagle dogs were randomly divided into a control group (MI was induced by permanent ligation of the left coronary artery, n = 8), an LL-TS group (MI with long-term intermittent LL-TS, n = 8), and a normal group (sham ligation without stimulation, n = 6). At the end of 6 weeks follow-up, LL-TS significantly reduced LV end-systolic and end-diastolic dimensions, improved ejection fraction and ratio of early (E) to late (A) peak mitral inflow velocity. LL-TS attenuated interstitial fibrosis and collagen degradation in the noninfarcted myocardium compared with the control group. Elevated level of MMP-9 and TGF-β1 in LV tissue and peripheral plasma were diminished in the LL-TS treated dogs. LL-TS improves cardiac function and prevents cardiac remodeling in the late stages after MI by downregulation of MMP-9 and TGF-β1 expression.

Atrial fibrillation: mechanisms, therapeutics, and future directions

Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia, affecting 1% to 2% of the general population. It is characterized by rapid and disorganized atrial activation leading to impaired atrial function, which can be diagnosed on an EKG by lack of a P-wave and irregular QRS complexes. AF is associated with increased morbidity and mortality and is a risk factor for embolic stroke and worsening heart failure. Current research on AF support and explore the hypothesis that initiation and maintenance of AF require pathophysiological remodeling of the atria, either specifically as in lone AF or secondary to other heart disease as in heart failure-associated AF. Remodeling in AF can be grouped into three categories that include: (i) electrical remodeling, which includes modulation of L-type Ca(2+) current, various K(+) currents and gap junction function; (ii) structural remodeling, which includes changes in tissues properties, size, and ultrastructure; and (iii) autonomic remodeling, including altered sympathovagal activity and hyperinnervation. Electrical, structural, and autonomic remodeling all contribute to creating an AF-prone substrate which is able to produce AF-associated electrical phenomena including a rapidly firing focus, complex multiple reentrant circuit or rotors. Although various remodeling events occur in AF, current AF therapies focus on ventricular rate and rhythm control strategies using pharmacotherapy and surgical interventions. Recent progress in the field has started to focus on the underlying substrate that drives and maintains AF (termed upstream therapies); however, much work is needed in this area. Here, we review current knowledge of AF mechanisms, therapies, and new areas of investigation.

The value of basic research insights into atrial fibrillation mechanisms as a guide to therapeutic innovation: a critical analysis

Atrial fibrillation (AF) is an extremely common clinical problem associated with increased morbidity and mortality. Current antiarrhythmic options include pharmacological, ablation, and surgical therapies, and have significantly improved clinical outcomes. However, their efficacy remains suboptimal, and their use is limited by a variety of potentially serious adverse effects. There is a clear need for improved therapeutic options. Several decades of research have substantially expanded our understanding of the basic mechanisms of AF. Ectopic firing and re-entrant activity have been identified as the predominant mechanisms for arrhythmia initiation and maintenance. However, it has become clear that the clinical factors predisposing to AF and the cellular and molecular mechanisms involved are extremely complex. Moreover, all AF-promoting and maintaining mechanisms are dynamically regulated and subject to remodelling caused by both AF and cardiovascular disease. Accordingly, the initial presentation and clinical progression of AF patients are enormously heterogeneous. An understanding of arrhythmia mechanisms is widely assumed to be the basis of therapeutic innovation, but while this assumption seems self-evident, we are not aware of any papers that have critically examined the practical contributions of basic research into AF mechanisms to arrhythmia management. Here, we review recent insights into the basic mechanisms of AF, critically analyse the role of basic research insights in the development of presently used anti-AF therapeutic options and assess the potential value of contemporary experimental discoveries for future therapeutic innovation. Finally, we highlight some of the important challenges to the translation of basic science findings to clinical application.

Chronic Intermittent Low-Level Stimulation of Tragus Reduces Cardiac Autonomic Remodeling and Ventricular Arrhythmia Inducibility in a Post-Infarction Canine Model

OBJECTIVES

This study investigated whether chronic low-level tragus stimulation (LL-TS) inhibits cardiac sympathetic remodeling and reduces ventricular arrhythmia inducibility in a post-infarction canine model.

BACKGROUND

Low-level vagal stimulation has been shown to suppress cardiac sympathetic activity, which plays an important role in ventricular arrhythmia after myocardial infarction (MI). Our previous studies reported a noninvasive approach to deliver vagal stimulation by transcutaneous stimulation at the tragus, where the auricular branch of the vagus nerve is located.

METHODS

Twenty-two beagles were randomized to the normal control (n = 6), MI (left anterior descending coronary artery ligation without LL-TS [n = 8]), and TS (MI plus LL-TS [n = 8]) groups. LL-TS was delivered 2 h each day at 80% below the threshold which slowed sinus rate.

