Riding the Waves: End-Tidal CO2 Monitoring

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End-Tidal CO2 monitoring has a variety of uses in the Emergency Department.  Whether used diagnostically or for monitoring of a patient’s physiology, clinicians must possess an understanding of the information that you can gather from EtCO2 waveform tracings. Knowing how to interpret the waveforms makes EtCO2 much more than a number, allowing the clinician to gain insight into minute to minute changes in a patients physiological state.

Respiratory Physiology

Fortunately, to understand waveform capnography, we don’t need an in-depth knowledge of respiratory physiology.  The most important things to remember are: 1) ambient air contains effectively 0% carbon dioxide whereas exhaled air contains about 4.4% carbon dioxide, and 2) the dead space in your airway does not participate in gas exchange.   

Knowing this, we can take a look at an end-tidal waveform and understand discrete parts of the wave and what they represent throughout the respiratory cycle.

Phase 0: This part of the curve in inspiration.  During this phase, the CO2 value should be 0 as the capnometer is detecting only ambient air which should contain effectively 0 CO2.  

Phase 1:  This part of the curve is the earliest part of expiration.  This initial expiration contains only dead space gas that has not mixed with any gas from the conducting airway, so there is no increase in CO2.  This plateau should have a gentle upslope or remain flat.  

Phase 2: This transitional part of the curve still represents early expiration.  During this phase gas from the upper airways that is low in CO2 ​mixes with gas from the conducting airways.  The amount of CO2 increases in the expired air increases as you approach the alveolar plateau.

Phase 3: This part of the curve is called the alveolar plateau.  It represents the steady-state diffusion of CO2 in the alveoli.  The peak of this plateau is the end-tidal CO2.  The angle of change between Phase 2 and Phase 3 of respiration is referred to as the alpha angle.

Phase 0: As expiration ends, the capnometer again senses ambient air with almost 0 CO2, and the waveform instantaneously drops to 0.  

Knowing this, the waveform can give us a lot of useful information.  In addition to the numerical end-tidal CO2 which is a function of metabolic rate and cardiac output, we get useful information about air movement in the airways and the length of time of inspiration and expiration.  This means that the waveforms can offer assistance with the diagnosis of lung pathology.

The shape of this waveform represents an asthma exacerbation or bronchospasm.  As bronchial constriction causes worsening obstruction the slope of Phase 2 gets smaller as it takes a longer time to clear out dead space gas.  Further, there is no true alveolar plateau as the dead space has not completely cleared by the end of expiration.  The alpha angle disappears, giving a classic sawtooth appearance.

This waveform represents emphysema.  Recall that in emphysema the pathophysiology is related to the destruction of alveoli and increased compliance of the lungs.  Because of the alveolar destruction, the surface for gas exchange is decreased, and the alveolar plateau becomes downsloping.

While these features may not be necessary to manage patients, it allows us to explore and understand the pathophysiology and how it aligns with what we see on our monitors.  However, there are times when the use of end-tidal CO2 monitoring is critical to patient management.

Intubation

End-tidal monitoring is the most accurate method for immediate confirmation of endotracheal tube placement.  Breath sounds, fogging or condensation in the ET tube, and chest rise may all be present in esophageal intubations.  Whereas the capnogram for esophageal intubation will quickly drop to a flatline.

Capnography is also particularly useful in the transport of intubated patients.  Transport is associated with a higher risk of tube displacement.  Continuous monitoring allows for rapid recognition of tube displacement.

Cardiac Arrest

As mentioned before, tidal CO2 is a function of metabolic rate and cardiac output, so capnography can be utilized during CPR to monitor the effectiveness of chest compressions.  Positive waveforms should be seen with high-quality compressions, and a gradual decline in end-tidal CO2 should be a signal that a fresh provider should perform compressions. An abrupt increase in end-tidal is an indicator of the return of spontaneous circulation (however this can also represent recent administration of bicarbonate). 

Procedural Sedation

There are a number of studies that prove that capnography during sedation detects apnea earlier, leads to earlier intervention, and decreases morbidity and mortality when compared to pulse oximetry monitoring. 

For some additional background, take a look and listen at our previous posts/podcasts on EtCO2.

References

  1. Long, Brit, Alex Koyfman, and Michael A. Vivirito. "Capnography in the emergency department: a review of uses, waveforms, and limitations." The Journal of emergency medicine 53.6 (2017): 829-842.

  2. Whitaker, D. K. "Time for capnography–everywhere." Anaesthesia 66.7 (2011): 544-549.

  3. Kelly, John J., et al. "Use of tube condensation as an indicator of endotracheal tube placement." Annals of emergency medicine 31.5 (1998): 575-578.

  4. Tobias, Joseph D., Amy Lynch, and Jeremy Garrett. "Alterations of end-tidal carbon dioxide during the intrahospital transport of children." Pediatric emergency care 12.4 (1996): 249-251.

  5. Neumar, Robert W., et al. "Part 1: executive summary: 2015 American Heart Association guidelines update for cardiopulmonary resuscitation and emergency cardiovascular care." Circulation 132.18_suppl_2 (2015): S315-S367.

  6. Waugh, Jonathan B., Chad A. Epps, and Yulia A. Khodneva. "Capnography enhances surveillance of respiratory events during procedural sedation: a meta-analysis." Journal of clinical anesthesia 23.3 (2011): 189-196.

  7. Godwin, Steven A., et al. "Clinical policy: procedural sedation and analgesia in the emergency department." Annals of emergency medicine 63.2 (2014): 247-258.

  8. Lightdale, Jenifer R., et al. "Microstream capnography improves patient monitoring during moderate sedation: a randomized, controlled trial." Pediatrics 117.6 (2006): e1170-e1178.

Authorship

  • Written by Dave Wilson, MD, PGY-1, University of Cincinnati Department of Emergency Medicine

  • Peer Review and Editing - Jeffery Hill, MD MEd, Associate Professor, University of Cincinnati Department of Emergency Medicine

Cite As

Wilson, D. Hill, J. Riding the Waves: End-Tidal CO2 Monitoring. TamingtheSRU. www.tamingthesru.com/blog/core-content/riding-the-waves-end-tidal-co2-monitoring. 7/2/2023.