ETCO2 Capnography

Capnography – non-invasive, continuous measurement of inhaled and exhaled CO2

End-tidal CO2 (EtCO2) – maximum CO2 at end of exhalation.



Normal Capnogram

During inspiration, CO2 is negligible and recorded at zero baseline.


  1. Phase 1 (A to B)
    1. exhalation indicated by the first
    2. represents gas exhaled from upper airways
  2. Phase 2 (B to C)
    1. rapid rise in CO2 concentration as anatomical dead space is replaced with alveolar gas
  3. Phase 3 (C to D )
    1. alveolar gas passes the CO2 sensor
    2. capnograph flattens out, “alveolar plateau”
    3. End-tidal CO2 = value taken at the end of exhalation
  4. Phase 4 (D to E)
    1. a rapid downward stroke
    2. fresh gas passing the sensor is essentially free of carbon dioxide



  • Hyperventilation
  • high RR reduces amount of carbon dioxide in the exhaled air
  • waveform with regular shape but with a plateau below normal (CO2 deficiency)
  • check for hyperventilation, decreased pulmonary perfusion, hypothermia and decreased metabolism


2. Hypoventilation

  • regular shape with plateau above normal
  • indicates increased EtCO2 secondary to hypoventilation, respiratory depressant drugs, or increased metabolismHypoventilation

3. Bronchospasm

  • waveform develops a “shark fin” shapeBronchospasm

4, Shallow breathing

  • even though blood and alveolar CO2 are elevated, EtCO2 will appear to decrease
  • When the patient takes a deep breath, the carbon dioxide in the airway system is exhaled and the EtCO2 level is elevatedShallow breathing

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