Blissfully Unaware…or Not Quite?

Posted by • August 17th, 2011

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?

3 Responses to “Blissfully Unaware…or Not Quite?”

  1. I am the wowan who started the Anesthesia Awareness Campaign in 1998! Dr. Avidan and I have met in person and in the media. Why not use all availabloe technology (BIS + end-tidal) all the time? This Campaign suggests that patient/consumers ask three questions: Do you HAVE brain activity monitoring (usually BIS); Do you USE brain activity monbitors; and Will you USE ONE ON ME? If any of these answers is no, take patient consumerism elsewhere! We also suggest that you demand a before-the-day-of-surgery anesthesia consult, and ask to have the use of paralytic drugs limited as much as possible. First-time awareness victims don’t wear signs saying “It’s goikng to be me!” Awareness can occur in populations not considered high risk. Use of a monitor sure beats being entombed in a corpse and then suffering PTSD, possibly for the rest of your life.

  2. Dr T says:

    The study that this article references suggests that there is NO DIFFERENCE in rates of awareness whether we use BIS or not… In fact, in this particular cohort there were MORE cases in the non-bis group, but of course this was not statistically significant and we shouldn’t pay too much heed.

    There is not a hospital in the western world where end-tidal measurement wouldn’t be used, the question is whether we should ADD bis… A costly device now shown not to improve care.

    Suggesting that people should reject anesthetists who use an evidence-based approach is ridiculous.

  3. Dr G says:

    In addition Carol it is conceivable that using depth of anaesthesia monitoring such as BIS may actually increase the incidence of awareness. Many Anaesthetists currently use BIS to use the least anaesthetic possible while achieving an ‘acceptable’ BIS score. Making recommendations to the public (such as yours), while the evidence remains inconclusive is both dangerous and may result in a lower standard of care.