Hypoxic guard systems do not prevent rapid hypoxic inspired mixture formation

Because a case report and theoretical mass balances suggested that hypoxic guard systems may not prevent the formation of hypoxic inspired mixtures (F_IO₂ ≤ 21 %) over the clinically used fresh gas flow (FGF) range, we measured F_IO₂ over a wide range of hypoxic guard limits for O₂/N₂O and O₂/air mixtures. After IRB approval, 16 ASA I–II patients received sevoflurane in either O₂/N₂O (n = 8) or O₂/air (n = 8) using a Zeus^® anesthesia machine in the conventional mode. After using an 8 L/min FGF with F_DO₂ = 25 % for 10 min, the following hypoxic guard limits were tested for 4 min each, expressed as [total FGF in L/min; F_DO₂ in %]: [0.3;85], [0.4;65], [0.5;50], [0.7;36], [0.85;30], [1.0;25], [1.25;25], [1.5;25], [2;25], [3;25], [5;25], and [8;25]. In between these [FGF;F_DO₂] combinations, 8 L/min FGF with 25 % O₂ was used for 4 min to return to the same baseline F_IO₂ (25 %) before the start of the next combination. This sequence was studied once in each patient receiving O₂/air (n = 8), but twice in each patient who received O₂/N₂O (n = 8) to examine the effect of decreasing N₂O uptake over time, resulting in three groups: early O₂/N₂O, late O₂/N₂O, and O₂/air group. The [FGF;F_DO₂]–F_IO₂ relationship was examined. The overall [FGF;F_DO₂]–F_IO₂ relationship in the three groups was similar. In all 1, 1.25, and 1.5 L/min FGF groups, F_IO₂ decreased below 21 % in all but one patient; this occurred within 1 min in at least one patient. In the 0.7 L/min O₂/air group and the 3 L/min late O₂/N₂O and O₂/air groups, F_IO₂ decreased below 21 % in one patient. Current hypoxic guard systems do not reliably prevent a hypoxic F_IO₂ with O₂/N₂O and O₂/air mixtures, particularly between 0.7 and 3 L/min.