Dependent atelectatic regions are poorly aerated while nondependent aerated regions are ventilated with relatively high tidal volume (HTV).
Bellani and colleagues [ 44] showed that, in a cohort of 10 mechanically ventilated patients with ARDS, inflammation was not confined to the regions with density abnormalities on the CT scan (non-aerated or poorly aerated regions), but also involved normally aerated regions, confirming previous data from quantitative lung CT scans in ARDS patients [ 17].
Attempts to 'recruit' atelectasis with positive pressure often overinflate aerated regions.
Concurrently, as ventilation is redistributed from dependent to nondependent lung, breath sound power is enhanced in nondependent, aerated regions as well.
Beyond contributing to the gas exchange impairment (depending on the perfusion), these "always" collapsed regions may also act as stress risers at their interface with aerated regions, though they are theoretically protected from the mechanical ventilation.
This evidence gives more credence to the concern for alveolar overdistention in the nondependent, aerated regions of the neonatal lung.
Conversely, quantification of lung volume based on chest CT image analyses includes all low attenuation areas and thus, it cannot distinguish between ventilated or trapped aerated regions.
Tsuchida et al. demonstrated that in a model of alveolar collapse and surfactant depletion the atelectasis regions per se were protected from alveolar damage, whereas alveoli from the aerated regions were more affected [ 15].
Previously, Rodrigues and colleagues [ 45] observed only moderate 18-FDG uptake in non-aerated and poorly aerated regions in blunt chest trauma patients who did not develop ARDS, while in subjects who subsequently developed ARDS, diffuse 18-FDG uptake was identified 1 to 3 days before clinical manifestation of disease.
We used a cut-off density between the aerated and the over-aerated region of -900 HU similar to Vieira et al. [ 29].
The volume of tissue in the non-aerated region expressed as a percentage of total lung tissue (VtissNA%), which is equivalent to the weight of the non-aerated region expressed as a percentage of total lung weight, was used as an index of derecruitment.
