Capnography Guide: End Tidal CO₂, Respiratory rate, Waveform shape

Capnography monitors the concentration or partial pressure of carbon dioxide (CO₂) in the respiratory gases. Although capnography has historically been used mostly by anaesthetists, it is becoming more common in different clinical settings, being used by nurses, paramedics and doctors non-anaesthetists in emergency departments, interventional rooms and recovery departments. In this article we talk about the 3 important aspects you must notice in capnography, to interpret and make clinical decisions.

In healthy individuals, a capnogram will have a similar rectangular shape. Any changes to the normal shape require adequate analysis and interpretation to determine the physiologic or pathologic cause.¹ These changes are often the earliest indication of ventilatory problems.³

There are 3 main aspects you must notice in capnography in order to make a correct interpretation and appropriate clinical decisions:

1. End Tidal CO₂
2. Respiratory Rate
3. Capnogram Waveform shape

Respiratory rate corresponds to the number of breaths per minute. It is also displayed as a number and normal values range between 12 and 20 breaths per minute. Capnography monitors are a useful technology that provides an accurate measurement of respiratory rate.⁷

It is important to learn how to interpret the waveform shape to make adequate clinical decisions. Carbon dioxide waveforms can be plotted against time or against expired volume. Time capnography is used more commonly in clinical practice. A time capnogram has two important segments: inspiratory and expiratory. The expiratory segment is further divided into three phases (I, II, III), and an occasional phase IV, based on the physiology of carbon dioxide evolution from the lungs and airways:¹

• Phase I: start of exhalation, CO₂ concentration is initially zero
• Phase II: CO₂ increases rapidly as alveolar gas exits the airway
• 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
• Phase IV: start of inhalation, CO₂ decreases to zero as atmospheric air enters the airway

Systematic approaches to carbon dioxide waveform interpretation have been published. These can possibly reduce errors of interpretation or clinical interventions.³ Baseline, trend, frequency, shape, height, and width of the waveform all provide useful clues as to the quality of ventilation. 

Long (2016) suggests a systematic process to analyse capnography waveforms, divided into 5 steps:²

1. Look for presence of exhaled CO₂: Is a waveform present?
2. Inspiratory baseline: Is there rebreathing?
3. Expiratory upstroke: What is the shape? (i.e. steep, sloping, or prolonged?)
4. Expiratory/alveolar plateau: Is it sloping, steep, or prolonged?
5. Inspiratory downstroke: Is it sloping, steep, or prolonged?

Capnography use is increasing in different clinical settings as professionals understand the importance of detecting respiratory compromise quickly and with a high degree of sensitivity. Noticing all 3 aspects of capnography interpretation, clinicians ensure they have all information required to make clinical decisions and intervene appropriately. 

etCO₂ > 45 mmHg

 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.