Capnography is the measurement of the partial pressure of carbon dioxide (CO2) in exhaled air.1 It provides real-time information on ventilation (elimination of CO2), perfusion (CO2 transportation in vasculature), and metabolism (production of CO2 via cellular metabolism).2 The technology was originally developed in the 1970s to monitor general anesthesia patients; however, its reach has since broadened, with numerous applications currently in use and in development for the emergency provider (EP).3
Capnography exists in two configurations: a mainstream device that attaches directly to the hub of an endotracheal tube (ETT) and a side-stream device that measure levels via nasal or nasal-oral cannula.1,3
Qualitative monitors use a colorimetric device that monitors the end-tidal CO2 (EtCO2) in exhaled gas and changes color depending on the amount of CO2 present.2,4 Expired CO2 and H20 form carbonic acid, causing the specially treated litmus paper inside the device to change from purple to yellow.2,4 Quantitative monitors display a capnogram, the waveform of expired CO2 as a function of time; as well as the capnometer, which depicts the numerical EtCO2 for each breath.4 In this overview, we will discuss the general interpretation of capnography and its specific uses in the ED.
Just like the various stages of an electrocardiogram represent different phases of the cardiac cycle, different phases of a capnogram correspond to different phases of the respiratory cycle. Knowing how to analyze and interpret each phase will contribute to the utility of capnography. While there has been considerable ambiguity in the terminology related to the capnogram,5-7 the most frequently referenced capnogram terminology consists of the following phases (Figure 1):
Phase I: represents beginning of exhalation, where the dead space is cleared from the upper airway.2 This should be zero unless the patient is rebreathing CO2-laden expired gas from either artificially increased dead space or hypoventilation.2,8 A precipitous rise in both the baseline and EtCO2 may indicate contamination of the sensor, such as with secretions or water vapor.2,6
Phase II: rapid rise in exhaled as the CO2 from the alveoli reaches the sensor.4 This rise should be steep, particularly when ventilation to perfusion (V/Q) is well matched. More V/Q heterogeneity, such as with COPD or asthma, leads to a more gradual slope.9 A more gradual phase 2 slope may also indicate a delay in CO2 delivery to the sampling site, such as with bronchospasm or ETT kinking.2
Phase III: the expiratory plateau, which represents the CO2 concentration approaching equilibrium from alveoli to nose. The plateau should be nearly horizontal.2 If all alveoli had the same pCO2, this plateau would be perfectly flat, but spatial and temporal mismatch in alveolar V/Q ratios result in variable exhaled CO2. When there is substantial V/Q heterogeneity, the slope of the plateau will increase.1,2,6