Prediction of Postoperative Pain by Measuring Nociception at the End of Surgery

Acute postoperative pain needs to be alleviated quickly to avoid sensitization in the postoperative period, since sensitization can lead to pain chronification. Severe acute postoperative pain is the most important risk factor in the development of persistent postoperative pain.

Unfortunately, the dose of opioids necessary to alleviate postoperative pain is highly variable, even between patients having been exposed to the same surgical procedure. Anesthesiologists usually try to prevent immediate postoperative pain by adjusting analgesic dosing at the end of surgery to the perceived need of analgesia. However, this approach is limited by the problem of respiratory depression induced by a too large dose of opiate analgesics. Studies have shown that despite these attempts many patients awake with moderate to severe pain.

A means of predicting immediate postoperative pain after surgery and the response to opiate analgesics would therefore be highly desirable. Many studies have tried to reveal predictive factors which can be evaluated before the start of the surgery, but these can explain only about 50% of the observed variability in postoperative pain intensity.

A different approach may be the evaluation of intraoperative nociception at the end of surgery to directly guide appropriate analgesia before the patient has regained consciousness. Measuring reflex pupil dilation after a standardized electrical stimulus is one method to measure opioid effect intra-operatively. This parameter may reflect opioid sensitivity, but not pain sensitivity.

The primary objective of this study is the evaluation of measures of nociception such as the pupillary dilation reflex or heart rate variability on acute pain after surgery.

In addition, pupillary dilation reflex and heart rate variability will be evaluated in their predictive performance of blood pressure and heart rate changes due to intubation.

The study is designed as a single centre, observational, descriptive study.

Inclusion criteria

• American Society of Anesthesiology (ASA) physical status less than 3
• Able to read and understand the information sheet and to sign and date the consent form
• Being scheduled for elective gynaecological intraperitoneal surgery (laparoscopy, laparoscopic hysterectomy, vaginal hysterectomy, abdominal hysterectomy) under general anesthesia
• Age>18

Exclusion criteria

• Regional anesthesia (epidural analgesia, multi-orifice wound catheter, or transversus abdominis plane block) used for postoperative analgesia
• Surgery performed under regional anesthesia
• Contraindication to a general anesthesia using propofol and sufentanil (e.g. necessity for rapid sequence induction)
• BMI >35 (limit of the equations used in the target controlled infusion device)
• Severe renal insufficiency precluding use of morphine (GFR<30 ml/min)