Pressure-volume curve does not predict steady-state lung volume in canine lavaae lung injury

To better understand strategies for recruiting and maintaining lung volume in acute lung injury, we examined relationships between steady-state lung volume and cumulative cyclic recruitment/ derecruitment volume history and the quasi-static pressure-volume curve, in an animal saline lavage lung injury model. Small-volume tidal pressure-volume loops performed after inflation from functional residual capacity demonstrated incremental, cyclic recruitment only if the peak pressure achieved exceeded the pressure at which the compliance increased (Pflex) on the pressure-volume curve, whereas loops performed after deflation from total lung capacity remained close to the envelope deflation curve. Recruitment continued to occur up to and beyond a peak inspiratory airway pressure of 40 cm H2O, as demonstrated by both the tidal loops and by computed tomography-derived lung volume data. Tidal-specific compliance was relatively constant across positive end-expiratory pressure levels after inflation from functional residual capacity, but peaked at moderate positive end-expiratory pressure after deflation from total lung capacity, further demonstrating the effects of volume history and providing experimental validation of the recruitment models of Hickling (AJRCCM 2001;163:69-78). These results support the interpretation of Pflex as pressure threshold for recruitment, but otherwise do not suggest a role for the pressure-volume curve in predicting steady-state lung volume.