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 de creasing 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 wi th the lower inflection point (LIP) and oxygenation changes. A modified aut omated 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 (Z EEP). V-DER was the volume loss between the curves recorded from PEEP and f rom ZEEP at the same Pel. Derecruitment occurred at each PEEP decrement, be ing 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 PEE P 15 cm H2O (rho = 0.9, p = 0.001), suggesting that compliance above LIP ma y reflect the amount of recruitable lung. Thus, alveolar closure in ALI occ urs over a wide range of pressures, and LIP is a poor predictor of alveolar closure.