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

# Significance of ETCO2 * PaCO2 is always higher than ETCO2—sometimes much higher. * The most common cause of a sudden drop in ETCO2 is hypotension, which causes decreased pulmonary artery (PA) pressure. Hemorrhage abruptly reduces pulmonary perfusion and increases alveolar dead space, quickly reducing ETCO2. Hemorrhage is a frequent cause of a rapid decrease in ETCO2. * Low PA pressure prevents perfusion of the upper parts of the lung, leading to increased alveolar dead space (DS). * A much less common cause of increased alveolar DS is pulmonary embolism. * A thromboembolus (DVT) abruptly reduces pulmonary perfusion and increases alveolar dead space, abruptly reducing ETCO2.