A systematic review of the effects of transcutaneous auricular vagus nerve stimulation on baroreflex sensitivity and heart rate variability in healthy subjects

'Transauricular vagus nerve stimulation' for prevention of postoperative delirium in elderly patients undergoing major surgery: a study protocol for a multicentre, participant-blinded and assessor-blinded, randomised, controlled trial

INTRODUCTION

Postoperative delirium (POD) is a frequent complication in elderly patients undergoing major surgery. Research has shown that neuroinflammation, postoperative pain and autonomic nervous system dysfunction play significant roles in its onset. Vagus nerve stimulation (VNS) has the potential to reduce inflammation, ease postoperative pain and aid in recovery by enhancing acetylcholine release and activating the cholinergic anti-inflammatory pathway. This study aims to assess the effectiveness and safety of transauricular VNS (ta-VNS) in preventing POD in elderly patients undergoing major surgery.

METHODS AND ANALYSIS

This multicentre, participant-blinded and assessor-blinded, randomised, parallel-group controlled trial will compare the incidence of POD in elderly patients undergoing major surgery who receive ta-VNS versus sham stimulation. A total of 300 eligible patients will be randomly assigned in a 1:1 ratio to either the active or sham stimulation group. The active stimulation group will receive electrical stimulation to the left cymba conchae at a frequency of 30 Hz and a pulse width of 250 µs, with a 30 s on/30 s off cycle. The intensity will start at 0.4V and be increased in 0.4V increments until a tingling sensation is felt, then adjusted to the highest tolerable level without pain. After obtaining informed consent and randomisation, the initial intervention will begin in the preoperative area and continue throughout the surgery. For the four postoperative days, the intervention will be administered twice daily in 2-hour sessions each morning and afternoon. The sham group will follow the same procedure, with electrodes placed on the left cymba conchae. After adjusting the stimulation intensity, the device will be switched off. The primary outcome is the incidence of POD from postoperative day 0 to day 7 or discharge. Secondary outcomes include the severity of POD, quality of recovery, sleep quality and adverse events.

ETHICS AND DISSEMINATION

The protocol was approved by Sir Run Run Shaw Hospital Affiliated to Zhejiang University School of Medicine on 9 January 2024 (Approval number: 20240014), and the trial was registered on the Chinese Clinical Trial Registry on 21 February 2024, prior to recruitment. The study will be performed according to the guidelines of the Declaration of Helsinki. Written informed consent will be obtained from all participants. The results will be submitted for publication in a refereed journal.

TRIAL REGISTRATION NUMBER

ChiCTR2400081078.

The Acute Effects of Varying Frequency and Pulse Width of Transcutaneous Auricular Vagus Nerve Stimulation on Heart Rate Variability in Healthy Adults: A Randomized Crossover Controlled Trial

Background/Objective: Heart rate variability (HRV) is a key biomarker of autonomic function, linked to morbidity and mortality across various diseases. Transcutaneous auricular vagus nerve stimulation (taVNS) shows therapeutic promise, but its effects on HRV and the influence of specific stimulation parameters remain unclear. This study investigated whether the acute effects of taVNS on HRV depend on combinations of stimulation frequency and pulse width. Methods: Seventy-eight healthy adults participated in seven randomized sessions, each testing one of six active taVNS protocols or an inactive sham condition applied to the cymba conchae of the left ear. The active protocols varied by frequency (10 Hz or 25 Hz) and pulse width (100 µs, 250 µs, or 500 µs). The sessions included 15 min of baseline, 15 min of taVNS or sham condition, and 10 min of recovery. HRV was calculated using the standard deviation of NN intervals (SDNN) and the root mean square of successive differences (RMSSD) from continuous ECG recordings. Results: The 10 Hz/250 µs, 10 Hz/500 µs, and 25 Hz/100 µs protocols significantly increased SDNN time series compared to the sham condition. Exploratory analysis revealed SDNN increases during the second 5 min of stimulation with the 10 Hz/500 µs protocol and during the first 5 min of recovery with the 10 Hz/250 µs and 25 Hz/100 µs protocols. No significant changes in the RMSSD were found for any protocol. Conclusions: TaVNS is safe in healthy adults, and specific frequency and pulse width combinations can acutely enhance overall HRV, as reflected in SDNN, but do not affect vagally mediated HRV, as reflected by the RMSSD. Future studies should optimize taVNS parameters to maximize physiological and clinical outcomes.

