• Ye Liu, Da-wei Liu, Yun Long, and Zhi-yi Xie.
• Department of Critical Care Medicine, China Academy of Medical Science, Beijing, China.
• Zhonghua Nei Ke Za Zhi. 2012 Aug 1;51(8):604-8.

ObjectiveTo investigate the relationship between end-tidal carbon dioxide with its related indicators and ventilation/perfusion of the acute respiratory distress syndrome (ARDS) lung, and to explore a feasible way to titrate positive end-expiratory pressure (PEEP) in clinical practice.MethodsFive mixed-breed dogs with oleic acid lung injury model were mechanically ventilated at a serial PEEP trial including a recruitment maneuver (RM) before each PEEP level changed. The value of blood dynamics, end-tidal carbon dioxide partial pressure (PetCO2) and arterial carbon dioxide pressure under different PEEP levels were recorded. Arterial end-tidal carbon dioxide gradient (Pa-etCO2) and dead space fraction (Vd/Vt%) were calculated. All dogs received CT scan. Lung volume under different pressure levels, and ratio and volume of alveolar closing pressure, collapsed alveoli, sufficiently and insufficiently ventilated alveoli were obtained. Alveolar opening and closing analysis were performed by non-liner regression equation.ResultsThe mean pressure when Vd/Vt% obtained lowest level were (11.2 ± 4.4) cm H2O (1 cm H2O = 0.098 kPa), which had no significant difference when compared to alveolar closing pressure [(11.5 ± 3.2) cm H2O](P > 0.05). The fraction of insufficiently ventilated and collapsed alveoli showed a significant linear correlation with the Vd/Vt% when PEEP was lower than P(min) (r = 0.632, P = 0.004). There was a linear correlation between the Vd/Vt% and the fraction of over-distended alveoli when PEEP was higher than P(min) (r = 0.770, P = 0.001).ConclusionsClosing pressure is in accordance with PEEP level after RM having reached the best ventilation/circulation ratio. The characteristics of lung collapse can be revealed by Vd/Vt% changes after RM. To titrate PEEP for the lowest Vd/Vt% after RM may be a feasible way to match the best ventilation and circulation effects of PEEP.

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