To the Editor:—
Ahmad et al. 1reported on the impact of electroencephalographic Bispectral Index (BIS) monitoring on fast tracking of gynecologic outpatients undergoing laparoscopic surgery. Although these authors concluded that the use of BIS monitoring did not have a significant effect on the ability to fast track outpatients at their institution, their seemingly “unexpected” findings are actually quite predictable given their study design.
In their article, the authors state, “sevoflurane was titrated to maintain a BIS value in the 50–60 range….” Although no actual BIS data were presented, the mean (± SD) concentration of sevoflurane was reported to be 2.14 ± 0.25%. Unfortunately, this concentration of sevoflurane is simply not consistent with BIS values of 50–60. In an earlier study involving a similar outpatient population, 2sevoflurane concentrations of 0.6–1.2% (0.9 ± 0.3%) were associated with BIS values in the range of 55–65 (62 ± 3)! In this earlier study, sevoflurane concentrations of 1.5–2% (1.8 ± 0.3%) were consistently associated with BIS values of less than 50 (42 ± 8). 2A more recent study by Hodgson and Liu, 3also published in the Journal, found a linear relationship between the end-tidal concentration of sevoflurane and the BIS value. With sevoflurane concentrations of 1–2%, the BIS values would be expected to range from 50 to 30.
An even more troublesome point is that when Ahmad et al. presented these study results, 4they stated that a BIS target range of 40–50 was maintained in their BIS monitored group. This was also verified when the authors were questioned about their study by Dr. Girish Joshi from the University of Texas Southwestern Medical Center at Dallas in the November 2001 issue of Anesthesiology News (New York, McMahon Publishers).
Of importance with respect to the negative findings reported by Ahmed et al. , the anesthetic, analgesic, and muscle relaxant drug dosages were all identical in both their control and BIS-monitored groups. It is obvious that the anesthesiologists caring for patients in the BIS group failed to use the BIS monitor to make decisions regarding their use of any of these anesthetic drugs. Therefore, the negative findings are totally predictable . Of interest, at least two more recent studies 5,6have confirmed our earlier findings 2that cerebral monitoring improves the titration of volatile anesthetics and, thereby, can facilitate a fast-track recovery.
Another flaw in the study design that could also have biased the results against BIS monitoring relates to the anesthetic management at the end of the operation. Although sevoflurane was discontinued when the laparoscope was removed from the abdominal cavity in both study groups, nitrous oxide was added in the BIS-monitored group to “maintain the BIS value below 60….” On the other hand, in the control group nitrous oxide was only added if the blood pressure and/or heart rate increased to more than 20% above the awake values! The authors failed to mention how many subjects received nitrous oxide in each group. Therefore, this practice would have biased the emergence data in favor of the control (vs. BIS-guided) group.
The criteria used for fast tracking was the standard Aldrete scoring system, and it is not clear that any effort was made by the investigators to implement a paradigm shift in the recovery process by educating the anesthesia personnel and nursing staff. In a recent study by Apfelbaum et al . 7a multidisciplinary educational program was believed to be critically important in increasing the PACU bypass rate and, thereby, facilitating the fast-tracking process.
Because the authors’ institution was apparently able to fast track more than 85% of their cases without using a cerebral monitor, the study was in fact seriously “underpowered” with respect to their primary outcome variable. Interestingly, the authors failed to present the actual “bypassed” (vs. nonbypassed) recovery times. The only data presented was the percentage of patients who actually bypassed the PACU after satisfying the bypass criteria during a 10-min observation period following discontinuation of the anesthetic drugs. Of interest, the mean length of their stay in the phase II recovery area averaged over 200 min.
Finally, the authors state that a recent study in a similar population reported fast-track rates of only 8–11%. However, in a study published in 2001, 8we reported fast tracking 80% of our gynecologic laparoscopy patients undergoing general anesthesia at a public teaching hospital in Dallas, Texas (Parkland Memorial Hospital).
Rather than clarifying the issue with regard to BIS monitoring and fast tracking, this article has simply “muddied the water.” We would agree that this study successfully “eliminated the possibility of contamination of the study results by anesthesiologists who learned from their experience using the BIS® monitor…,” because they obviously failed to change their standard anesthetic practice based on the information provided by the cerebral monitor. By requiring “strict adherence” to their standard anesthetic practices, it is not unexpected that they achieved a negative result. We believe the results of this study are misleading, and we strongly disagree with the authors’ conclusion that “this study clearly demonstrates that the application of the BIS® monitor does not have a significant impact on the ability to successfully fast track outpatients following general anesthesia….”