P006: INADVERTENT HYPOXIC INSPIRED GAS MIXTURES DURING LOW FLOW ANESTHESIA, INCLUDING HIGH FIRE RISK CASES
Samsun Lampotang, PhD, Monica Burisan Ortiz, Jonathan Wakim, Derek O'Hara, Nik Gravenstein, MD, Dave Lizdas; University of Florida
Introduction: Fresh gas flow (FGF) and the fraction of delivered oxygen (FDO2) set on an anesthesia machine are operator-selected based on a patient’s particular conditions, phase of the case, type of procedure, surgical fire risk and operator preference. The Joint Commission recommends FiO2 of ≤ 0.3, consistent with patient needs, to mitigate surgical fire risk which may lead some to use air (FiO2 0.21) to further reduce fire risk. Low flow anesthesia is recommended for multiple reasons, including preservation of costly inhaled agents and the environment. We explore whether low flow anesthesia at a FGF of 30% FDO2 or air (21% FDO2) results in inadvertent delivery of hypoxic inspired mixtures (FiO2 < 0.21) in the Aestiva (GE, Madison, WI) anesthesia machine. A secondary outcome of our study is to verify the FGF ≥ MV relationship.
Methods: We used human patient simulator (HPS; version B, CAE Healthcare/Medical Education Technologies Inc., Sarasota, Florida) to model a 70 kg male; therefore IRB approval was not required. We used an anesthesia machine (Aestiva S/5 with 7900 Ventilator, GE Healthcare, Madison, WI) to mechanically ventilate the intubated HPS during each trial. We set HPS CO2 production factor to 1.0, respiratory quotient to 0.8, and minute ventilation to 10 L/min. We integrated an Arduino R3 microcontroller with a 0.1% resolution oxygen analyzer (PSR 11-39-MD, Analytical Industries, Inc., Pomona, CA) calibrated with laboratory calibration gases to record FDO2 at the anesthesia machine inspiratory port. Each of 3 repeated trials lasted for 5 min. We automatically recorded data points every 0.5 sec. We set O2 consumption to 450 mL/min and we began with FGF of air (FDO2 = 0.21) equal to the MV. In subsequent trials, we decreased FGF by 0.5 L/min increments until we began to see a hypoxic inspired mixture (FiO2 < 0.21). As an additional element of the study, we delivered FGF values of 2 L/min and 5 L/min with a 7:1 ratio of air to oxygen to simulate Joint Commission recommendations of maintaining FDO2 no greater than 30% when there is surgical fire risk.
Results: Using air FGF and setting O2 consumption to 225 ml/min, a MV that is ≤ FGF was the most appropriate setting to prevent rebreathing over 5 minutes. FGF settings ≤ MV fell under the FiO2 value of 0.21 (corresponding to a hypoxic inspired mixture) during a timeframe of 5 minutes which. With FDO2 at 30%, MV set to 10 L/min, and O2 consumption set to 450 ml/min, significant dips in FiO2 were also observed; at 2 and 5 L/min FGF, FiO2 readings after 5 minutes were around ~0.19 and ~0.27 respectively.
Conclusions: Our preliminary results confirm that the minimum FGF must be equal to or greater than the MV to avoid rebreathing and therefore to prevent delivering a hypoxic inspired gas mixture when FGF is air. It is counterintuitive that oxygen inflows > 200 ml/min (≥metabolic O2 consumption) are not sufficient to prevent hypoxic inspired mixtures from developing.