Fig. 4. The risk reduction for severe nausea or vomiting, wound infection, and any complication associated with avoidance of nitrous oxide in selected subgroups, expressed as univariate odds ratio (95% confidence interval). The  vertical interrupted lines represent the overall risk reductions for the selected outcomes in the total study population. ASA = American Society of Anesthesiologists. * Any major complication includes wound infection, pneumonia, pneumothorax, myocardial infarction, thromboembolism, stroke, awareness, and death within 30 days of surgery.