Anaesthetic uptake and elimination: is there a difference between halothane and isoflurane in the dog?

In two groups of dogs, the uptake and elimination of halothane and isoflurane were studied using a closed-loop anaesthetic system which automatically controlled end-tidal halothane and isoflurane partial pressures at equi-MAC concentrations. Haemodynamic and respiratory variables were recorded and the anaesthetic partial pressures were measured in the inspired and expired air, as well as in the arterial, cerebrovenous and mixed venous blood. The controller delivered a higher inspired partial pressure of halothane than of isoflurane to compensate for the higher blood/gas partition coefficient. The partial pressures of halothane in the arterial, cerebrovenous and mixed venous blood increased at rates similar to those of isoflurane. During 160 min of uptake, the cerebral venous partial pressures remained significantly lower than the arterial partial pressures for both agents. On discontinuation of the anaesthetic, the partial pressures of halothane and isoflurane decreased at equal rates in arterial, cerebrovenous and mixed venous blood, and in end-tidal gas. It was concluded that the rate of uptake of isoflurane is more rapid than that of halothane from the alveolar space to the blood, but not from the blood to brain tissue. The rates of elimination from brain tissue and from blood were found to be similar for both agents.