title: Prevention of airway fires: do not overlook the expired oxygen concentration.
abstract: BACKGROUND
  It is generally accepted that when an ignition source is used the inspired oxygen concentration (FIO2) should be <30% in the breathing circuit to help prevent airway fires. The time and conditions required to reduce a high O2% in the breathing circuit to <30% has not yet been systematically studied.
  
  
  METHODS
  We evaluated the inspired and expired circuit oxygen concentration response times of an Aestiva Avance S/5 anesthesia machine to reach an FIO2 of <30% from a starting FIO2 of 100% and 60% after reducing the FIO2 to 21%. The circuit was connected to a human patient simulator which has a functional residual capacity of 2 L, total lung capacity of 2.8 L, an oxygen consumption of 200 mL/min, and respiratory quotient of 0.8. Fresh gas flow (FGF) inputs of 2 L/min and 5 L/min were chosen to represent a spectrum of typical clinical FGF rates. Minute ventilation was set at 4 L/min. Determining the requisite median time to reach an O2 concentration of <30% in the breathing circuit was the primary aim of the study.
  
  
  RESULTS
  The median times (1st-99th percent confidence interval) required to achieve inspiratory and expiratory oxygen concentrations of <30% with the extended circuit configuration when starting at 60% for 5 L FGFs were 35 (32-36) and 104 (88-122) seconds, respectively. With 2 L FGF, these median times increased to 303 (291-313) and 255 (232-278) seconds, respectively. A shortened circuit configuration (P = 0.006) and higher FGF flow rate (P < 0.0001) were noted to be factors decreasing the median time required to achieve an oxygen concentration of <30%.
  
  
  CONCLUSIONS
  Both inspired and expired circuit oxygen concentration may take minutes to decrease to <30% depending on circuit length, FGF rate, and starting circuit oxygen concentration. During the reduction in FIO2, the expiratory oxygen concentration may be >30% for a considerable time after the FIO2 is in a "safe" range. An increased expired oxygen concentration should also be considered an airway fire risk, and patient care protocols may need to be modified based on future studies.
authors.affiliation: From the Department of Anesthesiology, University of Florida College of Medicine, Gainesville, Florida.
authors.forename: Matthew
authors.initials: M
authors.lastname: Remz
authors.forename: Isaac
authors.initials: I
authors.lastname: Luria
authors.forename: Michael
authors.initials: M
authors.lastname: Gravenstein
authors.forename: Scott D
authors.initials: SD
authors.lastname: Rice
authors.forename: Timothy E
authors.initials: TE
authors.lastname: Morey
authors.forename: Nikolaus
authors.initials: N
authors.lastname: Gravenstein
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Rice
mesh_terms: D000764:Anesthesia, Closed-Circuit / Q000379:methods*; D018685:Anesthetics, Inhalation / Q000008:administration & dosage*; D003198:Computer Simulation; D004305:Dose-Response Relationship, Drug; D045853:Exhalation; D005740:Gases; D006801:Humans; D001239:Inhalation; D008348:Manikins; D010100:Oxygen / Q000008:administration & dosage*; D010101:Oxygen Consumption; D061214:Patient Safety; D012121:Respiration, Artificial / Q000295:instrumentation* / Q000379:methods*; D013997:Time Factors
chemical_list: D018685:Anesthetics, Inhalation; D005740:Gases; D010100:Oxygen