Measurement of alveolar derecruitment in patients with acute lung injury: computerized tomography versus pressure–volume curve

INTRODUCTION: Positive end-expiratory pressure (PEEP)-induced lung derecruitment can be assessed by a pressure–volume (P–V) curve method or by lung computed tomography (CT). However, only the first method can be used at the bedside. The aim of the study was to compare both methods for assessing alveolar derecruitment after the removal of PEEP in patients with acute lung injury or acute respiratory distress syndrome. METHODS: P–V curves (constant-flow method) and spiral CT scans of the whole lung were performed at PEEPs of 15 and 0 cmH(2)O in 19 patients with acute lung injury or acute respiratory distress syndrome. Alveolar derecruitment was defined as the difference in lung volume measured at an airway pressure of 15 cmH(2)O on P–V curves performed at PEEPs of 15 and 0 cmH(2)O, and as the difference in the CT volume of gas present in poorly aerated and nonaerated lung regions at PEEPs of 15 and 0 cmH(2)O. RESULTS: Alveolar derecruitments measured by the CT and P–V curve methods were 373 ± 250 and 345 ± 208 ml (p = 0.14), respectively. Measurements by both methods were tightly correlated (R = 0.82, p < 0.0001). The derecruited volume measured by the P–V curve method had a bias of -14 ml and limits of agreement of between -158 and +130 ml in comparison with the average derecruited volume of the CT and P–V curve methods. CONCLUSION: Alveolar derecruitment measured by the CT and P–V curve methods are tightly correlated. However, the large limits of agreement indicate that the P–V curve and the CT method are not interchangeable.