Capnography is a noninvasive method of monitoring exhaled CO2 and is used in many situations as a reflection of arterial CO2 concentration. Expired CO2 can be accurately measured using infrared spectrometry, mass spectrometry, and Raman scattering. Colorimetric CO2 detectors are useful as a qualitive indicator of the presence of CO2. Trending end-tidal CO2 and analysis of the capnographic wave form can provide information on changes in a patient's cardiopulmonary status or malfunction of equipment used to support a patient's cardiovascular or pulmonary systems. The accuracy of end-tidal CO2 measurements as a reflection of arterial CO2 tension in critically ill patients with respiratory or cardiovascular problems is limited. Changes in deadspace, respiratory rate and depth, positive end-expiratory pressure, areolar circulation, and cardiac output can significantly impact end-tidal CO2 values more than arterial CO2 concentrations. With marked changes in end-tidal CO2 that are of concern, arterial blood gas analysis should be obtained to determine if the end-tidal CO2 measurement is representatire of a change in arterial CO2 concentrations. Despite the limitations of end-tidal CO2 as a reflection of arterial CO2, it is still useful as a noninvasive monitor to alert clinicians of potential changes in a patient's cardiopulmonary condition.
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