Oleic acid lung injury: a morphometric analysis using computed tomography

Background: The oleic acid-induced lung injury (OAI) model is considered to represent the early phase of acute respiratory distress syndrome (ARDS). Its inherent properties are important for the design and the interpretation of interventional studies. The aim of this study was to describe the evolution of morphometric lung changes during OAI using computed tomography (CT) analysis. Furthermore, the effect of a temporary change in positive end-expiratory pressure (PEEP) was evaluated. Methods: Fifteen anaesthetized pigs were ventilated in volume-controlled mode with a baseline PEEP of 5 cm H2O. Helical CT scans were taken at baseline and 1 h after oleic acid injection. The PEEP was then either increased to 10 cm H2O (n¼5), decreased to 0 cm H2O (n¼5) or kept constant (n¼5) for 30min. For the next 30min, the baseline PEEP level was applied in all animals before the final CT scans 2 h after the induction of OAI. Dimensional and volumetric changes were determined from radiographical attenuation values. Results: There was a major decrease in gas volume and an increase in tissue volume within the first hour. A net increase in total lung volume, with a larger transverse area but no displacement of the diaphragm, was manifest after 2 h. A minor increase in volume of non-aerated lung, located to the caudal region, was observed during the second hour. The tidal volume was redistributed to the middle and apical regions. The temporary change in PEEP did not influence the morphological progress of OAI. Conclusion: Decreased gas volume and increased tissue volume are the dominating morphometric characteristics of oleic acid lung injury, occurring mainly within the first hour. With these changes manifest, the course of injury is not affected by a limited period of moderately changed PEEP during the second hour. The net increase of total lung volume suggests a predominance of oedema formation over airway and alveolar collapse.