Jamie Sleigh, Erin Mackay, Logan Voss, John Barnard, Stewart Allen.

Waikato Clinical School, University of Auckland, Hamilton, New Zealand

Background: During general anaesthesia titration of opioid dosage tends to be somewhat idiosyncratic. It would be useful to have a measure of the nociceptive state of the anaesthetised patient; because accurate opioid titration might make for a smoother recovery. Spindle activity is manifest as waxing and waning 8-14Hz oscillations in the electroencephalogram (EEG) signal. It is usually considered to be a sign of thalamic and cortical hyperpolarisation. Hagihira and others have suggested that the presence of spindle activity in the EEG might be a reasonable measure of adequate intra-operative nociceptive blockade (1)

Methods: After ethical approval and patient consent EEG data was collected during induction of general anaesthesia from 20 patients. Patients were randomized to recieve either 1 or 2.5mcg/kg fentanyl at induction. Sevoflurane was titrated to achieve a State Entropy of 30-60 at the point of incision. The change in spindle activity in the EEG was recorded at intubation and at surgical incision.

Results: The strongest stimulation occurred at laryngoscopy; with clear changes in spindle activity. However only 30% patients showed a decrease in spindle activity, whereas 70% showed an increase in activity. 25% showed an increase in the frequency of the spindle waves.

Conclusion: Change in spindle activity in the EEG may be an indicator of nociceptive stimulus. However the direction of the change in activity depends on the preexisting EEG context, and is not modified by modest doses of fentanyl.

Reference: (1) Hagihira et al. Electroencephalographic Bicoherence is sensitive to Noxious Stimlui during Isoflurane or Sevoflurane Anaesthesia. Anesthesiology 2004; 100:818-25.