Pupil diameter is used during anaesthesia to assess depth of anaesthes
ia and indicate cerebral hypoxia. This is especially so during cardiac
bypass when other autonomic signs cannot be monitored. We have used a
pupillometer to determine the effect of cardiopulmonary bypass on the
pupil. We have also investigated if any effect was caused by washout
of opioid from the central nervous system by allocating patients to on
e of two groups: in one the bypass pump was preloaded with fentanyl, i
n the other with 0.9% saline. Cardiopulmonary bypass caused pupil dila
tation of between 17% and 53%, which was unaffected by preloading the
bypass pump with fentanyl. This effect lasted for the duration of the
study, which ended 30 min after the start of cardiopulmonary bypass. S
ympathetic nervous system reflexes and hypothermia may account for thi
s observation, but further research is necessary to exclude other cont
ributory factors.