An increased body mass index is no risk factor for postoperative nausea and vomiting. A systematic review and results of original data.

Background: An increased Body Mass Index (BMI) is almost always mentioned as a fundamental risk factor for postoperative nausea (PN), vomiting (PV) or both (PONV). However, multivariate analyses were unable to detect any correlation. Therefore, we asked whether an increased BMI is really a risk factor for PONV. Methods: For the systematic review, a search of electronic databases and a detailed manual search of reviews were carried out. For the analysis of the original data, 587 adult patients from a randomised controlled antiemetic trial (RCT) who underwent general anaesthesia were allocated to four weight groups: Underweight (BMI<20), Normal Weight (BMI 20-25), Overweight (BMI 25-30) and Obesity (BMI≥30). Results: Four publications with original data were found. Two described a positive relationship, although not clearly supported by the data. Despite this, most narrative reviews claimed a positive correlation between obesity and PONV by quoting again narrative reviews or misquoting originals. In the RCT, the calculated underlying risk profile for PONV was comparable between all groups. Incidences (95% confidence intervals) of PONV were 45.8% (34.0; 57.6), 41.7 (36.5; 46.9), 47.8 (38.4; 57.1) and 44.1 (31.0; 57.1), for the groups Underweight, Normal Weight, Overweight and Obesity, respectively (P=0.69). The incidences of PN and PV also did not differ with P=0.76 and P= 0.36, respectively. Conclusion: Systematic search of the literature provides no evidence for a positive relationship. Furthermore, our data confirm that an increased BMI is not a risk factor for PONV. This negative finding is important as focussing on the relevant risk factors is needed to allow for an objective risk assessment of PONV.