The goal of this clinical trial is to learn whether different frequencies of transauricular vagus nerve stimulation (tVNS) can reduce postoperative pain and opioid consumption in adults undergoing abdominal or urologic surgery. It will also evaluate the effects of tVNS on recovery quality, autonomic function, and treatment safety. The main questions it aims to answer are:

Does transauricular vagus nerve stimulation reduce postoperative opioid consumption during the first 24 hours after surgery? Do different stimulation frequencies (high-frequency and low-frequency tVNS) produce different effects on postoperative pain and autonomic responses? Is transauricular vagus nerve stimulation safe and well tolerated in surgical patients?

Researchers will compare high-frequency tVNS, low-frequency tVNS, and sham stimulation (inactive stimulation) to determine their effects on postoperative pain and opioid requirements.

Participants will:

Be randomly assigned to receive high-frequency tVNS, low-frequency tVNS, or sham stimulation.

Receive two 30-minute stimulation sessions, one before anesthesia and one after surgery in the post-anesthesia care unit.

Undergo assessments of postoperative pain, opioid consumption, need for rescue analgesics, postoperative nausea and vomiting, heart rate changes, and quality of recovery.

Be monitored for adverse events related to the intervention during the study period.

A total of 174 participants will be enrolled in this single-center, randomized, double-blind, sham-controlled clinical trial.

Postoperative pain remains a major challenge in perioperative care and is associated with delayed recovery, increased healthcare utilization, and greater opioid consumption. Despite advances in multimodal analgesia, opioid medications continue to play a central role in postoperative pain management and may be associated with adverse effects such as nausea, vomiting, respiratory depression, and prolonged recovery. Therefore, the development of effective non-pharmacological adjunctive strategies is of considerable clinical interest.

Transauricular vagus nerve stimulation (tVNS) is a non-invasive neuromodulation technique that stimulates the auricular branch of the vagus nerve through the external ear. Experimental and clinical studies suggest that vagal stimulation may modulate pain perception through activation of central autonomic pathways, enhancement of descending inhibitory pain mechanisms, and regulation of inflammatory responses. Previous clinical trials have demonstrated promising effects of tVNS on acute and chronic pain conditions, including reductions in postoperative pain scores.

However, the optimal stimulation parameters for perioperative analgesia remain uncertain. In particular, the influence of stimulation frequency on clinical outcomes and autonomic responses has not been adequately investigated. Different stimulation frequencies may activate distinct neural pathways and produce different effects on pain modulation and autonomic regulation.

This study is designed to evaluate the effects of different frequencies of transauricular vagus nerve stimulation applied before and after surgery in adult patients undergoing elective surgical procedures. In addition to assessing postoperative opioid requirements, the study will explore the relationship between autonomic responses and postoperative recovery. The findings may contribute to the optimization of non-pharmacological analgesic strategies and support the incorporation of transauricular vagus nerve stimulation into multimodal perioperative pain management protocols.

Inclusion Criteria:

• Adults aged 18 to 59 years.
• Undergoing elective abdominal or urologic surgery.
• American Society of Anesthesiologists (ASA) physical status I-III.
• Able and willing to provide written informed consent.

Exclusion Criteria:

• History of neurological disorders.
• History of psychiatric disorders.
• Inability to understand study procedures.
• Pregnancy.
• Cognitive impairment.
• Communication difficulties that interfere with study assessments.
• Auricular dermatitis or skin lesions at the stimulation site.
• Pre-existing cardiac arrhythmia.
• Chronic opioid use.
• Implanted electronic medical devices, including pacemakers.

De-identified individual participant data underlying the results reported in publications arising from this study will be made available, including demographic characteristics, intervention allocation, primary and secondary outcome measures, and adverse event data. Data will be shared only after removal of all direct identifiers to protect participant confidentiality.

De-identified individual participant data and supporting study documents will be available beginning 3 months after publication of the primary study results and will remain available indefinitely, with no end date.

De-identified data will be available to qualified researchers who provide a methodologically sound research proposal. Requests should be submitted to the principal investigator. Access will be granted following review and approval of the proposed use and execution of a data-sharing agreement.

Participants, outcome assessors, and data analysts will remain blinded to treatment allocation throughout the study. The researcher responsible for administering the stimulation will not be blinded because intervention assignment is required for device setup and delivery.