Effects of fentanyl on the response of plasma beta-endorphin immunoreactivity to surgery

Beta-endorphin appears to play a definite role in the biologic response to stress and in the endogenous mechanism of pain perception. Opiates exogenously administered during surgery decrease or even suppress the activation of "stress hormones" such as ACTH and cortisol. In the present study, the authors tried to assess the effects of fentanyl administration on plasma beta-endorphin immunoreactivity PBE(ir) during surgical stress. In one group of nine patients, a standard enflurane-based general anesthetic technique without opiates was used for a staging laparotomy. A second group of ten patients undergoing the same type of surgery received fentanyl (10-20 micrograms/kg) as the primary anesthetic drug. In both groups, multiple blood samples were collected prior to, during, and after surgery, following the same time protocol. PBE(ir), plasma cortisol and, in five patients, plasma ACTH were determined by radioimmunoassay. There was no significant change in PBE(ir) in either group after anesthetic induction. Unlike the enflurane group, the fentanyl group did not demonstrate any significant increase from baseline in PBE(ir) during surgery. There was a significant group difference between enflurane and fentanyl in PBE(ir) levels for both "early" and "late" surgery values, but not for the "awake" values (recovery period) where both groups had elevated PBE(ir) levels. Plasma cortisol and plasma ACTH changes followed a trend similar to those of PBE(ir). The suppression of both cortisol and PBE(ir) responses during surgery after administration of fentanyl provides further evidence for the involvement of the endorphin system in the stress response and for its physiologic association with the hypothalamo pituitary axis.