RESULTS

At 2-month follow-up, LL-TS was found to significantly reduce ventricular arrhythmia inducibility (arrhythmia score: 1.8 ± 0.8 vs. 3.6 ± 0.7, p < 0.01, compared to the MI group), decreased left stellate ganglion (LSG) activity (frequency: 32 ± 15 vs. 112 ± 29 impulses/s; and amplitude: 0.15 ± 0.12 mV vs. 0.38 ± 0.12 mV, compared to MI group), and attenuated cardiac sympathetic remodeling induced by chronic MI. The nerve growth factor (NGF) protein was down-regulated, whereas the small conductance calcium-activated potassium channel type2 (SK2) protein was up-regulated in the LSG by chronic LL-TS.

CONCLUSIONS

Chronic LL-TS could reduce the ventricular arrhythmia inducibility, LSG neural activity and sympathetic neural remodeling in a post-infarction canine model. Down-regulation of NGF protein and up-regulation of SK2 protein in the LSG contribute to the salutary effects of LL-TS.

Intermittent left cervical vagal nerve stimulation damages the stellate ganglia and reduces the ventricular rate during sustained atrial fibrillation in ambulatory dogs

BACKGROUND

The effects of intermittent open-loop vagal nerve stimulation (VNS) on the ventricular rate (VR) during atrial fibrillation (AF) remain unclear.

OBJECTIVE

The purpose of this study was to test the hypothesis that VNS damages the stellate ganglion (SG) and improves VR control during persistent AF.

METHODS

We performed left cervical VNS in ambulatory dogs while recording the left SG nerve activity (SGNA) and vagal nerve activity. Tyrosine hydroxylase (TH) staining and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining were used to assess neuronal cell death in the SG.

RESULTS

We induced persistent AF by atrial pacing in 6 dogs, followed by intermittent VNS with short ON-time (14 seconds) and long OFF-time (66 seconds). The integrated SGNA and VR during AF were 4.84 mV·s (95% confidence interval [CI] 3.08-6.60 mV·s) and 142 beats/min (95% CI 116-168 beats/min), respectively. During AF, VNS reduced the integrated SGNA and VR, respectively, to 3.74 mV·s (95% CI 2.27-5.20 mV·s; P = .021) and 115 beats/min (95% CI 96-134 beats/min; P = .016) during 66-second OFF-time and to 4.07 mV·s (95% CI 2.42-5.72 mV·s; P = .037) and 114 beats/min (95% CI 83-146 beats/min; P = .039) during 3-minute OFF-time. VNS increased the frequencies of prolonged (>3 seconds) pauses during AF. TH staining showed large confluent areas of damage in the left SG, characterized by pyknotic nuclei, reduced TH staining, increased percentage of TH-negative ganglion cells, and positive TUNEL staining. Occasional TUNEL-positive ganglion cells were also observed in the right SG.

CONCLUSION

VNS damaged the SG, leading to reduced SGNA and better rate control during persistent AF.

Treatment of Atrial and Ventricular Arrhythmias Through Autonomic Modulation

This paper reviews the contribution of autonomic nervous system (ANS) modulation in the treatment of arrhythmias. Both the atria and ventricles are innervated by an extensive network of nerve fibers of parasympathetic and sympathetic origin. Both the parasympathetic and sympathetic nervous system exert arrhythmogenic electrophysiological effects on atrial and pulmonary vein myocardium, while in the ventricle the sympathetic nervous system plays a more dominant role in arrhythmogenesis. Identification of ANS activity is possible with nuclear imaging. This technique may provide further insight in mechanisms and treatment targets. Additionally, the myocardial effects of the intrinsic ANS can be identified through stimulation of the ganglionic plexuses. These can be ablated for the treatment of atrial fibrillation. New (non-) invasive treatment options targeting the extrinsic cardiac ANS, such as low-level tragus stimulation and renal denervation, provide interesting future treatment possibilities both for atrial fibrillation and ventricular arrhythmias. However, the first randomized trials have yet to be performed. Future clinical studies on modifying the ANS may not only improve the outcome of ablation therapy but may also advance our understanding of the manner in which the ANS interacts with the myocardium to modify arrhythmogenic triggers and substrate.

Continuous vagal nerve stimulation affects atrial neural remodeling and reduces atrial fibrillation inducibility in rabbits

BACKGROUND

The effects of continuous vagal nerve stimulation (VNS) on atrial neural remodeling during atrial fibrillation (AF) remain unclear.

OBJECTIVE

To test the hypothesis that VNS affects atrial neural remodeling and reduces AF inducibility.

