Written by: Shawn Luo, MD (NUEM ‘22) Edited by: Matt McCauley, MD (NUEM ‘21) Expert Commentary by: N. Seth Trueger, MD, MPH
Continuous waveform capnography has increasingly become the gold standard of ETT placement confirmation. However, capnography can provide additional valuable information, especially when managing critically ill or mechanically ventilated patients.
Normal Capnography
Phase I (inspiratory baseline) reflects inspired air, which is normally devoid of CO2.
Phase II (expiratory upstroke) is the transition between dead space to alveolar gas.
Phase III is the alveolar plateau. Traditionally, PCO2 of the last alveolar gas sampled at the airway opening is called the EtCO2. (normally 35-45 mmHg)
Phase 0 is the inspiratory downstroke, the beginning of the next inspiration
Figure 1. Normal Capnography Tracing (emDOCs.net)
EtCO2 is only one component of capnography. Measured at the end-peak of each waveform, it reflects alveolar CO2 content and is affected by alveolar ventilation, pulmonary perfusion, and CO2 production.
Figure 2. Factors affecting ETCO2 (EMSWorld)
EtCO2 - PaCO2 Correlation
Correlating EtCO2 and PaCO2 can be problematic, but in general, PaCO2 is almost always HIGHER than EtCO2. Normally the difference should be 2-5mmHg but the PaCO2-EtCO2 gradient is often increased due to increased alveolar dead space (high V/Q ratio), such as low cardiac output, cardiac arrest, pulmonary embolism, high PEEP ventilation.
Important Patterns
Let’s go through a few cases and learn some of the important capnography waveforms to recognize
Case 1: Capnography with Advanced Airway
An elderly gentleman with a history of COPD, CAD & CKD gets rushed into the trauma bay with respiratory distress and altered mental status. You gave him a trial of BiPAP for a few minutes without improvement.
You swiftly tubed the patient. It was not the easiest view, but you advance the ETT hoping for the best. Upon attaching the BVM to bag the patient, you saw this on capnography:
Figure 3. Case 1 (EMSWorld)
Oops, the ETT is in the esophagus, as evidenced by the low-level EtCO2 that quickly tapers off.
2. You remove the ETT, bag the patient up, and try again with a bougie. Afterward, you see…
Figure 4. Capnography with ETT in right main bronchus (EMSWorld)
This suggests a problem with ETT position, most often in the right main bronchus. Notice the irregular plateau--the initial right lung ventilation, followed by CO2 escaping from the left lung. Beware that capnography can sometimes still appear normal despite the right main bronchus placement.
3. You pull back the ETT a few cm and the CXR now confirms the tip is now above the carina. The patient’s capnography now looks like this:
Figure 5. Capnography showing obstruction or bronchospasm (SketchyMedicine)
Almost looks normal but notice the “shark fin” appearance, this is due to delayed exhalation, often seen in airway obstruction and bronchospasms such as COPD or asthma exacerbation.
4. You suction the patient and administer several bronchodilator nebs. The waveform now looks more normal:
Figure 6. Capnography showing normal waveform (SketchyMedicine)
5. However, just as you were about to get back to the workstation to call the ICU, the monitor alarms and you see this:
Figure 7. Sudden loss of capnography waveform (SketchyMedical)
Noticing the ETT still in place with good chest rise, you quickly check for a pulse. There is none.
6. You holler, push the code button and start ACLS with a team of clinicians. With CPR in progress, you notice this capnography:
Figure 8. Capnography during CPR (SketchyMedicine)
Initially, your patient’s EtCO2 was only 7, after coaching the compressor and improving CPR techniques, it increased to 14.
You are also aware that EtCO2 at 20min of CPR has prognostic values. EtCO2 <10 mmHg at 20 minutes suggests little chance of achieving ROSC and can be used as an adjunctive data point in the decision to terminate resuscitation.
7. Fortunate for your patient, during the 3rd round of ACLS, you notice the following:
Figure 9. ROSC on capnography (emDOCs.net)
This sudden jump in EtCO2 suggests ROSC. You stop the CPR and confirm that the patient indeed has a pulse.
8. As you are putting in orders for post-resuscitation care, you notice this:
Figure 10. Asynchronous breathing on capnography (SketchyMedical)
This curare cleft comes from the patient inhaling in between ventilator-delivered breaths and is usually a sign of asynchronous breathing. However, in the post-arrest scenario, it is a positive prognostic sign as your patient is breathing spontaneously. You excitedly call your mom, I meant MICU, about the incredible save.
Case 2: Capnography with Non-intubated Patient
You just hung up the phone with MICU when EMS brings you a young woman with a heroin overdose. She already received some intranasal Narcan from EMS but per EMS report patient is becoming sleepy again.
She mumbles a little as you shout her name, and as you put an end-tidal nasal cannula on her, you saw this:
Figure 11. Hypoventilation on capnography (emDOCs.net)
Noticing the low respiratory rate and high EtCO2 value, you recognize this is hypoventilation.
