Differences in the deflation limb of the pressure-volume curves in acute respiratory distress syndrome from pulmonary and extrapulmonary origin

Objective. To assess the differences in the deflation pressure-volume (PV) curves between acute respiratory distress syndrome from pulmonary (ARDSp) and extrapulmonary (ARDSe) origin. Design. Prospective study.Setting. Twenty-bed intensive care unit in an university hospital. Patients. Ten patients within the first 24 h from meeting ARDS criteria, classified as ARDSp or ARDSe in a clinical basis. Interventions. A deflation PV curve was recorded by means of decreasing steps of continuous positive airway pressure (CPAP) from 35 to 0 cmH(2)O.Results. The simultaneous recording of pressure at the airway opening (Pao), esophageal pressure (Pes) and volumes (V) allows us to trace the Pao-V, Pes-V and transpulmonary pressure (Ptp)-V curves. These data were fitted to a sigmoid model and ARDSp and ARDSe groups were compared. ARDSp has lower lung compliance and higher chest wall compliance than ARDSe (35.9+/-11.3 vs. 77.2+/-50.6 and 199.6+/-44.4 vs. 125.5+/-16.5 ml/cmH(2)O, respectively, P<0.05). The Pao-V curve in ARDSp is shifted down and right with respect to ARDSe. The Ptp-V curve shows a similar displacement. The Pes-V curve in the ARDSp group is, however, shifted to the left. When relative values (percentage to the maximum volume achieved at 35 cmH(2)O) are considered, these differences persist, but, in the Ptp-V curves, are only significant in the low-pressure range.Conclusions. Differences between ARDSp and ARDSe PV curves are present all along the pressure axis and are related to differences not only in the Pes-V curve, but also in the Ptp-V curve.