What is Capnography?
Capnography measures the concentration of carbon dioxide (CO2) in exhaled air, providing a real-time, continuous assessment of ventilation. The end-tidal CO2 (ETCO2) value represents the partial pressure of CO2 at the end of exhalation, which closely correlates with arterial CO2 levels. Normal ETCO2 ranges from 35 to 45 mmHg. Capnography provides both a numeric value and a waveform, and both are clinically important.
While pulse oximetry tells you about oxygenation (how much oxygen is in the blood), capnography tells you about ventilation (how effectively CO2 is being eliminated). A patient can have a normal SpO2 while retaining dangerous levels of CO2, making capnography essential for complete respiratory monitoring. Many EMS systems now consider capnography the standard of care for all intubated patients and increasingly for non-intubated patients as well.
Clinical Applications of Capnography
Endotracheal Tube Confirmation
Capnography is the gold standard for confirming endotracheal tube placement. A properly placed ET tube will produce a consistent ETCO2 waveform with values in the 35 to 45 mmHg range. Esophageal intubation will show an absence of CO2 or rapidly decreasing values (the small amount of CO2 in the stomach is quickly washed out). Continuous capnography also detects tube dislodgment during transport, which can be missed by auscultation alone in a noisy ambulance.
CPR Quality Assessment
During cardiac arrest, ETCO2 correlates with cardiac output generated by chest compressions. Higher ETCO2 values indicate more effective compressions. The American Heart Association recommends targeting an ETCO2 of at least 10 mmHg during CPR, and values below this suggest compressions need improvement. A sudden increase in ETCO2 during cardiac arrest (often to 40 mmHg or above) may indicate return of spontaneous circulation (ROSC), sometimes before a pulse is palpable.
Ventilation Monitoring
Capnography detects hypoventilation (rising ETCO2) and hyperventilation (falling ETCO2) in real time. This is critical for head injury patients, where both hypo- and hyperventilation worsen outcomes. Target ETCO2 of 35 to 45 mmHg for normal ventilation. For asthma and COPD patients, a rising ETCO2 indicates worsening bronchospasm and impending respiratory failure, even if the patient appears to be breathing adequately.
Reading the Capnography Waveform
The normal capnography waveform has a characteristic rectangular shape with four phases. An upsloping plateau ("shark fin" shape) suggests bronchospasm, as seen in asthma or COPD. A gradually rising baseline indicates rebreathing, which may be caused by an exhausted CO2 absorber or inadequate expiratory time. Loss of the waveform indicates apnea, tube dislodgment, or cardiac arrest. Understanding waveform patterns adds a powerful diagnostic dimension beyond the numeric ETCO2 value alone.