The Effect of Transcutaneous Auricular Vagus Nerve Stimulation (taVNS) on Cardiorespiratory Physiology: Four Waveforms Trial with Tragus vs Earlobe Stimulation

BackgroundTranscutaneous auricular vagus nerve stimulation (taVNS) depends on activating afferent vagal pathways, leading to central and physiological (parasympathetic) modulation. Explaining how taVNS changes cardiorespiratory physiology requires systematic characterization of the effects of electrode location and stimulation waveform using rigorous experimental controls. ObjectiveTo determine the acute effects of four taVNS stimulation configurations (left unilateral 25 Hz, 100 Hz, burst, and bilateral 25 Hz) on physiological markers of parasympathetic activation compared with configuration-matched earlobe stimulation in healthy subjects at rest. MethodsCustomized 8-mm ear-clip electrodes delivered monophasic 500 s pulses (anode anterior) to the tragus or a configuration-matched earlobe control across four stimulation configurations (left unilateral 25 Hz, 100 Hz, burst, and bilateral 25 Hz). Twenty-five healthy participants completed a randomized, single-blind, within-subject crossover study, undergoing both active and control stimulation for all four configurations (200 sessions total). Each session consisted of five 60-second stimulation blocks (1,000 stimuli total). Heart rate (HR), heart rate variability (HRV; RMSSD), and respiration were recorded continuously. Linear mixed-effects models compared physiological responses between tragus and earlobe stimulation during the first stimulation block (primary analysis), across 5-second HR intervals (secondary analysis), and averaged across all five stimulation blocks (post hoc analysis). ResultsDuring the first stimulation block, HRV was significantly higher during unilateral left 25 Hz tragus stimulation than during the matched earlobe control ({beta} = -10.34, t(33) = -2.63, p = .01, 95% CI [-18.05, -2.64]), consistent with increased parasympathetic activity. No significant differences were observed for HRV with the other stimulation configurations, or for HR or respiration under any configuration. Analysis of 5-second HR intervals likewise revealed no significant temporal effects for block one. Analysis of 5-second heart rate intervals across the five blocks identified differences between tragus and earlobe at discrete time points but did not show a consistent temporal pattern across the stimulation period. Across all five stimulation blocks, unilateral left 25 Hz tragus stimulation remained associated with higher HRV ({beta} = -6.56, t(124) = -3.85, p < .01, 95% CI [-9.90, -3.22]) and also produced an increase in HR relative to earlobe stimulation ({beta} = -1.05, t(24) = -2.57, p = .01, 95% CI [-1.85, -0.24]). Respiration was unaffected throughout. ConclusionIn this systematic study, unilateral left 25 Hz tragus stimulation produced physiological modulation consistent with vagal target engagement. These findings underscore the importance of rigorous dose-specific taVNS studies to establish reproducible physiological biomarkers.