Reversible causes of low EtCO2 in CPR

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Steve Haywood
Steve Haywood
Former Respiratory Therapist working to de-mystify the ventilator through hands-on workshops and online education. Additional interests include HFNC in CHF and COPD, Medical Education, and Tupperware.

The Pre-brief

A frozen cadaver was thawed and intubated. The EtCO2 reading went to 65mmHG! Carbon Dioxide was detected for 100 seconds! (1)

EtCO2 levels less than 10mmHg after 20 minutes of CPR indicate poor outcomes(2), but EtCO2 levels less than 10mmHg early in cardiac arrest should prompt you to look for a reversible cause.

In this post, we will review the reversible causes of low EtCO2 in cardiac arrest.

Poor Compressions

Adequate compressions should almost always be the first priority of the resuscitations, regardless of the EtCO2 level. We must assure adequate depth, rate, and location. 

  1. Rate – 100-120 compressions per minute. 
    1. Many compressors will go too fast when their adrenaline is present. Compression rates of 140/minute lead to very low survival rates, yet these rates are often achieved by overzealous staff. (3)
    2. Sing a song! “Stayin’ Alive” is the classic but many songs have the correct tempo.
  2. Depth 
    1. Make sure you rotate compressors often. Fatigue will lead to shallow compressions. 
    2. Offer a stool to the compressor to assure the shoulders are directly above the chest. On typical hospital beds, the compressor can kneel on the bed next to the patient. This technique is another way to get into an appropriate position for good compressions. 
    3. Use a backboard or place the bed in CPR mode. A soft bed will absorb the force of the compressions.
  3. Hand Positioning
    1. Disease processes like COPD and CHF can lead to movement of the heart from the normal anatomic position. If the heart is not in its usual place, then the hands of the compressor should not be in their usual place. 
    2. If EtCO2 levels are low early in the arrest, consider attempting a sub-xiphoid or 4 chamber view to assure the heart is actually being compressed. Move the compressor’s hand to the most appropriate position. 
    3. Though not always available, transesophageal echo can visually confirm the heart is compressed with each push. 

Poor Ventilations

If gas cannot make it from the lungs to the detector, no CO2 can be detected. The low EtCO2 may indicate issues with the mode of ventilation rather than data about survivability. 

1. Esophageal intubation. Intubation can be difficult during chest compressions. Some have even advocated “DO NOT INTUBATE IN CPR”. If the ETT is in the esophagus, there may still be CO2 present. The levels will be lower and the waveform will look more like a dome rather than a box.

If there is a concern for esophageal intubation, you can confirm with a video laryngoscope, or simply pull the tube and ventilate with a bag-valva-mask (BVM) or place a supraglottic airway (SGA). 

2. Poor BVM seal. 

      a. Use 2 person technique.

      b. Use the VE seal technique as described HERE

      c. If still having issues, consider placing an SGA.

3. Unseated SGA

      a. If a loud audible leak is heard from the mouth with every ventilation, your SGA may not be properly seated. 

Reviewer’s Comment: ***Gel-based LMA cuffs expand with body heat. The first few breaths will likely have an air leak from the mouth*** -Caleb Harrell 

      b. Consider replacing or using a BVM.

Obstructive Shock

If blood cannot get to the lungs, CO2 cannot get to the lungs. Consider reversible causes of obstructive shock if no other cause of low EtCO2 is found.

  1. Tamponade – Tamponade will collapse the RV thereby not allowing blood into the lungs. Evaluate with ultrasound and drain if present. 
  2. PE – A PE that causes cardiac arrest could sufficiently obstruct the right ventricle to drop the EtCO2. Read more about the Evaluation of PE in Cardiac Arrest HERE.
  3. Tension Pneumothorax can shift the mediastinum and not allow blood to exit the RV. Lung ultrasound is a quick and accurate way to evaluate for a pneumothorax. 

Equipment Malfunction

Condensation in the sampling line or an improperly calibrated monitor can lead to falsely low EtCO2 measurement. Condensation or secretions in the sampling line will likely cause the waveform to be completely flat. Some monitors require zeroing before use. If it attempts to zero while connected to the patient you could get a false low reading. Know your equipment beforehand to understand what is required to properly set up your monitor.  If no other cause of low EtCO2 is discovered, you can change out the in-line detector and re-zero your monitor. 

The Debrief

  • If the EtCO2 is low early in cardiac arrest, look for a reversible cause.
  • First, assure proper chest compressions and ventilations.
  • Next look for reversible causes of right ventricle obstruction.
  • Finally, consider an equipment malfunction.
  • If no reversible cause of low EtCO2 is discovered, the patient’s likelihood of survival is low, especially after 20 minutes of resuscitation.

References

  1. Reid C, Lewis A, Habig K, Burns B, Billson F, Kunkel S, Fisk W. Sustained life-like waveform capnography after human cadaveric tracheal intubation. Emerg Med J. 2015 Mar;32(3):232-3. doi: 10.1136/emermed-2013-203105. Epub 2013 Oct 8. PMID: 24105333.
  2. Sandroni C, De Santis P, D’Arrigo S. Capnography during cardiac arrest. Resuscitation. 2018 Nov;132:73-77. doi: 10.1016/j.resuscitation.2018.08.018. Epub 2018 Aug 22. PMID: 30142399.
  3. Duval S, Pepe PE, Aufderheide TP, Goodloe JM, Debaty G, Labarère J, Sugiyama A, Yannopoulos D. Optimal Combination of Compression Rate and Depth During Cardiopulmonary Resuscitation for Functionally Favorable Survival. JAMA Cardiol. 2019 Sep 1;4(9):900-908. doi: 10.1001/jamacardio.2019.2717. PMID: 31411632; PMCID: PMC6694399.

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