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Capnography – Not Just a Load of Hot Air

Capnography is the measurement of inhaled and exhaled carbon dioxide (CO2) concentration. The graphical illustration of CO2 within respired gases versus time is known as the capnogram. The instrument which is used to record this information is known as a capnograph

Physiology and it’s Importance in Capnography – A Brief Recap

Understanding the associated physiology as well as the constituents of both inspired and expired fractions of the respiratory cycle is key to understanding the capnogram. This understanding allows for not only recognition of normal, but any deviations from normal, that may be attained whilst using a capnograph to monitor your anaesthetised patient.

Key physiology:

  • Carbon dioxide (CO2) is produced as a waste product of normal aerobic metabolism performed within the patients tissues.
  • Gas exchange is the exchange of newly inspired oxygen (O2) for the metabolic waste product CO2 which is transported back to the lungs within the bloodstream. 
  • This exchange occurs at the level of the alveoli, which act as the principal site of gas exchange within the lungs.
  • The gas exhaled during early expiration is that from the anatomical dead space which does not participate in gas exchange and as a result early expiratory gas is usually devoid of CO2
  • Gas from the alveoli is exhaled during mid and late expiration. As a result, CO2 concentration increases throughout the remaining expiratory phase until a maximal level is reached at the end of expiration.
  • Following end expiration, the next inspiratory phase begins and the cycle repeats. 

Why use it?

When used alongside pulse oximetry, capnography provides continuous, non-invasive and real-time information relating to:

  • circulation (i.e. oxygenated blood being adequately delivered to the patient’s tissues, deoxygenated blood being delivered back to the lungs),
  • metabolism (i.e. CO2 being produced as a waste product of metabolism),
  • alveolar ventilation (i.e. the exchange of O2 and CO2 at the alveoli).

This allows the anaesthetist to make deductions into the clinical state of these three physiological processes in just the one reading. For this reason, capnography is considered a vital tool in the monitoring of anaesthetised small animal patients.

If the capnogram (see below) is considered to be normal in appearance then the maximal CO2 concentration reached during expiration, recorded as the partial pressure of CO2 (PECO2), can be taken as a representation of alveolar carbon dioxide tension which can in turn be used as an estimate of arterial carbon dioxide tension (PaCO2).

The Trace (Capnogram)

In the normal capnogram the trace attained is broken down into four adjoining phases (Figure. 1):

  • Phase I (inspiratory baseline) – reflects the CO2 concentration in inspired gas, which is devoid of CO2 in normal circumstances.
  • Phase II (expiratory upstroke) – reflects the CO2 concentration within expired gas from the anatomical dead space (which does not participate in gas exchange) and alveolar gas from the respiratory tree and alveoli.
  • Phase III (alveolar plateau) – reflects the end of expiration and the final expulsion of alveolar gas from the airways. The peak CO2 concentration reached at the end of this phase, known as end-tidal carbon dioxide (EtCO2), is usually displayed as the PECO2.
  • Phase 0 (inspiratory downstroke) – the return of CO2 concentration to baseline levels as the next inspiratory phase (Phase I) begins.


Figure 1.

The result of multiple normal respiratory cycles captured on a trace gives the appearance of a line of elephants holding tails (Figure. 2).


Figure 2.

Coming soon!

  • Capnography II – The ‘wiggly lines’. The meaning of the different capnography traces and a downloadable Capnography cheat-sheet.
Article by
Dr. Dan Cripwell
BSc (Hons) BVSc CertAVP (EM) PgCert (VPS) MRCVS

Veterinary Technical Advisor UK
RCVS Recognised Advanced Veterinary Practitioner

Originally published: Friday, 4th May 2018
Last updated: Tuesday, 15th May 2018

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