Understanding capnography waveforms

author: Vivian Deng
Understanding capnography waveforms

Waveform phases and analysis

The capnogram, or waveform generated by the ETCO2 monitor is one of the most useful tools available to assess a patient’s ventilation and airway status, and can provide information on metabolism and perfusion.

While reviewing the next steps, please refer to the capnogram picture/sample above.

Is there a waveform present?
• If there is no waveform, then an immediate patient assessment is needed.
• The presence of a waveform verifies a patent airway and that the patient is ventilating. The next steps will help determine the efficacy of ventilation.

Is the shape generally square (please refer to the circled area in the picture above)?
• If the shape is not generally square, then there is a perfusion, ventilation or mechanical issue.

Does the waveform start and end at “zero” baseline (A-B) along the horizontal access?
This segment is the beginning of exhalation

• The first gas to appear at the sampling point is the last gas that was inhaled into the conducting airways.
• This gas has not been subject to gas exchange and is essentially free of CO2, so remains at the zero baseline.
 • This baseline represents the gas occupying the anatomical dead space.
 • Anatomical dead space is the internal volume of the upper airways where no gas exchange takes place. This includes the nose, pharynx, trachea, and bronchi.
• Each wave should return to a zero baseline.
• A continuous rise off the horizontal baseline indicates rebreathing of expired CO2, or mechanical issues in ventilated patients.

Is there a notable expiratory up stroke (B-C) and inspiratory down stroke (D-E)?
The up stroke (B-C) represents gas that is a transition area composed partially of conducting airway volume and partially from alveoli (gas exchange).
• Here we see both a rise in CO2 concentration and accumulation of volume.
• Failure to demonstrate an upstroke is reflective of ineffective ventilation, either due to low tidal volume/ shallow breathing, or partial airway obstruction.

Inspiration (D-E) is marked by a rapid downward direction of the capnogram.
• This downward stroke corresponds to the fresh gas, which is essentially free of carbon dioxide that passes the CO2 sensor during inspiration.
• The capnogram will then remain at zero baseline throughout inspiration.
• Decreasing expiratory up stroke and inspiratory down stroke reflect decreasing ETCO2 levels. Assess for metabolic, perfusion, ventilation, or mechanical issues.

Is there an alveolar plateau (C-D)?

All of the gas passing by the CO2 sensor is alveolar gas which causes the capnogram to flatten out.
• Loss of the alveolar plateau means that there is ineffective ventilation, because CO2 is not clearing the airway, usually due to shallow breathing, or partial obstruction. Follow practice protocol for assessment and support of patient with potential respiratory compromise.

End-tidal concentration (D)
The ETCO2 value displayed on the monitor is the highest value measured during exhalation and usually occurs just prior to inspiration.