A LUNG COMPUTED TOMOGRAPHIC ASSESSMENT OF POSITIVE END-EXPIRATORY PRESSURE-INDUCED LUNG OVERDISTENSION

The aim of this study was to assess positive end-expiratory pressure ( PEEP)-induced lung overdistension and alveolar recruitment in six pati ents with acute lung injury (ALI) using a computed tomographic (CT) sc an method. Lung overdistension was first determined in six healthy vol unteers in whom CT sections were obtained at FRC and at TLC with a pos itive airway pressure of 30 cm H2O. In patients, lung volumes were qua ntified by the analysis of the frequency distribution of CT numbers on the entire lung at zero end-expiratory pressure (ZEEP) and PEEP. In h ealthy volunteers at FRC, the distribution of the density histograms w as monophasic with a peak at -791 +/- 12 Hounsfield units (HU). The lo west CT number observed was -912 HU. At TLC, lung volume increased by 79 +/- 35% and the peak CT number decreased to -886 +/- 26 HU. More th an 70% of the increase in lung volume was located below -900 HU, sugge sting that this value can be considered as the threshold separating no rmal aeration from overdistension. In patients with ALI, at ZEEP the d istribution of density histograms was either monophasic (n = 3) or bip hasic (n = 3). The mean CT number was -319 +/- 34 HU. At PEEP 13 +/- 3 cm H2O, lung volume increased by 47 +/- 19% whereas mean CT number de creased to -538 +/- 171 HU. PEEP induced a mean alveolar recruitment o f 320 +/- 160 mi and a mean lung overdistension of 238 +/- 320 mi. In conclusion, overdistended lung parenchyma of healthy volunteers is cha racterized by a CT number below -900 HU. This threshold can be used in patients with ALI for differentiating PEEP-induced alveolar recruitme nt from lung overdistension.