Computational modelling of cardiac control following myocardial infarction using an in silico patient cohort

Loss of cardiac physiological function following myocardial infarction (MI) is accompanied by neural adaptations in the baroreflex that are compensatory in the short term, but then become associated with long-term disease progression. One marker of these adaptations is decreased baroreflex sensitivity, a strong predictor of post-MI mortality. The relative contributions of cardiac remodelling and neural adaptation in the sensory, central brainstem and peripheral ganglionic loci to baroreflex sensitivity changes remain underexplored. We used a computational model-based approach that accounts for the short-term dynamics of closed-loop human cardiac control to integrate disparate experimental studies on neural adaptation following MI into a unified quantitative framework. We developed an ensemble of 59 distinct model parameterizations that account for the clinically observed heterogeneity of cardiac control in healthy individuals. We simulated an in silico cohort of 35,400 patients with MI, corresponding to six scenarios of one or more loci of neural adaptation coupled with cardiac remodelling. We evaluated the range of MI-induced shifts in arterial pressure, heart rate and baroreflex curve responses. Our results show that adaptation in any single neural locus coupled with cardiac remodelling is sufficient to account for the MI-induced haemodynamic and autonomic changes observed experimentally. Of the adaptation pathways, we found that individuals with central or peripheral vagal efferent adaptation and preserved baroreceptor gain could maintain high baroreflex sensitivity after ischaemic injury. These results suggest that there are a multitude of adaptive pathways for tuning the baroreflex circuit to shift cardiac control physiology, potentially explaining patient heterogeneity post-MI. KEY POINTS: Baroreflex sensitivity is a strong indicator of post-myocardial ischaemia survival and is variable among individuals. We fine-tuned a computational model ensemble based on physiological observations to develop an in silico patient cohort consistent with the range of baroreflex responses observed experimentally. Simulation and analysis of the in silico cohort show that individuals with a functional afferent pathway and the ability to adapt along the vagal efferent pathway can maintain baroreflex sensitivity post-cardiac ischaemia.

Altered leptin signaling and attenuated cardiac vagal activity in rats with type 2 diabetes

Introduction

The leading cause of death in type 2 diabetes mellitus (T2DM) patients is cardiovascular-related events, including myocardial infraction-induced ventricular arrhythmia. Previous studies have shown that T2DM-induced functional remodeling of cardiac vagal postganglionic (CVP) neurons contributes to ventricular arrhythmogenesis. As leptin resistance is common in T2DM patients, and CVP neurons are located in epicardial adipose pads, a tissue that secretes leptin, in this study we aimed to elucidate a correlation between leptin resistance and CVP neuronal dysfunction in T2DM.

Methods

A high fat-diet/low dose streptozotocin-induced T2DM rat model was used in this study to characterize T2DM-induced alterations in cardiac parasympathetic tone, determined by changes in baroreflex sensitivity and CVP neuronal excitability. The impact of leptin resistance on CVP neurons was also studied by examining the expression of leptin in epicardial adipose pads, and leptin receptors and uncoupling protein 2 (UCP2) in CVP neurons.

Results

T2DM rats exhibited diminished baroreflex sensitivity, and decreased CVP neuronal excitability, demonstrated by a reduced frequency of action potentials, diminished nAChR currents, and an attenuated response to nicotine stimulation. Additionally, compared to sham animals, the expression of leptin receptors and UCP2 in CVP neurons was reduced as early as 4 weeks post-T2DM although the leptin levels in epicardial adipose pads was increased during the progression of T2DM, which demonstrated the occurrence of leptin resistance in T2DM CVP neurons.

Conclusion

Cardiac parasympathetic dysfunction in T2DM rats is due, in part, to functional remodeling of CVP neurons. As leptin resistance develops as early as 4 weeks post-T2DM induction, diminished leptin receptors-UCP2 signaling may contribute to CVP neuronal dysregulation.

