Monitoring depth of anesthesia: with emphasis on the application of the bispectral index and the middle latency auditory evoked response to the prevention of recall.

AMONG the earliest systematic observations of the physiologic effects of anesthetic agents was John Snow’s description, in 1847, of the various stages of ether anesthesia. Although the focus has evolved somewhat, our interest in measures of the depth of anesthesia has persisted. Although the concern was initially largely one of avoiding the hazards of overdose, we have added a greater interest in the prevention of “underdosage. ” There is considerable interest in preventing potentially hazardous hemodynamic and movement responses and in preventing recall. The latter concern applies most particularly to the patient who has received neuromuscular blocking agents. The contemporary literature also indicates an interest in using depth of anesthesia monitors as a means of controlling cost. The hope is that precise titration of anesthetic agents, as guided by a monitor of anesthetic depth, can serve to avoid wastage of expensive anesthetics and expedite postanesthesia care unit or hospital discharge, or both. There have been several thoughtful discussions of the monitoring of depth of anesthesia and of the problem of awareness. Table 1 lists many of the techniques or devices that have been proposed or tested as methods for determining depth of anesthesia. A thorough discussion by Heier and Steen published in 1996 reviews the status of all but the most recent of those techniques. Briefly, the review leads to the conclusion that, although several techniques allow one to identify statistically significant differences in depth of anesthesia among defined anesthetic conditions for populations of patients, none has the sensitivity and specificity to allow the clinician to draw certain conclusions about depth of anesthesia in the individual patients for whom he or she treats. The then-available (1996) devices served as trend monitors of varying reliability but did not permit conclusive statements about depth of anesthesia in individual patients. The purpose of this review is to summarize the developments that postdate the articles cited previously. That progress has involved principally two depth-ofanesthesia monitoring methods: the Bispectral Index, known by the trademarked acronym BIS (Aspect Medical Systems Inc., Newton, MA); and the middle latency auditory evoked response (MLAER). The BIS is an empirically derived index that is dependent on a measure of the “coherence” among components of electroencephalography. The MLAER uses measurements of the amplitude and latency of the early cortical components of the auditory evoked response. This discussion will focus on developments related to those two methods. In addition, because of the interest on the part of the media, patients, practitioners, and investigators regarding the topic of awareness during anesthesia, the issue to which this review gives greatest attention is: Can the available monitors be used to prevent the occurrence of awareness during anesthesia? Professor and Chair, Department of Anesthesiology, University of California; Staff Anesthesiologist, Anesthesia Service, VA Medical Center, San Diego, California.