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

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. 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 micrograms.kg-1.min-1) or alfentanil (1.6 and 3.2 micrograms.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. 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. 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.