Feasibility and Psychophysiological Outcomes of a Real-World, Nonrandomized Sequential-Cohort Pilot Combining Transcutaneous Auricular Vagus Nerve Stimulation with Breath-Focused Biofeedback in Adolescents

Background/Objectives: The vagus nerve is a key neurophysiological pathway connecting the brain and body. Its functional state—vagal tone—is typically indexed by heart-rate variability (HRV) and inversely reflects physiological stress. Adolescents often exhibit reduced vagal tone and heightened stress reactivity. Schools offer a strategic setting for delivering stress-reduction interventions. This pilot study assessed the feasibility, safety, and preliminary efficacy of a school-based program combining HRV biofeedback with transcutaneous auricular vagus nerve stimulation (taVNS). Methods: Seventeen Vocational Education and Training (VET) students (ages 15–17) from AZITAIN-IFMPB (Eibar, Spain) completed six weekly 30-minute sessions. Each session included paced-breathing biofeedback (emWave®) combined with taVNS (Parasym™). Feasibility was assessed via recruitment, retention, acceptability, and adverse events. Physiological outcomes included HRV (analysed with Kubios) and hair cortisol concentration. State anxiety was assessed using the State–Trait Anxiety Inventory (STAI). Assessments were conducted at baseline, post-intervention, and at 7-week follow-up. Results: Recruitment reached 50%; retention in the second cohort was 75%. The intervention was well tolerated, with 80% acceptability and one mild adverse event. taVNS significantly increased parasympathetic activity (PNS index; F(1, 11) = 5.26, p = .043, η²ₚ = 0.32), and reduced state anxiety (STAI; F(1, 13) = 6.79, p = .022, η²ₚ = 0.34). Hair cortisol rose post-intervention (F(1, 12) = 11.75, p = .005, η²ₚ = 0.49). Conclusions: This pilot study demonstrated the feasibility of implementing the intervention in adolescents and provided preliminary estimates of its effects on stress-related outcomes; these findings invite cautious consideration of peripheral calming alongside central attentional engagement and will inform the design of a larger, controlled trial. Keywords: vagal tone – heart rate variability (HRV), transcutaneous auricular vagus nerve stimulation (taVNS), adolescent stress regulation, biofeedback intervention