Since the advent of clinical anesthesia, there has been a continuous need for evaluating the depth of anesthesia. Depth of anesthesia was always important in avoiding either overdose (toxicity) or “underdose” (administering less than absolutely necessary). The problem of overdosing has been partly solved by assignment of dedicated and specially trained doctors to provide anesthesia services, by accumulating more than a 100 years of experience in administering anesthesia, by using safer drugs (volatile and intravenous) with extremely smaller toxicity than the older ones and by recognizing and establishing the stages of anesthesia (such as Guedel stages) and the hemodynamic parameters as indirect but useful indices of the depth of anesthesia. The current trend of minimizing expenses of anesthesia calls for tight titration of drug administration.
The problem of “underdosing” has evolved over the years either from surgeons being unhappy from the “operating conditions” (the patient presenting unnecessary and potentially harmful hemodynamic responses, moving or coughing as a response to surgical stimulus, with no muscle relaxation which diminishes the access to the abdominal cavity for example) or from patients who some times recall intraoperative events and even remember experiencing pain. This phenomenon, called awareness during anesthesia, has been acknowledged as a failure of providing anesthesia, even though the providers would assure that the patient was as calm as usual and the doses were within “usual range”. This complication signals psychological sequelae for the patient and legal problems for the anesthesiologist.
Since the introduction of neurophysiologic monitoring and analysis to modern clinical practice, there has been a strenuous effort to invent indices that would predictably indicate the depth of anesthesia and point out those patients who are at increased risk of awareness during anesthesia. The Bispectral Index monitor, constructed by Aspect Medical Inc is based on EEG analysis and incorporates the empirical best clinical judgment that corresponds to certain EEG patterns. This index and many others attempt to enforce our clinical judgment with objective tools and hopefully provide better clinical outcome of either intravenous or inhalational anesthesia.
This special article provides information on the EEG analysis that produces the BIS Index and reviews the clinical experience with it’s use. It concludes that BIS index is the best almost real time clinical tool on hand so far, for evaluating the depth of anesthesia, even though it’s specificity and sensitivity are not yet approaching the ideal monitors requirements.