Makindo Medical Notes

📉 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.