The overall populations was divided into four groups (~ 50 patients each) according to the quartiles of the P ETCO 2/PaCO 2 (lowest ratio, the worst = group 1, highest ratio, the best = group 4). The P ETCO 2/PaCO 2, measured at 5 cmH 2O airway pressure, significantly decreased from mild to mild–moderate moderate–severe and severe ARDS. The source was a database in which we collected since 2003 all the patients enrolled in different CT scan studies. We retrospectively studied 200 patients with ARDS.

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).

Anatomical dead space equation