Comparison of alfentanil and fentanyl as supplements to induction of anaesthesia with thiopentone.

In 120 premedicated patients undergoing general surgery, anaesthesia was induced with thiopentone 3 mg kg-1, preceded by alfentanil 4.5, 9.0 or 13.5 micrograms kg-1 or fentanyl 1.5 micrograms kg-1. The largest alfentanil dose attenuated the arterial blood pressure response to laryngoscopy and intubation better than the smaller doses of alfentanil. Changes in frontal muscle electromyogram or plasma cortisol and prolactin levels were not dependent on the adjuvant used. After thiopentone, 30, 7 and 17% of the patients given alfentanil 9.0 and 13.5 micrograms kg-1 and fentanyl 1.5 micrograms kg-1, respectively, reacted to pinching of the lower abdomen. Patients given alfentanil 4.5 micrograms kg-1 did not tolerate the endotracheal tube after recovery from suxamethonium block and their heart rate was increased 12 min after alfentanil administration. We conclude that the antinociceptive effect of alfentanil is distinctly shorter than that of fentanyl. The analgesic potency of alfentanil is between one sixth and one ninth of that of fentanyl.