End-tidal to arterial PCO₂ ratio: a bedside meter of the overall gas exchanger performance

The physiological dead space is a strong indicator of severity and outcome of acute respiratory distress syndrome (ARDS). The "ideal" alveolar PCO₂, in equilibrium with pulmonary capillary PCO₂, is a central concept in the physiological dead space measurement. As it cannot be measured, it is surrogated by arterial PCO₂ which, unfortunately, may be far higher than ideal alveolar PCO₂, when the right-to-left venous admixture is present. The "ideal" alveolar PCO₂ equals the end-tidal PCO₂ (P_ETCO₂) only in absence of alveolar dead space. Therefore, in the perfect gas exchanger (alveolar dead space = 0, venous admixture = 0), the P_ETCO₂/PaCO₂ is 1, as P_ETCO₂, P_ACO₂ and PaCO₂ are equal. Our aim is to investigate if and at which extent the P_ETCO₂/PaCO₂, a comprehensive meter of the "gas exchanger" performance, is related to the anatomo physiological characteristics in ARDS.

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_ETCO₂/PaCO₂, measured at 5 cmH₂O 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_ETCO₂/PaCO₂ (lowest ratio, the worst = group 1, highest ratio, the best = group 4). The progressive increase P_ETCO₂/PaCO₂ 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 PaO₂/FiO₂ and PaCO₂. The improvement of P_ETCO₂/PaCO₂ was also associated with a significant decrease of physiological dead space and venous admixture. When PEEP was increased from 5 to 15 cmH₂O, the greatest improvement of non-aerated tissue, PaO₂ and venous admixture were observed in quartile 1 of P_ETCO₂/PaCO₂ and the worst deterioration of dead space in quartile 4.

The ratio P_ETCO₂/PaCO₂ is highly correlated with CT scan, physiological and clinical variables. It appears as an excellent measure of the overall "gas exchanger" status.