The aim of the study was to determine the size and location of homogeneous inflamed/noninflamed and inhomogeneous inflamed/noninflamed lung compartments and their association with acute respiratory distress syndrome (ARDS) severity.

In total, 20 ARDS patients underwent 5 and 45 cmH₂O computed tomography (CT) scans to measure lung recruitability. [¹⁸F]2-fluoro-2-deoxy-d-glucose ([¹⁸F]FDG) uptake and lung inhomogeneities were quantified with a positron emission tomography-CT scan at 10 cmH₂O. We defined four compartments with normal/abnormal [¹⁸F]FDG uptake and lung homogeneity.

The homogeneous compartment with normal [¹⁸F]FDG uptake was primarily composed of well-inflated tissue (80±16%), double-sized in nondependent lung (32±27% versus 16±17%, p<0.0001) and decreased in size from mild, moderate to severe ARDS (33±14%, 26±20% and 5±9% of the total lung volume, respectively, p=0.05). The homogeneous compartment with high [¹⁸F]FDG uptake was similarly distributed between the dependent and nondependent lung. The inhomogeneous compartment with normal [¹⁸F]FDG uptake represented 4% of the lung volume. The inhomogeneous compartment with high [¹⁸F]FDG uptake was preferentially located in the dependent lung (21±10% versus 12±10%, p<0.0001), mostly at the open/closed interfaces and related to recruitability (r²=0.53, p<0.001).

The homogeneous lung compartment with normal inflation and [¹⁸F]FDG uptake decreases with ARDS severity, while the inhomogeneous poorly/not inflated compartment increases. Most of the lung inhomogeneities are inflamed. A minor fraction of healthy tissue remains in severe ARDS.