Lung inhomogeneities, inflation and [18F]FDG uptake rate in ards
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KeywordsLung Volume Normal Lung Lung Parenchyma Stress Raiser Actual Classification
In ARDS lung parenchyma presents great variability in inflation, lung inhomogeneities and [18F]FDG uptake. In fact, inflation progressively decreases along the sternum-vertebral axis leading to further inhomogeneities that may act as “stress raiser”. That can activate a local inflammatory response leading to edema.
We aimed to examine the voxel by voxel relationship between [18F]FDG uptake and inhomogeneity according to the actual classification of ARDS.
20 ARDS patients underwent a PET-CT scan at 10 cmH2O. [18F]FDG uptake was determined with the graphical Patlak approach voxel by voxel. Lung inhomogeneities were determined by measuring the gas/tissue ratio in two contiguous lung regions. We defined inhomogeneities the fraction of lung volume whose inhomogeneities were greater than 1.61.
5 patients presented mild, 12 moderate and 3 severe ARDS. In mild and moderate ARDS a consistent lung fraction is homogeneous with a high [18F]FDG metabolic activity (53 ± 14% and 53 ± 20%). Inhomogeneous lung fraction with a higher [18F]FDG uptake increases from mild to severe (12 ± 3%, 16 ± 9% and 27 ± 11%). On the other hand, the homogeneous parenchyma with normal [18F]FDG uptake decreases in worse ARDS (33 ± 14%, 26 ± 20% and 5 ± 9%).
Our findings indicate that the actual classification of ARDS from mild to severe reflects the underlying pathophysiology. In fact, while a similar sized homogeneous and inflamed/metabolically more active compartment is present in all the ARDS patients, in mild ARDS it is associated with a consistent fraction of normal lung while in severe ARDS is primarily associated with inhomogeneous, inflamed/metabolically more active lung tissue.
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