Non-invasive vagus nerve stimulation decreases vagally mediated heart rate variability

The vagus nerve plays a critical role in balancing the body’s physiological functions, including the cardiovascular system. Measures of heart rate (HR) and its variability (HRV) may provide non-invasive proxies of vagal activity in humans, but transcutaneous auricular vagus nerve stimulation (taVNS) has produced mixed effects so far—limited by a lack of studies stimulating the right branch. Here, we used a randomized cross-over design to study the effects of taVNS on HR and HRV. To estimate how the side of the stimulation (left vs. right ear) affects cardiovascular function, we recorded an electrocardiogram in four sessions per person (factors: Stimulation × Side). To evaluate potential interactions with physiological states, we investigated three phases per session: baseline, during stimulation (taVNS vs. sham), and after consuming a milkshake (∼400 kcal) with concurrent stimulation. First, we found moderate evidence against an effect of taVNS on HR (BF10=0.21). Second, taVNS decreased HRV (multivariate p =.004) independent of physiological state with strong evidence for RMSSD (BF10=15.11) and HF-HRV (BF10=11.80). Third, taVNS-induced changes were comparable across sides and more strongly correlated (vs. sham), indicating similar cardiovascular effects independent of the stimulation side. We conclude that taVNS reduces HRV without altering HR, contradicting the common assumption that increased HRV indexes a heightened vagal tone. Instead, our results support a putative role of vagal afferent activation in arousal. Crucially, modulatory effects on the cardiovascular system can be safely elicited by taVNS on both sides, opening new options for treatment. Graphical Abstract Created with BioRender.com