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capnography waveform interpretation monitoring guideline ASA capnography
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Capnography - One-Page Note
Capnography is continuous graphical monitoring of carbon dioxide (CO₂) throughout the respiratory cycle. The numerical value displayed is end-tidal CO₂ (ETCO₂ or PetCO₂), the maximum expired CO₂ concentration at end-expiration.
Normal values
- ETCO₂: about 35-45 mmHg in a spontaneously breathing healthy adult.
- ETCO₂ is usually 2-5 mmHg lower than PaCO₂ because of physiologic alveolar dead space.
- A widening PaCO₂-ETCO₂ gradient suggests increased dead space, such as pulmonary embolism, low cardiac output, or severe lung disease.
Normal capnogram waveform
| Phase | Meaning |
|---|---|
| Phase 0 | Inspiration: CO₂ rapidly falls to baseline (near zero). |
| Phase I | Early expiration: anatomical dead-space gas, little or no CO₂. |
| Phase II | Expiratory upstroke: mixing of dead-space and alveolar gas. |
| Phase III | Alveolar plateau: predominantly alveolar gas; ETCO₂ is measured at its end. |
A normal waveform is nearly rectangular, with a sharp upstroke and a relatively flat alveolar plateau.
Common waveform abnormalities
| Pattern | Likely cause |
|---|---|
| “Shark-fin” upsloping plateau | Bronchospasm, asthma/COPD, partial airway obstruction, kinked endotracheal tube. |
| Raised baseline, not returning to zero | Rebreathing: exhausted CO₂ absorbent, faulty expiratory valve, inadequate fresh-gas flow. |
| Sudden fall in ETCO₂ | Reduced pulmonary perfusion or cardiac output, hypotension, pulmonary embolism, disconnection, leak, extubation. |
| Gradual increase in ETCO₂ | Hypoventilation, respiratory depression, increased CO₂ production, fever, malignant hyperthermia, rebreathing. |
| Low ETCO₂ with normal waveform | Hyperventilation, low cardiac output, hypothermia, reduced metabolism. |
| Flat trace / zero CO₂ | Apnea, esophageal intubation, circuit disconnection, complete airway obstruction, equipment failure. |
| Curare cleft in plateau | Spontaneous inspiratory effort during controlled ventilation, often due to inadequate neuromuscular blockade or light anesthesia. |
Major clinical uses
- Confirms tracheal intubation: persistent waveform with ventilation strongly supports tracheal rather than esophageal placement.
- Monitors ventilation: detects apnea and hypoventilation earlier than pulse oximetry, particularly when supplemental oxygen is being used.
- During procedural sedation and anesthesia: helps identify respiratory depression and airway obstruction early.
- CPR: continuous waveform confirms tube position and reflects pulmonary blood flow generated by chest compressions. A sudden sustained rise in ETCO₂ may indicate return of spontaneous circulation (ROSC).
- Ventilator and airway troubleshooting: detects obstruction, leaks, disconnection, rebreathing, and bronchospasm.
- Metabolic and circulatory assessment: ETCO₂ falls with reduced cardiac output or pulmonary perfusion and may rise with increased CO₂ production.
Capnography versus pulse oximetry
- Capnography measures ventilation: whether CO₂ is being exhaled.
- Pulse oximetry measures oxygenation: hemoglobin oxygen saturation.
- A patient receiving oxygen may remain well saturated for some time despite apnea. Capnography therefore gives earlier warning of hypoventilation.
Key precautions
Interpret ETCO₂ together with waveform, respiratory effort, oxygen saturation, clinical examination, and hemodynamic status. Low ETCO₂ does not always mean hyperventilation: it may reflect poor perfusion, severe dead space, or a technical problem.
Textbook support: Barash, Cullen, and Stoelting’s Clinical Anesthesia, 9e, p. 3741. The NCBI health technology assessment describes its role in anesthesia, airway confirmation, transport, and CPR. A recent systematic review on ETCO₂ as a severity marker in acute asthma is indexed as PMID 39617353.
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