Overdistension
Computed Tomography
The application of high levels of PEEP in patients with low lung recruitability can lead to overdistention of regions that are already aerated without opening the collapsed tissue (i.e., strain) [19]. CT scan is a unique tool to differentiate between recruitment and hyperinflation. Overdistension is defined by quantitative lung CT analysis as lung regions with a density threshold below - 900 Hounsfield units [18]. However, the quantitative computation of lung overdistension can be affected by the size of the voxel or the CT section thickness and by the type of reconstruction filter. In fact, a thick CT section (10 mm) underestimates the overinflated lung volume by 3-5% compared to thin CT sections (1.5 mm) [18]. The evaluation of hyperinflation can give information about the safety of our mechanical ventilation. In fact Terragni et al. have found that one-third of ARDS patients with large not aerated lung regions presented a substantial tidal hyperinflation, even when airway plateau pressure was between 28 and 30 cmH2O [40].
Recently Cressoni et al. have quantified the lung inhomogeneities by CT scan. They found that the extent of lung inhomogeneities increased with the severity of ARDS, correlated with the physiologic dead space, and it was associated with overall mortality. Interestingly they also demonstrated that in most of the patients, lung inhomogeneities decrease by increasing PEEP or airway pressure, suggesting a more homogeneous lung at higher pressure. However, in a small part of patients, lung inhomogeneity extent increased, instead of decreased while increasing PEEP or plateau pressure. This was due to the increase of poorly aerated tissue and increasing pressures, while in most patients where lung inhomogeneities decreased increasing pressure, the poorly aerated tissue also decreased [41].
Once again CT scan has provided physiological information to better understand the complex interaction between mechanical ventilation and ventilator-induced lung injury.
