Figure 1. Meta-analysis of randomized trials comparing incidence of PONV with or without omission of nitrous oxide. The odds ratios (OR) are plotted on the X axis on a logarithmic scale. The position of each solid square represents the OR of each trial. The horizontal line indicates the 95% confidence interval. In each trial, squares to the left of unity (OR = 1) imply that omission of nitrous oxide reduced PONV; squares to the right of unity imply that omission of nitrous oxide increased PONV.