Infrared Pupillometry During General Anesthesia to Predict Pain

The planned study is a prospective cohort study.

Patients undergoing arthroscopic knee surgery of any type (debridements, repair of anterior (ACL) or posterior cruciform ligaments (PCL), meniscectomies, medial (MCL) or lateral collateral ligament (LCL) repair, as well as microfracture surgery will be enrolled.

During anesthesia, infrared pupillometry will be used to assess whether a painful stimulus results in pupillary reflex dilation (PRD). The extend of the PRD may be an indicator how painful a stimulus is. The PRD is measurable under general anesthesia in spite of the fact that the patient does not "feel" or experience the painful stimulus.

The aim of the study is

1. to identify whether regional anesthesia techniques such as femoral and sciatic nerve blocks (performed before the start of anesthesia) will block the PRD evoked by a painful stimulus such electrical stimulation before procedure start. This would allow assessment of block success in the anesthetized patient.
2. to correlate the extent of the pupillary reflex dilation and hippus in the anesthetized patient during and at the end of surgery with early postoperative pain scores and subsequent analgesia requirements. Such correlation would allow to predict the amount of postoperative pain before the patients emerge from general anesthesia.

Knee arthroscopies are routinely performed under general anesthesia with or without regional anesthesia, depending on the anticipated extent of surgery and the preferences of the anesthesiologist and surgeon. The assessment of the PRD and/or hipppus before emergence would identify those patients who would benefit from an additional nerve block before emergence to achieve better pain control immediately postoperatively and to reduce the demand for postoperative analgesics.

The planned investigation tries to achieve two novel findings. First, the measurement of the PRD at the start of surgery will demonstrate whether a nerve block successfully blocks the initial surgical stimulus. There is currently no method available that can assess block success in the anesthetized patient. Secondly, the measurement at the end of the surgery tries to predict the amount of pain following emergence from surgery. Pain may be experienced by patients without nerve blocks as well as with nerve blocks, as the latter may have failed or incomplete blocks, or may had a more extensive procedure that cannot be completely covered by the block administered preoperatively. Identifying these patients may allow treatment to minimize discomfort following emergence.

Pupillary reflex dilation describes the dilation of the pupil following a painful stimulus. The dilation last several seconds. Pupillometry using an infrared pupillometer allows the quantification of the reflex dilation. The PRD can be given a numerical value that reflects the extent of the dilation. A recently published study demonstrated the correlation of the extend of the PRD with pain scores and opioid requirements postoperatively. However, this study was done in awake patients following surgery. The aim of this study is to predict analgesia requirements in patients still under general anesthesia, before they experience postoperative pain. We expect that infrared pupillometry during general anesthesia can predict pain following surgery. Once this has been established, this knowledge and its application would allow the anesthesiologist to initiate pain-relieving interventions such as additional nerve blocks or administration of analgesics before emergence of the patients from general anesthesia.

In addition to measuring the pupillary reflex dilation, we will also measure oscillations of pupil size before and after induction of anesthesia, as well as during recovery. These spontaneously occurring oscillations in pupil size, also called hippus, appear to be sensitive to anesthetics and opioids. However, the effect of these drugs on hippus has not been systematically addressed.