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01-09-2011 | Article

BIS no better than ETAC for prevention of intraoperative awareness


Free abstract

MedWire News: Surgical protocols incorporating electroencephalogram-derived bispectral index (BIS) are about as effective as those with standard monitoring of end-tidal anesthetic-agent concentration (ETAC) for the prevention of intraoperative awareness in high-risk patients.

Although differences in the percentage of patients showing intraoperative awareness were not significant, fewer cases in the ETAC group were observed, contrary to what was expected by the researchers.

"Similar to other structure protocols that have enjoyed widespread adoption, an ETAC protocol like the one used in this trial could be implemented for patients at high risk for awareness who are undergoing general anesthesia with a potent inhaled agent," say Michael Avidan (Washington University School of Medicine, St Louis, Missouri, USA) and co-authors in NEJM.

In total, 6041 patients at high risk for intraoperative awareness were randomly assigned to receive BIS-guided anesthesia, with an audible alert if the BIS value reached <40 or >60, on a scale of 0 (suppression of detectable brain activity) to 100 (awake) or ETAC-guided anesthesia, with an audible alert if the ETAC reached <0.7 or >1.3 minimum alveolar concentration. In addition to audible alerts, the protocols included structured education and checklists.

ETAC, currently the standard method for measuring intraoperative awareness, measures exhaled volatile agent as a correlate of brain concentration, while BIS reduces raw EEG data to unitless index.

"Depth-of-anesthesia monitors are controversial, largely because they perform poorly under some conditions, and data on their ability to prevent awareness are conflicting," said Gregory Cosby (Brigham and Women's Hospital, Boston, Massachusetts, USA) in an associated editorial.

During maintenance of anesthesia, BIS was less than 60 a median of 94.0% of the time, while ETAC was greater than 0.7 a median of 84.8% of the time. Both groups received a comparable amount of anesthesia and experienced similar rates of major postoperative adverse outcomes. No significant differences between the groups were seen for doses of sedative, hypnotic, opioid analgesics, or neuromuscular-blocking agents.

Analysis of the study findings showed a similar incidence of definite intraoperative awareness between the groups, with seven (0.24%) of 2861 patients reporting awareness in the BIS group compared with two (0.07%) of 2852 patients in the ETAC group.

Furthermore, a total of 19 (0.66%) patients had definite or possible intraoperative awareness in the BIS group compared with eight (0.28%) in the ETAC group.

The results also showed that 41% of cases of awareness occurred with ETAC or BIS values in the target ranges, suggesting that intraoperative awareness was not entirely preventable with either monitoring method.

Further analysis showed that patients who had definite or possible awareness had a median of one additional inclusion criterion and one additional pre-existing medical condition compared with patients who did not experience awareness.

Average 30-day mortality was 1.96% in the BIS group and 2.21% in the ETAC group. Median length of stay in hospital and in the intensive care unit was equal for both groups, at 7.0 days and 2.1 days, respectively.

"Whether these devices add value in this way remains to be seen," said Cosby.

By Ingrid Grasmo