Computed Tomography

Computed tomography (CT) scan has substantially improved our understanding of the inhomogeneous pattern of ARDS. Differently from chest X-ray, it avoids the superimposition of different structures and also allows accurate measurement of the extent of the abnormalities and of their density [7].

The classical morphological CT description includes the recognition of normally aerated lung, poorly inflated areas, characterized by increased density but with still recognizable vessels (ground-glass opacifications) and not-inflated areas with increased density and no recognizable vessels (consolidation) [8]. The increase in lung tissue and the reduction in lung gas volume due to the combination of alveolar flooding (edema), interstitial inflammation, and compression atelectasis are typical of the acute phase of ARDS.

In the early phase, ground-glass opacity has quite a distribution over the entire lung field, whereas consolidation is more prevalent in the dependent ones. In the late phase, over 2 weeks after the first insult, the reticular pattern is the single most frequent pattern and is more represented in the nondependent regions [9]. More than 70% of patients present with CT abnormalities at 6 months follow-up, characterized by fibrosis, air cysts, and bullae mainly located in the ventral, nondependent regions. They are more evident at CT scan than at chest radiography [10].

Furthermore, differences in pathophysiology between ARDS secondary to direct pulmonary insult and ARDS secondary to extrapulmonary disease reflect different CT morphological patterns. The former presents asymmetrical focal areas of consolidation, while extrapulmonary ARDS shows a symmetrical pattern with prevalence of ground-glass areas [11].

In the past decades, the CT quantitative analysis of poorly/normally aerated lung regions has led to the discovery that the ARDS lung is not stiff but “small” (the “baby lung”) and that the residual inflated lung is nearly normal. Moreover, it was showed that ARDS lung is very heavy, weighing 2-3 times that of a normal lung with a content of air decreased to values 20-30% of normal [12].

Then, CT proved that the acute exudative lesions in ARDS have a gravitationally dependent gradient of distribution, with more consolidation in the posterobasal regions, as a result of compressive gravitational forces [13]. In conclusion regional analysis of CT scan has allowed to demonstrate a nonhomogeneous distribution of densities since the gas content progressively decreases from top to bottom because each alveolus is compressed by the weight of the superimposed structures [14]. Accordingly this physical model, in order to inflate the collapsed lung regions and to maintain open these regions, mechanical ventilation should overcome the superimposed pressure generated by the lung mass and pleural pressure.