Concentrations and costs of a thirty second Priming technique with sevoflurane using the circle circuit

Priming the circuit for an application of inhalation agent depends on the time taken, the fresh gas flow (FGF) rate, the concentration setting of the agent on the vaporizer, and the priming technique. Based on previous studies, the priming time for a desirable concentration of sevoflurane varied between 30 seconds and 5 minutes. Remarkably, although the cost of priming with sevoflurane was lowest at 30 seconds, concentrations in the circuit were not clearly stated. The aim of this study was to test the success (and cost) of a 30-second priming technique to achieve a 4.5 per cent sevoflurane concentration in the circle circuit. Analyses were done on 20 samplings. Each time the adjustable pressure-limiting (APL) valve on the assembly was closed, the 2-liter reservoir bag emptied and the patient end occluded. For 30 seconds, the circuit was filled with oxygen (8 L x min(-1)) and 8 per cent sevoflurane. The pressure in the circuit was kept constant at 10 mbar by partially releasing the excess gas at the patient end. After 30 seconds, the sevoflurane concentration at the patient end of the circuit was analyzed and the highest concentration recorded. The cost of the sevoflurane used was then calculated. The sevoflurane concentration in all of the samples was more than 4.5 per cent (mean 6.40%; SD 0.30%). The average amount of liquid sevoflurane used was 1.8 ml at a cost of 42.30 baht (0.9 US dollars). A 30-second priming technique was sufficient to achieve the required concentration of sevoflurane in the circle circuit thus reducing costs for gaseous induction.