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

Background: The global inhomogeneity () index is a functional electrical impedance tomography (EIT) parameter which is used clinically to assess ventilation distribution. However, 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 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 ( ), analyzes images in functionally-defined lung regions, while our proposed measure ( ) is based on analysis in anatomically-defined regions. We additionally define a simulated comparison ( ) to determine the lower limit of the measure for a homogenous distribution of ventilation.

Results: As expected, the conventional [0.382 (0.088), median (interquartile range)] were significantly lower than the proposed [0.823 (0.152), P<0.05], and were much closer to the lower limit [0.343 (0.039)]. Both and were strongly correlated with arterial oxygen partial pressure to fractional inspired oxygen ratio (R=-0.88, P<0.05), whereas (R=0.23) was not. had a linear-regression slope 3.2 times that of suggesting a higher sensitivity to the changes in lung condition.

Conclusions: The proposed (or shortened as ) is an improved measure of ventilation inhomogeneity over GI, and better reflects portion of non-ventilated regions due to alveolar collapse or overdistension.