Capnography

Capnography is the continuous monitoring of exhaled carbon dioxide (CO2) concentration, displayed as both a numerical value (end-tidal CO2, or EtCO2) and a waveform over time. In airway care, it serves as the primary confirmation tool for tracheal tube placement and the most reliable bedside monitor of ventilation adequacy.

The capnography waveform displays CO2 concentration throughout the respiratory cycle. A normal waveform shows near-zero CO2 during inspiration, a rapid rise during early expiration as dead-space gas mixes with alveolar gas, a plateau representing alveolar CO2 (whose end-point is the EtCO2 value), and a rapid return to baseline with the next inspiration. Normal EtCO2 ranges from approximately 35-45 mmHg.

Clinical applications in airway management:

• Tube placement confirmation — a consistent waveform with appropriate EtCO2 values over multiple breaths is the most reliable confirmation that a tracheal tube is in the trachea, not the esophagus. An esophageal intubation produces no waveform or a rapidly diminishing one. This single application prevents one of the most dangerous airway complications.

• Ventilation monitoring — rising EtCO2 indicates hypoventilation or increased CO2 production; falling EtCO2 may indicate hyperventilation, decreasing cardiac output, or circuit disconnection. Capnography detects ventilation changes before pulse oximetry does.

• Cardiac arrest monitoring — during CPR, EtCO2 reflects the adequacy of chest compressions (which drive pulmonary blood flow and therefore CO2 delivery to the lungs). A sudden rise in EtCO2 during arrest may indicate return of spontaneous circulation.

• Waveform pattern recognition — abnormal waveform shapes indicate specific pathology. A sloped plateau suggests bronchospasm. A gradually rising baseline suggests rebreathing. A sudden loss of waveform suggests extubation, circuit disconnection, or cardiac arrest.

Capnography complements pulse oximetry but provides fundamentally different information. Pulse oximetry monitors oxygenation; capnography monitors ventilation. Together they cover both sides of gas exchange.

Related terms

• Ventilation — the process capnography monitors
• Oxygenation — the companion process monitored by pulse oximetry
• Airway — the passage whose integrity capnography confirms