“ P(a-et)CO2

If PaCOand PetCOare known, the gradient can be calculated. Normally, the gradient is less than 5 mm Hg; however, it can be increased with deadspace disease such as pulmonary embolism or decreased cardiac output.674 The presence of increased deadspace may also have some signi­ficance as a prognostic indicator in surgical patients, although further research is needed to confirm this.681 Interestingly, P(a-et)COmeasured after forced exhalation seems to be best for evaluation of acute pulmonary embolism,679  although this is not always prac­tical in the clinic.

Occasionally, PetCOmay actually be higher than PaCO2. The reasons for this are unclear but are most likely due to emptying of low V/Q units with long time constants at the end of expiration.


674. AARC Clinical Practice Guideline. Capnography/ capnometry during mechanical ventilation (2003 update). Respir. Care, 48:534-539, 2003.

679. Hatle, L., and Rokseth, R.: The arterial to end- expiratory carbon dioxide gradient in acute pulmonary embolism and other cardiopulmonary diseases. Chest, 66:352-357,1974.

681. Domsky, M., Wilson, R. F., and Heins, J.: Intra­operative end-tidal carbon dioxide values and derived calculations correlated with outcome: Prognosis and capnography. Crit. Care Med., 23:1497-1503, 1995. ”1








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