Patient outcome and brain state monitoring during general anesthesia

Abstract

General anesthesia is the reversible state of drug-induced unconsciousness. Conventional tracking of depth of anesthesia relied heavily on the changes in automatic and somatic responses to surgical stimulations. Recent development of intraoperative neurophysiologic recordings, such as the bispectral electroencephalo-graphic index and the auditory evoked potential, has provided real-time information on anesthetic drug effects, and may facilitate adjustment of anesthetic dosing. This thesis aimed to evaluate the impact of brain state monitoring on patient outcomes after general anesthesia.

Firstly, a randomized controlled trial was performed in 1,063 patients using auditory evoked potential recordings or routine clinical signs to guide anesthetic administration. Auditory evoked potential monitoring reduced anesthetic dosage by 15-30%. This was associated with quicker recovery, shorter hospital stay and superior quality of recovery. In addition, there was a lower incidence of postoperative nausea and vomiting (19.0% versus 47.9%, p < 0.001) and other infective complications (8.6% versus 11.3%, p = 0.028).

Secondly, in 921 elderly patients having major noncardiac surgery, a randomized controlled trial was conducted to compare the effect of bispectral index monitoring or routine care on postoperative cognitive outcome. Anesthetic dosage was reduced by 20-30% with bispectral index monitoring. Following surgery, the incidence of delirium was reduced in the monitoring group compared with routine anesthetic care (15.6% versus 24.1%, p = 0.010). The risk for postoperative cognitive dysfunction, measured by detailed neuropsychology test battery, was also decreased with bispectral index monitoring at 3 months after surgery (10.2% versus 14.7%, adjusted odds ratio (95% confidence intervals): 0.67 (0.32-0.98), p = 0.025).

Thirdly, a systematic review and meta-analysis was performed on 6 awareness trials involving 34,211 patients. Bispectral index monitoring was associated with a decreased risk of unintentional awareness during general anesthesia (0.13% in monitoring group versus 0.25% in controls), relative risk (95% confidential intervals): 0.54 (0.31-0.94), p = 0.028. The effect was most obvious in patients receiving intravenous anesthesia and those undergoing noncardiac surgery.

Finally, a long-term follow-up study was performed in 536 high-risk surgical patients participated in an awareness trial. By adopting bispectral index monitoring to avoid episodes of deep anesthesia, long-term mortality was decreased, adjusted hazard ratio (95% confidence intervals) 0.85 (0.74-0.98), p = 0.042. In addition, the rates of myocardial infarction and stroke were reduced when anesthetic was adjusted to maintain a bispectral index value within the therapeutic range.

The findings highlighted the utility of brain state monitoring in optimizing anesthetic administration. When unexpected periods of inadequate anesthesia was avoided, brain state monitoring was effective in the prevention of awareness during general anesthesia. Similarly, as unnecessarily deep level of anesthesia was prevented with monitoring, there was a substantial decrease in anesthetic exposure. Consequently, an improvement in the quality of recovery was observed with superior cognitive outcome and a decrease in long-term mortality and morbidity. The series of studies imply a greater role for anesthesia in determining patient outcome than previously acknowledged. The practice of brain state monitoring during general anesthesia should be encouraged to improve perioperative outcome, especially in high-risk patients undergoing complex major surgery.