Latency of pupillary reflex dilation during general anesthesia

1 Department of Anesthesia and Perioperative Care, University of California, San Francisco, California 94143-0648; 2 Department of Anesthesia, Royal Melbourne Hospital, Parkville, Victoria 3052, Australia; and 3 Outcomes Research Institute and Department of Anesthesiology and Pharmacology, University of Louisville, Louisville, Kentucky 40202 Submitted 2 February 2004 ; accepted in final form 24 March 2004 Areas of insensibility produced by neuraxial anesthesia or peripheral nerve blocks can be detected during general anesthesia by failure of noxious stimulation to trigger pupillary reflex dilation. We examined the latency of pupillary reflex dilation and the effect of fentanyl on the latency of reflex dilation during anesthesia in nine volunteers. We hypothesized that the reflex was generated by slowly conducting C nociceptive fibers and would be significantly delayed if a distal dermatome (L 4 ) was stimulated compared with a proximal dermatome (C 5 ). We also hypothesized that fentanyl would prolong the latency and alter the shape of the reflex. After induction of general anesthesia, pupillary reflex dilation was measured with an infrared pupillometer every 5 min after stimulations of the L 4 and C 5 dermatomes. Fentanyl (3 µg/kg) was then given intravenously. Pupillary reflex dilation latencies were calculated by examining each individual measurement. After 3 h, naloxone (400 µg) was given intravenously; anesthesia was then discontinued. Pupillary reflex dilation had a long latency and consisted of distinct early and late phases. No differences were found between latencies of reflex dilation after simulation of L 4 and C 5 dermatomes either before or after fentanyl administration. Fentanyl at high concentrations essentially eliminated pupillary reflex dilation; but over the 180-min observation period, first early and then late dilation returned. Fentanyl produced a small increase in the latency of the initial early dilation. We conclude that pupillary reflex dilation during anesthesia is not initiated by slowly conducting C fibers and that fentanyl depresses the reflex in a stereotypical manner. nociception; windup; pupil; opioids; sensory block Address for reprint requests and other correspondence: M. D. Larson, Dept. of Anesthesia and Perioperative Care, Univ. of California, San Francisco, CA 94143-0648 (E-mail: larsonm{at}anesthesia.ucsf.edu ; URL: http://www.or.org ).