Effect of Low-level Vagus Nerve Stimulation on Cardiac Remodeling in a Rapid Atrial Pacing–induced Canine Model of Atrial Fibrillation

Vagus Nerve Stimulation and Atrial Fibrillation: Revealing the Paradox

BACKGROUND AND OBJECTIVE

The cardiac autonomic nervous system (CANS) plays an important role in the pathophysiology of atrial fibrillation (AF). Cardiovascular disease can cause an imbalance within the CANS, which may contribute to the initiation and maintenance of AF. Increased understanding of neuromodulation of the CANS has resulted in novel emerging therapies to treat cardiac arrhythmias by targeting different circuits of the CANS. Regarding AF, neuromodulation therapies targeting the vagus nerve have yielded promising outcomes. However, targeting the vagus nerve can be both pro-arrhythmogenic and anti-arrhythmogenic. Currently, these opposing effects of vagus nerve stimulation (VNS) have not been clearly described. The aim of this review is therefore to discuss both pro-arrhythmogenic and anti-arrhythmogenic effects of VNS and recent advances in clinical practice and to provide future perspectives for VNS to treat AF.

MATERIALS AND METHODS

A comprehensive review of current literature on VNS and its pro-arrhythmogenic and anti-arrhythmogenic effects on atrial tissue was performed. Both experimental and clinical studies are reviewed and discussed separately.

RESULTS

VNS exhibits both pro-arrhythmogenic and anti-arrhythmogenic effects. The anatomical site and stimulation settings during VNS play a crucial role in determining its effect on cardiac electrophysiology. Since the last decade, there is accumulating evidence from experimental studies and randomized clinical studies that low-level VNS (LLVNS), below the bradycardia threshold, is an effective treatment for AF.

CONCLUSION

LLVNS is a promising novel therapeutic modality to treat AF and further research will further elucidate the underlying anti-arrhythmogenic mechanisms, optimal stimulation settings, and site to apply LLVNS.

Effects of transcutaneous auricular vagus nerve stimulation on cardiovascular autonomic control in health and disease

Autonomic nervous system (ANS) dysfunction is a well-known feature of cardiovascular diseases (CVDs). Studies on heart rate variability (HRV), a non-invasive method useful in investigating the status of cardiovascular autonomic control, have shown that a predominance of sympathetic modulation not only contributes to the progression of CVDs but has a pivotal role in their onset. Current therapies focus more on inhibition of sympathetic activity, but the presence of drug-resistant conditions and the invasiveness of some surgical procedures are an obstacle to complete therapeutic success. On the other hand, targeting the parasympathetic branch of the autonomic nervous system through invasive vagus nerve stimulation (VNS) has shown interesting results as alternative therapeutic approach for CVDs. However, the invasiveness and cost of the surgical procedure limit the clinical applicability of VNS and hinder the research on the physiological pathway involved. Transcutaneous stimulation of the auricular branch of the vagus nerve (tVNS) seems to represent an important non-invasive alternative with effects comparable to those of VNS with surgical implant. Thus, in the present narrative review, we illustrate the main studies on tVNS performed in healthy subjects and in three key examples of CVDs, namely heart failure, hypertension and atrial fibrillation, highlighting the neuromodulatory effects of this technique.

Application of Insertable Cardiac Monitor in Establishing a Dog Model of Atrial Fibrillation by High-Frequency Right Atrial Pacing

Background

On the base of traditional modeling with high-frequency atrial pacing, insertable cardiac monitor is innovatively used to track the incidence of atrial fibrillation (AF), to observe the efficiency and safety of establishment in AF model.

Material/Methods

Twelve adult beagle dogs were randomly divided into a blank control group and an AF model group. The thinnest available bipolar solid electrode lead-3830 was implanted in AF group, connected to AF pacemaker to establish a high-frequency atrial pacing model by AOO pacing mode, and the occurrence of AF was tracked in real time by an insertable cardiac monitor Reveal LINQ. The areas of the left and the right atrium were measured by echocardiography. In addition, the morphology of left atrial tissues was observed using light and electron microscopes.

