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Image Gallery: Capnography

Rachel Reed, DVM, DACVAA, University of Georgia

Anesthesiology & Pain Management

|May 2017|Peer Reviewed|Web-Exclusive

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Capnometry involves the use of a capnometer to measure the amount of carbon dioxide (CO2) in expired gas. CO2 is produced as a by-product of metabolism in living cells and carried via the blood to the lungs, where it diffuses into alveolar gas and is expired. The amount of CO2 in expired gas is a direct representation of the adequacy of alveolar ventilation and an indirect indication of cardiac output and metabolic rate. 

A normal end tidal CO2 (ETCO2) of 35 to 45 mm Hg is expected in the absence of hyper- or hypoventilation. Hyperventilation results in an abnormally low ETCO2 (<30 mm Hg), whereas hypoventilation results in an abnormally high ETCO2 (>50 mm Hg). A decrease in cardiac output or metabolic rate while alveolar ventilation remains unchanged results in a decrease in ETCO2. An increase in cardiac output or metabolic rate while alveolar ventilation remains unchanged results in an increase in ETCO2

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Capnography machines provide clinicians with a waveform (capnogram) created by expired CO2 as it is being measured by the machine. Understanding how to interpret capnograms can be useful in managing patients with hyper- and/or hypoventilation, along with identifying impending cardiac arrest and airway leaks, as well as identifying and troubleshooting complications associated with anesthetic equipment. Capnography arguably can provide more information about the status of patients than do other traditional methods of anesthesia monitoring. 

References and Author Information

For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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