Fires require fuel, oxygen, and a heat source. To minimize the risk of an airway fire, we can reduce the available oxygen. If the inspired oxygen concentration is 60 or 100%, how long does it take to get the ambient oxygen concentration <30%?
Dr. Mark J. Rice, MD, Department of Anesthesiology,University of Florida College of Medicine, Gainesville, FL and coauthors used an Aestiva Avance S/5™ anesthesia machine to determine how fresh gas flow, circuit length, and inspired oxygen determine time to reach an FiO2 <30% with an inspired oxygen concentration of 60 or 100%. Their work is published in this month’s issue of Anesthesia & Analgesia in the manuscript “Prevention of Airway Fires: Do Not Overlook the Expired Oxygen Concentration.”
The authors varied breathing circuit volume by fully extending and compressing circuit lengths. Initial oxygen concentration was set to 60% or 100%. Fresh gas flows were either 2 or 5 L/min. They used a human patient simulator with a functional residual capacity (FRC) of 2 L, total lung capacity of 2.8 L, an oxygen consumption of 200 mL/min, and respiratory quotient of 0.8. The median times to achieve inspiratory and expiratory oxygen concentrations of <30% with the extended circuit configuration when starting from 60% for 5 L FGF were 35 and 104 seconds, respectively. To reach <30% oxygen with the extended circuit configuration when starting from 60% and 2 L FGF took 303 sec for the inspiratory circuit and 255 seconds for the expiratory circuit. Times were longer when going from 100%. Times were less when a compressed, shortened circuit was used. With higher fresh gas flows, times to reach <30% were reduced.
Though the findings are not surprising, they do show the expected time course to lower both inspired and exhaled oxygen. What is also apparent is that though the inspired oxygen concentration is less than 30%, the exhaled oxygen concentration might still be higher. Given the disastrous consequences of an airway fire, one might wonder why we ever need to use high oxygen concentrations for most patients. Before oximeters were used, there might have been justification for using higher inspired oxygen concentrations. Why do we maintain oxygen concentrations greater than 30% in patients who are 100% saturated?