Postoperative Nausea and Vomiting and BIS Monitoring

Postoperative Nausea and Vomiting and BIS Monitoring To the Editor: In a recent publication (1), Nelskylä and colleagues have reported that titration of sevoflurane using the bispectral index (BIS) monitor decreased postoperative nausea and vomiting (PONV) after outpatient gynecologic laparoscopic surgery (1). Although the authors’ state that the “BIS helps to optimize the administration of volatile anesthetics,” they failed to demonstrate any anesthetic-sparing effect in the BIS-guided group. Interestingly, the BIS group had a “shorter emergence time” which they attributed to more precise administration of sevoflurane. The author’s were apparently unaware of a study published in 1997 by Song et al. (2) which demonstrated that titration of sevoflurane (or desflurane) using the BIS monitor produced an anesthetic-sparing effect which resulted in a faster emergence after ambulatory anesthesia. More disturbing, are the inconsistencies between the current study and an earlier study published in 1999 by the same group of investigators (3). Consistent with our earlier findings (2), Yli-Hankala and colleagues (3) reported that “BIS monitoring decreased consumption of both propofol and sevoflurane” and hastened the immediate recovery after anesthesia. While the investigators hypothesize that “patients receiving too large and too small concentrations of sevoflurane” were at increased risk of PONV, they failed to provide any data regarding difference in the intraoperative variability of the end-tidal anesthetic concentrations or the BIS values between the two study groups. Although the use of postoperative opioids was allegedly similar in the two study groups, they did not provide information on the exact dosages of parenteral (and oral) opioid analgesics used during the perioperative period. Without this information, it is unclear how these investigators ruled out this potentially confounding variable. They also dismissed differences in smoking history as an explanation for their findings. However, it is worth noting that 31% of the patients in the BIS group had a history of smoking compared to only 13% in the control group, and the incidence of PONV in the smokers was less than half that of the non-smoking patients. Given the well-known association between opioid usage, smoking history and PONV (4,5), it would seem that the authors’ findings could have been explained by factors (e.g., opioid usage, demographic characteristics) unrelated to BIS monitoring. Paul F. White, PhD, MD