Acute Cardiovascular Effects of Transcutaneous Auricular Vagus Nerve Stimulation

Introduction

Cardiovascular disease (CVD) remains the leading cause of mortality worldwide, with arterial hypertension representing the most significant modifiable risk factor (Lim et al., 2012; Mills et al., 2020). While clinical manifestations of arterial hypertension typically emerge in later life, the underlying pathophysiological mechanisms, particularly autonomic dysfunction, begin decades earlier.

Autonomic imbalance, characterised by sympathetic overactivity and diminished parasympathetic tone, not only precedes sustained arterial hypertension but also independently predicts future cardiovascular risk, even in normotensive individuals (He et al., 2023; Thayer et al., 2010). Reduced heart rate variability (HRV), a non-invasive marker of parasympathetic activity, has been consistently associated with increased cardiovascular morbidity across diverse populations (Task Force, 1996).

Critically, young apparently healthy adults with suboptimal lifestyle factors, including physical inactivity, poor dietary habits, and chronic stress, frequently exhibit reduced HRV and altered sympathovagal balance (Liao et al., 1998). These subclinical autonomic changes represent an early, potentially reversible stage in the cardiovascular disease continuum, suggesting that interventions targeting autonomic balance may prevent or delay progression to overt disease (Goldstein et al., 2011).

The vagus nerve, the main parasympathetic pathway, exerts multiple cardioprotective effects, including heart rate deceleration, baroreflex enhancement, reduced vascular tone, and anti-inflammatory activity (Thayer \& Sternberg, 2006). Transcutaneous auricular vagus nerve stimulation (taVNS) has emerged as a non-invasive method to enhance vagal activity by delivering electrical stimulation to the auricular branch of the vagus nerve via surface electrodes placed on the tragus or cymba conchae (Badran et al., 2018). Neuroimaging studies confirm that taVNS activates central vagal projections, including the nucleus tractus solitarius, the primary relay station for cardiovascular autonomic control (Frangos et al., 2015).

Preliminary research demonstrates that acute taVNS sessions increase HRV, enhance baroreflex sensitivity, and reduce sympathetic vascular tone in healthy adults (Clancy et al., 2014; De Couck et al., 2017), while emerging evidence suggests clinically meaningful reductions in blood pressure (BP) in hypertensive patients (Mbikyo et al., 2024).

Despite these promising findings, significant knowledge gaps remain. Most studies have examined clinical populations with pre-existing autonomic abnormalities, making it difficult to isolate primary taVNS mechanisms from disease-related compensatory responses. Additionally, chronic intervention protocols preclude detailed characterisation of immediate autonomic and hemodynamic changes. Conducting mechanistic studies in healthy populations offers critical advantages: absence of confounding medications and disease adaptations enables clearer identification of taVNS-induced autonomic and hemodynamic changes, while establishing baseline response patterns provides an essential reference framework for interpreting clinical population responses and informing preventive interventions. Investigation of acute responses permits precise temporal mapping of physiological changes, distinguishing primary mechanisms from downstream consequences, enables efficient optimisation of stimulation parameters, and provides biological plausibility for chronic effects while identifying potential responders to therapy.

Therefore, this study proposes a randomised, sham-controlled crossover study to systematically characterise acute cardiovascular and autonomic responses to a single 60-minute taVNS session in healthy young adults. Using continuous non-invasive BP monitoring and detailed HRV analysis, this study will establish whether taVNS produces acute, measurable changes in BP, heart rate, and autonomic balance in individuals with normal baseline function. We will elucidate the temporal dynamics of taVNS-induced effects, characterise the mechanistic pathways distinguishing cardiac, hemodynamic, and autonomic contributions, and evaluate the specificity of active stimulation versus sham conditions. By establishing baseline physiological response patterns and elucidating acute mechanisms in a well-controlled population, our findings will lay the groundwork for subsequent investigations in at-risk and hypertensive individuals, ultimately contributing to evidence-based, personalised autonomic modulation therapy for cardiovascular disease prevention and management.