A scanographic assessment of pulmonary morphology in acute lung injury - Significance of the lower inflection point detected on the lung pressure-volume curve

The goal of this study was to assess lung morphology in patients with acute lung injury according to the presence or the absence of a lower inflection point (LIP) on the lung pressure-volume (P-V) curve and to compare the eff ects of positive end-expiratory pressure (PEEP). Eight patients with and si x without an LIP underwent a spiral thoracic CT scan performed at zero end- expiratory pressure (ZEEP) and at two levels of PEEP: PEEP1 = LIP + 2 cm H2 O and PEEP2 = LIP + 7 cm H2O, or PEEP1 = 10 cm H2O and PEEP2 = 15 cm H2O in the absence of an LIP. The volumes of air and tissue within the lungs were measured from the gas-tissue ratio and the volumes of overdistended and no rmally, poorly, and nonaerated lung areas were determined by the analysis o f the frequency histogram distribution. In the ZEEP condition, although tot al lung volume, volume of gas, and volume of tissue were similar in both gr oups, the percentage of normally aerated lung was lower (24 +/- 22% versus 55 +/- 12%, p < 0.05) and the percentage of poorly aerated lung was greater (40 +/- 12% versus 23 +/- 8%, p < 0.05) in patients with an LIP than in pa tients without an LIP. Lung density histograms of patients with an LIP show ed a unimodal distribution with a peak at 7 Hounsfield units (HU). Lung den sity histograms of patients without an LIP had a bimodal distribution, with a first peak at -727 HU and a second peak at 27 HU. Total respiratory syst em and lung compliances were lower in patients with an LIP whereas all othe r cardiorespiratory parameters were similar in the two groups. In both grou ps, PEEP induced an alveolar recruitment that was associated with lung over distension only in patients without an LIP. The amount of lung overdistensi on was related to the volume of lung parenchyma, characterized by a CT numb er less than -800 HU before PEEP implementation (y = 0.52x + 4, R = 0.87, a nd p < 0.0001). This study shows that the presence or the absence of an LIP on the lung P-V curve is associated with differences in lung morphology. I n patients without an LIP on the lung P-V curve, normally aerated lung area s coexist with nonaerated lung areas and increasing levels of PEEP result i n lung overdistension rather than in additional alveolar recruitment. In pa tients with an LIP, air and tissue are more homogeneously distributed withi n the lungs and increasing levels of PEEP result in additional alveolar rec ruitment without lung overdistention.