Lung regions identified with CT improve the value of global inhomogeneity index measured with electrical impedance tomography

摘要

Background: The global inhomogeneity ( GI ) index is a functional electrical impedance tomography (EIT) parameter which is used clinically to assess ventilation distribution. However, GI may underestimate the actual heterogeneity when the size of lung regions is underestimated. We propose a novel method to use anatomical information to correct the GI index calculation. Methods: EIT measurements were performed at the level of the fifth intercostal space in six patients with acute respiratory distress syndrome. The thorax and lungs were segmented automatically from serial individual CT scans. The anatomically derived lung regions were calculated in EIT images from simulating a homogeneous ventilation distribution in a finite element model. The conventional approach ( GI meas,func ), analyzes images in functionally-defined lung regions, while our proposed measure ( GI meas,anat ) is based on analysis in anatomically-defined regions. We additionally define a simulated comparison ( GI sim,anat ) to determine the lower limit of the GI measure for a homogenous distribution of ventilation. Results: As expected, the conventional GI meas,func [0.382 (0.088), median (interquartile range)] were significantly lower than the proposed GI meas,anat [0.823 (0.152), P0.05], and were much closer to the lower limit GI sim,anat [0.343 (0.039)]. Both GI meas,anat and GI meas,func were strongly correlated with arterial oxygen partial pressure to fractional inspired oxygen ratio (R=?0.88, P0.05), whereas GI sim,anat (R=0.23) was not. GI meas,anat had a linear-regression slope 3.2 times that of GI meas,func suggesting a higher sensitivity to the changes in lung condition. Conclusions: The proposed GI meas,anat (or shortened as GI anat ) is an improved measure of ventilation inhomogeneity over GI, and better reflects portion of non-ventilated regions due to alveolar collapse or overdistension.