Blissfully Unaware…or Not Quite?

Published - Written by Rena Xu

Imagine one day waking up to find that you are strapped to a narrow table. Strangers wearing masks loom over you with scalpels in their hands.  You know what is happening – but there is nothing you can do about it.

More than the premise of lurid science fiction films, intra-operative awareness is a serious medical concern for patients undergoing surgery.  It affects up to 1% of high-risk patients; as many as 70% of those who are affected later develop post-traumatic stress disorder.

Given this risk, scientists have looked to decrease the rate of intra-operative awareness by monitoring the level of anesthesia.  But this begs the question: how should the level of anesthesia be monitored?

In a paper published this week in NEJM, Avidan et al. report the results of BAG-RECALL, a trial that investigated the relative effectiveness of two different monitoring systems for decreasing intra-operative awareness.  The first approach was to use a Bispectral Index (BIS) monitor.  This system uses electroencephalogram (EEG) signals to calculate a score from 100 (awake) to 0; the score reflects the patient’s level of consciousness.  The alternative approach was to monitor the end-tidal anesthetic concentration (ETAC); this is a conventional estimate of the degree to which patients are anesthetized.

BAG-RECALL was a prospective study conducted at three centers in the United States and Canada.  More than 6,000 high-risk patients were randomized to receive monitoring by either BIS or ETAC.  Only the anesthesia practitioners were made aware of the assignments; the study was blinded for patients, post-operative interviewers, and other data analysts.

Out of all the patients enrolled in the study, 49 reported intra-operative memories.  Experts determined that 9 of these patients had experienced definite awareness, and 27 patients had experienced either definite or possible awareness.  Unexpectedly, the ETAC cohort actually had a lower incidence of awareness: compared to 7 out of 2,861 patients in the BIS cohort, only 2 out of 2,852 patients in the ETAC cohort experienced definite awareness (p=0.98; difference, 0.17%; 95% confidence interval, -0.03% to 0.38%).  These results suggested there was no statistically significant difference in the rate of intra-operative awareness between the two approaches.

The results of the BAG-RECALL trial are in keeping with those of previous studies.  A 1,900-patient, single-center trial called B-Unaware also found no reduction in intra-operative awareness using BIS monitoring as compared with ETAC.

NEJM deputy editor Dr. Mary Beth Hamel states of BAG-RECALL, “This large, well conducted trial did not detect any advantage of BIS monitoring for prevention of intra-operative awareness over less costly ETAC monitoring and provides valuable evidence to help guide clinical care.”

In a corresponding editorial, Dr. Gregory Crosby of the Department of Anesthesiology, Perioperative, and Pain Medicine at Brigham and Women’s Hospital in Boston writes, “It is unreasonable to expect any such [clinical] monitor to unfailingly detect conscious awareness — a specific and still mysterious property of the brain and mind — and neither patients nor physicians should think otherwise. Notwithstanding this and other weaknesses of current devices, a window into the anesthetized brain, albeit a foggy one, may still be useful, in conjunction with information from other monitors, in operating rooms, endoscopy suites, and critical care units as a generic, all-purpose index of the brain’s response to powerfully sedating drugs.”

What best practices have you observed for monitoring levels of intra-operative sedation?  Will the results of the BAG-RECALL trial influence your use of commercial monitoring systems like BIS?