Pentobarbitone formation during thiopentone infusion

The plasma concentrations and elimination half-life of pentobarbitone were determined in 14 surgical patients receiving a continuous, exponentially decreasing, infusion of thiopentone (mean total dose, 1.05 g; SD 0.34; mean duration of infusion 2.4 hours, SD 0.7) as the primary anaesthetic agent. The plasma pentobarbitone concentration increased gradually, to reach a maximum of 1.49 micrograms/ml (SD 0.61) at the end of the thiopentone infusion, which was 15.5 per cent (SD 6.04) of the plasma thiopentone concentration. The elimination half-life of pentobarbitone measured over the following 70 hours in nine of the patients was 34.3 hours (SD 8.2), which is within the range of values reported previously in several studies in which pentobarbitone was administered directly to volunteers. It was concluded that the formation of this active metabolite during 2-3 hour thiopentone infusions was unlikely to be of clinical relevance, but that significant concentrations may occur with longer thiopentone infusions.