Computed tomography in adult respiratory distress syndrome: what has it taught us?
Computed tomography (CT) has played an important role in improving our knowledge of the pathophysiology of the adult respiratory distress syndrome (ARDS), and in determining the morphological and functional relationships of different manoeuvres commonly used in the therapeutic management of this syndrome (changes in body position, application of positive end-expiratory pressure (PEEP) and mechanical ventilation). During the early phase of the disease, the ARDS lung is characterized by a homogenous alteration of the vascular permeability. Thus, oedema accumulates evenly in all lung regions with a nongravitational distribution (homogenous lung). The increased lung weight, due to increased oedema, causes a collapse of the lung regions along the vertical axis, through the transmission of hydrostatic forces (compression atelectasis). