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

To determine if the previously described single-step O2/N2O fresh gas flow (FGF) sequence could be combined with a simple desflurane vaporizer (FD) sequence to maintain the end-expired desflurane (FAdes) at 4.5% with the anesthesia delivery unit machine (ADU Anesthesia Machine(R); General Electric, Helsinki, Finland). Prospective randomized clinical study. Onze Lieve Vrouw Hospital, Aalst, Belgium, a large teaching hospital. 42 ASA physical status I and II patients requiring general endotracheal anesthesia and controlled mechanical ventilation. In 18 patients undergoing general anesthesia with controlled mechanical ventilation, FD was determined to maintain FAdes 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 FD schedule that approached this observed FD pattern with the fewest possible adjustments in another 24 patients. FD of 6.5% for 15 minutes followed by 5.5% thereafter approximated the observed FD 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 FAdes increased above 4.9%, FD was decreased in 5 patients after 23 minutes (0.5% decrement once or twice); in two patients, FD was temporarily increased. In one patient, FGF was temporarily increased because the bellows volume became insufficient. One O2/N2O rotameter FGF setting change from 6 to 0.7 L per minute after three minutes and one desflurane FD 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.