METHODS

Twenty rabbits were randomly divided into two groups: rapid atrial pacing (RAP) group and RAP with VNS group. AF inducibility studies and atrial histologic analyses were performed after 4 weeks.

RESULTS

Five rabbits of RAP group (5/10) in the RAP group developed sustained AF. None of rabbits in RAP with VNS group had developed AF. The incidence of sustained AF in VNS group was significant lower than that in rapid pacing group (P<.01). Treatment with VNS resulted in a significant reduction in atrial neural remodeling and AF duration (P<.01).

CONCLUSIONS

Atrial neural remodeling plays an important role in the initiation and maintenance of AF. Modulating autonomic nerve function with VNS can contribute to AF control.

Central-peripheral neural network interactions evoked by vagus nerve stimulation: functional consequences on control of cardiac function

Using vagus nerve stimulation (VNS), we sought to determine the contribution of vagal afferents to efferent control of cardiac function. In anesthetized dogs, the right and left cervical vagosympathetic trunks were stimulated in the intact state, following ipsilateral or contralateral vagus nerve transection (VNTx), and then following bilateral VNTx. Stimulations were performed at currents from 0.25 to 4.0 mA, frequencies from 2 to 30 Hz, and a 500-μs pulse width. Right or left VNS evoked significantly greater current- and frequency-dependent suppression of chronotropic, inotropic, and lusitropic function subsequent to sequential VNTx. Bradycardia threshold was defined as the current first required for a 5% decrease in heart rate. The threshold for the right vs. left vagus-induced bradycardia in the intact state (2.91 ± 0.18 and 3.47 ± 0.20 mA, respectively) decreased significantly with right VNTx (1.69 ± 0.17 mA for right and 3.04 ± 0.27 mA for left) and decreased further following bilateral VNTx (1.29 ± 0.16 mA for right and 1.74 ± 0.19 mA for left). Similar effects were observed following left VNTx. The thresholds for afferent-mediated effects on cardiac parameters were 0.62 ± 0.04 and 0.65 ± 0.06 mA with right and left VNS, respectively, and were reflected primarily as augmentation. Afferent-mediated tachycardias were maintained following β-blockade but were eliminated by VNTx. The increased effectiveness and decrease in bradycardia threshold with sequential VNTx suggest that 1) vagal afferents inhibit centrally mediated parasympathetic efferent outflow and 2) the ipsilateral and contralateral vagi exert a substantial buffering capacity. The intact threshold reflects the interaction between multiple levels of the cardiac neural hierarchy.

Spinal cord stimulation suppresses focal rapid firing-induced atrial fibrillation by inhibiting atrial ganglionated plexus activity

OBJECTIVE

This study was designed to demonstrate that spinal cord stimulation (SCS) could suppress high-frequency stimulation (HFS)-induced focal atrial fibrillation (AF) at atrial and pulmonary vein (PV) sites by inhibiting atrial ganglionated plexus (GP) activity.

METHODS

Multielectrode catheters were attached to atria and all PV sites. SCS was performed at the T1-T5 spinal region for 1 hour. At the baseline state and the end of 1 hour of SCS, 40 milliseconds of HFS was delivered 2 milliseconds after atrial pacing to determine the AF threshold at each site. One electrode was attached to the superior left GP so that HFS to this site induced sinus rate slowing. Microelectrodes inserted into the anterior right GP recorded neural firing.

RESULTS

SCS induced a significant increase in AF threshold at all sites (all P < 0.05). The sinus rate slowing response induced by superior left GP stimulation was blunted by SCS (17% ± 3.6% vs. 39% ± 3.8%, P < 0.05). The frequency (32 ± 4 vs. 87 ± 6 impulses per minute, P < 0.05) and amplitude (0.16 ± 0.02 vs. 0.42 ± 0.04 mv, P < 0.05) of the neural activity recorded from the anterior right GP were markedly inhibited by SCS.

CONCLUSIONS

SCS may prevent episodic AF caused by rapid PV and non-PV firing through modulating GP activity.

Low-level transcutaneous electrical vagus nerve stimulation suppresses atrial fibrillation

BACKGROUND

Transcutaneous low-level tragus electrical stimulation (LLTS) suppresses atrial fibrillation (AF) in canines.

OBJECTIVES

This study examined the antiarrhythmic and anti-inflammatory effects of LLTS in humans.

METHODS

Patients with paroxysmal AF who presented for AF ablation were randomized to either 1 h of LLTS (n = 20) or sham control (n = 20). Attaching a flat metal clip onto the tragus produced LLTS (20 Hz) in the right ear (50% lower than the voltage slowing the sinus rate). Under general anesthesia, AF was induced by burst atrial pacing at baseline and after 1 h of LLTS or sham treatment. Blood samples from the coronary sinus and the femoral vein were collected at those time points and then analyzed for inflammatory cytokines, including tumor necrosis factor alpha and C-reactive protein, using a multiplex immunoassay.

