Uptake and distribution of halothane in dog blood

The uptake and distribution of halothane in dog blood were studied by analysing the whole blood and plasma concentrations of halothane in samples of arterial and mixed venous blood at different times after the induction of anesthesia with constant inspired halothane levels of 1.0, 1.5, 2.0, and 2.5%, respectively. In general a steady state was reached 2-3 h after induction. Comparison of the arterial blood concentration calculated from end-tidal halothane partial pressure with that determined by direct analysis of the blood indicated that halothane in the alveoli and halothane in arterial blood are not in thermodynamic equilibrium. The arterial halothane concentrations calculated from the end-tidal halothane partial pressure assuming equilibrium is much higher than those found experimentally; thus end-tidal halothane partial pressure is not a true measure of blood halothane concentration. The distribution of halothane between the plasma and cells in blood appeared to be sufficiently rapid to be independent of the approach to the steady state.