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.
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Hendrickx JF, De Cooman S, Vandeput DM, Van Alphen J, Coddens J, Deloof T, De Wolf AM. Air–oxygen mixtures in circle systems. J Clin Anesth. 2001;13:461–4.
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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.
Jan Hendrickx, Philip Peyton, and Andre De Wolf are members of the NAVAt group.
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Appendix 1: Anesthesia machine standards
ASTM (American Society for Testing and Materials) standard F1850-00, clause 51.13.1 protection against accidental delivery of hypoxic gas mixtures
The anesthesia workstation shall be provided with a device to protect against an operator selected delivery of a mixture of oxygen and nitrous oxide having an oxygen concentration below 21 % oxygen in the fresh gas or in the inspiratory gas. If an override mechanism is provided to permit operator selection of oxygen concentration below 21 %, the activation of this mechanism shall be continuously indicated.
International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC) Standard EN60601-2-13, clause 51.102.2 Anaesthetic gas delivery system
*51.102.3 Protection against selection of an oxygen concentration below that of ambient air the anesthetic gas delivery system shall be provided with means to prevent the unintentional selection of a mixture of oxygen and nitrous oxide having an oxygen concentration below that of ambient air. If an operator-selected override mechanism is provided, its activation shall be clearly indicated. Compliance is checked by visual inspection and functional testing.
Appendix 2: Reconstruction of mass balances in case report
By assuming instantaneous gas mixing between the lung and anesthesia circuit, only one O2 and one N2O concentration has to be considered in the determination of mass balances. Let us assume that the combined volume of the circuit and the lungs is ≈5 L and that gases sampled by the gas analyzer are returned to the circuit. Analogous to what happened in the patient presented in the case report, the 5 L system contains 25 % O2 and 75 % N2O, or ≈1,250 mL O2 and ≈3,750 mL N2O just before lowering the FGF to 950 mL/min. The amounts added per minute with the lower FGF of 950 mL/min and an FDO2 of 27 % are 256 mL O2 and 694 mL N2O. The amounts removed per minute by the patient are ≈200 mL O2/min (approximate oxygen consumption by a healthy patient during general anesthesia) and almost no N2O (because N2O uptake after 6 h has almost ceased). Subtracting the amounts taken up by the patient from those added by the FGF yields a balance of +56 mL O2 and +694 mL N2O/min. Adding the latter amounts to those present in the circuit and lungs just before lowering the FGF yields 1,306 mL O2 (=1,250 + 56) and 3,750 mL N2O (=4,444 + 694), which results in an FO2 of 22.7 %. After scavenging the amount of gas administered in excess of patient uptake, 1,136 mL O2 and 3,864 mL N2O remain present in the circuit and lungs after 1 min, and the same mass balances as described during the first minute can be repeated for the next minute and so on. FO2 can be calculated to decrease from 25 to 22.7, 20.7, 19.0 and 17.5 % after 1, 2, 3, and 4 min, respectively—a course that resembles that of the FIO2 in our patient (Fig. 1).
Analogous mass balances can be used to calculate the predicted time course of change in FO2 in the system for a range of FGF, with VO2 = 0.2 L/min, VN2O = 0.1 L/min, and system volume = 5 L (Fig. 3).
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De Cooman, S., Schollaert, C., Hendrickx, J.F.A. et al. Hypoxic guard systems do not prevent rapid hypoxic inspired mixture formation. J Clin Monit Comput 29, 491–497 (2015). https://doi.org/10.1007/s10877-014-9626-y
- Machine standards
- Hypoxic guard system
- Hypoxic mixtures