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# Significance of ETCO2 * PaCO2 is always higher than ETCO2—sometimes much higher * The commonest cause of a sudden drop in ETCO2 is hypotension, which causes a decrease in pulmonary artery (PA) pressure. Hemorrhage suddenly reduces pulmonary perfusion and increases alveolar dead space, abruptly de...

# Significance of ETCO2 * PaCO2 is always higher than ETCO2—sometimes much higher * The commonest cause of a sudden drop in ETCO2 is hypotension, which causes a decrease in pulmonary artery (PA) pressure. Hemorrhage suddenly reduces pulmonary perfusion and increases alveolar dead space, abruptly decreasing ETCO2. Hemorrhage is a common cause of a rapid decrease in ETCO2. * Low PA pressure prevents perfusion of the upper parts of the lung, creating increased alveolar dead space (DS). * A much less common cause of increased alveolar DS is pulmonary embolism. * A thromboembolus (DVT) suddenly reduces pulmonary perfusion and increases alveolar dead space, abruptly reducing ETCO2. # Why does increased alveolar dead space reduce the ETCO2? * The gas in alveolar dead space does not contain CO2. * Consequently, alveolar dead space gas dilutes the CO2 from the perfused alveoli. * This dilution of exhaled CO2 during the "alveolar plateau" is the key concept.