The effect of epidural administration of alfentanil on intra‐operative intravenous alfentanil requirements during nitrous oxide‐oxygen‐alfentanil anaesthesia for lower abdominal surgery

The effects of epidural administration of alfentanil on the intravenous alfentanil dose requirements and the plasma concentrations required to suppress responses to surgical stimulation during nitrous oxide‐oxygen‐alfentanil anaesthesia in 20 patients undergoing lower abdominal surgery were studied. Before induction of anaesthesia, patients in one group (E) received an epidural injection of 1mg alfentanil, followed by an epidural infusion of alfentanil 0.2mg.h‐1 until skin closure, whilst patients in the other group (C, control) received a continuous infusion of sodium chloride via a sham catheter in order to blind the main investigator to the treatment. Anaesthesia was induced and maintained with nitrous oxide (66%) in oxygen and a‘target’‐controlled intravenous infusion of alfentanil. During surgery, the‘target’alfentanil concentration was increased or decreased according to patients’responses. The number of responses to surgical stimulation was smaller in patients from group E (median 1, range 0‐3) than in patients from group C (median 4, range 1–15; p < 0.005), even though the alfentanil intravenous infusion rates were smaller in group E [mean (SD): 1.6(0.5)μg.kg‐1min‐1] than in group C [2.9(1.2)μg.kg‐1min‐1, p < 0.02]. Both the lowest concentrations associated with no response [133(40)ng.ml‐1]and the highest concentrations associated with a response [155(65)ng.ml‐1] in group E were lower than those in group C [238(100) ng.ml‐1, p < 0.01 and 334(163) ng.ml‐1, p < 0.05, respectively]. We concluded that epidural administration of alfentanil reduces intravenous alfentanil requirements during nitrous oxide‐oxygen‐alfentanil anaesthesia for lower abdominal surgery. The results indicate a spinal mechanism of action of epidural alfentanil.