Incidence and regional distribution of lung overinflation during mechanical ventilation with positive end-expiratory pressure

Objective: In patients with acute lung injury, alveolar recruitment resulting from positive end-expiratory pressure (PEEP) may be associated with overinflation of previously aerated lung regions. The aim of this study was to assess the incidence and regional distribution of lung overinflation resulting from mechanical ventilation with PEEP. Design. Reanalysis with a specific software including a color-coding system of quantitative lung computed tomography data obtained in four previous prospective studies. Setting. A 20-bed surgical intensive care unit of a Parisian university hospital. Patients. Thirty-two patients with acute lung injury in whom computed tomography of the whole lung was obtained at zero end-expiratory pressure (ZEEP) and PEEP 15 cm H2O. Interventions. None. Measurements and Main Results: Total lung recruitment was measured as the reaeration of poorly aerated (computed tomography attenuations ranging between -500 and -100 Hounsfield units) and nonaerated (computed tomography attenuations 2:-100 Hounsfield units) lung areas, and overinflation was measured as the lung volume characterized by computed tomography attenuations less than or equal to-900 Hounsfield units. PEEP was associated with a significant alveolar recruitment (423 +/- 178 mL). Concomitantly, a lung overinflation of 123 +/- 138 mL was found in 14 patients (44%). In eight patients without chronic obstructive pulmonary disease, lung overinflation was predominantly found in nondependent lung regions located beneath the dome of diaphragm. In six patients with a past history of chronic obstructive pulmonary disease, PEEP increased the volume of emphysematous areas present in apical lung regions and produced an overinflation of nondependent lung regions located beneath the dome of diaphragm. Conclusion. Lung overinflation resulting from mechanical ventilation with PEEP is observed in more than one third of patients with acute lung injury lying supine and predominates in caudal and nondependent lung regions. Furthermore, in patients with a history of chronic obstructive pulmonary disease, PEEP markedly increases the volume of emphysematous lung regions.