I.V. Anaesthesia and EEG burst suppression in rats: Bolus injections and closed-loop infusions

We describe a system for monitoring and controlling i.v. anaesthesia in rats using burst suppression ratio (BSR) detection in the extradural EEG. After bolus injection, peak BSR values of 95% were achieved with propofol 8 mg kg^-1, etomidate 3.5 mg kg^-1 and alphaxalone 4.5 mg kg^-1. Thiopental 32 mg kg^-1 produced a peak BSR of 70% (larger doses were not tolerated). Recovery was fastest with propofol, followed by etomidate and alphaxalone with equal duration, and slowest with thiopental. In further experiments, a closed-loop infusion system maintained BSR accurately at targets of 30%, 50%, 70% or 90% for 60 min with propofol or etomidate. During these experiments the infusion rates were found to decrease with time, more so with etomidate (approximately 40%) than with propofol (approximately 20%). Recovery times were 2-3 times longer with etomidate than with propofol. This model demonstrated differences between i.v. anaesthetics and may be useful in screening new compounds in preclinical development.