The cerebral and systemic kinetics of thiopentone and propofol in halothane anaesthetized sheep

The cerebral and systemic kinetics of intravenous thiopentone (250 mg over 2 minutes, n = 5) and propofol (100 mg over 2 minutes, n = 6) were determin ed in sheep anaesthetized with halothane (2.0%) and mechanically ventilated to an end-expired carbon dioxide tension of 40 mmHg. The sheep were previo usly instrumented with arterial and sagittal sinus (effluent from the brain ) blood sampling catheters. Systemic kinetics were inferred from the time-c ourse of the arterial blood concentrations, and cerebral kinetics from the time-course of the arterio-sagittal sinus concentration difference across t he brain. Under halothane anaesthesia, the peak arterial concentrations of each drug occurred at the end of the two-minute infusion, and was 42.3 mg/l and 12.3 mg/l for thiopentone and propofol, respectively. Propofol had a s ignificantly larger systemic clearance (3.19 l/min) than thiopentone (0.99 l/min). The brain concentrations of propofol equilibrated more slowly with the arterial concentrations than those of thiopentone. The extraction ratio across the brain neat the end of the infusions (1.5 min) were 0.85 and 0.4 6 respectively. These data were also compared to analogous previously publi shed data for initially conscious sheep. The systemic kinetics of thiopento ne were little affected by halothane anaesthesia. For propofol, halothane a naesthesia was associated with a statistically significant reduction in cle arance (50% of awake), a slower initial half-life (247% of awake), and the emergence of a second slower half-life in some sheep. The cerebral kinetics of both drugs were subtly altered by halothane anaesthesia.