Alveolar derecruitment at decremental positive end-expiratory pressure levels in acute lung injury - Comparison with the lower inflection point, oxygenation, and compliance

We examined the hypothesis that recording multiple elastic pressure-volume (Pel/V) curves and calculating alveolar derecruitment (V-DER) induced by decreasing positive end-expiratory pressure (PEEP) may allow determination of alveolar closing pressures, thus helping to select the optimal PEEP level. V-DER measured in 16 patients with acute lung injury (ALI) was compared with the lower inflection point (LIP) and oxygenation changes. A modified automated method was used to record multiple Pel/V curves at low constant flow. PEEP was decreased in 5-cm H2O steps, from 20 or 15 cm H2O to 0 cm H2O (ZEEP). V-DER was the volume loss between the curves recorded from PEEP and from ZEEP at the same Pel. Derecruitment occurred at each PEEP decrement, being spread almost uniformly over the 20/15 to 0 cm H2O range. V-DER was not correlated with LIP. V-DER changes correlated with Pa-O2/FIO2 changes (rho = 0.6, p = 0.02). Linear compliance at ZEEP was correlated to V-DER at PEEP 15 cm H2O (rho = 0.9, p = 0.001), suggesting that compliance above LIP may reflect the amount of recruitable lung. Thus, alveolar closure in ALI occurs over a wide range of pressures, and LIP is a poor predictor of alveolar closure.