Figure 5. The propofol-alfentanil concentration isobologram obtained by Vuyk et al. [ 19,] for a 50% probability of no response to peritoneal opening using total intravenous anesthesia without nitrous oxide (solid line) as compared with the propofol-alfentanil concentration isobologram obtained in our study with nitrous oxide for a 50% probability of no response to cutaneous nerve stimulation (shown as individual data points and a dashed line). There are similar synergistic effects observed. The curve obtained by Vuyk et al. [ 19 ] is shifted to the right compared with our results, as would be expected in the absence of nitrous oxide. Note that the axes are reversed compared with Figure 4.