Pressure-Volume Relationship in the Injured Lung

Acute lung injury (ALI) is known to be associated with abnormal mechanical properties of the respiratory system with hallmark features of a reduction in functional residual capacity (FRC) and a reduction in static compliance of the respiratory system [1]. Measurements of the inspiratory pressure-volume (P-V) curves of the respiratory system have been hence used in mechanically ventilated patients with ALI as a means of assessing their status and progress [1] and to optimize the use of positive end-expiratory pressure (PEEP) [2] and of mechanical ventilation [3–8]. However, although FRC and static compliance of the respiratory system are known to be reduced in patients with ALI [4], the question remains whether this simply reflects the downward displacement along the P-V relationship described in normal humans [9] or an overall change of the characteristic P-V relationship of the respiratory system [10].