What is end tidal CO2 and it's significance
Reading File
Finding Sources
Searching PubMed
"capnography" AND "carbon dioxide"
Searching the Web
end-tidal CO2 capnography clinical significance guideline
Reading File
Reading File
Reading Web Page
https://www.ncbi.nlm.nih.gov/books/NBK539754
End-tidal CO₂ (ETCO₂) is the partial pressure or concentration of carbon dioxide measured at the very end of exhalation. At this point, exhaled gas most closely reflects gas from the alveoli. It is measured continuously by capnography, which provides both a number, in mmHg, and a waveform called a capnogram.
- Typical ETCO₂: about 35-45 mmHg
- In normal lungs and circulation, ETCO₂ is usually about 2-5 mmHg lower than arterial PaCO₂, because of physiologic dead space.
Why it is significant
ETCO₂ gives real-time information about three linked processes:
-
Ventilation
- Rising ETCO₂ suggests hypoventilation, apnea developing, airway obstruction, or excessive CO₂ production.
- Falling ETCO₂ suggests hyperventilation or reduced delivery of CO₂ to the lungs.
-
Pulmonary perfusion and cardiac output
- CO₂ must be delivered by blood to the lungs before it can be exhaled.
- A sudden marked fall in ETCO₂ can occur with severe hypotension, pulmonary embolism, cardiac arrest, or poor-quality chest compressions.
- During CPR, a rising ETCO₂ may indicate improving perfusion; a sudden sustained increase can suggest return of spontaneous circulation.
-
Airway patency and correct endotracheal tube placement
- Persistent, regular capnographic waveforms after intubation strongly support tracheal placement.
- Loss or sudden disappearance of the waveform may indicate apnea, circuit disconnection, ventilator failure, extubation, or tube displacement. Continuous waveform capnography is particularly useful because cardiac arrest may produce low ETCO₂ despite a correctly placed tube. Tintinalli's Emergency Medicine, p. 225.
Common interpretation
| Finding | Possible implication |
|---|---|
| ETCO₂ >45-50 mmHg or progressively rising | Hypoventilation, respiratory depression from sedatives/opioids, airway obstruction, inadequate ventilator rate |
| ETCO₂ <35 mmHg | Hyperventilation, reduced cardiac output/perfusion, pulmonary embolism, leak, disconnection |
| Abrupt fall to near zero | Apnea, disconnected circuit, extubation, severe loss of perfusion |
| “Shark-fin” capnogram | Bronchospasm or obstructive airway disease, such as asthma/COPD |
| Gradual rise during sedation | Early respiratory depression, often before oxygen saturation falls, especially if supplemental oxygen is used |
ETCO₂ versus pulse oximetry
Pulse oximetry answers: “Is the blood oxygenated?”
Capnography answers: “Is the patient ventilating and moving CO₂?”
Capnography answers: “Is the patient ventilating and moving CO₂?”
A sedated patient receiving oxygen can remain well saturated for some time despite hypoventilation or apnea. ETCO₂ and the capnogram can reveal that problem earlier.
Important limitation
ETCO₂ is not always a reliable substitute for arterial PaCO₂. The gap between PaCO₂ and ETCO₂ widens when alveolar dead space increases, for example in severe COPD/asthma, pulmonary embolism, shock, or low cardiac output. Therefore, in unstable or severely ill patients, interpret ETCO₂ trends alongside the clinical picture and, when needed, arterial blood gases.
This is a shared conversation. Sign in to Orris to start your own chat.