RESULTS

There were no differences in baseline characteristics between the 2 groups. Pacing-induced AF duration decreased significantly by 6.3 ± 1.9 min compared with baseline in the LLTS group, but not in the control subjects (p = 0.002 for comparison between groups). AF cycle length increased significantly from baseline by 28.8 ± 6.5 ms in the LLTS group, but not in control subjects (p = 0.0002 for comparison between groups). Systemic (femoral vein) but not coronary sinus tumor necrosis factor (TNF)-alpha and C-reactive protein levels decreased significantly only in the LLTS group.

CONCLUSIONS

LLTS suppresses AF and decreases inflammatory cytokines in patients with paroxysmal AF. Our results support the emerging paradigm of neuromodulation to treat AF.

Device-based autonomic modulation in arrhythmia patients: the role of vagal nerve stimulation

OPINION STATEMENT

Vagal nerve stimulation (VNS) has shown promise as an adjunctive therapy for management of cardiac arrhythmias by targeting the cardiac parasympathetic nervous system. VNS has been evaluated in the setting of ischemia-driven ventricular arrhythmias and atrial arrhythmias, as well as a treatment option for heart failure. As better understanding of the complexities of the cardiac autonomic nervous system is obtained, vagal nerve stimulation will likely become a powerful tool in the current cardiovascular therapeutic armamentarium.

Intermittent low-level vagosympathetic nerve trunk stimulation inhibits ganglionated plexi activity to prevent atrial fibrillation

BACKGROUND

The cardiac autonomic nervous system (CANS) plays a role in the occurrence and persistence of atrial fibrillation (AF). Low-level vagosympathetic nerve stimulation (LL-VNS) has been shown to inhibit the occurrence of AF.

OBJECTIVE

The novel objective of this study was to compare the effects of intermittent low- level vagosympathetic nerve stimulation (I-VNS) and continuous low-level vagosympathetic nerve stimulation (C-VNS).

METHODS

19 beagles were randomly divided into 3 groups: Group A, rapid left atrial appendage pacing for 6 hours; Group B, rapid atrial pacing (RAP) for 6 hours and C-VNS (20 Hz, interval 0.1 ms, square wave) with 50% threshold voltage strength; Group C, RAP for 6 hours and I-VNS (continuously recurring cycles of 30-second ON, 30-second OFF). The atrial monophasic action potential (MAP) and the effective refractory periods (ERP) of the atrium and the pulmonary veins were measured at baseline, 1 hour, 3 hours and 6 hours after the experiment began. After the experiment, tyrosine hydroxylase (TH) and choline acetyl transferase (CHAT) expression levels in the anterior right ganglionated plexi (ARGP) from each group were measured.

RESULTS

Inter-group comparisons of MAP and ERP demonstrated that Group A was significantly different from Groups B and C (P < 0.05), while the difference between Groups B and C was not significant (P > 0.05). The MAP and ERP in Group A gradually decreased, reaching a minimum at 6 hours, but no significant changes were observed in Groups B and C. When compared to Group A, both Groups B and C had reduced TH and CHAT expression.

CONCLUSIONS

During the occurrence and development of AF, I-VNS could protect the cardiovascular system, possibly replacing C-VNS. Additionally, both I-VNS and C-VNS inhibited ganglionated plexus (GP) activity during the AF prevention.

Cervical vagal nerve stimulation activates the stellate ganglion in ambulatory dogs

Background and Objectives

Recent studies showed that, in addition to parasympathetic nerves, cervical vagal nerves contained significant sympathetic nerves. We hypothesized that cervical vagal nerve stimulation (VNS) may capture the sympathetic nerves within the vagal nerve and activate the stellate ganglion.

Materials and Methods

We recorded left stellate ganglion nerve activity (SGNA), left thoracic vagal nerve activity (VNA), and subcutaneous electrocardiogram in seven dogs during left cervical VNS with 30 seconds on-time and 30 seconds off time. We then compared the SGNA between VNS on and off times.

Results

Cervical VNS at moderate (0.75 mA) output induced large SGNA, elevated heart rate (HR), and reduced HR variability, suggesting sympathetic activation. Further increase of the VNS output to >1.5 mA increased SGNA but did not significantly increase the HR, suggesting simultaneous sympathetic and parasympathetic activation. The differences of integrated SGNA and integrated VNA between VNS on and off times (ΔSGNA) increased progressively from 5.2 mV-s {95% confidence interval (CI): 1.25-9.06, p=0.018, n=7} at 1.0 mA to 13.7 mV-s (CI: 5.97-21.43, p=0.005, n=7) at 1.5 mA. The difference in HR (ΔHR, bpm) between on and off times was 5.8 bpm (CI: 0.28-11.29, p=0.042, n=7) at 1.0 mA and 5.3 bpm (CI 1.92 to 12.61, p=0.122, n=7) at 1.5 mA.

