Autonomic effects of epidural and intravenous fentanyl

Abstract

Background. We tested the hypothesis that there is greater suppression of autonomic reflexes during general anaesthesia when fentanyl is administered epidurally than when it is given intravenously. Methods. Ten volunteers were anaesthetized with desflurane. Noxious stimuli of variable intensity were then delivered by tetanic electrical stimuli. Heart rate, arterial pressure, and pupillary dilation in response to these stimuli defined nociception. Seven of these volunteers participated twice using a crossover design: they received i.v. fentanyl on one study day and epidurally on the other. Autonomic responses to alternative tetanic stimuli at L4 and C5 dermatomes were measured every 5 min for 3 h after fentanyl administration Results. After a brief redistribution period, plasma fentanyl concentrations were virtually identical on both days. After stimulation of the L4 dermatome only, block of pupillary reflex dilation was greater by 47 (22)% after epidural fentanyl compared with i.v. fentanyl. Time to maximal depression of reflex dilation after L4 stimulation was 41 (13) min. Arterial pressure and heart rate decreased after fentanyl by either route but there were no differences observed between L4 and C5 stimulations. Conclusion. We conclude that during general anaesthesia, epidural fentanyl enhances antinociception by a spinal mechanism which can be detected by pupillary dilation but not by changes in arterial pressure or heart rate. © The Board of Management and Trustees of the British Journal of Anaesthesia 2007.