Comparison of anaesthetic and kinetic properties of thiobutabarbital, butabarbital and hexobarbital after intravenous threshold doses in the male rat.

Due to the exceptionally long duration of action of thiobutbarbital the anaesthetic properties of this barbiturate was reinvestigated with an intravenous threshold technique using butabarbital and hexobarbital as references. Adult male rats were used. The criterion of anaesthesia was a burst suppression in the EEG of 1 sec. or more (the "silent second" = SS). The dose which induced the criterion was used as a threshold. The barbiturates were infused with different rates to obtain dose rate curves. After induction of the threshold criterion the animals were either killed and different tissue concentrations were analyzed with a HPLC method or allowed to survive and duration of SS and duration of loss of righting reflex were recorded. With hexobarbital, duration of SS and of loss of righting reflex increased significantly with increasing dose rate. With increasing rates of thiobutabarbital and butabarbital there was in both cases a stepwise increase in duration of SS. At sacrifice, after induction of SS with slow rates brain concentrations of both thiobutabarbital and butabarbital were lower than values recorded after higher rates. The change between the two concentrations was abrupt and occurred at a rate of 20 mg/kg/min. with thiobutabarbital and at the rate of 1.25 mg/kg/min. with butabarbital. This phenomenon was the reverse of acute tolerance which was recorded with hexobarbital and can thus be denoted acute supersensitivity. A kinetic analysis of serum, muscle and fat indicated considerable differences between the barbiturates. As indicated by mortality figures the induction of acute supersensitivity could be potentially dangerous.