Emerging Uses of Capnometry in Emergency Medicine

The Physiologic Basis for Capnometry Capnometry is based on a discovery by chemist Joseph Black, who in 1875 noted the properties of a gas released during exhalation that he called “fixed air.” That gas—carbon dioxide—is produced as a consequence of cellular metabolism, a waste product of the process of oxygen and glucose combining to produce energy. Carbon dioxide exits the body via the lungs. The amount of CO2 in exhaled breath reflects cardiac output and pulmonary blood flow as the gas is transported by the venous system to the right side of the heart and then pumped into the lungs by the right ventricle. Carbon dioxide is involved not only in oxidative cellular metabolism, but also in gas transport and ventilation. Capnometers measure the concentration of CO2 exhaled at the end of the breath, commonly known as the end-tidal breath CO2 (ETCO2). When outside the normal range, high or low, it indicates a problem with ventilation that requires immediate action. As long as the heart is beating and blood is flowing, CO2 is delivered continuously to the lungs for exhalation. Anything that interferes with normal ventilation quickly changes ETCO2, even as SPO2 —the indirect measurement of oxygen saturation in the blood—levels remain high. Thus, CO2 is a better, more sensitive and more rapid indicator of ventilation problems than SPO2 measurements.