Volume Capnography in the Intensive Care Unit: Physiological Principles, Measurements, and Calculations.

Volume capnography provides a noninvasive and continuous display of the fractional concentration or partial pressure of expired carbon dioxide versus exhaled volume. Derived measurements and calculations include the end-tidal, mean alveolar, and mixed expired carbon dioxide partial pressure (Pco2 ); the volume of CO2 exhaled per breath and per minute; the airway, alveolar, and physiologic dead space volume; the physiologic dead space to tidal volume ratio; and expired minute ventilation, dead space ventilation, and alveolar ventilation. Thus, volume capnography provides a wealth of information about both respiratory and cardiovascular function, and it has many potential applications in critically ill patients. The effective use of volume capnography, however, requires a thorough understanding of its underlying physiological principles and the meaning and significance of derived measurements and calculations.