A c u t e r e s p i r a t o r y d i s t r e s s s y n d r o m e ( A R D S ) i scharacterized by increased elastance of the lung and respiratory system (1). Depending on the precipitating factor, pulmonary vs. extrapulmonary, the distribution of pathologic findings and altered respiratory mechanics in the lung is heterogeneous (2). Portions of the lung can be collapsed and/or fluid-filled while others are well-aerated. In order to facilitate gas exchange, recruitment maneuvers are sometimes employed in patients with ARDS (3). These can be performed with a constant high pressure inspiratory hold for 30-40 seconds (4,5) or via stepwise recruitment by increasing the positive end-expiratory pressure (PEEP) (6), typically followed by application of PEEP at a higher level than the previous baseline to maintain aeration of the recruited lung units (7). Recruitment maneuvers have been proposed as useful tools in managing patients with ARDS in order to add previously non-participatory lung units to gas exchange as well as to assess disease severity. The potential effectiveness of recruitment maneuvers in incorporating previously collapsed lung units into gas exchange or in improving the distension of previously poorly aerated pulmonary units differs from patient to patient consequent to the variability in etiology of the lung injury and the heterogeneity of lung parenchyma (8).A recent study by Chiumello and colleagues compared computed tomographic (CT) to respiratory mechanics methodologies for the measurement of the effectiveness of lung recruitment in ARDS (9). For the CT method, CT scans of the lungs at two levels of inflation were used to assess the effectiveness of increased PEEP for recruitment of new lung units to gas exchange (8). For this method, pulmonary voxels are classified into groups based on relative density as measured by CT Hounsfield units (HU). By established convention, regions with HU of >−100 are designated as having no aeration, −100 to −500 poorly aerated, −500 to −900 good aeration, and −900 to −1,000 as over-distended. Comparing the number of voxels in each group in CT scans performed at 5 cmH 2 O PEEP and 15 cmH 2 O PEEP established recruitment as the change in mass of non-ventilated lung between the CT scans.By contrast, respiratory mechanics methods for determining recruitment measure the change in total lung volume of a single breath at different levels of PEEP compared with the anticipated change in volume if compliance were unchanged. One method (termed the EELV-Cst, RS method by Chiumello et al.) measures the static compliance of the respiratory system for one breath at 5 cmH 2 O PEEP and calculates the anticipated lung volume at 15 cmH 2 O PEEP if compliance were stable. The expected increase is compared with measured end-expiratory lung volume by helium dilution at 15 and 5 cmH 2 O PEEP. The lung volume measured at PEEP 15 cmH 2 O in excess of what was predicted from compliance measurement is attributed to recruitment (10). A related method of slow-flow pressure-volume curves generated...