Development and performance of a two-step desflurane-O₂/N₂O fresh gas flow sequence

Study Objective: To determine if the previously described single-step O₂/N₂O fresh gas flow (FGF) sequence could be combined with a simple desflurane vaporizer (F_D) sequence to maintain the end-expired desflurane (F_Ades) at 4.5% with the anesthesia delivery unit machine (ADU Anesthesia Machine(R); General Electric, Helsinki, Finland). Design: Prospective randomized clinical study. Setting: Onze Lieve Vrouw Hospital, Aalst, Belgium, a large teaching hospital. Patients: 42 ASA physical status I and II patients requiring general endotracheal anesthesia and controlled mechanical ventilation. Interventions: In 18 patients undergoing general anesthesia with controlled mechanical ventilation, F_D was determined to maintain F_Ades at 4.5% with O₂/N₂O FGF of two and 4 L per minute for three minutes and 0.3 and 0.4 L per minute thereafter. Using the same FGF sequence, we prospectively tested the F_D schedule that approached this observed F_D pattern with the fewest possible adjustments in another 24 patients. Main Results: F_D of 6.5% for 15 minutes followed by 5.5% thereafter approximated the observed F_D course well. When it was prospectively tested, the median (25th, 75th percentiles) performance error was -1% (-5.1%, 5.2%); absolute performance error, 7.1% (3.9%, 9.5%); divergence, -6.6% per hour (23.1%, 3.1%/h); and wobble, 2.2% (1.8%, 3.2%). Because F_Ades increased above 4.9%, F_D was decreased in 5 patients after 23 minutes (0.5% decrement once or twice); in two patients, F_D was temporarily increased. In one patient, FGF was temporarily increased because the bellows volume became insufficient. Conclusions: One O₂/N₂O rotameter FGF setting change from 6 to 0.7 L per minute after three minutes and one desflurane F_D change from 6.5% to 5.5% after 15 minutes maintained anesthetic gas concentrations within predictable and clinically acceptable limits during the first 20 minutes.