Assessment of PEEP-induced reopening of collapsed lung regions in acute lung injury: are one or three CT sections representative of the entire lung?

Objectives: To study whether PEEP-induced reopening of collapsed lung regions - defined as the decrease in nonaerated lung volume measured on a single or three computerized tomographic (CT) sections - is representative of the decrease in overall nonaerated lung volume. Design: Review of 39 CT scans obtained in consecutive patients with Acute Lung Injury. Settings: Fourteen-bed surgical intensive care unit of a University Hospital. Measurements and results: PEEP-induced decrease in nonaerated lung volume was measured in 39 patients with ALI on a single juxtadiaphragmatic CT section, on three CT sections - apical, hilar, and juxtadiaphragmatic - and on contiguous apex-to-diaphragm CT sections. The percentage of decrease in nonaerated lung volume following PEEP, was compared between one, three and all CT sections using a linear regression analysis and Bland and Altman's method. The decrease in nonaerated lung volume measured on a single and three CT sections was significantly correlated with the de-crease in nonaerated lung volume measured on all CT sections: R = 0.83, P < 0.0001 for one CT section and R = 0.92, P < 0.0001 for three CT sections. However, measurements performed on a single CT section were poorly representative of the overall lung: bias -6 %, limits of agreement ranging between -37 % and +25 %. Measurements performed on three CT sections overestimated by 11 % the overall decrease in nonaerated lung volume: bias -11 %, limits of agreement ranging between -29 % and +7 %. Conclusions: PEEP-induced reopening of collapsed lung regions measured on a single or three CT sections sensibly differs from the reopening of collapsed lung regions measured on the overall lung. The inhomogeneous distribution of PEEP-induced reopening of collapsed lung regions along the cephalocaudal axis probably explains these discrepancies.