Fig. 1

Assessment of negative or positive responses to inhibit a CO₂ pneumoperitoneum under a predetermined Ce of remifentanil using the Dixon “up-and-down” method in 22 consecutive male patients (a) and 22 consecutive female patients (b). Horizontal bars represent crossover midpoints (negative to positive). The EC₅₀ values of remifentanil required to inhibit the CO₂ pneumoperitoneum in the male group and the female group were 5.00 ± 0.52 ng/ml and 4.17 ± 0.38 ng/ml, respectively. EC₅₀, median effective concentration