The term capnography refers to the noninvasive measurement of the partial pressure of carbon dioxide (CO2) in exhaled breath expressed as the CO2 concentration over time. The relationship of CO2 concentration to time is graphically represented by the CO2 waveform, or capnogram (figure 1). Changes in the shape of the capnogram are diagnostic of disease conditions, while changes in end-tidal CO2 (EtCO2), the maximum CO2 concentration at the end of each tidal breath, can be used to assess disease severity and response to treatment. Capnography is also the most reliable indicator that an endotracheal tube is placed in the trachea after intubation.
Oxygenation and ventilation are distinct physiologic functions that must be assessed in both intubated and spontaneously breathing patients. Pulse oximetry provides instantaneous feedback about oxygenation. Capnography provides instantaneous information about ventilation (how effectively CO2 is being eliminated by the pulmonary system), perfusion (how effectively CO2 is being transported through the vascular system), and metabolism (how effectively CO2 is being produced by cellular metabolism).
Capnography became a routine part of anesthesia practice in Europe in the 1970s and in the United States in the 1980s. It is now part of the standard of care for all patients receiving general anesthesia and is an emerging standard of care in emergency medical services, emergency medicine, and intensive care.
This topic review will discuss the basic physiology and interpretation of capnography and its use in the emergency department.
PRINCIPLES OF OPERATION
Carbon dioxide (CO2) monitors measure gas concentration, or partial pressure, using one of two configurations: mainstream or sidestream. Mainstream devices measure respiratory gas (in this case CO2) directly from the airway, with the sensor located on the airway adapter at the hub of the endotracheal tube. Sidestream devices measure respiratory gas via nasal or nasal-oral cannula by aspirating a small sample from the exhaled breath through the cannula tubing to a sensor located inside the monitor (picture 1).