What are the normal end‑tidal carbon dioxide (ETCO₂) values on a capnography monitor, and how should abnormal readings be interpreted and managed?

Normal End-Tidal CO₂ Values and Clinical Interpretation

Normal end-tidal CO₂ (ETCO₂) values range from 35-45 mmHg (4.7-6.0 kPa), and you should target normocapnia in mechanically ventilated patients, particularly after cardiac arrest. 1

Normal Values and Monitoring Standards

ETCO₂ monitoring is considered the optimal method for confirming endotracheal tube placement and should be continuous with waveform capnography 2. The most recent European Resuscitation Council guidelines (2025) explicitly recommend targeting a partial pressure of carbon dioxide of 4.7-6.0 kPa (approximately 35-45 mmHg) in adults with return of spontaneous circulation (ROSC) after cardiac arrest 1.

Key Normal Parameters:

• Resting normal: 35-45 mmHg (4.7-6.0 kPa)
• During exercise: May increase to >20 mmHg above baseline in healthy individuals 3
• Post-cardiac arrest target: 35-45 mmHg (normocapnia) 4, 1

Abnormal Values and Clinical Significance

Low ETCO₂ (<35 mmHg)

During CPR, persistently low ETCO₂ values (<15 mmHg) indicate poor chest compression quality or predict poor outcomes 5:

• <10-15 mmHg during CPR: Strongly associated with failure to achieve ROSC after 15-20 minutes of advanced life support 5
• Focus efforts on improving chest compression quality and avoiding excessive ventilation 5
• Sudden sustained rise in ETCO₂: Often precedes identification of ROSC 5

In shock states, low ETCO₂ correlates with:

• Elevated lactate levels
• Increased transfusion requirements
• Higher mortality
• Serves as a rapid bedside marker of systemic hypoperfusion 6
• Threshold <35 mmHg (4.7 kPa): High index of suspicion for hemorrhagic shock and need for blood transfusion in trauma patients 7

High ETCO₂ (>45 mmHg)

Values >50 mmHg indicate hypoventilation and potential respiratory compromise 8:

• During procedural sedation: ETCO₂ >50 mmHg, absent waveform, or absolute change from baseline >10 mmHg suggests respiratory depression 8
• In exercise testing: Values >35 mmHg indicate possible gas exchange abnormality; >50 mmHg indicate likely pulmonary abnormality 3

Critical Caveats and Pitfalls

When ETCO₂ is Unreliable

ETCO₂ should NOT be used as a substitute for arterial PCO₂ in patients with cardiopulmonary disease 3. The gradient between ETCO₂ and arterial PCO₂ is highly variable and unpredictable in:

• Cardiac arrest patients: Initial ETCO₂ varies widely based on arrest etiology (higher in asphyxial vs. primary cardiac arrest) 5
• Critically ill patients: 79% of prehospital intubated patients had PCO₂-ETCO₂ gradient >0.67 kPa, making capnography unreliable for guiding ventilation 9
• Ventilation-perfusion mismatch: Any cardiorespiratory failure alters V/Q ratios, impairing CO₂ elimination 10
• ECMO patients: The membrane oxygenator removes CO₂ and bypasses lung circulation, leading to abnormally low ETCO₂ 11

Medication Effects

Interpret ETCO₂ with caution for 1-2 minutes after medication administration 5:

• Epinephrine and vasoconstrictors: Transiently decrease ETCO₂
• Sodium bicarbonate: Transiently increases ETCO₂

Technical Considerations

• Pulse oximetry limitations: Can overestimate true oxygen saturation in people with darker skin tones 1
• Low-flow states: Cause poor signal quality 1
• Requires waveform capnography: Colorimetric detectors are acceptable only in low-volume hospitals as a substitute 2

Management Algorithm

During Procedural Sedation 12, 8:

1. Monitor ETCO₂ continuously with capnography
2. If ETCO₂ >50 mmHg, absent waveform, or change >10 mmHg from baseline → provide verbal/tactile stimulation
3. If no response → initiate airway support

During CPR 5:

1. If ETCO₂ consistently <15 mmHg → improve chest compression quality
2. Sudden sustained rise in ETCO₂ → check for ROSC
3. Do NOT use ETCO₂ alone to terminate resuscitation (insufficient pediatric data for threshold)

Post-Cardiac Arrest 1:

1. Obtain arterial blood gases
2. Target normocapnia: 35-45 mmHg (4.7-6.0 kPa)
3. Monitor ETCO₂ continuously but verify with arterial blood gases
4. In hypothermic patients, monitor PCO₂ frequently as hypocapnia may occur

In Shock States 6, 7:

1. ETCO₂ <35 mmHg → suspect hypoperfusion/hemorrhage
2. Correlate with lactate levels
3. Consider need for blood transfusion in trauma patients

Bottom line: Use ETCO₂ as a continuous monitoring tool for trends and immediate changes, but always confirm with arterial blood gases when making critical ventilation decisions in critically ill patients.