Figure 1. The concentration-effect relations of the combination of propofol and alfentanil for laryngoscopy, intubation, and opening of the peritoneum. The curves were obtained by logistic regression of the response-no-response data versus the corresponding measured blood propofol concentrations and the corresponding natural logarithm of the measured plasma alfentanil concentrations for the three stimuli separately. The displayed curves represent alfentanil and propofol concentrations associated with a 50% probability of no response. Because laryngoscopy was performed more than once, the highest alfentanil concentration associated with a response and the lowest alfentanil concentration without a response were used in the regression. The curve for laryngoscopy is described by the equation: EC50of alfentanil = e sup (5.588788 - 0.2256.Cprop), where Cprop= the blood propofol concentration; R2= 0.27. The curve for intubation is described by the equation: EC50of alfentanil = e sup (6.279797 - 0.31562.Cprop); R2= 0.41. The curve for the opening of the peritoneum is described by the equation: EC50of alfentanil = e sup (6.412278 - 0.42692.Cprop); R2= 0.38.