Capnography Monitoring

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

Clinical Interpretation of Capnography:
End Tidal CO2

One of the aspects professionals must notice is End Tidal CO2 (EtCO2), corresponding to the level of carbon dioxide that is released at the end of an exhaled breath. This is displayed as numerical value, which should range between 35 and 40 mmHg or 4.0 – 5.7 kPa.1, 4, 5 Continuous monitoring of EtCO2 has been demonstrated to show greater sensitivity for detecting episodes of respiratory compromise.6

Clinical Interpretation of Capnography:
Respiratory rate

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

Clinical Interpretation of Capnography:
Waveform Shape

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. 

Diagnosis8 Values8 Interventions8
Bradypneic hypoventilation SpO2 Normal Reassess patient and continue sedation
etCO2 Increases
RR Decreases
Waveform Increased amplitude and width
       
  SpO2 Decreases

Reasses patient

Cease drug administration or reduce dosing

Asses for airway obstruction

Consider supplemental oxygen

etCO2 Increases
RR Decreases
Waveform Increased amplitude and width

Waveform8

etCO2 > 45 mmHg

Want to learn more about capnography in different clinical situations?

About the author

My name is Andreia Trigo RN BSc MSc, I am a nurse consultant with over a decade of experience in anaesthesia, sedation and pain management.

This involves patient care, as well as lecturing at post grad level on these topics, presenting at conferences and co-developing a very successful sedation course at SedateUK. My passion for creating safer environments for patients and professionals led me to collaborate with Medtronic and share my knowledge and expertise with our professional community.

 

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.

  • 1. Gelman (2013) Capnography outside the operating room. Anesthesiology. Available at https://anesthesiology.pubs.asahq.org/article.aspx?articleid=2034665

  • 2. Long (2016) Interpreting Waveform Capnography: Pearls and Pitfalls. EmDocs. Available at http://www.emdocs.net/interpreting-waveform-capnography-pearls-and-pitfalls/

  • 3. Farquharson et al (2019) A systematic approach to capnography waveforms. RT Magazine. Available at https://www.rtmagazine.com/department-management/clinical/systematic-approach-capnography-waveforms/

  • 4. Sullivan (2019) 5 things to know about capnography. EMS1. Available at https://www.ems1.com/ems-products/capnography/articles/5-things-to-know-about-capnography-Hr5ETRdXzCoU3fLH/

  • 5. Richardson (2016) Capnography for Monitoring End-Tidal CO2 in Hospital and Pre-hospital Settings: A Health Technology Assessment. Canadian Agency for Drugs and Technologies in Health. Available at https://www.ncbi.nlm.nih.gov/books/NBK362376/

  • 6. Overdyk, F. J., Carter, R., Maddox, R. R., Callura, J., Herrin, A. E., & Henriquez, C. Continuous oximetry/capnometry monitoring reveals frequent desaturation and bradypnea during patient-controlled analgesia. Anesth Analg. 2007;105(2):412-418.

  • 7. Wheatley I (2018) Respiratory rate 3: how to take an accurate measurement. Nursing Times. https://www.nursingtimes.net/clinical-archive/respiratory-clinical-archive/respiratory-rate-3-how-to-take-an-accurate-measurement-25-06-2018/

  • 8. Krauss B, Hess DR. Capnography for procedural sedation and analgesia in the emergency department. Ann Emerg Med. 2007;50(2):172-81. Epub Jan. 12, 2007