The patients qualified for abdominal surgery were enrolled in the study. All patients were premedicated with oral midazolam (7.5 mg). Epidural cannulations were placed by anesthesiologists before general anesthesia. The catheters' placement procedure was conducted under local anesthesia and according to the local protocol for the infections' prevention. The epidural infusion (solution of 0.2 % ropivacaine 5 ml with 25 to 50 mcg sufentanil in 50 ml of 0,9% NaCl) was started with a 5 mL bolus of the mentioned solution a few minutes before skin incision. The continuous infusion was maintained throughout the surgery at the rate of 3-12 mL/h. The patients were induced into general endotracheal anesthesia according to a standardized protocol, with propofol 1-3 mg/kg, fentanyl 1-2 mcg/kg, and rocuronium bromide 0.6 mg/kg. Anaesthesia was continued by using sevoflurane or desflurane MAC 1 to maintain mean arterial pressure with a value of +/- 20% of the original value. The postoperatively patients were monitored in the PACU (Post Anesthesia Care Unit) for 1 hour and then transferred to the intensive care unit or surgical unit according to their clinical status and co-morbidities. Whole blood samples (2.0 ml) were collected to measure sufentanil concentrations - during the epidural infusion and up to 72 hours after its cessation. Vital parameters (e.g. blood pressure, saturation, heart rate, respiratory rate, and pain score) were monitored at regular intervals. The epidural sufentanil and ropivacaine infusion were continued after surgery as long as was necessary.