Conclusion

Intermittent cervical VNS may selectively capture the sympathetic components of the vagal nerve and excite the stellate ganglion at moderate output. Increasing the output may result in simultaneously sympathetic and parasympathetic capture.

The role of the autonomic ganglia in atrial fibrillation

Recent experimental and clinical studies have shown that the epicardial autonomic ganglia play an important role in the initiation and maintenance of atrial fibrillation (AF). In this review, we present the current data on the role of the autonomic ganglia in the pathogenesis of AF and discuss potential therapeutic implications. Experimental studies have demonstrated that acute autonomic remodeling may play a crucial role in AF maintenance in the very early stages. The benefit of adding ablation of the autonomic ganglia to the standard pulmonary vein (PV) isolation procedure for patients with paroxysmal AF is supported by both experimental and clinical data. The interruption of axons from these hyperactive autonomic ganglia to the PV myocardial sleeves may be an important factor in the success of PV isolation procedures. The vagus nerve exerts an inhibitory control over the autonomic ganglia and attenuation or loss of this control may allow these ganglia to become hyperactive. Autonomic neuromodulation using low-level vagus nerve stimulation inhibits the activity of the autonomic ganglia and reverses acute electrical atrial remodeling during rapid atrial pacing and may provide an alternative non-ablative approach for the treatment of AF, especially in the early stages. This notion is supported by a preliminary human study. Further studies are warranted to confirm these findings.

Interventional and device-based autonomic modulation in heart failure

"Heart failure is an increasingly prevalent disease with high mortality and public health burden. It is associated with autonomic imbalance characterized by sympathetic hyperactivity and parasympathetic hypoactivity. Evolving novel interventional and device-based therapies have sought to restore autonomic balance by neuromodulation. Results of preclinical animal studies and early clinical trials have demonstrated the safety and efficacy of these therapies in heart failure. This article discusses specific neuromodulatory treatment modalities individually-spinal cord stimulation, vagus nerve stimulation, baroreceptor activation therapy, and renal sympathetic nerve denervation."

Remodeling of stellate ganglion neurons after spatially targeted myocardial infarction: Neuropeptide and morphologic changes

BACKGROUND

Myocardial infarction (MI) induces remodeling in stellate ganglion neurons (SGNs).

OBJECTIVE

We investigated whether infarct site has any impact on the laterality of morphologic changes or neuropeptide expression in stellate ganglia.

METHODS

Yorkshire pigs underwent left circumflex coronary artery (LCX; n = 6) or right coronary artery (RCA; n = 6) occlusion to create left- and right-sided MI, respectively (control: n = 10). At 5 ± 1 weeks after MI, left and right stellate ganglia (LSG and RSG, respectively) were collected to determine neuronal size, as well as tyrosine hydroxylase (TH) and neuropeptide Y immunoreactivity.

RESULTS

Compared with control, LCX and RCA MIs increased mean neuronal size in the LSG (451 ± 25 vs 650 ± 34 vs 577 ± 55 μm(2), respectively; P = .0012) and RSG (433 ± 22 vs 646 ± 42 vs 530 ± 41 μm(2), respectively; P = .002). TH immunoreactivity was present in the majority of SGNs. Both LCX and RCA MIs were associated with significant decreases in the percentage of TH-negative SGNs, from 2.58% ± 0.2% in controls to 1.26% ± 0.3% and 0.7% ± 0.3% in animals with LCX and RCA MI, respectively, for LSG (P = .001) and from 3.02% ± 0.4% in controls to 1.36% ± 0.3% and 0.68% ± 0.2% in LCX and RCA MI, respectively, for RSG (P = .002). Both TH-negative and TH-positive neurons increased in size after LCX and RCA MI. Neuropeptide Y immunoreactivity was also increased significantly by LCX and RCA MI in both ganglia.

CONCLUSION

Left- and right-sided MIs equally induced morphologic and neurochemical changes in LSG and RSG neurons, independent of infarct site. These data indicate that afferent signals transduced after MI result in bilateral changes and provide a rationale for bilateral interventions targeting the sympathetic chain for arrhythmia modulation.

