End-Tidal CO₂ (ETCO₂): What the Evidence Actually Supports?

ETCO₂ is one of the most versatile monitors in emergency medicine—but only if we use it for what it truly reflects.

Capnography has become ubiquitous in modern EDs: on monitors, ventilators, transport stretchers, and sedation carts. Yet despite widespread use, ETCO₂ is frequently misinterpreted as a simple ventilation number, which it is not.

This post summarizes where ETCO₂ adds strong, evidence-based value in the ED, where it should be interpreted cautiously, and where it should not be used as a surrogate for arterial CO₂.

What ETCO₂ Physiologically Represents

ETCO₂ reflects the interaction of three physiologic domains:

1. Ventilation – alveolar ventilation

2. Perfusion – pulmonary blood flow / cardiac output

3. Metabolism – CO₂ production

Any disturbance in one domain alters ETCO₂.
This is precisely why ETCO₂ is powerful in emergency care—and why it is dangerous when interpreted in isolation.

Evidence-Based Uses of ETCO₂ in the ED

1. Airway Confirmation and Continuous Airway Monitoring

Highest level of clinical consensus.

• Continuous adequate waveform capnography is the most specific method for confirming tracheal intubation, even in transport settings.

• No or bad waveform on monitors doesn’t confirm anything, like in arrest scenario.

• Loss of ETCO₂ waveform reliably identifies:

  • Esophageal intubation

  • Tube displacement

  • Circuit disconnection

Evidence

• Donald & Paterson, EMJ, 2006

• Long et al., J Emerg Med, 2017

• Sandroni et al., Resuscitation, 2018

➡️ In the ED, ETCO₂ is mandatory for airway safety, not optional.

2. Cardiac Arrest and CPR Quality

During CPR, ETCO₂ primarily reflects pulmonary blood flow, making it a real-time surrogate of cardiac output.

Key evidence-based roles:

• ETCO₂ <10 mmHg → poor CPR quality / poor prognosis

• Rising ETCO₂ → improving chest compressions

• Sudden increase in ETCO₂ → ROSC until proven otherwise

Evidence

• Sandroni et al., Resuscitation, 2018

• Long et al., J Emerg Med, 2017

No other bedside monitor provides such immediate feedback during resuscitation.

3. Shock and Undifferentiated Hypotension

In shock states, ETCO₂ correlates more strongly with perfusion than ventilation.

• Low ETCO₂ is associated with:

  • High lactate

  • Low MAP and cardiac output

  • Increased mortality

Prospective ED studies demonstrate that ETCO₂ on arrival predicts in-hospital mortality in non-traumatic shock.

Evidence

• Kheng & Rahman, Int J Emerg Med, 2012

• Ekka et al., Turkish J Emerg Med, 2025

• López-Izquierdo et al., AJEM, 2024 & 2025

➡️ In shock, ETCO₂ is best used as a resuscitation trend marker, not a target.

4. Procedural Sedation and Early Detection of Hypoventilation

Multiple ED and peri-procedural studies show that ETCO₂ detects respiratory depression earlier than pulse oximetry.

• ETCO₂ rises or waveform loss often precedes:

  • Desaturation

  • Clinical apnea

Evidence

• Miner et al., Acad Emerg Med, 2002

• Burton et al., Acad Emerg Med, 2006

• Camacho et al., J PeriAnesthesia Nurs, 2021

This makes ETCO₂ a safety monitor, not merely a respiratory parameter.

5. Metabolic Acidosis and Respiratory Compensation (Adjunct Only)

ETCO₂ correlates modestly with bicarbonate and pH in conditions such as DKA and sepsis, but correlation is population-level, not patient-specific.

• Reported correlation coefficients typically range from r ≈ 0.3–0.6

• ETCO₂ frequently underestimates PaCO₂ in critically ill patients due to:

  • Shock

  • Increased dead space

  • V/Q mismatch

Evidence

• Kartal et al., Am J Emerg Med, 2011

• Taghizadieh et al., J Cardiovasc Thorac Res, 2016

• Aminiahidashti et al., Emergency (Tehran), 2018

➡️ ETCO₂ can screen for metabolic acidosis, but cannot replace ABG/VBG for ventilatory decisions.

