Capnography Waveform Interpretation


  • Capnography waveform interpretation can be used for diagnosis and ventilator-trouble shooting
  • The CO2 waveform can be analyzed for 5 characteristics:–Height–Frequency–Rhythm–Baseline–Shape


4 phases

  • Phase I (inspiratory baseline) reflects inspired gas, which is normally devoid of carbon dioxide.
  • Phase II (expiratory upstroke) is the transition between VDana, which does not participate in gas exchange, and alveolar gas from the respiratory bronchioles and alveoli.
  • Phase III is the alveolar plateau. Traditionally, PCO2 of the last alveolar gas sampled at the airway opening is called the PETCO2.
  • Phase 0  is the inspiratory downstroke, the beginning of the next inspiration

Other features:

  • Normal end-tidal PCO2 is approximately: 38 mmHg or 5%
  • the alpha angle is the transition from Phase II to Phase III
  • the beta angle is the transition from Phase III to Phase I (the start of inspiration)
  • an additional phase IV (terminal upstroke before phase 0) may be seen in pregnancy

ETCO2 only represents alveolar CO2 when a relatively horizontal plateau phase (phase III) is seen.



  • Ventilator disconnection
  • Airway misplaced – extubation, oesophageal intubation
  • Capnograph not connected to circuit
  • Respiratory/Cardiac arrest
  • Apnoea test in brain death dead patient
  • Capnongraphy obstruction


CO2 Production

  • Fever
  • Sodium bicarbonate
  • Tourniquet release
  • Venous CO2 embolism
  • Overfeeding

Pulmonary perfusion

  • Increased cardiac output
  • Increased blood pressure

Alveolar ventilation

  • Hypoventilation
  • Bronchial intubation
  • Partial airway obstruction
  • Rebreathing

Apparatus malfunctioning

  • Exhausted CO2 absorber
  • Inadequate fresh gas flows
  • Leaks in ventilator tubing
  • Ventilator malfunctioning


CO2 production

  • Hypothermia

Pulmonary perfusion

  • Reduced cardiac output
  • Hypotension
  • Hypovolaemia
  • Pulmonary embolism
  • Cardiac arrest

Alveolar ventilation

  • Hyperventilation
  • Apnoea
  • Total airway obstruction (note high airway pressures)
  • Extubation

Apparatus malfunctioning

  • Circuit disconnection (note low airway pressures)
  • Leaks in sampling tube
  • Ventilator malfunctioning


  • Kinked ET tube
  • CO2 analyzer defective
  • Total disconnection
  • Ventilator defective


  • Calibration error
  • CO2 absorber saturated (check capnograph with room air)
  • Water drops in analyzer or condensation in airway adapter


  • ROSC during cardiac arrest
  • correction of ET tube obstruction


  • CO2 rebreathing (e.g. soda lime exhaustion)
  • Contamination of CO2 monitor (sudden elevation of base line and top line)
  •  Inspiratory valve malfunction (elevation of the base line, prolongation of down stroke, prolongation of phase III)


See Figure 2 of Kodali 2013 and Interpret your capnogram at Capnography.com

  • esophageal intubation (<6 waveforms of decreasing height)
  • right main bronchus intubation (ETCO2 can increase, decrease or stay the same, can also cause a bifid capnogram)
  • curare cleft (partially paralysed patient on mechanical ventilation)
  • cardiogenic oscillations
  • camel hump (seen in patients in lateral position)
  • Rebreathing capnogram of Mapleson D circuit
  • phase IV in pregnancy
  • Dilution of expiratory gases by the forward flow of fresh gases during the later part of expiration when expiratory flow rate decreases below the forward gas flow rate
  • sometimes see reverse phase 3 slope seen in patients with emphysema (alveolar destruction leads to rapid delivery of CO2 to airways)
  • Sticking inspiratory valve
  • expiratory valve malfunction
  • mandatory versus spontaneously triggered breaths
  • dual capnogram in lung transplants
  • air leak
  • malignant hyperthermia
  • air / oxygen dilution during mask sampling of spontaneously breathing patients

References and Links


Journal articles

  • Blanch L, Romero PV, Lucangelo U. Volumetric capnography in the mechanically ventilated patient. Minerva Anestesiol. 2006 Jun;72(6):577-85. PMID: 16682932.
  • Kodali BS. Capnography outside the operating rooms. Anesthesiology. 2013 Jan;118(1):192-201. PMID: 23221867.
  • Thompson JE, Jaffe MB. Capnographic waveforms in the mechanically ventilated patient. Respir Care. 2005 Jan;50(1):100-8; discussion 108-9.PMID: 15636648.

FOAM and web resources

CCC 700 6

Critical Care


Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of three amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

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