Abstract
Horses breathing spontaneously under halothane anaesthesia were subjected to expiratory resistance by the introduction of a water-trap into the expiratory limb of a circle absorber. Resistances of 10 and 20 cm H2O produced no significant increase in Pao2 (P > 0.05) during halothane/air and halothane/oxygen anaesthesia. The imposition of resistance was associated with an increase in Paco2and a significant increase in mixed venous Pco2. In three animals subjected to 20 cm H2O resistance under halothane/air anaesthesia, the cardiac output was reduced (P<0.01). It was concluded that the indiscriminate application of end-expiratory pressure has no place in routine equine anaesthesia.

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