Objective: To determine whether predictable alveolar concentrations of sevoflurane are reliably produced in dogs when liquid sevoflurane is injected into closed circuit breathing systems, as calculated by Lowe's square-root-of-time anaesthetic uptake model, and to confirm the validity of the model using soda lime and calcium hydroxide lime.
Study design: Prospective clinical study.
Animals: Eleven healthy dogs with a mean body mass of 34 +/- 9 kg scheduled for pelvic limb orthopaedic surgery.
Materials and methods: Following pre-anaesthetic medication, anaesthesia was induced with propofol and maintained with sevoflurane in a closed circle system. Epidural anaesthesia was performed with morphine and bupivacaine. Liquid sevoflurane was injected into the circuit by syringe, using dosages and time intervals derived from Lowe's square-root-of-time anaesthetic uptake model. The target alveolar concentration chosen was 1.1 x MAC (2.6% end-tidal sevoflurane). Either soda lime (group S; n = 6) or calcium hydroxide lime (Amsorb; group A; n = 5) were used for CO(2) absorption. Sevoflurane concentration and the respiratory gas composition were measured with an infrared gas analyser.
Results: End-tidal sevoflurane concentrations were close to the predicted value of 2.6% at 9 minutes (2.53 +/- 0.1% group S; 2.60 +/- 0.26% group A) and 16 minutes (2.55 +/- 0.30 group S; 2.52 +/- 0.28% group A) but declined thereafter to reach 50% (group S) and 64% (group A) of the predicted value at 121 minutes. There was a constant trend towards higher end-tidal sevoflurane concentrations in group A but the difference was not statistically significant.
Conclusions: The square-root-of-time model leads to significantly lower alveolar concentrations than expected, suggesting that the rate of sevoflurane uptake in dogs declines less rapidly than predicted. The use of Amsorb tends to reduce the deviation from predicted concentrations.
Clinical relevance: The model used in this study provided only an approximate guide to the volume of liquid sevoflurane required. Consequently, the definitive dose schedule must be based on measured anaesthetic concentrations and clinical monitoring.