False Negative BIS? Maybe, Maybe Not!

Abstract

To the Editor: I have read with interest the recent case report of unintended intraoperative awareness by Mychaskiw et al. (1). Unfortunately, the paucity of data presented renders impossible any interpretation of this case or of how one might alter one’s practice to avoid similar occurrences. Notably, the primary anesthetic in this case was sevoflurane and nitrous oxide, yet end-tidal airway concentrations of these anesthetics are not presented. The minimum alveolar anesthetic concentration (MAC) of sevoflurane/nitrous oxide in a comparable age group has been reported as 1.3% Atm (2). Mollestad et al. (3) found a zero percent incidence of awareness in patients equilibrated at one MAC. Chortkoff et al. (4) demonstrated that even emotionally charged verbal information was not subject to recall above a desflurane concentration double that of MAC-Awake, a value that can be calculated to be approximately 0.68% Atm (5). It seems very unlikely then that this patient was close to equilibration with the reported inspired concentration of 2% sevoflurane plus nitrous oxide. Further, I can not interpret an isolated Bispectral index (BIS) value of 47. The BIS is a continuously variable value that is updated every 2 s, much like a digital heart rate or an arterial pressure, except that BIS seems to have more moment-to-moment variability. Without a trend plot of the BIS over time, it is not reasonable to assume that a single number well characterizes it over the entire period of many minutes when awareness might have occurred. No data are provided in the report regarding signal quality. The BIS monitor continuously provides an index of signal quality to help prevent clinicians from inadvertently exercising the first law of computer science: “Garbage In: Garbage Out”(6). Unfortunately, the period of awareness in this case was during sternal splitting, a time of frequent application of electrosurgical currents. Without reporting the concurrent signal quality index from the monitor, it is possible that the entire BIS record was artifact, not physiologic. Finally, interesting case reports need not be deprived of plots of actual BIS or signal data because the Aspect A-2000 monitor (Aspect Medical Systems, Newton, MA) has a nonvolatile memory that can store approximately a month’s duration of case data for download and plotting. Ira Rampil, MS, MD

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