Isoflurane, halothane, and regional cerebral blood flow at various levels of Pa(CO₂) in rabbits

The effects of halothane and isoflurane on regional cerebral blood flow (CBF) were studied in 18 New Zealand White rabbits anesthetized with nitrous oxide (N₂O) and morphine sulfate (MS) at three different levels of Pa(CO₂). CBF was measured using the hydrogen clearance technique. Monitored variables were intracranial pressure (ICP), central venous pressure, heart rate, mean arterial pressure, electroencephalogram, arterial blood gases, end-tidal (ET) volatile anesthetic, and ET CO₂. Addition of 1 MAC halothane to the N₂O/MS background anesthetic caused flow to increase significantly in all three regions studied (cortex, dorsal hippocampus, white matter) at all three levels of Pa(CO₂) (low: 20-25 mmHg; normal: 35-40 mmHg; high: 50-55 mmHg). Addition of 1 MAC isoflurane to the background anesthetic caused CBF to decrease significantly in all regions during hypocapnia. During normocapnia, CBF was unchanged with the addition of 1 MAC isoflurane in all regions and during hypercapnia, CBF increased significantly only in the dorsal hippocampus following addition of 1 MAC isoflurane to the MS/N₂O background anesthetic. Volatile anesthetic administration was associated with significant, although small, increases in ICP at all Pa(CO₂) levels. We conclude that 1 MAC concentrations of halothane and isoflurane have opposite effects on CBF when added to a N₂O/MS anesthetic during hypocapnia and that the effects of isoflurane on regional CBF are dependent on Pa(CO₂) in rabbits under the anesthetic conditions of this experiment.