End-tidal to arterial PCO(2) ratio: a bedside meter of the overall gas exchanger performance

BACKGROUND: The physiological dead space is a strong indicator of severity and outcome of acute respiratory distress syndrome (ARDS). The “ideal” alveolar PCO(2), in equilibrium with pulmonary capillary PCO(2), is a central concept in the physiological dead space measurement. 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) equals the end-tidal PCO(2) (P(ET)CO(2)) only in absence of alveolar dead space. Therefore, in the perfect gas exchanger (alveolar dead space = 0, venous admixture = 0), the P(ET)CO(2)/PaCO(2) is 1, as P(ET)CO(2), P(A)CO(2) and PaCO(2) are equal. Our aim is to investigate if and at which extent the P(ET)CO(2)/PaCO(2), a comprehensive meter of the “gas exchanger” performance, is related to the anatomo physiological characteristics in ARDS. RESULTS: We retrospectively studied 200 patients with ARDS. The source was a database in which we collected since 2003 all the patients enrolled in different CT scan studies. The P(ET)CO(2)/PaCO(2), measured at 5 cmH(2)O airway pressure, significantly decreased from mild to mild–moderate moderate–severe and severe ARDS. The overall populations was divided into four groups (~ 50 patients each) according to the quartiles of the P(ET)CO(2)/PaCO(2) (lowest ratio, the worst = group 1, highest ratio, the best = group 4). The progressive increase P(ET)CO(2)/PaCO(2) from quartile 1 to 4 (i.e., the progressive approach to the “perfect” gas exchanger value of 1.0) was associated with a significant decrease of non-aerated tissue, inohomogeneity index and increase of well-aerated tissue. The respiratory system elastance significantly improved from quartile 1 to 4, as well as the PaO(2)/FiO(2) and PaCO(2). The improvement of P(ET)CO(2)/PaCO(2) was also associated with a significant decrease of physiological dead space and venous admixture. When PEEP was increased from 5 to 15 cmH(2)O, the greatest improvement of non-aerated tissue, PaO(2) and venous admixture were observed in quartile 1 of P(ET)CO(2)/PaCO(2) and the worst deterioration of dead space in quartile 4. CONCLUSION: The ratio P(ET)CO(2)/PaCO(2) is highly correlated with CT scan, physiological and clinical variables. It appears as an excellent measure of the overall “gas exchanger” status.