Pharmacodynamics of alfentanil as a supplement to propofol or nitrous oxide for lower abdominal surgery in female patients

Abstract

Background: Although propofol and alfentanil are given in combination in clinical practice, the pharmacodynamic interaction between these drugs has not been described. Methods: The pharmacodynamics of alfentanil when given as a supplement to propofol were studied in 10 ASA physical status 1 female patients (group P) undergoing lower abdominal surgery and compared to the pharmacodynamics of alfentanil when given as a supplement to nitrous oxide (group N, n = 10). Anesthesia was induced by either computer-controlled infusion of propofol and alfentanil at target concentrations of 3 μg/ml and 100 ng/ml (group P) or computer-controlled infusion of 400 ng/ml alfentanil as a supplement to nitrous oxide and oxygen (ratio 2:1; group N). The target concentration of alfentanil was varied to patient responses, and the nitrous oxide and propofol concentrations were maintained constant. A sigmoid Emax model was fitted to response/no response data versus plasma alfentanil concentrations at intubation, skin incision, and the opening of the peritoneum in both groups and for the intraabdominal part of surgery in the individual patients. In addition, the speed of recovery in both groups was determined by a deletion-of-p's test. Results: The EC50 (the concentration at which, with a 50% probability, the patients did not respond to the surgical stimuli) of alfentanil during propofol anesthesia was 92 ng/ml for intubation, 55 ng/ml for skin incision, 84 ng/ml for the opening of the peritoneum, and 66 ± 38 ng/ml (mean ± SD) for the intraabdominal part of surgery. The corresponding values during nitrous oxide anesthesia were significantly higher: 429 ng/ml for intubation, 101 ng/ml for skin incision, and 206 ± 65 ng/ml for the intraabdominal part of surgery (P < 0.001). The speed of recovery was similar in both groups. Conclusions: The alfentanil requirements in ASA physical status 1 female patients undergoing lower abdominal surgery are less when given as a supplement to propofol (4 μg/ml) compared to 66% N2O.