Pain sensation and concentration time profiles of predicted hydromorphone and sufentanil in a representative patient. Hydromorphone concentration axis (i.e., left y-axis) is limited to 10 ng/ml although the maximum predicted hydromorphone plasma concentration during the patient-controlled analgesia (PCA) study phase was 17.6 ng/ml. Ce HM = hydromorphone effect-site concentration; Cp HM = hydromorphone plasma concentration; Cp SUF = sufentanil plasma concentration; NRS = numerical rating scale ratings under inspiration; TCI = target-controlled infusion; TCI-PCA = PCA with TCI.