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A1081
October 13, 2018
10/13/2018 1:15:00 PM - 10/13/2018 2:45:00 PM
Room North, Room 21
Inadvertent Hypoxic Inspired Gas Mixtures During Simulated Low Flow Anesthesia, High Fire Risk Cases
Samsun Lampotang, Ph.D., Monica Bursian Ortiz, Student, Johnathon Wakim, Student, Derek O'Hara, Student, David Lizdas, B.S., Anthony DeStephens, M.Eng., Nikolaus Gravenstein, M.D.
Univ of FL College of Med.-Faculty, Gainesville, Florida, United States
Disclosures: S. Lampotang: None. M. Bursian Ortiz: None. J. Wakim: None. D. O'Hara: None. D. Lizdas: None. A. DeStephens: None. N. Gravenstein: None.
Introduction: Hypoxic- inspired gas mixtures develop with low- flow anesthesia, e.g., 1-2 L/min, when the FGF is air.1,2 The Joint Commission Sentinel Event Alert (SEA) 29 recommends, consistent with patient needs, a fraction of inspired oxygen (FiO2) < 0.3, if there is surgical fire risk.3 SEA 29 should have specified the fraction of delivered oxygen, FDO2 < 0.3, instead of FiO2; we provide supporting data below.

Methods: Using an anesthesia machine (Aestiva S/5 with 7900 Ventilator;, GE Healthcare, Madison, WI), we simulated mechanical ventilation (volume -controlled) with FDO2 of 0.3 or 0.21 (air) of an oxygen-consuming Human Patient Simulator (HPS,; version B;, CAE Healthcare, Sarasota, FL) modeling a 70- kg male. Ventilation parameters were: VT 500 mL, RR 20 and 12, and I:E 1:2. On the HPS, we set the CO2 production factor to 1.0, respiratory quotient to 0.8, and O2 consumption to 225 and then 450 mL/min. We integrated an Arduino R3 microcontroller with a 0.1% resolution- calibrated oxygen analyzer (PSR 11-39-MD;, Analytical Industries, Inc., Pomona, CA) to record FiO2 every 0.5 s at the anesthesia machine inspiratory port. We began with a FGF of air equal to the MV. In subsequent trials, we decreased FGF by 0.5- L/min increments until we observed a hypoxic- inspired mixture (FiO2 < 0.21). Next, wWe then delivered FGF values of 2, 5, and 10 L/min with a 7:1 ratio of air to oxygen (0.3 FDO2) to simulate what would happen if we used FDO2, instead of FiO2, when following SEA 29.

Results: With air FGF values set below MV (6 L/min), FiO2 fell below the FDO2 of 0.21, delivering a hypoxic- inspired gas mixture (FiO2 < 0.21) at FGFs of 5.5, 5, 4.5, and 4 L/min. Similarly, with 0.3 FDO2 instead of air, significant dips in FiO2 were observed for trials with FGF

< MV (10 L/min); at 2 and 5 L/min FGF, FiO2 readings after 5 minutes were around ~0.19 and ~0.27, respectively. A FGF of 10 L/min, equivalent to the MV, yielded FiO2 = FDO2 value at 0.3.

Discussion: Our results demonstrate that a hypoxic- inspired gas mixture can develop with low- flow anesthesia (FGF 2 L/min) and FDO2 set at 0.3 for high- fire- risk cases. Our results confirm the FGF ≥ MV heuristic to avoid rebreathing and to ensure that FiO2 = FDO2. We also demonstrate that FiO2 is dependent on the FGF/MV ratio, varies over time, and can therefore be harder to set and maintain than FDO2. From a fire risk perspective, especially considering nasal cannulae where FDO2 is usually (but incorrectly and unsafely) 1.0 (100% O2) when using a traditional anesthesia machine auxiliary O2 flowmeter, FDO2 is the parameter of concern. Using the heuristic that FiO2 increases by 0.04 for every liter perL/ minute of O2, for a 2 L/min O2 inflow, FiO2 would be 0.29 but FDO2 would be 1.0 (> 0.3). Because O2 is denser than air, it can sink instead of completely mixing with air and create a pool of O2-enriched air with FO2 > 0.3 that promotes fires (the oxidizer leg of the fire triangle).

Traditional hypoxic guards have been considered to fail when hypoxic- inspired gas mixtures are delivered.1,2 Anesthesia providers must distinguish between FDO2 and FiO2 because “what you set is not always what you get.” Hypoxic guards are designed to control FDO2 with regard to the ratio of N2O to O2, and will not prevent FiO2 from becoming hypoxic. FiO2 is heavily influenced by the FGF/MV ratio (Figure 1).

References:

1.

J Clin Monit Comput 2014;29(4):491-497. doi:10.1007/s10877-014-9626-y

2.

J Clin Monit Comput 2016;30:251. https://doi.org/10.1007/s10877-015-9710-y

3.

https://www.jointcommission.org/assets/1/18/SEA_29.PDF



Figure 1

Copyright © 2018 American Society of Anesthesiologists