SK channels and ventricular arrhythmias in heart failure

Small-conductance Ca(2+)-activated K(+) (SK) currents are important in the repolarization of normal atrial (but not ventricular) cardiomyocytes. However, recent studies showed that the SK currents are upregulated in failing ventricular cardiomyocytes, along with increased SK channel protein expression and enhanced sensitivity to intracellular Ca(2+). The SK channel activation may be either anti-arrhythmic or pro-arrhythmic, depending on the underlying clinical situations. While the SK channel is a new target of anti-arrhythmic therapy, drug safety is still one of the major concerns.

Maladaptive autonomic regulation in PTSD accelerates physiological aging

A core manifestation of post-traumatic stress disorder (PTSD) is a disconnection between physiological state and psychological or behavioral processes necessary to adequately respond to environmental demands. Patients with PTSD experience abnormal oscillations in autonomic states supporting either fight and flight behaviors or withdrawal, immobilization, and dissociation without an intervening “calm” state that would provide opportunities for positive social interactions. This defensive autonomic disposition is adaptive in dangerous and life threatening situations, but in the context of every-day life may lead to significant psychosocial distress and deteriorating social relationships. The perpetuation of these maladaptive autonomic responses may contribute to the development of comorbid mental health issues such as depression, loneliness, and hostility that further modify the nature of cardiovascular behavior in the context of internal and external stressors. Over time, changes in autonomic, endocrine, and immune function contribute to deteriorating health, which is potently expressed in brain dysfunction and cardiovascular disease. In this theoretical review paper, we present an overview of the literature on the chronic health effects of PTSD. We discuss the brain networks underlying PTSD in the context of autonomic efferent and afferent contributions and how disruption of these networks leads to poor health outcomes. Finally, we discuss treatment approaches based on our theoretical model of PTSD.

Low-level vagosympathetic trunk stimulation inhibits atrial fibrillation in a rabbit model of obstructive sleep apnea

BACKGROUND

Atrial fibrillation (AF) is highly associated with obstructive sleep apnea (OSA) in which AF is triggered by hyperactivity of the cardiac autonomic nervous system. Previous studies showed that low-level vagosympathetic trunk stimulation (LLVS), at voltages not slowing sinus rate or AV conduction, inhibits AF by suppressing the cardiac autonomic nervous system.

OBJECTIVE

The purpose of this study was to investigate whether LLVS delivered at the right vagosympathetic trunk suppresses AF in a rabbit model of OSA.

METHODS

Eleven rabbits received a tracheostomy under general anesthesia. The endotracheal tube was clamped at end expiration for 1 minute to simulate OSA. Over a period of 4 hours, OSA was delivered every 6 minutes. Effective refractory period (ERP), blood pressure, intraesophageal pressure, and blood gases (O2, CO2, pH) were measured before and after each episode of OSA. AF duration and ERP were measured by programmed stimulation. Group 1 rabbits (n = 6) received LLVS (50% below that which slowed the sinus rate) in the first 3 hours. Group 2 rabbits (n = 5) only received OSA.

RESULTS

Group 1 ERP began to lengthen progressively from the second hour compared to group 2. AF duration increased in the first hour for both groups but began to shorten progressively after the first hour in group 1 rabbits. Blood pH, O2 or CO2 level, intraesophageal pressure, and hypertensive response during OSA were not different between the 2 groups.

CONCLUSION

LLVS is capable of suppressing ERP shortening and AF induced by OSA. LLVS may serve as a new therapeutic approach to treat OSA-induced AF.

The use of low-level electromagnetic fields to suppress atrial fibrillation

BACKGROUND

Extremely low-level electromagnetic fields have been proposed to cause significant changes in neural networks.

OBJECTIVE

We sought to investigate whether low-level electromagnetic fields can suppress atrial fibrillation (AF).

METHODS

In 17 pentobarbital anesthetized dogs, bilateral thoracotomies allowed the placement of multielectrode catheters in both atria and at all pulmonary veins. AF was induced by rapid atrial pacing (RAP) or programmed atrial extrastimulation. At baseline and end of each hour of RAP, during sinus rhythm, atrial programmed stimulation gave both the effective refractory period (ERP) and the width of the window of vulnerability. The latter was a measure of AF inducibility. Microelectrodes inserted into the anterior right ganglionated plexi recorded neural firing. Helmholtz coils were powered by a function generator inducing an electromagnetic field (EMF; 0.034 μG, 0.952 Hz). The study sample was divided into 2 groups: group 1 (n = 7)-application of EMF to both cervical vagal trunks; group 2 (n = 10)-application of EMF across the chest so that the heart was located in the center of the coil.

RESULTS

In group 1, EMF induced a progressive increase in AF threshold at all pulmonary vein and atrial sites (all P < .05). In group 2, the atrial ERP progressively shortened and ERP dispersion and window of vulnerability progressively increased (P < .05 compared to baseline values) during 3 hours of RAP and then returned to baseline values during 3 hours of combined application of RAP and EMF (P < .05 compared to the end of the third hour of RAP). The frequency and amplitude of the neural activity recorded from the anterior right ganglionated plexi were markedly suppressed by EMF in both groups.

