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

There is considerable current interest in the issue of awareness. The concern that, in our patients, unnecessary anxiety about the risk of awareness and unrealistic expectations about the ability of the BIS monitor to prevent the phenomenon have developed has already been discussed in ANESTHESIOLOGY.34,35 It has also been asserted that careful, prospective study with subsequent peer-reviewed publication will be necessary to establish the effectiveness of any putative awareness-prevention device.35 The peer-reviewed literature does not support the notion that any commercially available monitor can serve to prevent awareness, although it indicates that useful trend-monitoring of depth of anesthesia and titration of depth of sedation can be accomplished with the BIS.10,11 Furthermore, even in the event of the development of a device that reliably identifies anesthetic states representing a high risk for awareness, episodes of awareness still may occur. The first reason is that depth of anesthesia at any moment is probably the sum of the effects of the anesthetic agents being administered and the prevailing degree of stimulus-related arousal. Even a monitor that meets the stringent specificity conditions suggested above might 'fail,' in the context of light anesthesia with minimal surgical stimulus, in the event of a sudden increase in the intensity of stimulus. The second is that there will continue to be situations in which the clinician is limited by failing hemodynamics from administering the anesthetic agents that are otherwise warranted. It is unrealistic to expect any monitor to be proof-positive against the occurrence of awareness.