End-tidal CO2 ("EtCO2") monitoring is a noninvasive technique that measures the partial pressure or maximal concentration of carbon dioxide (CO2) at the end of an exhaled breath, which is expressed as a percentage of CO2 or mmHg.

The normal values are 5% to 6% CO2, which is equivalent to 35-45 mmHg. CO2 reflects cardiac output (CO) and pulmonary blood flow as the gas is transported by the venous system to the right side of the heart and then pumped to the lungs by the right ventricles. When CO2 diffuses out of the lungs into the exhaled air, a device called capnometer measures the partial pressure or maximal concentration of CO2 at the end of exhalation.

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.

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 (see "Pulse oximetry"). 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 (see "Monitoring during anesthesia", section on 'Capnography') and is an emerging standard of care in emergency medical services, emergency medicine, and intensive care.