EFFECTS OF REMIFENTANIL, A NEW SHORT-ACTING OPIOID, ON CEREBRAL BLOOD-FLOW, BRAIN ELECTRICAL-ACTIVITY, AND INTRACRANIAL-PRESSURE IN DOGS ANESTHETIZED WITH ISOFLURANE AND NITROUS-OXIDE

Background: A new short-acting opioid, remifentanil, is metabolized by esterase activity in blood and tissue. It is important to know whether remifentanil may decrease the time to recovery of opioid-induced cardiovascular and cerebral effects compared to that of other short-acting agents such as alfentanil. Methods: Baseline measures were made during 1% end-tidal isoflurane and 50% N2O in oxygen in dogs. Approximately equipotent low- and high-dose remifentanil (0.5 and 1.0 mug . kg-1 . min-1) or alfentanil (1.6 and 3.2 mug . kg-1 . min-1) were infused for 30 min each (total infusion time 60 min) followed by a 30-min recovery period. Blood pressure, heart rate, and intracranial pressure were recorded continuously. Electroencephalogram measurements were made using aperiodic analysis, and regional cerebral blood flow using radioactive microspheres. Results: Both remifentanil and alfentanil decreased blood pressure and heart rate 25-30%. Cortex, hippocampus, and caudate blood flow decreased 40-50% during opioid infusion, but flow changes in lower brain regions were modest or absent. The electroencephalogram showed a shift from low-amplitude, high-frequency activity during baseline to high-amplitude, low-frequency activity during opioid infusion. During a 30-min recovery period, heart rate, electroencephalogram, and regional cerebral blood flow recovered to baseline levels in remifentanil- but not in alfentanil-treated dogs. Blood pressure and intracranial pressure decreased during opioid infusion and increased above baseline levels during the recovery period in remifentanil-treated dogs. Conclusions: These results show that the cardiovascular and cerebral effects of remifentanil and alfentanil are similar but that recovery of these parameters occurs sooner following remifentanil.