Patm, atmospheric pressure; EELV, end-expiratory lung volumemeasured by using the nitrogen technique; …We designed a multicenter study with the primary objective of investigating theprecision (reproducibility) of the nitrogen washout/washin technique for EELVmeasurement in patients with ALI/ARDS at two PEEP levels, including a high level, with asmall variation in oxygen concentration (10%). Our secondary objective was to evaluatethe accuracy of the technique by comparing PEEP-induced changes (��) in lung volumewith the nitrogen technique or the PEEP-volume above the FRC measured with passivespirometry. As PEEP-volume is relatively easy to measure accurately with a calibratedpneumotachograph, it may therefore be considered a “gold standard.” Because we expectedpossible discrepancies between the two techniques, we also compared the measured changesin lung volume (��EELV and ��PEEP-volume) with the minimal predicted increase inlung volume, computed from static compliance (Cstat) at low PEEP and��PEEP. The minimal predicted increase in lung volume was considered thesmallest-volume increase that can occur. We have also used this method to evaluatealveolar recruitment, as described elsewhere [11].Material and methodsThis was a multicenter study performed in five French medical intensive care units atthe Henri Mondor University Hospital in Cr��teil, European Georges PompidouUniversity Hospital in Paris, Angers University Hospital in Angers, l’Archet 1University Hospital in Nice, and Charles Nicolle University Hospital in Rouen. Incompliance with French legislation, the institutional review board of the Henri MondorUniversity Hospital approved the protocol for all centers and waived the need forinformed consent, as PEEP optimization was considered part of standard care. Thepatients or next of kin received information about the study.PatientsPatients were enrolled if they met the standard criteria for acute lung injury (ALI) [12]: partial pressure of arterial oxygen over fraction of inspired oxygen(PaO2/FiO2) less than 300 mm Hg, bilateral pulmonaryinfiltrates on the chest radiograph, and no clinical evidence of left atrialhypertension. Most patients had ARDS, defined as PaO2/FiO2 lessthan 200 mm Hg. Exclusion criteria were age younger than 18 years, pregnancy, historyof chronic obstructive pulmonary disease and/or lung surgery, and hemodynamicinstability, defined as an increase in vasoactive drug (epinephrine, norepinephrine)dosages in the last 6 hours.