Comparison of the Acute Effects of Auricular Vagus Nerve Stimulation and Deep Breathing Exercise on the Autonomic Nervous System Activity and Biomechanical Properties of the Muscle in Healthy People

Background/Objectives: We aimed to examine the acute effects of deep breathing exercise and transcutaneous auricular vagus nerve stimulation (taVNS) on autonomic nervous system activation and the characteristics of certain muscle groups and to compare these two methods. Methods: 60 healthy adults between the ages of 18 and 45 were randomly divided into two groups to receive a single session of taVNS and deep breathing exercises. Acute measurements of pulse, blood pressure, perceived stress scale, autonomic activity, and muscle properties were performed before and after the application. Results: A significant decrease was detected in the findings regarding the perceived stress scale, pulse, and blood pressure values as a result of a single session application in both groups (p < 0.05). In addition, it was determined that the findings regarding autonomic measurement values increased in favor of the parasympathetic nervous system in both groups (p < 0.05). In measurements of the structural properties of the muscle, the stiffness values of the muscles examined in both groups decreased (p < 0.05), while the findings regarding relaxation increased (p < 0.05), except for the masseter in the deep breathing (DB) group. As a result of the comparative statistical evaluation between the groups, the increase in parasympathetic activity was found to be greater in the DB group according to root mean square of differences in successive RR intervals (RMSSD), the percent of differences in adjacent RR intervals > 50 ms (pNN50), and stress index parameters (p < 0.05). In the measurements made with the Myoton®PRO device, the increase in the relaxation value was higher in the gastrocnemius muscle of the VNS group (p < 0.05). Conclusions: It has been observed that both methods can increase parasympathetic activity and muscle relaxation in healthy people in a single session. However, DB appears to be slightly superior in increasing parasympathetic activity, and VNS appears to be slightly superior in increasing relaxation.

Feasibility of trancutaneous auricular vagus nerve stimulation in Black and Hispanic/Latino people with peripheral neuropathy

Introduction

Peripheral neuropathy (PN) is the most common neurodegenerative disorder, and the primary causes are chemotherapy-induced peripheral neuropathy (CIPN) and diabetic neuropathy (DN). Transcutaneous auricular vagus nerve stimulation (taVNS) is a promising non-pharmacological and non-invasive intervention that targets key pathways involved with PN. However, research is needed to determine the feasibility, acceptability, and effects of taVNS in people with PN. It is also critical that this research on taVNS include the perspectives of Black and Hispanic/Latino patients, who are often underrepresented in research.

Methods

This research was comprised of two consecutive studies: a survey and a pilot randomized sham-controlled trial (RCT). The survey assessed symptom burden, management strategies, and interest in taVNS among CIPN patients. The pilot RCT evaluated the feasibility, acceptability, and preliminary effects of taVNS in Black and Hispanic/Latino patients with CIPN or diabetic neuropathy. Participants were recruited from the University of Miami medical system, with culturally sensitive approaches to enhance minority participation.

Results

The survey included 62 respondents, 78% Black or Hispanic/Latino, revealing high symptom burden and significant interest in taVNS (82% expressed moderate to high interest). The pilot RCT enrolled 28 participants, achieving a 42% recruitment rate and 86% retention. taVNS was well tolerated, with no significant adverse effects. Preliminary data indicated a decrease in neuropathic symptoms and an increased heart rate variability (HRV) during active taVNS, suggesting autonomic modulation. Tingling sensation and pain decreased by median values of 2.0 and 1.5, respectively. Additionally, the median values for standard deviation of the RR interval increased from 34.9 (CI = 21.6-44.8) at baseline to 44.8 (CI = 26.5-50.3) during intervention. Exit interviews highlighted positive participant experiences and identified potential barriers, such as protocol length and distrust in medical research.

