BACKGROUND: Estimation of the mu-agonist opioid effect in anesthetized and 
paralyzed patients is often imprecise and can be obscured by concomitant 
administration of drugs that affect the sympathetic nervous system, such as 
beta-adrenergic blocking agents. As an alternative to hemodynamic measures of 
opioid effect, the authors tested the hypothesis that the pupillary light reflex 
or pupillary reflex dilation correlated with alfentanil concentrations during 
isoflurane anesthesia.
METHODS: Six volunteers were anesthetized on 4 days with 0.8% isoflurane. 
Alfentanil was administered intravenously to target total plasma concentrations 
of 0, 25, 50, and 100 ng/ml. A 5-s tetanic electrical stimulus was applied to 
the skin. Pupil size and the pupillary light reflex were recorded before and 
after alfentanil administration, and before and for 8 min after the stimulus.
RESULTS: Alfentanil exponentially impaired reflex pupillary dilation, decreasing 
the maximum response amplitude from 5 mm at 0 ng/ml, to 2.3 mm at 25 ng/ml, to 
1.0 mm at 50 ng/ml, and finally to 0.2 mm at 100 ng/ml. In contrast, only the 
highest concentration of alfentanil depressed the dilation of the pupil in the 
first 2 s after the stimulus. Alfentanil administration had no effect on the 
pupillary light reflex.
CONCLUSIONS: Dilation of the pupil in response to a noxious stimulus is a 
measure of opioid effect in isoflurane-anesthetized volunteers. In contrast, the 
pupillary light reflex is unaffected by alfentanil during isoflurane anesthesia. 
These data suggest that stimulus-induced pupillary dilation may be used to 
evaluate the analgesic component of a combined volatile and opioid anesthetic.