The meta-analysis by Fernandez-Guisasola et al [1] found that the omission of nitrous oxide significantly reduced postoperative nausea and vomiting (PONV): a pooled relative risk of 0.80, 95% CI 0.71-0.90, p=0.0003. The ability to predict risk is necessary to reduce PONV rates. However, anaesthetists’ subjective patient assessment may not be accurate.

During a prospective audit in our hospital, we evaluated the anaesthetist’s subjective risk assessment relative to the risk stratification system proposed by Apfel [2]. This model identifies four primary risk factors during inhalational anaesthesia: female sex, non-smoking status, a history of PONV or motion sickness and opioid use. The Apfel model, using four risk factors, predicted PONV risk across groups of patients without loss of discriminating power [2,3].

One hundred and twenty five patients undergoing general anaesthesia for gynaecological surgery were included in our audit. Pre-operatively anaesthetists were asked, based on their subjective assessment, to classify patients as low, moderate or high risk. This was recorded, along with the presence of Apfel risk factors, using a questionnaire. Risk was underestimated when compared with the Apfel scoring system (Table 1). The Apfel model predicts that with three or more risk factors the incidence of PONV is greater than 61%. In our audit, 51% of our patients had three or four factors, yet only 17% of patients were considered high risk and 25% were classified as low risk by our anaesthetists.

A risk score dependent approach effectively reduces departmental incidence of PONV [4]. However, accurate risk assessment is necessary and our audit suggests that anaesthetists may be inaccurate when subjectively estimating risk. Although no model can predict which patients ultimately have PONV, they can provide a rational basis for a PONV strategy. Incorporating models such as the Apfel score should be used to guide department anti-emetic guidelines.

A Roberts

P Barclay

M Stott

Liverpool Women’s Hospital

Liverpool, UK

E-mail: alunroberts@doctors.net.uk

No external funding and no competing interests declared

References

1      Fernandez-Guisasola J, Gomez-Arnau JI, Cabrera Y, Garcia del Valle S. Association between nitrous oxide and the incidence of postoperative nausea and vomiting in adults: a systematic review and meta-analysis. Anaesthesia 2010; 65: 379-87.

2      Apfel CC, Laara 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

3      Gan TJ, Meyer T, Apfel CC, Chung F, Davis PJ, Eubanks S, Kovac A, Philip BK, Sessler DI, Temo J, Tramer MR, Watcha M. Consensus guidelines for managing postoperative nausea and vomiting. Anesthesia and Analgesia 2003; 97: 62-71

4      Pierre S, Corno G, Benais H, Apfel CC. A risk score dependent antiemetic approach effectively reduces post operative nausea and vomiting – a continuous quality improvement initiative. Canadian Journal of Anesthesia 2004; 51: 320-5

Table 1 Anaesthetists subjective assessment of PONV risk versus the number of Apfel factors present. Values are numbers.

We appreciate the interesting comments of Dr Roberts et al about our study [1]. The consequences of a specific anaesthetic procedure should be judged on an overall basis but anaesthetist´s subjective assessment is hardly ever a reliable method.

We concluded that avoiding nitrous oxide does reduce the risk of postoperative nausea and vomiting (PONV), especially in women. The origin of PONV depends on a number of variables that go beyond anaesthetic or surgical factors and there are still many issues to be resolved, especially with regard to molecular and genetic aspects [2]; thus, the impact of any intervention depends critically on the patient´s baseline risk. The identification of the degree of PONV risk for each patient in a specific surgical setting (using the Apfel model or other) helps in decision-making, involving questions such as the anaesthetic technique and anti-emetic medication [3]. For example, consensus has been reached with respect to the view that anti-emetic prophylaxis should be reserved for patients at moderate or high risk for PONV [4].

J Fernández-Guisasola, Y Cabrera

Anaesthesia Department, Hospital Rúber, Madrid, Spain

jfguisasola@telefonica.net

1. Fernández-Guisasola J, Gómez-Arnau JI, Cabrera Y, et al. Association between nitrous oxide and the incidence of postoperative nausea and vomiting in adults: a systematic review and meta-analysis. Anaesthesia 2010;65: 379-87.

1. Candiotti KA, Birnbach DJ, Lubarsky DA et al. The impact of pharmacogenomics on postoperative nausea and vomiting: do CYP2D6 allele copy number and polymorphisms affect the success or failure of ondansetron prophylaxis? Anesthesiology 2005; 102:543-9.

1. Apfel CC, Korttila K, Abdalla M et al; IMPACT Investigators. A factorial trial of six interventions for the prevention of postoperative nausea and vomiting. The New England Journal of Medicine 2004; 350:2441-51.

1. Gan TJ. Risk factors for postoperative nausea and vomiting. Anesthesia & Analgesia 2006; 102:1884-98.