We use expandable circle breathing systems (Compact™, Intersurgical, Berkshire, UK) in our paediatric general anaesthesia magnetic resonance imaging (MRI) lists. We noticed that our circuits were not always fully extended until just before the final slide into the MRI scanner and that following expansion there was often a significant reduction in the measured vapour concentrations. Circuit expansion increases the circuit volume and without adjustment of gas flow rates and vapour concentration causes the vapour concentrations to fall, risking accidental light anaesthesia. The reduction in vapour concentration will depend on the proportional change in volume of the patient/circuit system and the fresh gas flow rates. This effect can be exaggerated in paediatric patients.
We modelled a situation using a 1‐l reservoir bag as an artificial lung, oxygen 100% at a rate of 6 l.min‐1 and sevoflurane 8%. Mechanical ventilation was set to 24 breaths per min and tidal volume 200 ml. After 3 min, a steady concentration of sevoflurane 7.4% was achieved. We then reduced the fresh gas flow rate to 1 l.min ‐1 and fully expanded both limbs of the circuit. Ventilator settings were unchanged. The sevoflurane concentration fell over the next 2 min to 2.4%—a 75% reduction. This is consistent with the manufacturer’s information that the circuit expands approximately five times its compressed length (and therefore volume).
The manufacturer’s instructions for use advise expanding the circuit to the length required before inducing general anaesthesia primarily to assess circuit compliance and check for leaks, but we consider the risk of unplanned reduction in vapour concentration should also be highlighted.
We thank Mr S. Duggan, Product Manager, Intersurgical Ltd., for his assistance. No external funding or competing interests declared.