Acute respiratory distress syndrome: lessons from computed tomography of the whole lung

This review aims to show how computed tomography of the whole lung has modified our view of acute respiratory distress syndrome, and why it impacts on the optimization of the ventilatory strategy.

Computed tomography allows an accurate assessment of the volumes of gas and lung tissue, respectively, and lung aeration. If computed tomographic sections are contiguous from the apex to the lung base, quantitative analysis can be performed either on the whole lung or, regionally, at the lobar level. Analysis requires a manual delineation of lung parenchyma and is facilitated by software, including a color-coding system that allows direct visualization of overinflated, normally aerated, poorly aerated, and nonaerated lung regions. In addition, lung recruitment can be measured as the amount of gas that penetrates poorly aerated and nonaerated lung regions after the application of positive intrathoracic pressure.

The lung in acute respiratory distress syndrome is characterized by a marked increase in lung tissue and a massive loss of aeration. The former is homogeneously distributed, although with a slight predominance in the upper lobes, whereas the latter is heterogeneously distributed. The lower lobes are essentially nonaerated, whereas the upper lobes may remain normally aerated, despite a substantial increase in regional lung tissue. The overall lung volume and the cephalocaudal lung dimensions are reduced primarily at the expense of the lower lobes, which are externally compressed by the heart and abdominal content when the patient is in the supine position. Two opposite radiologic presentations, corresponding to different lung morphologies, can be observed. In patients with focal computed tomographic attenuations, frontal chest radiography generally shows bilateral opacities in the lower quadrants and may remain normal, particularly when the lower lobes are entirely atelectatic. In patients with diffuse computed tomographic attenuations, the typical radiologic presentation of "white lungs" is observed. If these patients lie supine, lung volume is preserved in the upper lobes and reduced in the lower lobes, although the loss of aeration is equally distributed between the upper and lower lobes. This observation does not support the "opening and collapse concept" described as the "sponge model." In fact, interstitial edema, alveolar flooding, or both, not collapse, are histologically present in all regions of the lung in acute respiratory distress syndrome. Compression atelectasis is observed only in caudal parts of the lung, where external forces (such as cardiac weight, abdominal pressure, and pleural effusion) tend to squeeze the lower lobes. When a positive intrathoracic pressure is applied to patients with focal acute respiratory distress syndrome, poorly aerated and nonaerated lung regions are recruited, whereas lung regions that are normally aerated at zero end-expiratory pressure tend to be rapidly overinflated, increasing the risk of ventilator-induced lung injury.

Selection of the optimal positive end-expiratory pressure level should not only consider optimizing alveolar recruitment, it should also focus on limiting lung overinflation and counterbalancing compression of the lower lobes by maneuvers such as appropriate body positioning. Prone and semirecumbent positions facilitate the reaeration of dependent and caudal lung regions by partially relieving cardiac and abdominal compression and may improve gas exchange.