Thiopental Reduces Halothane MAC in Rats

The anesthetic contribution of specific plasma concentrations of thiopental has not been previously defined in laboratory animals. The plasma thiopental concentrations needed to reduce the anesthetic requirement for halothane by fractions of the minimum alveolar anesthetic concentration (MAC) were assessed in the rat. After steady-state thiopental plasma concentrations were established with a constant infusion, the tail-clamp technique was used to determine control MAC and the MAC of halothane with increasing concentrations of thiopental. We observed progressive reductions in halothane MAC. This required logarithmic increases in thiopental concentration rather than linear ones. A nonlinear reduction in anesthetic requirement was noted with an approximate 50% reduction in MAC at a thiopental plasma concentration of 7.4 micrograms/mL and an approximate 90% reduction at 32 micrograms/mL. Thiopental appears to provide essentially complete anesthesia in the rat model with a logarithmic contribution of increasing plasma concentrations.