Anesth Analg 2002;95:496
© 2002 International Anesthesia Research Society
LETTERS TO THE EDITOR
Postoperative Nausea and Vomiting and BIS Monitoring
Paul F. White, PhD MD
Professor & Holder of the Margaret Milam McDermott Distinguished Chair in Anesthesiology, Department of Anesthesiology and Pain Management, University of Texas Southwestern Medical Center, Dallas, Texas
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.
References
- Nelskyla KA, Yli-Hankala AM, Puro PH, Korttila KT. Sevoflurane titration using bispectral index decreases postoperative vomiting in phase II recovery after ambulatory surgery. Anesth Analg 2001; 93: 1165–9.[Abstract/Free Full Text]
- Song D, Joshi GP, White PF. Titration of volatile anesthetics using bispectral index facilitates recovery after ambulatory anesthesia. Anesthesiology 1997; 87: 842–8.[Web of Science][Medline]
- Yli-Hankala AM, Vakkuri A, Annila P, Korttila KT. EEG bispectral index monitoring in sevoflurane or propofol anesthesia: analysis of direct costs and immediate recovery. Acta Anaesthesiol Scand 1999; 43: 545–9.[Web of Science][Medline]
- Apfel C, Greim C, Haubitz I, et al. The discriminating power of a risk score for postoperative vomiting in adults undergoing various types of surgery. Acta Anaesthesiol Scand 1998; 42: 502–9.[Web of Science][Medline]
- Apfel C, Läärä E, Koivuranta M, et al. A simplified risk score for predicting postoperative nausea and vomiting conclusions from cross-validations between two centers. Anesthesiology 1999; 91: 693–700.[Web of Science][Medline]
Response
Kaisa Nelskylä, MD Ph.D,
Helena Puro, MD,
Kari Korttila, MD PhD, FRCA, and
Arvi Yli-Hankala, MD PhD
Department of Anaesthesia and Intensive Care, Helsinki University Central Hospital, Helsinki, Finland
University of Tampere Medical School, Tampere University Hospital, Dept. of Anaesthesia and Critical Care, Tampere, Finland
In Response: We appreciate Dr White’s continued interest in our article after its publication (1). We apologize for not quoting the study by Song et al. (2) as one of our references, as during the editing process of our article we were asked to keep the word count and the number of references as small as possible. Comparing the two studies Song et al. (2) and Yli-Hankala et al. (3) with our present study, it is noticeable that the study protocols were different, e.g., in terms of how to administer sevoflurane. In the protocol used by Song et al., the investigators could almost freely adjust inhaled anesthetics in order to keep mean arterial pressure within 20% of baseline and heart rate < 100 bpm. However, their upper limit to end-tidal sevoflurane was 2% and they gave significantly more mivacurium in the BIS-titrated sevoflurane group when compared to sevoflurane given without BIS-titration. Song et al. only studied patients undergoing laparoscopic tubal ligation and they all received droperidol at the end of anesthesia to prevent PONV. They present a power analysis for sample size without a standard deviation of the primary end point and it would have been important to see the 95% confidence interval of their results as they studied only 15 patients/group. Investigators in the Yli-Hankala et al. study were free to use any concentration of sevoflurane. In the present study (1) administration of sevoflurane, opioids and rocuronium were more precisely controlled. There was no difference in the amounts of fentanyl or alfentanil given between the groups. We have already given the amounts in detail in the 4th paragraph of the results section of our paper.
We agree with Dr White that it is important to present the estimated risk on PONV for each group studied when answering the question if one anesthetic technique is associated with less PONV than another technique. Such a risk, i.e., the probability of patients to have PONV, should be presented using a validated risk score, such as the simplified risk score validated by Apfel et al. (4). With this validated and widely accepted score one can predict how likely a patient will have PONV. The score is calculated using 4 risk factors; female gender, previous history of PONV or motion sickness, nonsmoking and the perioperative use of opioid analgesics. If a patient has 0, 1, 2, 3 or 4 of these risk factors she/he has a 10%, 21%, 39%, 61% or 79% probability to have PONV, respectively (4). The score was not presented in our paper (1) or in the paper by Song et al. (2). As noticed by Dr White 10 of our patients in the BIS-guided group and 4 patients in the control group were smokers. However, it has already been mentioned in our paper that the difference was not statistically significant between the groups. When we tabulate the Apfel scores to our patients (Table 1), we can see that there is no difference in the calculated risk for PONV between our study groups, i.e., the comparison between our BIS-guided sevoflurane and control group is warranted.
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Table 1. Table 1. Number of Patients with Different Risk Factors for PONV According to Apfel (4) and the Calculated Risk for PONV in BIS-Guided Sevoflurane Anesthesia and in Control Group
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In summary, neither confounding variables nor differences in risk for PONV between our study groups can explain our findings. BIS-guided sevoflurane anesthesia was associated with faster orientation, faster ability to drink and better psychomotor performance at 30 min after anesthesia. The BIS-guided patients had significantly less vomiting (16%) than patients anesthetized without BIS-monitoring. As Dr White points out it is important that in future studies on PONV, whether they deal with different antiemetics, anesthetic techniques or devices, the investigators should show that the risk of PONV is similar in each group among studies. The use of Apfel score has been validated for this purpose and is highly recommended to be used in all PONV research (5).
References
- Nelskylä KA, Yli-Hankala AM, Puro PH, Korttila KT. Sevoflurane titration using bispectral index decreases postoperative vomiting in phase II recovery after ambulatory surgery. Anesth Analg 2001; 93: 1165–9.
- Song D, Joshi GP, White PF. Titration of volatile anesthetics using bispectral index facilitates recovery after ambulatory anesthesia. Anesthesiology 1997; 87: 842–8.
- Yli-Hankala A, Vakkuri A, Annila P, Korttila K. EEG bispectral index monitoring in sevoflurane or propofol anesthesia: analysis of direct costs and immediate recovery. Acta Anaesthesiol Scand 1999; 43: 545–9.
- Apfel CC, Läärä E, Koivuranta M, Greim CA, Roewer N. A simplified risk score for predicting postoperative nausea and vomiting: Conclusions from cross-validations between two centers. Anesthesiology 1999; 91: 693–700.
- Apfel CC, Kranke P, Eberhart LHJ, Roos A, Roewer N. Comparison of predictive models for postoperative nausea and vomiting. Brit J Anaesth 2002; 88: 234–40.[Abstract/Free Full Text]
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