Capnography: End Tidal CO₂ Monitoring Facts 

End tidal CO₂ monitoring is represented as a number and a graph on a monitor. The number is called capnometry, which is the partial pressure of CO₂ detected at the end of exhalation, ranging between 35 - 45 mm Hg or 4.0 – 5.7 kPa. The waveform is called capnograph and shows how much CO₂ is present at each phase of the respiratory cycle.^1,3

End tidal CO₂ (EtCO₂) monitoring is the fastest indicator of ventilatory compromise. Whilst capnography assesses ventilation, which is the movement of air in and out of the lungs; pulse oximetry assesses oxygenation which is the amount of oxygen that is bound to red blood cells. 

When monitoring End Tidal CO₂, there are 3 aspects to consider: the EtCO₂ value, the waveform shape and the respiratory rate (RR). Whilst it is fairly easy to interpret numerical values for EtCO₂ and RR, interpretation of the waveform shape requires specific knowledge discussed below.

A normal capnogram always has the following features:

• The waveform shape always starts at zero and returns to zero
• A maximum CO₂ is reached with each breath, corresponding to EtCO₂
• The amplitude of the waveform depends on EtCO₂ concentration
• The width of the waveform depends on expiratory time
• There is a similar shape for subjects with normal lung function
• The waveform has four phases:

1. Phase I: start of exhalation, CO₂ concentration is initially zero
2. Phase II: CO₂ increases rapidly as alveolar gas exits the airway
3. Phase III: CO₂ concentration is relatively constant (reflects the concentration of CO₂ in the alveolar gas). This phase ends with a value of maximum CO₂ concentration
4. Phase IV: start of inhalation, CO₂ decreases to zero as atmospheric air enters the airway

The following image is taken from SedateUK

Although it has been historically used by anaesthetists during general anaesthesia, End Tidal CO₂ monitoring is becoming more common in other clinical areas like emergency environments, critical care, during sedation practice, in ambulatory settings and in post-operative recovery units. Several guidelines have supported the use of capnography outside the theatre environment:

ALS Guidelines (2015) mention “Waveform capnography must be used to confirm and continually monitor tracheal tube placement, and may be used to monitor the quality of CPR and to provide an early indication of return of spontaneous circulation (ROSC).”⁶

House of Delegates of ASA (2011) “During moderate or deep sedation, the adequacy of ventilation shall be evaluated by continual observation of qualitative clinical signs and monitoring for the presence of exhaled carbon dioxide unless precluded or invalidated by the nature of the patient, procedure, or equipment.”⁷

AAGBI (2015) “Continuous capnography should be used for all patients undergoing moderate or deep sedation, and should be available wherever any patients undergoing anaesthesia or moderate or deep sedation are recovered.”⁸

The wider use of EtCO₂ monitoring in different clinical areas reflects its importance as a monitoring tool that gives indication into three crucial aspects: the patient’s airway patency, breathing adequacy and circulatory status. Professionals are able to identify potential breathing complications (such as airway obstruction, hyperventilation, hypoventilation, or apnoea) and respond accordingly with a change in clinical management.³ As it provides real time, ongoing data, it prompts early diagnosis, allowing for clinical interventions to be adjusted according to monitor trends and preventing further deterioration.^1,3

 Visit our capnography tutorial and take the quiz! 

The content of this article is written by a blogger with whom Medtronic has a relationship. However, the contents represent the personal objective views, comments and techniques of the blogger and are not statements from Medtronic. To the extent this material might contain images of patients or any material where a copyright is held by a third party, all necessary written permissions from the patient or copyright holder, as applicable, with respect to use, distribution or copying of such images or copyrighted materials has been obtained by the blogger.