Understanding Capnography

Capnography is considered standard practice for nurses to monitor carbon dioxide (CO2) levels in the critical care environments. Here we will look at:

  • What is capnography?
  • What is end-tidal carbon dioxide (ETCO2)?
  • Clinical factors that may affect the accuracy and reliability of capnography readings.

What is Capnography?

The waveform capnography represents the amount of exhaled carbon dioxide (CO2). Capnography continuously measures and displays exhaled carbon dioxide throughout the respiratory cycle which can assist with the assessment of ventilation.

Explanation of end-tidal carbon dioxide (ETCO2) waveform?

Phases of the capnography waveform by EMDocs:

  1. Phase I is the inspiratory baseline, which is due to inspired gas with low levels of CO2.
  2. Phase II is the beginning of expiration which occurs when the anatomic dead space and alveolar gas from the alveoli/bronchioles transition.
    a. The transition from phase II to III is the alpha angle.
    b. The alpha angle can be used to assess the ventilation/perfusion of the lung. V/Q mismatches will have an alpha angle greater than 90 degrees.
  3. Phase III is the alveolar plateau, where the last of the alveolar gas is sampled. This is normally the PETCO2 (also known as End Expiration measured ETCO2).
    a. The transition from phase III to 0 is the beta angle.
    b. The beta angle can be used to assess rebreathing. If rebreathing occurs, the angle is greater than 90 degrees.
  4. This is actually phase 0, reflecting the inspiratory downstroke and the beginning of inspiration.

What are ‘normal’ ranges‘ for end-tidal carbon dioxide (ETCO2)?: 35-45 mmHg.

Capnography Uses:

  • Intubation and ETT placement
  • Ventilation monitoring
  • Procedural ventilation monitoring (non intubated)
  • During cardiac arrest for quality of compressions and return of spontaneous circulation

Benefits: rather than taking endless blood samples, if the patient has an ETT or trache then capnography will provide end-tidal carbon dioxide (ETCO2). Use the data trend for ETCO2, along with SPO2 and ventilator information.

Mind The Gap: Arterial to End-Tidal CO2 Gradient which under normal physiologic conditions, the difference between arterial PaCO2(from ABG) and alveolar end-tidal carbon dioxide (ETCO2 from capnograph) is 2-5 mmHg.

  • COAD (causing incomplete alveolar emptying)
  • ARDS (causing a ventilation-perfusion mismatch)
  • A leak in the sampling system or around the ET tube

Tips & Tricks: Baseline (Phase 1) should be 0, any deviation from this should warrant further checks:

  • Calibration error during set up: Action is to recalibrate
  • Capnography ‘window’ adaptor in ventilator tubing has condensation, water drops or sputum effecting the reading: Action is to change the adaptor.

Terminology Reminder: arterial blood gas analysis provides partial pressure of carbon dioxide in arterial blood (PaCO2).

Other methods to monitor Carbon dioxide (CO2) for confirmation of ETT position in the trachea during intubation: There are emergency tracheal intubation monitors, which use infrared technology and provide digital recording only, no waveform. Another option is colorific devices which use a pH detector for presence of CO2, no number or waveform is generated.

Additional Resources

Capnography website.

Deranged Physiology. (2019). Abnormal capnography waveforms and their interpretation.

Kodali, B.S. (2013). Capnography outside the operating rooms. Anesthesiology. 118 (1), p. 192

Respironics. (2012). Capnography Reference Handbook.

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