Awareness Under Anesthesia –
After reviewing the available literature I feel that the value of BIS or similar monitors as a device for detecting or preventing intraoperative awareness is unproven. The mechanism by which patients have recollection during surgery is not known. In some cases it is related to demonstrably inadequate administered concentrations of anesthetic drug. However there are a number of case reports in which recollection has occurred in the presence of what appears to have been adequate anesthetic delivery. Both of the large prospective randomized trials on the use of BIS to reduce intraoperative awareness had awareness reported in the BIS monitored group or when the BIS value was less than 60. One of these trials has been published (1), the other trial is in progress and only abstract data is available (2). Retrospective reviews identifying cases of awareness that might have been detected had brain function monitoring been used are speculative at best and can only be used as a guide for future studies. I believe that the ASA guidelines at present provide the best, most unbiased statement of how to address the problem of awareness under anesthesia.
Brain function monitoring as an aid for anesthesia administration –
There is a growing body of evidence that, when used in conjunction with anesthetic drugs to which it is sensitive, brain function monitoring devices such as BIS may improve aspects of anesthetic administration. The most systematic review of this is contained in a recent Cochrane Collaboration meta-analysis of 20 studies with 4056 participants that met their quality assessment criteria (they noted that seven additional studies were still undergoing assessment for possible future inclusion) (3). The studies they used were limited to men and women, aged 18 or over, in which BIS was compared to traditional clinical signs as a guide for the dose of anesthetic. There was significant variation in how the studies were conducted. However, in general the use of BIS resulted in significantly less inhalational agent or less propofol, a reduction in early recovery time and a reduction in anesthetic drug cost. The absolute reductions were fairly small, but the concept that information provided by BIS may be useful in the conduct of certain types of anesthetics is established. Whether this benefit is worth the cost is highly dependent on the clinical situation.
In this and multiple prior entries I have attempted to provide a balanced discussion of a rapidly developing field of anesthesia investigation. I am not an expert in this area. However I have read extensively and use brain electrical activity monitoring for many patients in my practice. I have expressed my opinions in this and earlier entries, but many thoughtful colleagues disagree with the positions I have taken. Nothing in this Weblog should be construed as the official position or policy of the Department of Anesthesiology and Critical Care at the University of Pennsylvania. Emerging information over the next several years may force me to make a total reconsideration of the material presented here. Finally I have frequently referred to a brain function monitor manufactured by Aspect Corporation. These references are only for the purposes of identification since at present their brand of brain function monitor is the most widely used, it is the one that I use in my practice and it is the subject of most of the studies that are available. There is no intent to infringe on any copyrights held by Aspect Corporation.
1) Myles PS: Bispectral index monitoring to prevent awareness during anaesthesia: the B Aware randomized controlled trial. Lancet 2004;363:1757-63
2) Finkel KJ et al: Sensitivity of BIS and MAC in the B-Unaware Trial. ASA Annual Meeting, San Francisco A728
3) Punjasawadwong Y et al: Bispectral index for improving anaesthetic delivery and post operative recovery. Cochrane Database of Systematic Reviews 2007(4). Art No: CD003843. DOI:10.1022/14651858.CD003843.pub2
David S. Smith, M.D., Ph.D.