CAPNOGRAPHY IN CRITICAL-CARE MEDICINE

Capnography is a noninvasive method of monitoring exhaled CO2 and is u sed in many situations as a reflection of arterial CO2 concentration. Expired CO2 can be accurately measured using infrared spectrometry, ma ss spectrometry, and Raman scattering. Colorimetric CO2 detectors are useful as a qualitive indicator of the presence of CO2. Trending end-t idal CO2 and analysis of the capnographic wave form can provide inform ation on changes in a patient's cardiopulmonary status or malfunction of equipment used to support a patient's cardiovascular or pulmonary s ystems. The accuracy of end-tidal CO2 measurements as a reflection of arterial CO2 tension in critically ill patients with respiratory or ca rdiovascular problems is limited. Changes in deadspace, respiratory ra te and depth. positive end-expiratory pressure, aveolar circulation. a nd cardiac output can significantly impact end-tidal CO2 values more t han 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 representative of a cha nge in arterial CO2 concentrations. Despite the limitations of end-tid al CO2 as a reflection of arterial CO2, it is still useful as a noninv asive monitor to alert clinicians of potential changes in a patient's cardiopulmonary condition.