Background and Goal of Study
There is controversy about the impact of depth of anaesthesia on post-operative outcomes1. We tested the hypothesis that episodes of light anaesthesia may give rise to pain sensitisation and elevated levels of post-operative pain.
Material and methods
60 women (ASA 1 & 2) undergoing wide local excision of breast tissue and sentinel node biopsy took part in the study. All received a Propofol induction and Sevoflurane (between 1 and 1.5 MAC) in 50% nitrous oxide maintenance along with Atracurium. Depth of anaesthesia was monitored using BIS (Covidien Vista). The anaesthetist delivering the anaesthetic was blind to the BIS index. The anaesthetist was told that the patient was light if the BIS exceeded 55, and deep if it fell below 30. BIS was recorded each minute following induction to completion of suturing.
Patients were followed up at 3 days, 6 weeks and 3 months using the McGill pain questionnaire, a visual analogue pain scale and a present pain intensity Likert scale.
Results and discussion
Significant correlations were found between (1) the BIS post induction, (2) during the 5-minute epoch following initial incision, and pain measures at 6 weeks (r=0.43, p=0.002; r=0.285, p=0.039), but not at the other two time points.
The patient sample was divided at a BIS of 50 creating deep and light groups. An independent t-test demonstrated significant differences between these on Present Pain Intensity (t=2.42, p=0.019) and approaching significance on the McGill Pain Questionnaire (t=1.90, p=0.063) and on the VAS (t=1.83, p=0.073) at 6 weeks.
This supports the hypothesis that light anaesthesia may allow sensitisation to pain at 6 weeks. Why no relationship at 2 days and 3 months? At 2 days, biological variables predominate in influencing pain perception, including analgesic dose, so psychological influences were masked. At 3 months most patients were pain-free, leaving no scope for significant correlation.
Conclusion
We found a positive relationship between anaesthetic lightness and post-operative pain at 6 weeks following breast surgery.
These findings suggest that depth of anaesthesia monitoring and responsive adjustment of anaesthetic dose may reduce post-operative pain. They support the theory that intra-operative events may be processed by the patient under light anaesthesia and have post-operative effects.
References
1.	Monk, T. Anesthetic depth is a predictor of mortality. Anesthesiology 2010; 112:1070–2

Depth of anaesthesia predicts post-operative pain at six weeks following wide local breast tissue excision and sentinel node biopsy