The Effect of Remifentanil on the Objectively Measured Pressure Pain Threshold

This is a single-arm study in which all patients will follow the same protocol. Study participation begins on the day of surgery, immediately prior to anesthesia induction, when the primary measurements are performed, and ends 24 hours postoperatively. During this period, analgesic consumption and adverse events will be monitored.

Intervention 1: Pressure pain threshold under remifentanil infusion. Pressure pain threshold will be assessed using an algometer at baseline (0 ng/mL) and during stepwise increases in target-controlled intravenous remifentanil (Ultiva®) concentrations of 1, 2, and 3 ng/mL. Measurements will be obtained at each concentration level after achieving steady-state conditions.

Intervention 2: Postoperative analgesic consumption and adverse events. Perioperative and postoperative analgesic drug consumption will be recorded. All adverse events will be systematically monitored and documented.

Upon arrival in the operating room, the patient will undergo standard monitoring, including non-invasive blood pressure, electrocardiography (ECG), pulse oximetry (SpO₂), bispectral index (BIS™), and train-of-four (TOF) monitoring.

An intravenous (IV) line will be placed, and crystalloid infusion will be initiated.

Mechanical pain threshold will then be assessed using a Medoc algometer. Measurements will be performed on the dorsum of the non-dominant hand. Four measurements will be obtained at different target plasma concentrations of remifentanil(Ultiva®): 0, 1, 2, and 3 ng/mL. Remifentanil will be administered using a target-controlled infusion (TCI) system based on the Minto pharmacokinetic model.

Following completion of these measurements, standardized intravenous anesthesia will be induced and maintained using propofol (propolipid®), remifentanil, and rocuronium (B.Braun®).

Total perioperative consumption of remifentanil and propofol will be recorded.

At the end of the surgical procedure, intravenous acetaminophen 1 g and a 4 mg morphine bolus will be administered.

Upon arrival in the post-anesthesia care unit (PACU), pain will be assessed using the visual analogue scale (VAS). During the first 24 postoperative hours, analgesia will be provided via a patient-controlled analgesia (PCA) pump delivering 2 mg intravenous HCL morphine (Sterop®) boluses on demand.

The PCA settings will follow a 2/10/20 regimen: 2 mg morphine per demand dose, a 10-minute lockout interval, and a maximum dose of 20 mg over 4 hours.

The use of acetaminophen as adjunct analgesia is permitted; however, corticosteroids, nonsteroidal anti-inflammatory drugs (NSAIDs), piritramide, and magnesium are prohibited.

Pain scores (VAS) will be reassessed at 12 and 24 hours postoperatively.

All adverse events (e.g., nausea, delirium) will be monitored and recorded during the 24-hour postoperative period.

Change in mechanical pain threshold measured objectively using an algometer (AlgoMed®) at increasing plasma concentrations of remifentanil (0, 1, 2, and 3 ng/mL) during the preoperative period.

The pain threshold is defined as the pressure (in kilopascals) at which the patient first reports pain during a standardized mechanical stimulus.

Correlation between preoperative pain threshold and total postoperative morphine consumption (particularly within the first 24 hours using patient-controlled analgesia).

Postoperative pain scores assessed using standard pain scales in the post-anesthesia care unit.

Intraoperative opioid (remifentanil) consumption in a standardized anesthesia setting.

Post operative opioid (PCA, morphine) consumption recorded within the first 24H post surgery.