Understanding capnography

What capnography measures (and why it matters)

• Capnography displays carbon dioxide (CO2) in exhaled gas over time as a waveform (capnogram) and a number (end‑tidal CO2, ETCO2).
• ETCO2 is the CO2 at the end of expiration; it is a useful bedside indicator of ventilation and (indirectly) circulation and metabolism.
• Continuous capnography is a core safety monitor for any patient with an advanced airway (e.g., tracheal tube, supraglottic airway) and during sedation where ventilation may be depressed.
• Capnography can detect problems earlier than pulse oximetry (oxygen saturation may stay normal for a while even if ventilation stops).

Key definitions for new starters

• ETCO2: the numeric value at the end of the expiratory phase; typically displayed in kPa in the UK.
• Capnogram: the waveform showing CO2 through the respiratory cycle.
• Dead space: ventilated but not perfused (or poorly perfused) lung; increases the gap between arterial CO2 and ETCO2.
• Rebreathing: inhaling previously exhaled CO2 (e.g., faulty circuit/valves or inadequate fresh gas flow).

Normal capnogram: what you should expect to see

• Baseline near zero during inspiration (inspired gas should contain little/no CO2).
• Rapid upstroke as exhalation begins, then a near-flat “alveolar plateau”.
• ETCO2 is measured at the end of the plateau (end expiration).
• A sharp downstroke back to zero with inspiration.

Typical ETCO2 values (context matters)

• Most anaesthetised, ventilated adults: ETCO2 often around 4.5–5.5 kPa (varies with patient and targets).
• Higher ETCO2 suggests hypoventilation, increased CO2 production, or rebreathing (check the whole clinical picture).
• Lower ETCO2 suggests hyperventilation, reduced pulmonary blood flow (e.g., hypotension), disconnection/leak, or cardiac arrest.
• Always interpret alongside respiratory rate, tidal volume, airway pressures, SpO2, blood pressure, and clinical assessment.

First-time scenarios: pattern recognition and immediate actions

• Sudden loss of trace (flat line): think disconnection, extubation, complete obstruction, or cardiac arrest. Act immediately: check patient, check circuit/airway, start ventilation with 100% O2, call for help.
• Gradual fall in ETCO2: consider reduced cardiac output/hypotension, increasing leak, or hyperventilation. Check BP/pulse, circuit integrity, ventilator settings.
• Rising ETCO2: consider hypoventilation (low minute ventilation), rebreathing, increased CO2 production (fever, shivering), or malignant hyperthermia (if rapid rise with other signs). Increase ventilation while investigating cause.
• Baseline not returning to zero: suggests rebreathing (e.g., exhausted soda lime, faulty expiratory valve, inadequate fresh gas flow, insufficient washout). Check valves, absorber, fresh gas flow, circuit setup.
• “Shark fin” (slanted upstroke/plateau): suggests airflow obstruction/bronchospasm or kinked tube. Treat bronchospasm (deepen anaesthesia, bronchodilator), check tube patency and circuit.
• Small trace/low amplitude: may be leak around airway (e.g., uncuffed tube, cuff leak), partial disconnection, or sampling line issue. Check cuff pressure, connections, sampling line.

Confirming tracheal intubation: how capnography helps

• Sustained CO2 waveform over several breaths is the most reliable sign of tracheal placement in a perfusing patient.
• If ETCO2 is absent/very low after intubation: assume oesophageal intubation until proven otherwise, but consider low cardiac output/cardiac arrest as an alternative explanation.
• In cardiac arrest, ETCO2 may be low; you still expect some CO2 if the tube is in the trachea and compressions are effective.
• Always combine capnography with clinical checks (chest rise, auscultation, tube depth, airway pressures) and oxygenation.

Capnography in CPR and peri-arrest

• ETCO2 reflects pulmonary blood flow during CPR; low values can indicate poor-quality compressions or severe low output.
• A sudden sustained rise in ETCO2 during CPR can indicate return of spontaneous circulation (ROSC).
• If ETCO2 is persistently very low, check: airway/circuit, compression quality, ventilation rate (avoid hyperventilation), and reversible causes.

Practical setup tips (sidestream vs mainstream)

• Sidestream sampling: a small sample is aspirated via a sampling line to the analyser; common in anaesthesia machines. Watch for water/secretions blocking the line and delays in waveform.
• Mainstream sampling: sensor sits in the airway; faster response but adds dead space/weight and is less common in theatre anaesthesia.
• Keep sampling lines unkinked and connected; use water traps if available; replace blocked lines promptly.
• If the trace looks wrong, first check the patient and the airway, then the sampling line and monitor.

Safe approach when the capnogram changes: a simple routine

• Look at the patient first: chest movement, colour, airway sounds, bag feel, consciousness level (if sedated).
• Check the airway and circuit: tube position, connections, filter/HME, valves, ventilator settings, oxygen supply.
• Check physiology: BP/HR, temperature, signs of bronchospasm, pneumothorax, pulmonary embolism, or low cardiac output.
• Escalate early: call for help if loss of trace, rapid deterioration, or uncertainty.