Figure 4. The alfentanil concentration-effect relations in the individual patients for the intraabdominal part of surgery when alfentanil was given as a supplement to a target propofol concentration of 4 micro gram/ml. The mean measured blood propofol concentrations were 5.1, 4.4, 5.1, 4.4, 7.5, 5.6, and 6.3 micro gram/ml in patients 1-7, respectively (see Table 4). The curves were determined by logistic regression of response-no-response data versus the corresponding measured plasma alfentanil concentrations, as shown beneath the curves. In patient 1, there was no overlap between response and no-response data; in this patient, the alfentanil concentration associated with a 50% probability of no response was determined by the midrange between the highest plasma alfentanil concentration with a response and the lowest plasma alfentanil concentration without a response. In patient 5, no responses occurred, and therefore in this patient the concentration-response relation could therefore not be determined. Filled circles = concentrations of alfentanil associated with a 50% probability (EC50s) of no response.
