Dead space and lung dishomogeneity

We developed a software to analyze the disomogeneity of the lung parenchyma, by comparing a given lung voxel with its neighboring voxels. If the gas fraction (a surrogate of”distension”) was equal, the ratio between the single voxel and its surroundings would be equal to 1 (“thus perfect homogeneity”) while the disomogeneity was expressed by ratios progressively greater than 1. we performed the “homogeneity” analysis in 110 patients with ALI/ARDS at two pressure levels compared to normal subjects, and we described the association of the disomogeity extent, acting as a stress riser, with relevant physiological variables and outcome. We found a weak relationship between the “stress raisers” (expressed both as average, extent or intensity) and oxygen-dependent variables (PaO2/FIO2 and shunt and with respiratory system compliance. Of note, however, an association was found between the stress raisers, expressed either as extent (r2 = 0.34), severity (0.204) or average (r2 = 0.37) and the dead spaces (this variable is highly predictive of outcome and it is not associated with any other CT scan variable).