The autonomic effects of transcutaneous auricular nerve stimulation at different sites on the external auricle of the ear

Transcutaneous auricular nerve stimulation (tANS) applied to specific parts of the external ear has positive health effects in both healthy volunteers and patient groups. However, due to differences between studies in ear stimulation sites and extent of effect on autonomic variables, it is challenging to determine what part of the external ear is the optimum site for electrode placement. This study investigated the autonomic effects of bilateral tANS at four different sites on the external auricle of the ear: tragus, cymba concha, helix and earlobe. tANS was performed using modified surface electrodes connected to a transcutaneous electrical nerve stimulation (TENS) machine. Participants (n = 24) each underwent four sessions where a 15 minute period of stimulation (pulse width: 200 s; pulse frequency: 30 Hz; current: adjusted to sensory threshold) was applied bilaterally to either the tragus, cymba concha, helix or earlobe. Heart rate and blood pressure were continuously recorded during 10-minute baseline, 15-minute stimulation and 10-minute recovery periods. Heart rate variability (HRV) was derived. Results showed that regardless of site, stimulation significantly influenced measures of HRV. Baseline LF/HF ratio predicted change in LF/HF ratio during stimulation for each site and for all sites combined. Response (i.e. change in LF/HF ratio between baseline and stimulation) was closely linked with measures reflecting starting autonomic function. This demonstrates the importance of evaluating how autonomic function is modulated by tANS in individual participants (as well as the whole group). These findings have key implications for adopting a tailored approach when considering the therapeutic/clinical applications of tANS.