Derivation and Prospective Testing of a Two-step Sevoflurane-O 2 -N 2 O Low Fresh Gas Flow Sequence

Simple vaporiser setting (FD) 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 A sevo) at 1.3% using the fewest possible number of F D adjustments with a previously described O 2 -N 2 O FGF sequence that allows early FGF reduction to 0.7 l.min −1 . In 18 ASA physical status I to II patients, FD was determined to maintain F A sevo at 1.3% with 2 l.min −1 O 2 and 4 l.min −1 N 2 O 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 A sevo 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 O 2 /N 2 O 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.