Hypoxic guard systems do not prevent rapid hypoxic inspired mixture formation

Abstract

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.

Appendices

Appendix 1: Anesthesia machine standards

1.1 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.

1.2 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 F_DO₂ 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 F_IO₂ 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).