How ETCO2 is Measured and What It Indicates
Measurement Technique
ETCO2 is most reliably measured using waveform capnography, which employs an infrared measuring cell to analyze CO2 content in expired gases and displays the CO2 concentration over time as a capnographic waveform. 1, 2
Technical Methods
Sidestream capnography uses continuous aspiration of patient gases through sampling tubing to an infrared analyzer, with conventional systems using 200 ml/min aspiration rates and newer microstream systems using only 30 ml/min 1, 3
Sampling interfaces vary by patient ventilation status:
Waveform visualization is essential—the capnogram peaks during expiration and troughs during inspiration, with specific patterns identifying respiratory abnormalities 1
Microstream capnometry provides more accurate measurements in spontaneously breathing, non-intubated patients compared to conventional sidestream devices (median arterial-to-end-tidal difference of 4.4 mmHg vs 7 mmHg, p=0.02) 3
What ETCO2 Indicates
Physiologic Determinants
ETCO2 reflects three primary physiologic processes: CO2 production from cellular metabolism, alveolar ventilation, and pulmonary blood flow. 1, 2
During low-flow states with relatively fixed minute ventilation, pulmonary blood flow becomes the primary determinant of ETCO2 values 1, 2
In cardiac arrest, ETCO2 levels directly reflect the cardiac output generated by chest compressions, making it a real-time indicator of CPR quality 1, 2
Normal Values and Clinical Interpretation
Values >50 mmHg indicate hypoventilation and potential respiratory compromise 2
An absolute change from baseline >10 mmHg is clinically significant and may indicate respiratory depression before oxygen desaturation occurs 2
In non-intubated patients, ETCO2 typically measures 3.5 mmHg lower than arterial PaCO2 (r²=0.772), with stronger correlation in hypocapnic patients (r²=0.838) 4
Clinical Applications
During CPR: If PETCO2 <10 mmHg after 20 minutes of CPR in intubated patients, this may be considered as one component of a multimodal approach to decide when to end resuscitative efforts, though it should not be used in isolation 1, 2
For CPR quality monitoring: If PETCO2 <10 mmHg during active resuscitation, immediately attempt to improve chest compression quality 1, 2
For sedation monitoring: Capnography detects hypoventilation earlier than pulse oximetry or visual observation, particularly valuable during deep sedation when ventilation cannot be directly observed 1
For ventilated patients: ETCO2 monitoring can detect hypoventilation before clinical signs become apparent and should be continuously monitored during patient transport 2
Critical Technical Considerations
Factors Causing Falsely Low ETCO2
- Bronchospasm or mucous plugging of the endotracheal tube 1, 2
- Kinking of the endotracheal tube 1, 2
- Alveolar fluid in the endotracheal tube 1, 2
- Hyperventilation 1, 2
- Air leak in the airway 1, 2
- Mouth breathing or nasal cannula occlusion 2
Important Limitations
In non-intubated patients (bag-mask ventilation or supraglottic airways), ETCO2 may not consistently reflect the true value, making measurements less reliable for prognostication 1
Supplemental oxygen administration delays detection of hypoventilation by maintaining oxygen saturation despite rising CO2 levels 1
All prognostication studies during cardiac arrest included only intubated patients—a specific ETCO2 cutoff value should NOT be used as an indication to end resuscitative efforts in non-intubated patients (Class III: Harm) 1
Confirmation of Endotracheal Tube Placement
Waveform capnography is the gold standard for confirming and monitoring correct endotracheal tube position (Class I, LOE C-LD), while fogging alone is insufficient 2