This study seeks to evaluate the efficacy of transcranial direct current stimulation (tDCS) and Afferent Vagus nerve simulation (VNS) in treating acute postoperative pain of patients in the post-anesthesia care unit after general surgery.

Study outcomes will include changes in pain levels and vital signs. For patients who are admitted after surgery, opioid consumption in the 24 hours after surgery and time from surgery to discharge are recorded as secondary outcomes.

Patients will be divided in the following comparison groups:

• Group +/+ will receive ten minutes of active tDCS followed by ten minutes of active VNS.
• Group +/- will receive ten minutes of active tDCS followed by ten minutes of sham VNS.
• Group -/+ will receive ten minutes of sham tDCS followed by ten minutes of active VNS.
• Group -/- will receive ten minutes of sham tDCS followed by ten minutes of sham VNS.

Two nonpharmacologic interventions have shown promise in treatment of chronic pain. Afferent Vagus nerve simulation (VNS) can modulate nociception, even though the exact mechanism is unclear. Clinically, it has found uses in chronic pain as well as early evidence of potential use in postoperative pain. Alternatively, transcranial direct current stimulation (tDCS) is a form of non-invasive and polarity-specific technique of modulating neuronal circuitry using a weak electric current delivered by a battery. tDCS has also been shown to modulate chronic pain and has been shown to produce EEG signatures similar to those seen in states of adequate intraoperative analgesia.

This study seeks to evaluate the efficacy of tDCS and VNS in treating acute postoperative pain of patients in the post-anesthesia care unit after general surgery by randomizing 200 subjects in 4 groups:

• Group +/+ will receive ten minutes of active tDCS followed by ten minutes of active VNS.
• Group +/- will receive ten minutes of active tDCS followed by ten minutes of sham VNS.
• Group -/+ will receive ten minutes of sham tDCS followed by ten minutes of active VNS.
• Group -/- will receive ten minutes of sham tDCS followed by ten minutes of sham VNS.

Patient's vital signs and pain level will be obtained before the procedure and after. Patients that have to stay in hospital will also be evaluated by opioid consumption and time from surgery to discharge.

Inclusion Criteria:

• Adults aged 18 years or above with the capacity to provide informed consent who are undergoing an elective surgery requiring general anesthesia

Exclusion Criteria:

• cardiac bradyarrhythmia: taking beta-blockers;
• patients with metal implants around the chest, neck, or head (e.g. implantable pacemakers and defibrillators, deep brain stimulation devices);
• pregnancy
• history of epilepsy or seizures.
• patients undergoing surgical procedures involving the head, neck or spine, or those requiring the use of cardiopulmonary bypass

Data will be shared in aggregate form. There is no plan to share individual participant data (IPD) with researchers not involved with the study design. Several posthoc analyses are being planned for retrospective analysis of the data obtained.

Patients will be assigned a Study ID that will determine the assigned procedure. At the study's completion the blind will be broken, and data will be turned over for analysis.