Methods for quantitative evaluation of alveolar structure during in vivo microscopy.

In vivo fibred confocal laser scanning microscopy allows an evaluation of differences in alveolar mechanics between healthy and acutely injured lungs during mechanical ventilation. The aim of this study was to develop new methods for a quantitative analysis of microscopic images in a murine model of experimental acute lung injury (ALI) and to assess the methods' portability to a large animal model. Differences observed in ALI compared to healthy lungs were: reduction of air-filled areas, increase of heterogeneity and increase of shape irregularity. Three indices were developed: the volume air index (VAI) applies an integral over specific signal intensities, the heterogeneity index (HI) and the Heywood circularity index (CI) comprise variances in size and shape of alveolar structures. The differences between healthy and ALI conditions were found to be significant for all of the used indices (VAI: 0.648 vs. 0.443 (p < 0.05), HI: 0.852 vs. 1.348 (p < 0.001) and CI: 1.56 vs. 1.66 (p < 0.001)). The portability of these algorithms to a porcine model was confirmed reaching similar results (VAI: 0.50 vs. 0.35, p < 0.05; HI: 0.62 vs. 1.83, p < 0.05; CI: 1.56 vs. 1.63, p < 0.001). VAI, HI and CI may help to quantify microscopic images of changes in alveolar structure after experimental ALI.