Tidal changes on CT and progression of ARDS

Cereda, Maurizio; Xin, Yi; Hamedani, Hooman; Bellani, Giacomo; Kadlecek, Stephen; Clapp, Justin; Guerra, Luca; Meeder, Natalie; Rajaei, Jennia; Tustison, Nicholas J; Gee, James C; Kavanagh, Brian P; Rizi, Rahim R

Secondary lung injury initiated by tidal inflation worsens outcome of acute respiratory distress syndrome (ARDS). 1 However, because consensus criteria for ARDS select populations of patients with heterogeneous characteristics, predicting the effect of mechanical ventilation on injury trajectory is difficult. 2 Diagnosis of ARDS 3 is largely based on plain (two-dimensional) chest radiography. A major advance came with the characterisation by CT, of the ventral 'baby lung', 4 which provided the rationale for use of low tidal volume 1 and prone positioning. 5 Quantitative study of ARDS using CT has been mostly limited to large regions of lung, 6 with limited analysis of inflation at very high resolution. 7 An alternative approach to pulmonary CT analysis pairs individual voxels in end-inspiratory (EI, table 1) and end-expiratory (EE) images, 8 and images are warped in order to superimpose anatomical structures that may be distorted by respiration or disease progression. 9 10 This approach permits detection of hyperinflation (low EI density) and air trapping (low EE density) in emphysematous lungs that is not otherwise detectable. 11 We previously reported that the spatial propagation of experimental lung injury was driven by tidal inflation, 12 and therefore hypothesised that in early acute lung injury, areas of lung with unstable inflation (detected by superimposed EI and EE CT scans) are 'at risk' for injury progression.