Boosting Exercise cApaCity With autOnomic Neuromodulation

Background A cardinal feature of acute tissue injury/systemic inflammation is the development of autonomic dysfunction, characterised by sustained sympathetic hyperactivity and loss of vagal function. Elevated resting heart rate is repeatedly associated with both cardiovascular morbidity and all-cause mortality in the general population. Vagal tone, which declines variably during ageing processes, is the chief determinant of resting heart rate. Healthy ageing is characterised by lower sympathetic and preserved vagal activity, reflected by lower resting heart rate. The gold-standard measure of vagal tone is heart rate recovery (HRR), obtainable from exercise testing. Loss of vagal tone in humans - as quantified by HRR - is associated with lower peak aerobic exercise capacity, a major barrier to rehabilitation. Persistent sympathetic hyperactivity downregulates G-protein coupled β-receptor recycling. This impairs cardiac function, lowers exercise capacity and increases systemic inflammation, thereby promoting cardiometabolic disease. Exercise tolerance may be enhanced by dedicated exercise training programmes, even in healthy individuals. Greater exercise capacity is linked with lower risks of cardiovascular disease, Type 2 diabetes, malignancy, osteoporosis, depression and premature death.

Although in healthy ageing cardiac vagal tone is predominantly determined by genetic factors, with strong associations identified between HRR and various genes expressed by key autonomic pathway components, vagal neuromodulation techniques have gained traction. Laboratory and clinical translational data have shown restoring (or preventing the loss of) vagal activity may therefore offer a therapeutic target to help optimise cardiopulmonary fitness and improve health outcomes. Stimulation of the dorsal vagal motor nucleus (DVMN) neurones using an optogenetic approach increases exercise capacity in rats. In heart failure patents, vagus nerve stimulation applied for 15 days was reported to increase the distance of the 6-min walk test. Proof-of-concept studies, primarily in human volunteers, using non-invasive electrical stimulation of either the auricular branch or the cervical bundle of the vagus nerve suggest that maintenance, or augmentation, of vagal activity is feasible, effective and associated with minimal side effects. Although transcutaneous stimulation of auricular nerves is readily accessible, studies lack focus on autonomic activation and have yet to examine effects on exercise capacity.

Here, the investigators describe the protocol for a randomised controlled crossover trial, hypothesizing that bilateral transauricular nerve stimulation (TAN) augments exercise capacity in human volunteers. Furthermore, the investigators aim to identify whether TAN augments exercise capacity in tandem with autonomic changes that are mechanistically implicated in favourably modulating cardiopulmonary fitness.

Recruitment/screening Potential participants (including NHS patients with no specific disease) will be found through posters displayed on NHS premises. After potential participants contact the research team, two independent members of the research team at the clinical research facility shall be involved - one will obtain informed consent while the other will deliver the interventional protocol. The member obtaining consent will call the potential participant and screen them against the eligibility criteria before asking for consent. All eligible participants that undergo screening will be recorded on a screening log on the Investigator Site File, even if they decline the study. Participants will be given a participant information sheet (PIS) one week in advance of the first day of intervention. Written informed consent must be obtained before the first intervention.

Informed consent The principal investigator or suitably qualified nominee will obtain written informed consent from each participant, including provision of a PIS accompanied by the relevant consent form, and an explanation of the aims, methods, anticipated benefits and potential hazards of the trial. All potential participants are free to refuse to enter the trial or to withdraw at any time during the trial, for any reason. If new safety information results in significant changes in the risk/benefit assessment, the PIS and consent form will be reviewed and/or amended accordingly. Participants will be informed that, as defined by the UK Policy Framework for Health and Social Care Research, all documentation will be stored for a minimum of 20 years at the end of the study.

Sample size Consensus CPET guidelines recommend that the VO2peak should be used to assess exercise capacity. The investigators require 28 participants [alpha=0.05; 1-beta=0.9], based on the experimental intervention increasing VO2peak (from each individual's own baseline) by the same degree as high intensity exercise training (which increases VO2peak by 0.51 l·min-1; 95% ci: 0.43 to 0.60 l min-1, SD 0.9). The sample size allows for a dropout rate of 10% per group.

Randomisation Participants are randomly assigned (1:1) by block randomisation to receive active or sham stimulation first to ensure a balance in sample size across groups over time (Power Analysis \& Sample Size (PASS) 2021, Utah, USA). Randomisation is performed one week before the first intervention, with a random block size of four. A randomisation sequence is created by a biostatistician who did not participate in the implementation or statistical analysis of trial. Allocation concealment is achieved through sequentially numbered, opaque, sealed envelopes.

