Absent ETCO2 during CPR suggests oesophageal intubation or circuit fault until proven otherwise. Always correlate with clinical picture.
Normal waveform
- Square-ish expiratory plateau (Phase II to III)
- ETCO2 typically 4 to 5 kPa (35 to 45 mmHg) in stable anaesthesia
- Sudden drop: disconnect, extubation, massive embolism
Low ETCO2 causes
- Hyperventilation, reduced cardiac output
- Pulmonary embolism, CPR with poor perfusion
- Leak in circuit or sampling line
Normal trace
- Square plateau, ETCO2 4 to 5 kPa
- Ventilation and perfusion adequate
- Stable anaesthesia
Abnormal pattern
- Rising: hypoventilation, rebreathing, MH
- Falling: low output, embolism, bronchospasm
- Sloped: obstruction or bronchospasm
Absent trace
- Oesophageal intubation until excluded
- Disconnection or circuit fault
- Cardiac arrest or no output
Troubleshooting
1
1. Confirm trace
Capnograph present before relying on SpO2 alone for tube placement.
Capnograph present before relying on SpO2 alone for tube placement.
↓
2
2. Zero and calibrate
Check sampling line, water trap, and connection at filter.
Check sampling line, water trap, and connection at filter.
↓
3
3. Interpret trend
Rising ETCO2: hypoventilation, malignant hyperthermia, rebreathing. Falling: hypotension, embolism, bronchospasm.
Rising ETCO2: hypoventilation, malignant hyperthermia, rebreathing. Falling: hypotension, embolism, bronchospasm.
↓
4
4. Act
Adjust ventilation, treat cause, confirm airway if waveform lost.
Adjust ventilation, treat cause, confirm airway if waveform lost.
Phase I to IV (brief)
- I dead space gas, II mixing, III alveolar plateau, IV inspiration
Pitfalls
- Confusing rebreathing with low fresh gas flow
- Ignoring gradual rise (MH, sepsis)
MedPrep - www.medprep.uk - Educational revision aid only. Not clinical guidance. Doses are adult examples; check the BNF, current guidelines and local policy, and seek senior advice. Nov 2026