Our aim is to investigate if and at which extent the P ETCO 2/PaCO 2, a comprehensive meter of the “gas exchanger” performance, is related to the anatomo physiological characteristics in ARDS. Therefore, in the perfect gas exchanger (alveolar dead space = 0, venous admixture = 0), the P ETCO 2/PaCO 2 is 1, as P ETCO 2, P ACO 2 and PaCO 2 are equal. The “ideal” alveolar PCO 2 equals the end-tidal PCO 2 (P ETCO 2) only in absence of alveolar dead space.
As it cannot be measured, it is surrogated by arterial PCO 2 which, unfortunately, may be far higher than ideal alveolar PCO 2, when the right-to-left venous admixture is present. The “ideal” alveolar PCO 2, in equilibrium with pulmonary capillary PCO 2, is a central concept in the physiological dead space measurement. The physiological dead space is a strong indicator of severity and outcome of acute respiratory distress syndrome (ARDS).
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