2. But very soon she becomes even less responsive and the waveform changed again:
Figure 12. Airway obstruction on capnography (emDOCs.net)
The inconsistent, interrupted breaths suggest airway obstruction, while the segments without waveform suggest apnea. You have to act fast.
3. By then your nurse has already secured an IV, so you pushed some Narcan. However, in the heat of the moment, you gave the whole syringe. The patient quickly woke up crying and shaking.
Figure 13. Hyperventilating on capnography (emDOCs.net)
She was quite upset and hyperventilating. The waveform reveals a high respiratory rate and relatively low EtCO2.
As much as you are a little embarrassed by putting the patient into florid withdrawal, you know it could have been a lot worse. Walking away from the shift, you think about how many times capnography has assisted you during those critical moments. “Hey, perhaps we should buy a capnography instead of a baby monitor,” you ask your wife at dinner.
Additional Resources
This website provides a tutorial and quiz on some of the basic capnography waveforms.
References
American Heart Association. 2019 American Heart Association Focused Update on Advanced Cardiovascular Life Support. Circulation. 2019; 140(24). https://doi.org/10.1161/CIR.0000000000000732
Brit Long. Interpreting Waveform Capnography: Pearls and Pitfalls. emDOCs.net. www.emdocs.net/interpreting-waveform-capnography-pearls-and-pitfalls/, accessed May 12, 2020
Capnography.com, accessed May 12, 2020
Kodali BS. Capnography outside the operating rooms. Anesthesiology. 2013 Jan;118(1):192-201. PMID: 23221867.
Long, Koyfman & Vivirito. Capnography in the Emergency Department: A Review of Uses, Waveforms, and Limitations. Clinical Reviews in Emergency Medicine. 2017; 53(6). https://doi.org/10.1016/j.jemermed.2017.08.026
Nassar & Schmidt, Capnography During Critical Illness. CHEST. 2016; 249(2). https://doi.org/10.1378/chest.15-1369
Sketchymedicine.com/2016/08/waveform-capnography, accessed May 13, 2020
Wampler, D. A. Capnography as a Clinical Tool. EMS World. www.emsworld.com/article/10287447/capnography-clinical-tool. June 28, 2011. Accessed May 13, 2020
Expert Commentary
This is a nice review of many of the intermediate and qualitative uses of ETCO2 in the ED. For novices, I recommend a few basic places to start:
Confirmation of intubation. Color change is good but it’s just litmus paper and gets easily defeated by vomit. Also, in low output states, it may not pick up. Further, colorimetric capnographs require persistent change over 6 breaths, not just a single change. Waveform capnography uses mass spec or IR spec to detect CO2 molecules. There are so many uses, it’s good to have, I don’t see why some are resistant to use this better plastic adapter connected to the monitor vs the other, worse, plastic adapter.
a. The mistake I have seen here is assuming a lack of waveform is due to low cardiac output, ie there’s no waveform because the patient is being coded, not because of esophageal intubation. There is always *some* CO2 coming out if there is effective CPR; if there isn’t, the tube is in the wrong place. If you really don’t believe it, check with good VL but a flatline = esophagus.
2. Procedural sedation. There’s lots of good work and some debate about absolute or relative CO2 changes or qualitative waveform changes that might predict impending apnea, but for me, the best use is that I can just glance at the monitor for a second or two and see yes, the patient is breathing. No more staring at the chest debating whether I see chest rise, etc. It’s like supervising a junior trainee during laryngoscopy with VL: it’s anxiolysis for me.
a. Using ketamine? Chest movement or other signs of respiratory effort without ETCO2 waveform means laryngospasm. Jaw thrust, bag, succinylcholine (stop when better).
3. Cardiac arrest.
a. Quality of CPR. Higher number means more output. Can mean the compressor needs to fix their technique, or more often, is tiring out and needs a swap.
b. ROSC. There can be a big jump (eg from 15 to 40) when ROSC occurs. Very helpful.
c. Ending a code. 20 mins into a code, if it’s <10 during good CPR, the patient is unlikely to survive. I try to view this as confirming what we know – it’s time to end the code. The mistake here is to not end a code that should otherwise end because the ETCO2 is above 10; it doesn’t work like that, it’s a 1-way test.
4. Leak. One waveform shape I wanted to add that I find helpful: if the downstroke kinda dribbles down like a messy staircase, it’s a leak. Can be an incomplete connection (eg tubing to the vent) or the balloon is too empty or full.
Seth Trueger, MD, MPH
Assistant Professor of Emergency Medicine
Department of Emergency Medicine
Northwestern University
How To Cite This Post:
[Peer-Reviewed, Web Publication] Luo, S., McCauley M. (2021, Sept 9). Basic Capnography Interpretation. [NUEM Blog. Expert Commentary by Trueger N.S]. Retrieved from http://www.nuemblog.com/blog/capnography