Where the Evidence Warns Against Over-Interpretation

❌ Using ETCO₂ as a PaCO₂ Surrogate in Critical Illness

Large ICU, trauma, and transport studies show:

• PaCO₂–ETCO₂ gradient ≥10 mmHg in 40–60% of critically ill patients

• Gradient widens unpredictably in shock and lung injury

Evidence

• Cooper et al., Prehosp Disaster Med, 2013

• Campion et al., J Trauma Acute Care Surg, 2019

• Martin-Gill et al., Prehospital Emerg Care, 2024

ETCO₂ should not be used alone to judge adequacy of ventilation in:

• Shock

• Severe metabolic acidosis

• ARDS

• Multi-organ failure

Practical ; Take-Home

• Always use ETCO₂ for airway confirmation and CPR

• Trust trends, not isolated values

• Low ETCO₂ in shock = poor perfusion until proven otherwise

• Do not use ETCO₂ alone to guide ventilatory compensation

• ABG/VBG remain essential in critically ill patients

In the emergency department, ETCO₂ is most powerful when used as a trend-based, physiology-informed monitor. It saves lives when interpreted in context—and misleads when treated as just another number.

References

1. Miner JR, Heegaard W, Plummer D. End-tidal carbon dioxide monitoring during procedural sedation. Acad Emerg Med. 2002;9(4):275-280. doi:10.1197/aemj.9.4.275

2. Burton JH, Harrah JD, Germann CA, Dillon DC. Does end-tidal carbon dioxide monitoring detect respiratory events prior to current sedation monitoring practices? Acad Emerg Med. 2006;13(5):500-504. doi:10.1197/j.aem.2005.12.017

3. Donald MJ, Paterson B. End tidal carbon dioxide monitoring in prehospital and retrieval medicine: a review. Emerg Med J. 2006;23(9):728-730. doi:10.1136/emj.2006.037184

4. Long B, Koyfman A, Vivirito MA. Capnography in the emergency department: a review of uses, waveforms, and limitations. J Emerg Med. 2017;53(6):829-842. doi:10.1016/j.jemermed.2017.08.026

5. Sandroni C, De Santis P, D’Arrigo S. Capnography during cardiac arrest. Resuscitation. 2018;132:73-77. doi:10.1016/j.resuscitation.2018.08.018

6. Kheng CP, Rahman NH. The use of end-tidal carbon dioxide monitoring in patients with hypotension in the emergency department. Int J Emerg Med. 2012;5:31. doi:10.1186/1865-1380-5-31

7. Ekka M, Sreekumar A, Aggarwal P, et al. Utility of end-tidal carbon dioxide monitoring in predicting in-hospital mortality in nontraumatic shock in the emergency department. Turk J Emerg Med. 2025;25(3):199-207. doi:10.4103/tjem.tjem_223_24

8. López-Izquierdo R, Martín-Rodríguez F, Sanz-García A, et al. Utility of noninvasive capnography and perfusion index in adult emergency department patients. Am J Emerg Med. 2024;79:85-90. doi:10.1016/j.ajem.2024.02.017

9. López-Izquierdo R, Martín-Rodríguez F, Cuadrillero R, et al. National Early Warning Score 2 plus capnography and perfusion index to estimate poor outcomes in emergency departments. Am J Emerg Med. 2025;90:16-22. doi:10.1016/j.ajem.2025.01.011

10. Kartal M, Eray O, Rinnert S, et al. ETCO₂: a predictive tool for excluding metabolic acidosis in emergency department patients. Am J Emerg Med. 2011;29(1):65-69. doi:10.1016/j.ajem.2009.01.018

11. Taghizadieh A, Pouraghaei M, Moharamzadeh P, et al. Correlation between end-tidal CO₂ and arterial blood gas parameters in patients with metabolic acidosis. J Cardiovasc Thorac Res. 2016;8(3):98-101. doi:10.15171/jcvtr.2016.19

12. Aminiahidashti H, Shafiee S, Kiasari AZ, Sazgar M. Applications of end-tidal carbon dioxide monitoring in emergency department; a narrative review. Emergency (Tehran). 2018;6(1):e5.

13. Cooper CJ, Kraatz JJ, Kubiak DW, et al. The relationship between end-tidal CO₂ and arterial CO₂ in critically ill trauma patients. Prehosp Disaster Med. 2013;28(1):87-93. doi:10.1017/S1049023X1200164X

14. Campion EM, Peltan ID, Brown SM, et al. Discordance between end-tidal and arterial carbon dioxide measurements in critically ill patients. J Trauma Acute Care Surg. 2019;87(5):1038-1045. doi:10.1097/TA.0000000000002465

15. Martin-Gill C, Guyette FX, Rittenberger JC, et al. Accuracy of end-tidal carbon dioxide in critically ill and injured patients during transport. Prehosp Emerg Care. 2024;28(1):1-9. doi:10.1080/10903127.2023.2243127