The effect of a 50% inspired mixture of nitrous oxide on arterial oxygen tension in spontaneously breathing horses anaesthetised with halothane

Abstract

The effect of nitrous oxide (N 2 O) on arterial partial pressure of oxygen (P a O 2 ) was evaluated in 20 adult horses anaesthetised with halothane. A fresh gas flow rate of 20ml/kg/min, comprising a 1:1 N 2 O/oxygen (O 2 ) mixture, was supplied via the rotameter flowmeters of an anaesthetic machine to a large animal breathing system. The horses breathed spontaneously from the circuit immediately after endotracheal intubation. Ten horses were subsequently positioned in lateral recumbency and ten in dorsal recumbency. A further twenty adult horses were anaesthetised with halothane and acted as controls; halothane in 20mls/kg/min of O 2 being supplied to the same breathing system. Fifty percent NO caused significant decreases in P a O 2 for horses in lateral and dorsal recumbency. However when administered to horses in lateral recumbency it did not promote arterial hypoxaemia. There was a higher risk of intraopera- tive arterial hypoxaemia (P a O 2 < 8.6kPa) associated with its use in spontaneously breathing horses in dorsal recumbency. Arterial hypoxaemia occurred in all horses during the first fifteen minutes of recovery but when N 2 O was discontinued, halothane in oxygen supplied to the breathing circuit for five minutes at a flow rate of 20ml/kg/minute was sufficient to ensure that diffusion hypoxia did not occur. The magnitude of the hypoxaemia was not signficantly different between the groups. The time taken to adopt sternal recumbency was significantly shorter in the horses that had received N 2 O.