Conclusion

The findings underscore the need for novel CIPN treatments and demonstrate the feasibility of conducting taVNS research in historically underrepresented populations. High interest in taVNS and successful recruitment and retention rates suggest that culturally sensitive approaches can enhance minority participation in clinical trials. These findings will be used to develop a large clinical trial to determine the efficacy of repeated taVNS in a diverse cohort.

Clinical Trial Registration

https://clinicaltrials.gov, identifier (NCT05896202).

Personalized auricular vagus nerve stimulation: beat-to-beat deceleration dominates in systole-gated stimulation during inspiration - a pilot study

Neuromodulation comes into focus as a non-pharmacological therapy with the vagus nerve as modulation target. The auricular vagus nerve stimulation (aVNS) has emerged to treat chronic diseases while re-establishing the sympathovagal balance and activating parasympathetic anti-inflammatory pathways. aVNS leads still to over and under-stimulation and is limited in therapeutic efficiency. A potential avenue is personalization of aVNS based on time-varying cardiorespiratory rhythms of the human body. In the pilot study, we propose personalized cardiac-gated aVNS and evaluate its effects on the instantaneous beat-to-beat intervals (RR intervals). Modulation of RR is expected to reveal the aVNS efficiency since the efferent cardiac branch of the stimulated afferent vagus nerve governs the instantaneous RR. Five healthy subjects were subjected to aVNS. Each subject underwent two 25-min sessions. The first session started with the non-gated open-loop aVNS, followed by the systole-gated closed-loop aVNS, then the non-gated, diastole-gated, and non-gated aVNS, each for 5min. In the second session, systole and diastole gated aVNS were interchanged. Changes in RR are analysed by comparing the prolongation of RR intervals with respect to the proceeding RR interval where aVNS took place. These RR changes are considered as a function of the personalized stimulation onset, the stimulation angle starting with R peak. The influence of the respiration phases is considered on the cardiovagal modulation. The results show that the systole-gated aVNS tends to prolong and shorten RR when stimulated after and before the R peak, respectively. The later in time is the stimulation onset within the diastole-gated aVNS, the longer tends to be the subsequent RR interval. The tendency of the RR prolongation raises with increasing stimulation angle and then gradually levels off with increasing delay of the considered RR interval from the one where aVNS took place. The slope of this rise is larger for the systole-gated than diastole-gated aVNS. When considering individual respiration phases, the inspiratory systole-gated aVNS seems to show the largest slope values and thus the largest cardiovagal modulatory capacity of the personalized time-gated aVNS. This pilot study indicates aVNS capacity to modulate the heartbeat and thus the parasympathetic activity which is attenuated in chronic diseases. The modulation is highest for the systole-gated aVNS during inspiration.

Treating heart failure by targeting the vagus nerve

Increased sympathetic and reduced parasympathetic nerve activity is associated with disease progression and poor outcomes in patients with chronic heart failure. The demonstration that markers of autonomic imbalance and vagal dysfunction, such as reduced heart rate variability and baroreflex sensitivity, hold prognostic value in patients with chronic heart failure despite modern therapies encourages the research for neuromodulation strategies targeting the vagus nerve. However, the approaches tested so far have yielded inconclusive results. This review aims to summarize the current knowledge about the role of the parasympathetic nervous system in chronic heart failure, describing the pathophysiological background, the methods of assessment, and the rationale, limits, and future perspectives of parasympathetic stimulation either by drugs or bioelectronic devices.

Effects of auricular vagus nerve stimulation and electrical earlobe stimulation on motor-evoked potential changes induced by paired associative stimulation

