Bispectral index in pediatrics: fashion or a new tool?

Abstract

The bispectral index (BIS, Aspect Medical Systems, Inc. Newton, MA) is becoming the gold standard for assessing depth of anesthesia in adult patients. Indeed, BIS values are highly correlated with the concentration of hypnotic drugs such as propofol or sevoflurane (1–8). The BIS is derived from a large database of EEG traces obtained in adult patients under various conditions of anesthesia (9). This database is regularly updated and the corresponding software is changed accordingly. The calculation of BIS is kept secret, but the elements included into the algorithm are known. Basically, the value displayed takes into account the frequency of EEG signal (the proportion of rapid to slow waves decreases with increasing hypnotic concentration), the synchronization of the waveforms (from virtually no synchronization in awake patient to higher degree of synchronization with increasing depth of anesthesia), and the percentage of burst suppression (the latter being observed at very deep level of anesthesia). The BIS is displayed as a dimensionless number between 0 (deep anesthesia) and 100 (awake), with 40–60 being suitable for surgical anesthesia (10). The BIS has well-known limitations. First, the BIS is more or less insensitive to narcotics, thus it reflects the hypnotic state which is only one component of anesthesia. Therefore it is not surprising that the specificity of BIS for predicting the response to noxious stimuli (such as surgical incision or laryngoscopy) under balanced anesthesia is low. Second, the BIS algorithm was validated mainly against propofol or volatile agents currently in use (isoflurane, desflurane and sevoflurane). BIS values are not correlated with ketamine plasma concentrations and are much higher at a given MAC values of halothane compared with sevoflurane (11). The latter drawbacks are not surprising as EEG traces obtained with these agents are very different from those integrated in the database (i.e. propofol and more recent volatile agents) (12–14). The BIS monitor was developed to assess depth of anesthesia and to decrease the incidence of awareness. The latter objective is achieved (15). Indeed, the B-Aware randomized controlled trial demonstrated that BISguided anesthesia reduces the risk of awareness in at-risk adult surgical patients undergoing relaxant general anesthesia. Other potential benefits in adults include decreased drug consumption, decreased postoperative incidence of nausea and vomiting (16) and quicker recovery time compared with non BIS-guided anesthesia (17). Having said that, one can understand easily that the BIS was conceived as an adult tool, not a pediatric one. Thus, the preliminary questions are: does it work in pediatric patients and if the answer is positive, what could be the interest of this device in pediatric patients? As indicated above, the BIS is only derived from adult EEG traces while EEG traces in young children differ from adult traces. Roughly, from infancy to adulthood, the amplitude of EEG decreases and dominant frequency increases (in other words, EEG becomes composed of faster waves of smaller amplitude with increasing age). However, the effects of anesthetic agents on EEG tracings are comparable throughout life. BIS values recorded in pediatric patients are inversely correlated with endtidal sevoflurane (18–20) and isoflurane concentrations (21) as in adults. BIS values are better Correspondence to: Pr Isabelle Murat, Service d’Anesthesie-Reanimation, Hopital d’enfants Armand Trousseau, 26 avenue du Dr Arnold Netter, 75571 Paris cedex 12, France (email: isabelle. murat@trs.aphp.fr). Pediatric Anesthesia 2005 15: 177–180 doi:10.1111/j.1460-9592.2004.01564.x