Fig. 2.  Variable slope E  ^maxsigmoidal dose–response curves for bupivacaine and ropivacaine generated by nonlinear regression. Data points with error bars are shown as mean (SD). The horizontal axis is on a logarithmic scale. Antilog values for dose are shown below log(dose) values to aid interpretation. ED⁵⁰was defined as the dose of the drug that reduces pain score to 50% of baseline at 30 min, and ED⁹⁰was defined as the dose of the drug that reduces pain score to 90% of baseline at 30 min. The values for log(ED⁵⁰) were different for the two local anesthetics, but the values for the Hill coefficient and log(ED⁹⁰) were similar.