CONCLUSION

Pulsed EMF applied to the vagal trunks or noninvasively across the chest can significantly reverse AF inducibility.

Vagus nerve stimulation reduces cardiac electrical instability assessed by quantitative T-wave alternans analysis in patients with drug-resistant focal epilepsy

OBJECTIVE

The cardiac component of risk for sudden unexpected death in epilepsy (SUDEP) and alterations in cardiac risk by vagus nerve stimulation (VNS) are not well understood. We determined changes in T-wave alternans (TWA), a proven noninvasive marker of risk for sudden cardiac death in patients with cardiovascular disease, and heart rate variability (HRV), an indicator of autonomic function, in association with VNS in patients with drug-resistant focal epilepsy.

METHODS

Ambulatory 24-h electrocardiograms (N = 9: ages 29-63, six males) were analyzed.

RESULTS

Mean TWA during the interictal period was 37 ± 3.1 μV (mean ± SEM) in lead V1 for nine patients monitored following implantation of the VNS system (n = 7) or battery change (n = 2). Of these, six patients also monitored prior to implantation (n = 5) or battery change (n = 1) showed abnormally high TWA levels pre-VNS (60.0 ± 4.3 μV), which were significantly reduced by 24.3 μV (to 35.7 ± 4.8 μV, p = 0.02) after VNS settings were adjusted for desired clinical response. TWA in four (67%) of the six patients was reduced in association with VNS to levels below the 47-μV cut point of abnormality. The decrease in TWA was correlated with VNS intensity (r = 0.88, p < 0.02). In addition, low-frequency HRV was reduced by 60% (805.61 ± 253.96 to 323.49 ± 102.74 msec(2) , p = 0.05) and low-to high-frequency HRV ratio by 32% (3.34 ± 0.57 to 2.26 ± 0.31, p = 0.025), indicating a change in autonomic balance in favor of parasympathetic dominance.

SIGNIFICANCE

This is the first report that elevated levels of TWA in patients with drug-refractory partial-onset seizures were reduced in association with VNS, potentially by improving sympathetic/parasympathetic balance. VNS may have a cardioprotective role at stimulation settings typically used for seizure control. These findings indicate the utility of TWA for tracking improvement in cardiac status in this population.

Estimating sympathetic tone by recording subcutaneous nerve activity in ambulatory dogs

INTRODUCTION

We tested the hypothesis that subcutaneous nerve activity (SCNA) of the thorax correlates with the stellate ganglion nerve activity (SGNA) and can be used to estimate the sympathetic tone.

METHODS AND RESULTS

We implanted radio transmitters in 11 ambulatory dogs to record left SGNA, left thoracic vagal nerve activity (VNA), and left thoracic SCNA, including 3 with simultaneous video monitoring and nerve recording. Two additional dogs were studied under general anesthesia with apamin injected into the right stellate ganglion while the right SGNA and the right SCNA were recorded. There was a significant positive correlation between integrated SGNA (iSGNA) and integrated SCNA (iSCNA) in the first 7 ambulatory dogs, with correlation coefficient of 0.70 (95% confidence interval [CI] 0.61-0.84, P < 0.05 for each dog). Tachycardia episodes (heart rate exceeding 150 bpm for ≥3 seconds) were invariably preceded by SGNA and SCNA. There was circadian variation of both SCNA and SGNA. Crosstalk was ruled out because SGNA, VNA, and SCNA bursts had different timing and activation patterns. In an eighth dog, closely spaced bipolar subcutaneous electrodes also recorded SCNA, but with reduced signal to noise ratio. Video monitoring in additional 3 dogs showed that movement was not a cause of high frequency SCNA. The right SGNA correlated strongly with right SCNA and heart rate in 2 anesthetized dogs after apamin injection into the right stellate ganglion.

CONCLUSIONS

SCNA recorded by bipolar subcutaneous electrodes correlates with the SGNA and can be used to estimate the sympathetic tone.

Effects of low-intensity autonomic nerve stimulation on atrial electrophysiology

Background and Objectives

The cardiac autonomic nervous system is an emerging target for therapeutic control of atrial fibrillation (AF). We evaluated the effects of low-intensity autonomic nerve stimulation (LI-ANS) on atrial electrophysiology, AF vulnerability, and neural remodeling.

