Derivation and prospective testing of a two-step sevoflurane-O ₂-N₂O low fresh gas flow sequence

Simple vaporiser setting (F_D) and fresh gas flow (FGF) sequences make the practice of low-flow anaesthesia not only possible but also easy to achieve. We sought to derive a sevoflurane F_D sequence that maintains the end-expired sevoflurane concentration (F_Asevo) at 1.3% using the fewest possible number of F_D adjustments with a previously described O₂-N₂O FGF sequence that allows early FGF reduction to 0.7 l.min^-1. In 18 ASA physical status I to II patients, F _D was determined to maintain F_Asevo at 1.3% with 2 l.min^-1 O₂ and 4 l.min^-1 N₂O FGF for three minutes, and with 0.3 and 0.4 l.min^-1 thereafter. Using the same FGF sequence, the F_D schedule that approached the 1.3% F _Asevo pattern with the fewest possible adjustments was prospectively tested in another 18 patients. The following F_D sequence approximated the F_D course well: 2% from zero to three minutes, 2.6% from three to 15 minutes and 2.0% after 15 minutes. When prospectively tested, median (25th; 75th percentile) performance error was 0.8 (-2.9; 5.9)%, absolute performance error 6.7 (3.3; 10.6)%, divergence 18.2 (-5.6; 27.4)%.h^-1 and wobble 4.4 (1.7; 8.1)%. In one patient, FGF had to be temporarily increased for four minutes. One O₂/N₂O rotameter FGF setting change from 6 to 0.7 l.min^-1 at three minutes and two sevoflurane F _D changes at three and 15 minutes maintained predictable anaesthetic gas concentrations during the first 45 minutes in all but one patient in our study.