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 (PETCO2) only in absence of alveolar dead space. Therefore, in the perfect gas exchanger (alveolar dead space = 0, venous admixture = 0), the PETCO2/PaCO2 is 1, as PETCO2, PACO2 and PaCO2 are equal. Our aim is to investigate if and at which extent the PETCO2/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 PETCO2/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 PETCO2/PaCO2 (lowest ratio, the worst = group 1, highest ratio, the best = group 4). The progressive increase PETCO2/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 PETCO2/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 PETCO2/PaCO2 and the worst deterioration of dead space in quartile 4. The ratio PETCO2/PaCO2 is highly correlated with CT scan, physiological and clinical variables. It appears as an excellent measure of the overall “gas exchanger” status.

中文翻译：

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潮气末与动脉PCO _2的比值：整体气体交换器性能的床头仪表

生理上的死腔是严重急性呼吸窘迫综合征（ARDS）严重程度和预后的重要指标。与肺毛细血管PCO2平衡的“理想”肺泡PCO2是生理死腔测量中的核心概念。由于无法测量，因此被动脉PCO2替代，不幸的是，当存在从右到左的静脉混合气时，它可能远高于理想的肺泡PCO2。仅在没有肺泡死腔的情况下，“理想的”肺泡PCO2才等于潮气末PCO2（PETCO2）。因此，在理想的气体交换器中（肺泡死腔= 0，静脉混合气= 0），PETCO2 / PaCO2为1，因为PETCO2，PACO2和PaCO2相等。我们的目的是调查PETCO2 / PaCO2（一种全面的“气体交换器”性能指标）在什么程度上以及在什么程度上发挥作用，与ARDS的解剖生理特征有关。我们回顾性研究了200例ARDS患者。资料来源是我们自2003年以来收集的所有参与不同CT扫描研究的患者的数据库。PETCO2 / PaCO2在5 cmH2O气道压力下测量，从轻度降至中度-中度-重度和重度ARDS显着降低。根据PETCO2 / PaCO2的四分位数，将总体人群分为四组（每组约50位患者）（最低比例，最差=第1组，最高比例，最好=第4组）。PETCO2 / PaCO2从四分位数1逐渐增加到4（即，逐步达到“完美”气体交换器值为1.0）与未充气组织的显着减少，不均一性指数和充气良好的组织的增加有关。呼吸系统弹性从四分位数1改善到4，以及PaO2 / FiO2和PaCO2。PETCO2 / PaCO2的改善还与生理学死腔和静脉内混合物的显着减少有关。当PEEP从5 cmH2O增加到15 cmH2O时，在PETCO2 / PaCO2的四分之一中观察到未充气组织，PaO2和静脉混合物的最大改善，在四分位数4中死区的恶化最严重。PETCO2/ PaCO2的比率很高与CT扫描，生理和临床变量相关。它似乎是衡量整体“气体交换器”状态的极好方法。当PEEP从5 cmH2O增加到15 cmH2O时，在PETCO2 / PaCO2的四分之一中观察到未充气组织，PaO2和静脉混合物的最大改善，在四分位数4中死区的恶化最严重。PETCO2/ PaCO2的比率很高与CT扫描，生理和临床变量相关。它似乎是衡量整体“气体交换器”状态的极好方法。当PEEP从5 cmH2O增加到15 cmH2O时，在PETCO2 / PaCO2的四分之一中观察到未充气组织，PaO2和静脉混合物的最大改善，在四分位数4中死区的恶化最严重。PETCO2/ PaCO2的比率很高与CT扫描，生理和临床变量相关。它似乎是衡量整体“气体交换器”状态的极好方法。