Background: The pupillary light reflex often is evaluated in the perianesthetic period to assess drug effects and brainstem function. Mild hypothermia alone, or combined with isoflurane, does not impair pupillary responses. Although perioperative hyperthermia is less common than hypothermia, abnormal increases in core temperature remain an important thermal disturbance. Accordingly, we evaluated the pupillary effects of hyperthermia alone and of hyperthermia combined with isoflurane and enflurane. Additionally, we determined the effects of nitrous oxide on pupillary responses. Methods: The pupillary light reflex was evaluated in 31 non-surgical volunteers participating in concurrent thermoregulatory studies. Pupillary reflexes were quantified using a portable infrared pupillometer during 1) hyperthermia alone (n=9); 2) hyperthermia with 0.8% and 1.2% end-tidal isoflurane (n=8); 3) hyperthermia with 1.7% end-tidal enflurane (n=5); and, 4) inhalation of 60% nitrous oxide (n = 9). Results: Mild hyperthermia alone (core temperature 38.5 0.3 C) produced no clinically significant change in the pupillary light reflex. Pupillary responses were markedly decreased by 0.8% isoflurane, 1.2% isoflurane, and 1.7% enflurane when the volunteers were normothermic. Mild hyperthermia combined with isoflurane or enflurane dilated the pupil and increased the amplitude of the light reflex. Sixty percent nitrous oxide decreased the pupillary reflex only 26 4%. Conclusion: Anesthetic-induced inhibition of the pupillary response to light is partially reversed by core hyperthermia. In contrast to enflurane and isoflurane, 60% nitrous oxide has little effect on the pupil.
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