4. Breathing System

4.1 Calibrate O2 monitor(s)

  • Calibrate to read 21% (sensor in room air)
  • Set low O2 alarm limit to more than 21% and make sure an alarm occurs
  • Reinstall sensor - flush circuit with O2 flush; verify monitor reads close to 100%
  • cannot calibrate - faulty fuel cell
  • crossover between O2 supply and other gas - hospital piping or in gas machine
  • wrong gas in central supply / cylinder

4.2 Check initial status of breathing system

  • Circuit complete, undamaged, unobstructed
  • Verify CO2 absorbent is adequate
  • Install PEEP valve if needed
  • Attach gas sampling line to circuit
  • Set all flows to zero (or minimum)


  • manufacturing defects causing leaks or obstruction
  • foreign object obstructing e.g. sampling port cap inside elbow
  • PEEP valve on inspiratory side

4.3 Check function of unidirectional valves

  • Temporarily remove circuit from absorber
  • Check inspiratory valve
    • Attach a spare reservoir bag to the inspiratory outlet of the absorber and occlude expiratory inlet with thumb
    • Fill bag with O2 flush until pressure gauge reads 40 cm H2O - pressure holds
    • Squeeze and release test bag - pressure rises but returns to 40 cm H2O - no loss



  • missing, incompetent valves or incorrectly seated
  • Check expiratory valve
    • Attach spare reservoir bag to the expiratory inlet of the absorber and occlude inspiratory outlet with thumb
    • Pressurize with O2 flush to 40 cm H2O and bag on expiratory inlet stays deflated

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