Pupil response to alfentanil and fentanyl

The pupillary response to alfentanil, fentanyl and a saline placebo was measured in patients anaesthetised in a standard manner with halothane, to determine whether the pupil diameters changed in a way which could be distinguished from background activity. Measurements were made to the nearest 0.1 mm using a purpose‐built pupillometer. The response to the drugs can be clearly distinguished from the background activity seen in the placebo group. The time courses of constriction caused by the two drugs are significantly different from the placebo group responses. The time to maximum to response with alfentanil is 4 minutes compared with 8 minutes for fentanyl. Both drugs produced at least 35% reduction in mean pupil diameter compared with the placebo group. The duration of the response to alfentanil was 25 minutes, whereas the fentanyl response lasted more than one hour.