Waveform capnography measures the CO₂ produced at the end of the expiratory phase (end-tidal CO₂) and is generally written as ETCO₂. Since cardiac output accounts for the majority of CO₂ that is transported to the lungs, ETCO₂ is well correlated with cardiac output as long as ventilation is stable.

Waveform capnography allows confirmation of accurate ET tube placement and continuously provides feedback. Alternatives to waveform capnography include a CO₂ or esophageal detector or ultrasound (with an experienced technician).

End-Tidal CO₂ Should be Between 35 and 40 mm Hg

Monitor Displaying Vital Signs and Capnography

Using capnography to evaluate or confirm the placement of other advanced airways is not as well studied as it is for ET tubes. However, capnography is likely to help determine if there is effective ventilation with such an airway. When capnography is used with an ET tube, it detects infrared light emitted through the tube, representing CO₂. It can easily detect low end-tidal CO₂, suggestive of esophageal intubation and is sufficiently sensitive to monitor for a dislodged ET tube. It is useful for a patient in cardiac arrest and can monitor the effectiveness of CPR due to its ability to detect very low levels of CO₂. Capnography and monitoring arterial pressure and central venous saturation of oxygen may be useful in optimizing CPR, managing vasopressors, and determining ROSC.

Comprehensive waveform capnography provides both a waveform and objective number measurements. Normal expiration displays characteristic waveforms that indicate both the inspiratory and expiratory phases. The peak is the end-tidal CO₂.

Normal Waveform, Adequate Compression, and ET Tube Dislodgement Capnography

When patients have sufficient circulation, waveform capnography can denote the appropriate placement of advanced airways. Generally, the ETCO₂ should range between 35 and 40 mm Hg.

When patients are in cardiac arrest, waveform capnography can denote the efficacy of CPR chest compression as well as ROSC. ETCO₂ of at least 10 mm Hg (> 20 mm Hg is ideal) indicates good CPR effort and the ability to achieve ROSC. A sustained ETCO₂ of 40 mm Hg or higher suggests ROSC has been achieved. On the other hand, an ETCO₂ of 0 can indicate that the advanced airway is placed incorrectly or that there is no gas exchange.