Dept Of Anesthesiology & Critical Care, University Of Pennsylvania School Of Medicine

A personal view about intraoperative brain function monitoring

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.

References --

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.

Considerations before adopting new standards for monitors

            BIS and other Brain Function Monitors (BFM) have value but a careful analysis of the unbiased research that was not funded by industry does not support their routine use.  It is important to understand the history of the ASA Practice Advisory on the subject of awareness under anesthesia.  In appointing an independent group to review the literature, many of the key supporters of this technology were felt to have significant conflicts of interest related to both research support and honorarium for speaking from Aspect Medical.  Therefore, the debate about this technology must be viewed in the context of the scientific evidence and potential financial motivators.  After publication of the Practice Advisory, the American Society of Anesthesiologists concluded that the evidence on the value of the technology was insufficient and that a study should be conducted which was funded independently of Aspect Medical.  The ASA has now funded two large-scale studies on the use of brain electrical activity monitoring to decrease intra op awareness; the results of these studies are pending.  An additional trial without industry sponsorship has been completed and the results should be published shortly.  The findings of this trial will further our understanding of how this monitoring modality adds to what is currently available.

            Are their disadvantages or risk to adding this additional monitor?  This can be extrapolated to the question of whether all monitoring is good.  When I trained, the vast majority of patients with coronary artery disease undergoing major surgery had pulmonary artery catheters.  In fact, randomized trials were planned and could not be conducted based upon beliefs by the practitioners that it was unethical to randomize patients.  Propensity analysis was required of cohort studies to finally justify studying this monitor, and there are now several studies in different venues which suggest no benefit and potential harm.  As noted in other sections of the webblog there is a lot we do not know about depth of anesthesia and consciousness, and intra-operative recollection.  We also do not know how the BIS and other BFM respond to surgery and anesthetic drugs.  I therefore believe that there are indications for the BIS monitor, but that for the vast majority of cases we should focus on ensuring our patients are receiving adequate drug concentration (e.g. ET inhalational agent analyzers and good functioning IVs for TIVA), and that we do not rely on information from such monitors as false assurance that all is well.

Lee A Fleisher, M.D, FACC.

Robert Dunning Dripps Professor and Chair, Department of Anesthesiology and Critical Care, University of Pennsylvania

How useful does a monitor need to be before it is considered worthy of routine intraoperative use?

Take for example, body temperature monitoring.  It is rare to make intraoperative clinical decisions based on the body temperature monitor, yet it is used for virtually every case.  Do we really need neuromuscular blockade monitors whenever a muscle relaxant is given, but no one would suggest that they not be used?  The use of train of four monitoring took a number of years to become a standard even at the institution where the concept was developed (WA Kofke, personal communication).  There now exist monitors that provide significant information about the interaction of certain commonly used types of anesthetic drugs with neurons that appear to be important for aspects of the phenomena we call general anesthesia.  Studies have consistently shown that these monitors improve administration of many types of general anesthetics as indicated by shorter time to wake up, less time in the PACU and often less overall dose of anesthetic drug.  As a bonus these monitors may also provide a warning about an increased potential for some types of intraoperative awareness.  Are the current generations of brain activity monitors true “depth of anesthesia monitors?”  My answer to this is no.  Will the current generation of brain activity monitors warn against all situations in which awareness during anesthesia possible?  Again my answer is no.  However despite their limitations, the data provided by these devices, when interpreted appropriately, may provide useful information that, in my experience, frequently improves the precision of anesthesia delivery, improves patient through put by decreasing the incidence of delayed awakening, decreases the dose of anesthetic used and might also warn against the potential for awareness.  However, as with other monitors, I have been in clinical situations in which BIS monitoring was unhelpful, misleading and even distracting.

David S. Smith, M.D., Ph.D.