Bias/blinding Participants and investigators are blinded to treatment allocations throughout data collection and analysis. Participants and investigators are unable to see the device settings. Participants are instructed by the investigators that the stimulation is imperceptible after determination of the prescription dose. Participants and investigators are also masked to the Holter data, which will be analysed offline by an independent trial statistician not involved with patient recruitment/device application who is masked to the treatment allocations. Interventions are performed at the same time of day to minimise the possible influence of circadian variations.

Data collection Clinical data will be recorded. CPET data will be extracted from the cycle ergometer software (Zan, nSpire). Heart rate data will be analysed using Kubios HRV Premium [version 3.5.0]. Plasma will be stored at -80ºC having been obtained after both intervention periods for exploratory analyses of biomarkers related to autonomic activation. These data will be collected by a member of the research team not involved with administering the interventions, and analysed by an independent member of the research team - both will be masked to the treatment allocations. Data will be entered electronically on a secure database (REDCAP).

Statistical analysis Analyses will be conducted using the intention-to-treat principle, where all participants with a recorded outcome are analysed according to the treatment group to which they were randomised. Baseline participant characteristics will be presented and stratified according to treatment allocation. The primary outcome (absolute change in VO2peak) will be analysed using repeat-measures analysis of variance (intervention allocation x intervention period) (before versus after TAN/sham). For secondary outcomes, heart rate, CPET, HRV data and plasma acetylcholine levels will be compared in each group using repeat-measures analysis of variance. Sensitivity analysis for variables that may affect exercise capacity and/or autonomic measures will also be conducted. The significance level will be set at 0.05. A full statistical analysis plan will be developed prior to analysis and the end of the study.

Trial management/data monitoring The Sponsor organisation is Queen Mary University of London (QMUL). Daily trial management will be coordinated by a trial management group consisting of the Chief Investigator and his/her support staff. An independent study committee will oversee the trial, including assessing the safety of the intervention, reviewing relevant new external evidence, and monitoring the overall conduct of the trial. This committee consists of an independent clinical trialist, lay representative and an independent Chair.

Adverse events Prespecified and any other adverse events directly related to the trial intervention will be recorded.

Confidentiality Information related to participants will be kept confidential and managed in accordance with the Data Protection Act (UK), NHS Caldecott Principles (UK), the Research Governance Framework for Health and Social Care (UK), and the conditions of Research Ethics Committee Approval, or corresponding legislation or approvals for a particular participating country or site. The principal investigator will maintain in strict confidence trial documents (e.g. written consent forms) and confidentiality.

Discussion Exercise capacity serves as an excellent preoperative prognosticator of an individual's ability to cope with the physiological (and pathophysiological) processes underlying ageing. Experimental animal studies and human exercise performance data have shown intact vagal tone enhances exercise capacity. However, acute tissue injury and systemic inflammation disrupt autonomic control. This proof-of-concept study aims to demonstrate the safety, feasibility and efficacy of TAN in augmenting cardiopulmonary fitness in tandem with autonomic changes that are mechanistically associated with favourable modulation of exercise capacity. These healthy volunteer data may offer key insights into potentially accelerating functional rehabilitation.

Experimental data indicate that cardiac vagal activity is a direct determinant of exercise capacity. Persistent autonomic dysfunction directly impairs cardiopulmonary fitness and therefore muscle mass and strength; key determinants of functional recovery cannot be increased. Furthermore, disruption of autonomic-mediated immunoregulatory mechanisms promotes a pro-inflammatory state whereby mechanisms controlling the resolution of inflammation are suppressed. The development and persistence of autonomic dysfunction therefore directly impairs cardiopulmonary fitness and rehabilitation. Therefore, individuals at high risk of morbidity and mortality are characterized by reduced vagal tone and slow heart rate recovery following incremental exercise.

Studies of non-invasive autonomic neuromodulation conducted in healthy volunteers and are susceptible to bias. The investigators utilised a double-blind, open crossover. randomised controlled design to address these issues. Although participants have varying fitness levels before the study, each individual serves as their own control. The use of a designated sham stimulation device is critical to minimise participant and investigator biases. Far more invasive studies have successfully recruited patients into trials where sham procedures served as the key control in cardiac and noncardiac orthopaedic surgery.

Assessing the effects of TAN on exercise capacity in healthy volunteers is broadly generalisable to the general population. Provided that the investigators can confirm in this setting that autonomic changes after TAN occur by measuring HRV, the low cost, non-invasive device used suggests a very low potential risk-to-benefit ratio. Systematic review identified TAN stimulation parameters that alter HRV, although there is no certainty that there may be minimum thresholds for the duration and/or intensity required to be clinically effective. Nonetheless, a 30-minute period of daily tragal stimulation has not been associated with any major adverse events.