Fig. 9. Remifentanil equivalents versus  blood propofol concentrations associated with a 50% probability of no response to intraabdominal surgical stimuli, from this study (filled circles ) and from the study by Vuyk et al.  1(open circles ). The curve represents a mechanistic function (see  Appendix) fitted to the data by unweighted least-squares nonlinear analysis described by the equation:where C^50,rem,eqis the remifentanil equivalent associated with a 50% probability of no response and C^propis the mean blood propofol concentration calculated in each patient. The C^alf,50s from the study by Vuyk et al.  1were transformed to C^50,rem,eq, by estimation of the potency of remifentanil relative to that of alfentanil for suppression of responses to lower abdominal surgery as an additional parameter (C^50,rem,eq= C^50,alf/31.1).