• anaesthesia;
  • respiratory minute volume;
  • sevoflurane;
  • Stephens anaesthetic machine;
  • vaporizer in-circle



To investigate the sevoflurane concentrations produced within the Stephens anaesthetic machine circuit (vaporizer in-circle system) at different fresh gas flow rates (FGFRs), temperatures, vaporizer settings and vaporizer sleeve positions when used to anaesthetize dogs of different body sizes.

Study design

Experimental non-blinded studies.


Eighteen mixed breed dogs, weights 4–39 kg.


Anaesthetic induction with propofol was followed by maintenance with sevoflurane in oxygen via the Stephens anaesthetic machine. In study 1, the vaporizer setting, temperature and circuit FGFRs were altered with the vaporizer sleeve down (n = 3), or in separate experiments, up (n = 3). Delivered (Fi′SEVO) and expired sevoflurane concentrations were recorded. Study 2 determined the vaporizer settings (sleeve up) required to achieve predetermined multiples of minimal alveolar concentration (MAC) of Fi′SEVO when sevoflurane was delivered to dogs (n = 12) of different bodyweights and at different FGFRs.


Delivered concentrations of sevoflurane were sufficient to maintain anaesthesia in all dogs, regardless of bodyweight, FGFR, vaporizer temperature and sleeve position. Fi′SEVO increased with increasing temperature, when the vaporizer sleeve was down, when vaporizer setting was increased and when FGFR was decreased. As the FGFR increased or the dog's bodyweight decreased, higher vaporizer settings were required to produce the same Fi′SEVO. The median Stephens vaporizer settings to achieve an Fi′SEVO of 1.3 MAC ranged from 4.3 to 5.0 for a small dog (1–10 kg), 2.5 to 5.6 for a medium dog (15–25 kg) and 2.5 to 3.5 for a large dog (30–40 kg), depending on the FGFR.

Conclusion and clinical relevance

The Stephens anaesthetic machine can deliver to dogs, weighing 4 kg and above, concentrations of sevoflurane sufficient or in excess of that required to maintain anaesthesia, at temperatures from 10 to 35 °C, FGFRs of 1 to 5 times the patient's estimated metabolic oxygen requirement and at any vaporizer sleeve position.