In October, 2005, the American Society of Anesthesiologists (ASA) published a Practice Advisory for Intraoperative Awareness and Brain Function Monitoring (Practice Advisory). A task force composed of anesthesiologists, methodologists, and consultants, some of whom disclosed financial relationships with the brain-monitoring device companies, prepared the Practice Advisory. The primary issue addressed in the Practice Advisory is whether use of clinical techniques, conventional monitoring systems, or brain-function monitors to assess the depth of anesthesia will reduce the occurrence of intraoperative awareness. The Practice Advisory summarizes the most pertinent information on brain-function monitoring and provides an in-depth review of the literature. We recommend that all anesthesia providers review this document in its entirety.

One of the key points of the Practice Advisory is that “intraoperative awareness cannot be measured during the intraoperative phase of general anesthesia, since the recall component of awareness can only be determined postoperatively by obtaining information directly from the patient.” Essentially, the term “intraoperative awareness” is commonly used to mean explicit recall. Brain-function monitors attempt only to monitor depth of anesthesia, defined by the ASA as “a continuum of progressive central nervous system depression and decreased responsiveness to stimulation.” Calling such monitors, which monitor brain function or brain activity, “awareness monitors” is misleading.

Published studies have indicated that the incidence of intraoperative recall may be as high as 1 to 2 per 1,000 patients receiving general anesthetics. One
study concluded that the use of brain-function monitors could reduce this incidence by as much as 80%.

Intraoperative recall is an important event. Some patients who have had unplanned intraoperative awareness have suffered from posttraumatic stress disorder, primarily relating to the circumstance of being aware during surgery, and possibly in pain, without the ability, because of paralysis, to convey their predicament. Unfortunately, the ability to measure the presence and impact of implicit memory is beyond our current technology.

Traditionally, anesthesia providers evaluated heart rate, blood pressure, purposeful movement, papillary responses, lid reflex, sweating, and tearing and monitored the patient with an end-tidal analyzer to assess whether anesthesia depth was adequate. At present, no clinical trials or other comparative studies have examined the effect of clinical assessment or conventional monitoring on the incidence of intraoperative awareness; however, some cases have been documented in which patients had intraoperative awareness in the absence of observed increases in heart rate and blood pressure. In a consensus opinion, the Practice Advisory recommends continued use of these physiologic parameters to assess anesthetic depth in all patients.

The major devices currently being marketed to assess anesthetic depth use forehead electrodes to record electroencephalographic (EEG) activity with or without electromyographic (EMG) information, or they use auditory evoked potentials (AEP). An analog EEG signal is processed via various public or proprietary algorithms applied to one or more of the following categories of data: frequency, amplitude, latency, and phase-relationship. A unitless number, often referred to as an “index,” is generated. It typically is scaled from 0 to 100, and it represents the spectrum of states of consciousness. Zero corresponds with an isoelectric EEG or absent middle-latency AEP, and 100 corresponds with a state of complete wakefulness.