Hypoxic guard systems do not prevent rapid hypoxic inspired mixture formation

• Published in: Journal of clinical monitoring and computing 2015-08, Vol.29 (4), p.491-497
• Main Authors: De Cooman, Sofie, Schollaert, Caroline, Hendrickx, Jan F. A., Peyton, Philip J., Van Zundert, Tom, De Wolf, Andre M.
• Format: Article
• Language: English
• Subjects: Aged; Air; Anesthesia - methods; Anesthesia, Closed-Circuit - methods; Anesthesia, Inhalation - methods; Anesthesiology; Anesthetics; Anesthetics, Inhalation; Critical Care Medicine; Female; Formations; Gas flow; Guards; Health Sciences; Humans; Hypoxia - diagnosis; Hypoxia - prevention & control; Intensive; Medicine; Medicine & Public Health; Methyl Ethers - therapeutic use; Middle Aged; Monitoring; Monitoring, Intraoperative - instrumentation; Monitoring, Intraoperative - methods; Nitrous oxides; Original Research; Oxygen - chemistry; Patients; Receiving; Reproducibility of Results; Statistics for Life Sciences; Time Factors; Uptakes
• ISSN: 1387-1307; 1573-2614
• DOI: 10.1007/s10877-014-9626-y

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