📉 Capnography = continuous, non-invasive measurement of CO₂ in respiratory gases.
It is an essential monitor in anaesthesia, critical care, sedation, and resuscitation.
Provides information on ventilation, perfusion, and metabolism.
🧾 About
- Measures concentration or partial pressure of CO₂ in exhaled breath.
- End-tidal CO₂ (EtCO₂) = maximum CO₂ at end of expiration (normally 35–45 mmHg).
- Does not require an ETT: can be measured with supraglottic airways, facemask (good seal), or nasal cannula.
- Can be displayed as:
- 📊 Number only (e.g. portable Emma device).
- 📈 Waveform capnogram (ventilator/monitor display).
- CO₂ scrubber (soda lime) prevents rebreathing → failure causes ↑ inspired CO₂.
⚒️ Uses
- ✅ Confirm tracheal intubation (gold standard vs oesophageal intubation).
- ✅ Monitor ventilation in anaesthesia, ICU, and procedural sedation.
- ✅ Cardiac arrest:
- EtCO₂ > 10–20 mmHg = adequate compressions.
- Sudden ↑ EtCO₂ = ROSC (before pulse detectable).
- EtCO₂ > 20 mmHg after 20 min → better prognosis.
- 🧠 Traumatic brain injury: avoid hyperventilation-induced ↓EtCO₂, which ↓ cerebral perfusion pressure.
- 🦠 Emerging role in sepsis: EtCO₂ may correlate with prognosis.
- 🏥 Other: sleep studies, OSA monitoring.
📈 Capnogram Waveform
- A → Start of exhalation (baseline, no CO₂).
- B → Dead space gas mixed with alveolar CO₂.
- C → Expiration of alveolar gas.
- D → Alveolar plateau (reflects V/Q status).
“Shark fin” appearance = bronchospasm/asthma/COPD.
- E → EtCO₂ (highest point, reported value).
- F → Inspiration begins (rapid fall back to baseline).
📊 Causes of High EtCO₂
- ⬆️ Metabolism: malignant hyperthermia, fever, shivering, sepsis.
- ⬇️ Ventilation: hypoventilation, equipment malfunction (e.g. exhausted soda lime).
📊 Causes of Low EtCO₂
- ⬆️ Ventilation: hyperventilation.
- ⬇️ Circulation: cardiac arrest, PE, shock, severe hypotension.
- ⬇️ Metabolism: hypothermia, metabolic suppression.
🚫 No EtCO₂ Trace
- ❌ Oesophageal intubation (initial trace may appear briefly).
- ❌ CO₂ embolism (e.g. laparoscopic insufflation into vessel/organ → sudden loss of trace + cardiovascular collapse).
- ❌ Equipment failure / disconnection.
📚 References
- Thompson JE, Jaffe MB. Capnographic waveforms in mechanically ventilated patients. Respir Care. 2005;50(1):100-109.
- Long B, Koyfman A, Vivirito MA. Capnography in the emergency department. J Emerg Med. 2017;53(6):829-842.
