The acute respiratory distress syndrome (ARDS) is a frequent and severe form of acute respiratory failure. Mechanical ventilation is the cornerstone of treatment but it may induce a specific form of lung injury (Ventilator induced Lung Injury) responsible for superimposed morbidity and mortality. Lung volume is dramatically decreased during ARDS. Lung volume measurements remained limited to clinical research until recently when the nitrogen washout/washin technique has been adapted for bedside use and implemented in an intensive care ventilator. The aim of this work was to test the nitrogen washout/washin method in clinical conditions of ARDS treatment with high Positive End Expiratory Pressure (PEEP) and high oxygen fraction (FiO2). Once this preliminary validation study was realised, we used the technique to assess the amount of lung recruitment induced by PEEP and positioning.We showed in the first study that accuracy and reproducibility of the technique were acceptable. PEEP and FiO2 had a minor influence on measurements. In the second study, the recruitment induced by a “maximal recruitment” PEEP set to obtain a plateau pressure between 28-32 cmH2O has been estimated using end-expiratory lung volume (EELV) measurements. A significant correlation was found between the recruitment measured on Pressure/Volume curves and the recruitment estimated comparing the predicted minimal increase in lung volume and the true increase in EELV.In a third study, we have evaluated the concomitant effects of verticalization on EELV and oxygenation following a change from supine to semi recumbent, seated and back to supine position. In this third study, verticalization (seated position) resulted in a significant concomitant increase in lung volume and oxygenation. Interestingly, patients responding to verticalization had lower EELV at baseline than non-responders. Only the group of patients increasing their PaO2/FiO2>20% during verticalization had a significant increase in their EELV compared to non-responders.These three studies confirmed the feasibility of the technique, easier than the gold standard techniques (helium dilution or CT scan), and offering both research and clinical perspectives. This technique should also allow an easier approach of the strain induced by ventilation and assess the risk of ventilation induced lung injury.
