Patient maintained alfentanil target-controlled infusion for analgesia during extracorporeal shock wave lithotripsy

The purpose of this study was to determine whether alfentanil given by a pharmacokinetic-based target controlled infusion (TCI) system under patient control is a suitable analgesic technique for extracorporeal shock wave lithotripsy (ESWL).

The design was an open, unblinded, noncomparative, prospective study. Forty outpatients undergoing ESWL were given patient maintained alfentanil TCI. Pain, nausea and sedation were assessed every 300 shocks. Vital signs were recorded every three minutes, pulse oximetry and electrocardiography being monitored continuously. Blood alfentanil concentration was measured for comparison with the predicted value.

Alfentanil consumption (median 1.34 mg, range 0.8-3.6) and measured levels following treatment (median 60 ng.ml-1, range 15.6-134.3) varied widely. The precision of the TCI system and the median prediction error (bias) were both 49%. The median of pain scores recorded during treatment was 4 (range 0-8). The median respiration rate was 15 bpm (range 10-23), three patients required oxygen (SaO2 < 92%) cardiovascular measurements were stable and there was no excessive sedation. The incidence of nausea was 15%. All patients were ready for hospital discharge within one hour following treatment.

Patient maintained alfentanil TCI provides good analgesia for ESWL in the majority of patients with little sedation. Respiratory depression is uncommon but supplementary oxygen should be given prophylactically. There is considerable interindividual variation in demand for alfentanil indicating the usefulness of the patient control method. The TCI system underestimated alfentanil blood concentrations but this did not affect its clinical usefulness.