Objective: To measure the concentrations of desflurane at different sites of the anesthesia circuit by varying fresh gas flow (FGF) rates but fixing expired concentration.

Material and Method: Thirty ASA PS I-II adult patients were enrolled in this crossover study. After induction of anesthesia and ten minutes of wash-in period, the flow meters of oxygen and air were then adjusted to maintain FiO2 at 0.3 with the random sequences of FGF rates at 0.5, 1 and 2 L.min-1. Desflurane vaporizer was adjusted to obtain 5% end-tidal desflurane concentration (FeDES) throughout the study period. After FeDES reached the target and was stable for 20 minutes, inspired concentration of desflurane (FiDES) and delivered desflurane concentration at fresh gas outlet (FdDES) were measured. Lastly, the consumption of desflurane was calculated.

Results: FdDES was higher than FiDES in every FGF rates. FdDES at FGF 0.5 L.min-1 (6.13±0.12) was significantly higher than FdDES at 1 and 2 L.min-1 (5.68±0.08, 5.54±0.07, respectively), but not significantly different between FGF 1 and 2 L.min-1. FeDES/FdDES at FGF 0.5, 1 and 2 L.min-1 were 0.82±0.014, 0.88±0.012 and 0.87±0.011, respectively. There was no significant difference of FeDES/FdDES between FGF 1 and 2 L.min-1, but there was significant difference between FGF 1 and 0.5 L.min-1 with the p-value <0.001. The calculated liquid desflurane consumption per hour at FGF rate of 0.5, 1 and 2 L.min-1 were 8.77±0.17, 16.28±0.24 and 31.73±0.41 mL.hr-1.

Conclusion: Using FGF 2 L.min-1 has no advantage over FGF 1 L.min-1, because they both have the similar FdDES. Regarding at FGF 0.5 and 1 L.min-1, the delivered concentration has to be increased to obtain the desired expired concentration with more intense at FGF 0.5 L.min-1 because there are more discrepancies between FdDES and FeDES.

Keywords: Low flow anesthesia, Desflurane, Concentration, Consumption