Paired associative stimulation (PAS) is a combination of transcranial magnetic stimulation (TMS) and peripheral nerve stimulation (PNS). PAS can induce long-term potentiation (LTP)-like plasticity in humans, manifested as motor-evoked potential (MEP) enhancement. We have developed a variant of PAS ("high-PAS"), which consists of high-frequency PNS and high-intensity TMS and targets spinal plasticity and promotes rehabilitation after spinal cord injury (SCI). Vagus nerve stimulation (VNS) promotes LTP-like plasticity and enhances recovery in SCI and stroke in humans and animals when combined with repetitive motor training. We combined high-PAS with simultaneous noninvasive transcutaneous auricular VNS (aVNS) to determine if aVNS enhances the extent of PAS-induced MEP amplitude increase. Sixteen healthy participants were stimulated for 20 min in four different sessions (PAS, PAS + aVNS, PAS + shamVNS, and aVNS) in a randomized single-blind setup. MEPs were measured before, immediately after, and at 30, 60, and 90 min post-stimulation. Stimulation protocols with PAS significantly potentiated MEPs (p = 0.005) when compared with aVNS (p = 0.642). Although not significant, MEP enhancement observed after PAS (43.5%) is further increased by aVNS (49.7%) and electrical earlobe stimulation (63.9%). Our aVNS setup failed to significantly enhance the effect of PAS, but sham VNS revealed a trend towards enhanced plasticity. Optimization of auricular VNS stimulation setup is required for possible tests of patients with SCI.

Impact of optimized transcutaneous auricular vagus nerve stimulation on cardiac autonomic profile in healthy subjects and heart failure patients

Objective.To determine the optimal frequency and site of stimulation for transcutaneous vagus nerve stimulation (tVNS) to induce acute changes in the autonomic profile (heart rate (HR), heart rate variability (HRV)) in healthy subjects (HS) and patients with heart failure (HF).Approach.We designed three single-blind, randomized, cross-over studies: (1) to compare the acute effect of left tVNS at 25 Hz and 10 Hz (n= 29, age 60 ± 7 years), (2) to compare the acute effect of left and right tVNS at the best frequency identified in study 1 (n= 28 age 61 ± 7 years), and (3) to compare the acute effect of the identified optimal stimulation protocol with sham stimulation in HS and HF patients (n= 30, age 59 ± 5 years, andn= 32, age 63 ± 7 years, respectively).Main results.In study 1, left tragus stimulation at 25 Hz was more effective than stimulation at 10 Hz in decreasing HR (-1.0 ± 1.2 bpm,p< 0.001 and -0.5 ± 1.6 bpm, respectively) and inducing vagal effects (significant increase in RMSSD, and HF power). In study 2, the HR reduction was greater with left than right tragus stimulation (-0.9 ± 1.5 bpm,p< 0.01 and -0.3 ± 1.4 bpm, respectively). In study 3 in HS, left tVNS at 25 Hz significantly reduced HR, whereas sham stimulation did not (-1.1 ± 1.2 bpm,p< 0.01 and -0.2 ± 2.9 bpm, respectively). In HF patients, both active and sham stimulation produced negligible effects.Significance.Left tVNS at 25 Hz is effective in acute modulation of cardiovascular autonomic control (HR, HRV) in HS but not in HF patients (NCT05789147).

Vagus nerve stimulation for cardiovascular diseases: Is there light at the end of the tunnel?

Autonomic dysfunction and chronic inflammation contribute to the pathogenesis and progression of several cardiovascular diseases (CVD), such as heart failure with preserved ejection fraction, atherosclerotic CVD, pulmonary arterial hypertension, and atrial fibrillation. The vagus nerve provides parasympathetic innervation to the heart, vessels, and lungs, and is also implicated in the neural control of inflammation through a neuroimmune pathway involving the spleen. Stimulation of the vagus nerve (VNS) can in principle restore autonomic balance and suppress inflammation, with potential therapeutic benefits in these diseases. Although VNS ameliorated CVD in several animal models, early human studies have demonstrated variable efficacy. The purpose of this review is to discuss the rationale behind the use of VNS in the treatment of CVD, to critically review animal and human studies of VNS in CVD, and to propose possible means to overcome the challenges in the clinical translation of VNS in CVD.

State- and frequency-dependence in autonomic rebalance mediated by intradermal auricular electroacupuncture stimulation

Background

Vagus nerve stimulation (VNS) improves diseases such as refractory epilepsy and treatment-resistant depression, likely by rebalancing the autonomic nervous system (ANS). Intradermal auricular electro-acupuncture stimulation (iaES) produces similar effects. The aim of this study was to determine the effects of different iaES frequencies on the parasympathetic and sympathetic divisions in different states of ANS imbalance.

