Postoperative Nausea and Vomiting and BIS Monitoring

Abstract

Postoperative Nausea and Vomiting and BIS Monitoring 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.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 GroupIn 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). Kaisa Nelskylä, MD, Ph.D Helena Puro, MD Kari Korttila, MD, PhD, FRCA Arvi Yli-Hankala, MD, PhD