Subjects and Methods

Fourteen dogs were subjected to 3 hours rapid atrial pacing (RAP, 5 Hz) and concomitant high frequency LI-ANS (20 Hz, at voltages 40% below the threshold) as follows: no autonomic stimulation (control, n=3); or right cervical vagus nerve (RVN, n=6), anterior right ganglionated plexi (ARGP, n=3), and superior left ganglionated plexi (SLGP, n=2) stimulation. Programmed and burst atrial pacing were performed at baseline and at the end of each hour to determine atrial effective refractory period (ERP), window of vulnerability (WOV), and inducibility of sustained AF.

Results

Atrial ERP was significantly shortened by 3 hours RAP (in control group, ΔERP=-47.9±8.9%, p=0.032), and RAP-induced ERP shortening was attenuated by LI-ANS (in LI-ANS group, ΔERP=-15.4±5.9%, p=0.019; vs. control, p=0.035). Neither WOV for AF nor AF inducibility changed significantly during 3 hours RAP with simultaneous LI-ANS. There was no significant difference between the control and LI-ANS group in nerve density and sprouting evaluated by anti-tyrosine hydroxylase and anti-growth associated protein-43 staining. Among the various sites for LI-ANS, the ARGP-stimulation group showed marginally lower ΔWOV (p=0.077) and lower nerve sprouting (p=0.065) compared to the RVN-stimulation group.

Conclusion

Low-intensity autonomic nerve stimulation significantly attenuated the shortening of atrial ERP caused by RAP. ARGP may be a better target for LI-ANS than RVN for the purpose of suppressing atrial remodeling in AF.

Vagus nerve stimulation initiated late during ischemia, but not reperfusion, exerts cardioprotection via amelioration of cardiac mitochondrial dysfunction

BACKGROUND

We previously reported that vagus nerve stimulation (VNS) applied immediately at the onset of cardiac ischemia provides cardioprotection against cardiac ischemic-reperfusion (I/R) injury.

OBJECTIVE

This study aimed to determine whether VNS applied during ischemia or at the onset of reperfusion exerts differential cardioprotection against cardiac I/R injury.

METHODS

Twenty-eight swine (25-30 kg) were randomized into 4 groups: Control (sham-operated, no VNS), VNS-ischemia (VNS applied during ischemia), VNS-reperfusion (VNS applied during reperfusion), and VNS-ischemia+atropine (VNS applied during ischemia with 1 mg/kg atropine administration). Ischemia was induced by left anterior descending (LAD) coronary artery occlusion for 60 minutes, followed by 120 minutes of reperfusion. VNS was applied either 30 minutes after LAD coronary artery occlusion or at the onset of reperfusion and continued until the end of reperfusion. Cardiac function, infarct size, myocardial levels of connexin 43, cytochrome c, tumor necrosis factor α, and interleukin 4, and cardiac mitochondrial function were determined.

RESULTS

VNS applied 30 minutes after LAD coronary artery occlusion, but not at reperfusion, markedly reduced ventricular fibrillation incidence and infarct size (~59%), improved cardiac function; attenuated cardiac mitochondrial reactive oxygen species production, depolarization, swelling, and cytochrome c release; and increased the amount of phosphorylated connexin 43 and interleukin 4 as compared with the Control group. These beneficial effects of VNS were abolished by atropine.

CONCLUSION

VNS could provide significant cardioprotective effects even when initiated later during ischemia, but was not effective after reperfusion. These findings indicate the importance of timing of VNS initiation and warrant the potential clinical application of VNS in protecting myocardium at risk of I/R injury.

Role of the autonomic nervous system in modulating cardiac arrhythmias

The autonomic nervous system plays an important role in the modulation of cardiac electrophysiology and arrhythmogenesis. Decades of research has contributed to a better understanding of the anatomy and physiology of cardiac autonomic nervous system and provided evidence supporting the relationship of autonomic tone to clinically significant arrhythmias. The mechanisms by which autonomic activation is arrhythmogenic or antiarrhythmic are complex and different for specific arrhythmias. In atrial fibrillation, simultaneous sympathetic and parasympathetic activations are the most common trigger. In contrast, in ventricular fibrillation in the setting of cardiac ischemia, sympathetic activation is proarrhythmic, whereas parasympathetic activation is antiarrhythmic. In inherited arrhythmia syndromes, sympathetic stimulation precipitates ventricular tachyarrhythmias and sudden cardiac death except in Brugada and J-wave syndromes where it can prevent them. The identification of specific autonomic triggers in different arrhythmias has brought the idea of modulating autonomic activities for both preventing and treating these arrhythmias. This has been achieved by either neural ablation or stimulation. Neural modulation as a treatment for arrhythmias has been well established in certain diseases, such as long QT syndrome. However, in most other arrhythmia diseases, it is still an emerging modality and under investigation. Recent preliminary trials have yielded encouraging results. Further larger-scale clinical studies are necessary before widespread application can be recommended.