Methods

We measured heart rate variability (HRV) and heart rate (HR) of non-modeled (normal) rats with the treatment of various frequencies to determine the optimal iaES frequency. The optimized iaES frequency was then applied to ANS imbalance model rats to elucidate its effects.

Results

30 Hz and 100 Hz iaES clearly affected HRV and HR in normal rats. 30 Hz iaES increased HRV, and decreased HR. 100 Hz iaES decreased HRV, and increased HR. In sympathetic excited state rats, 30 Hz iaES increased HRV. 100 Hz iaES increased HRV, and decreased HR. In parasympathetic excited state rats, 30 Hz and 100 Hz iaES decreased HRV. In sympathetic inhibited state rats, 30 Hz iaES decreased HRV, while 100 Hz iaES decreased HR. In parasympathetic inhibited rats, 30 Hz iaES decreased HR and 100 Hz iaES increased HRV.

Conclusion

30 Hz and 100 Hz iaES contribute to ANS rebalance by increasing vagal and sympathetic activity with different amplifications. The 30 Hz iaES exhibited positive effects in all the imbalanced states. 100 Hz iaES suppressed the sympathetic arm in sympathetic excitation and sympathetic/parasympathetic inhibition and suppressed the vagal arm and promoted the sympathetic arm in parasympathetic excitation and normal states.

A framework for the interpretation of heart rate variability applied to transcutaneous auricular vagus nerve stimulation and osteopathic manipulation

Reports on autonomic responses to transcutaneous auricular vagus nerve stimulation (taVNS) and osteopathic manipulative techniques have been equivocal, partly due to inconsistent interpretation of heart rate variability (HRV). We developed a mechanistic framework for the interpretation of HRV based on a model of sinus node automaticity that considers autonomic effects on Phase 3 repolarization and Phase 4 depolarization of the sinoatrial action potential. The model was applied to HRV parameters calculated from ECG recordings (healthy adult humans, both genders) before (30 min), during (15 min), and after (30 min) a time control intervention (rest, n = 23), taVNS (10 Hz, 300 μs, 1-2 mA, cymba concha, left ear, n = 12), or occipitoatlantal decompression (OA-D, n = 14). The experimental protocol was repeated on 3 consecutive days. The model simulation revealed that low frequency (LF) HRV best predicts sympathetic tone when calculated from heart rate time series, while high frequency (HF) HRV best predicts parasympathetic tone when calculated from heart period time series. Applying our model to the HRV responses to taVNS and OA-D, revealed that taVNS increases cardiac parasympathetic tone, while OA-D elicits a mild decrease in cardiac sympathetic tone.

VNS improves VSMC metabolism and arteriogenesis in infarcted hearts through m/n-AChR-Akt-SDF-1α in adult male rats

Vagal nerve stimulation (VNS) provides a novel therapeutic strategy for injured hearts by activating cholinergic anti-inflammatory pathways. However, little information is available on the metabolic pattern and arteriogenesis of VSMCs after MI. VNS has been shown to stimulate the expression of CPT1α, CPT1β, Glut1, Glut4 and SDF-1α in coronary VSMCs, decreasing the number of CD68-positive macrophages while increasing CD206-positive macrophages in the infarcted hearts, leading to a decrease in TNF-α and IL-1β accompanied by a reduced ratio of CD68- and CD206-positive cells, which were dramatically abolished by atropine and mecamylamine in vivo. Knockdown of SDF-1α substantially abrogated the effect of VNS on macrophagecell alteration and inflammatory factors in infarcted hearts. Mechanistically, ACh induced SDF-1α expression in VSMCs in a dose-dependent manner. Conversely, atropine, mecamylamine, and a PI3K/Akt inhibitor completely eliminated the effect of ACh on SDF-1α expression. Functionally, VNS promoted arteriogenesis and improved left ventricular performance, which could be abolished by Ad-shSDF-1α. Thus, VNS altered the VSMC metabolism pattern and arteriogenesis to repair the infarcted heart by inducing SDF-1α expression, which was associated with the m/nAChR-Akt signaling pathway.