Results

The insertable cardiac monitor Reveal LINQ sensitively, conveniently, accurately, efficiently, and dynamically recorded the AF load in AF group. After the successfully establishment of the AF mode, the area of the left atrium and right atrium were significantly enlarged compared with that before modeling by echocardiography. Furthermore, the area of the left atrium and right atrium in the AF group were larger than that in the control group. The morphological observation of the left atrium tissues in both groups prompted the reconstruction of the atrial structure in dogs in the AF group.

Conclusions

The application of insertable cardiac monitor Reveal LINQ improves the efficiency of monitoring the AF load, with high sensitivity, convenience, and accuracy.

The role of low-level vagus nerve stimulation in cardiac therapy

Introduction: Cardiovascular diseases are accompanied by autonomic nervous system (ANS) imbalance which is characterized by decreased vagal tone. Preclinical and clinical studies have revealed that increasing vagal activity via vagus nerve stimulation (VNS) could protect the heart. Based on these studies, VNS has emerged as a potential non-pharmaceutical treatment strategy. Although it's still difficult to find the optimal stimulus parameters, however, in arrhythmia model, it is reported that low-level VNS (LL-VNS) exacts paradoxical effects from the high-level VNS. Thus, the concept of LL-VNS is introduced. Areas covered: Animal and human studies have discussed the safety and efficacy of VNS and LL-VNS, and this review will discuss the research data in cardiovascular diseases, including atrial arrhythmia, ventricular arrhythmia, ischemia/reperfusion injury, heart failure, and hypertension. Expert opinion: In this regard, various clinical studies have been performed to verify the safety and efficacy of VNS. It is shown that VNS is well-tolerated and safe, but the results of its efficacy are conflicting, which may well block the translational process of VNS. The appearance of LL-VNS brings new idea and inspiration, suggesting an important role of subthreshold stimulation. A better understanding of the LL-VNS will contribute to translational research of VNS.

Selective ablation of the ligament of Marshall attenuates atrial electrical remodeling in a short-term rapid atrial pacing canine model

INTRODUCTION

Cardiac sympathetic activation facilitates atrial electrical remodeling during atrial fibrillation (AF). Selective ablation of the distal part of the ligament of Marshall (LOM_LSPV ) could decrease cardiac sympathetic innervation. This study aimed to investigate the effects of LOM_LSPV ablation on atrial electrical remodeling in a short-term rapid atrial pacing (RAP) model.

METHODS

In 16 anesthetized dogs, 6 hours of RAP (20 Hz, 2 × threshold) was delivered before LOM_LSPV ablation (group 1, N  =  8) or after (group 2, N  =  8). Heart rate variability (HRV), serum norepinephrine (NE), atrial electrophysiological indices were analyzed. Six times of burst pacing (20 Hz, 2 × threshold, lasting for 5 seconds, were performed to induce AF, the number of episodes and the duration of AF were compared.

RESULTS

LOM_LSPV ablation decreased sympathetic indices of HRV and serum NE. Atrial effective refractory period (ERP) was shortened during RAP in both groups with higher reduction degrees in group 1. In group 1, the shortening of atrial ERP, elevating of ERP dispersion and sum of window of vulnerability (ΣWOV), facilitating of AF induced by RAP were subsequently reversed by LOM_LSPV ablation. In group 2, LOM_LSPV ablation prolonged atrial ERP, decreased ΣWOV, eliminated AF induction. The subsequent RAP failed to alter these indices. Histological studies showed abundant sympathetic nerve fibers in LOM_LSPV .

CONCLUSION

LOM_LSPV ablation could inhibit atrial electrical remodeling during short-term RAP by reducing the cardiac sympathetic activity. LOM_LSPV may be a potential target in AF ablation, especially in patients with highly cardiac sympathetic activation or atrial electrical remodeling.