Hypoxic guard systems do not prevent rapid hypoxic inspired mixture formation
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Because a case report and theoretical mass balances suggested that hypoxic guard systems may not prevent the formation of hypoxic inspired mixtures (FIO2 ≤ 21 %) over the clinically used fresh gas flow (FGF) range, we measured FIO2 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 FDO2 = 25 % for 10 min, the following hypoxic guard limits were tested for 4 min each, expressed as [total FGF in L/min; FDO2 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;FDO2] combinations, 8 L/min FGF with 25 % O2 was used for 4 min to return to the same baseline FIO2 (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;FDO2]–FIO2 relationship was examined. The overall [FGF;FDO2]–FIO2 relationship in the three groups was similar. In all 1, 1.25, and 1.5 L/min FGF groups, FIO2 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, FIO2 decreased below 21 % in one patient. Current hypoxic guard systems do not reliably prevent a hypoxic FIO2 with O2/N2O and O2/air mixtures, particularly between 0.7 and 3 L/min.
KeywordsMachine standards Hypoxic guard system Hypoxia Hypoxic mixtures Rebreathing
The experiments comply with the current laws of Belgium—all patients were enrolled at the OLV hospital, Aalst, Belgium. The volunteer on the accompanying video had given consent to both the experiment and publication of the video.
Conflict of interest
Jan Hendrickx has received lecture fees, travel support, equipment loans, and support for the NAVAT meetings from AbbVie, Acertys, Air Liquide, Allied healthcare, Armstrong Medical, Baxter, Draeger, GE, Hospithera, Heinen und Lowensein, Intersurgical, Maquet, MDMS, MEDEC, Micropore. Molecular, NWS, Philips, Quantum Medical.
Supplementary material 1 (MPG 135950 kb) Electronic supplementary material: Breathing air from a circle breathing system with a 1 L/min FGF. A volunteer is breathing air from a circle breathing system with 8 L/min FGF: after switching the flow to 1 L/min, FIO2 decreases to 12 % within 2 min
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