Background: Immediate postoperative pain could be prevented by the administration of long-lasting analgesics before the end of the anaesthesia. However, to prevent over or underdosing of analgesics under anaesthesia, tools are required to estimate the analgesia-nociception balance.
Objective: We investigated whether the pupillary dilation reflex (PDR) and the nociceptive flexion reflex (NFR) at the end of general anaesthesia correlate with immediate postoperative pain, as a sign of analgesic underdosing, and with delayed tracheal extubation as a sign of analgesic overdosing.
Design: Prospective observational study.
Setting: Klinikum im Friedrichshain, Berlin, Germany, from May 2013 to April 2015.
Patients: A total of 110 patients scheduled for primary hip arthroplasty under general anaesthesia.
Observations: Psychometric and clinical data were obtained preoperatively. The PDR and the NFR were assessed preoperatively and at the end of anaesthesia. Shortly after extubation of the trachea, patients rated their pain intensity.
Endpoints: The primary endpoint was the immediate postoperative numeric rating scale pain intensity (0 to 10) and the secondary endpoint was the length of the time interval between reflex measurements and tracheal extubation.
Results: PDR correlated significantly with the immediate postoperative pain intensity (Spearman's ρ = -0.28, P < 0.01). PDR and NFR correlated significantly with the time interval until extubation (ρ = 0.33/0.26, both P < 0.01). Multivariable linear regression models were able to predict 38% of the variability of immediate postoperative pain intensity and 44% of the variability of the time interval until extubation. The intraoperative nociceptive reflex thresholds were the strongest factors in both multivariable models, only exceeded by the preoperative reflex measurements in the model for postoperative pain.
Conclusion: The investigated nociceptive reflexes reflected the analgesia-nociception balance under general anaesthesia. The preoperative reflexes provide additional information about individual subjective pain sensitivity.
Trial registration: Deutsches Register Klinischer Studien, DRKS (registration number DRKS00000665).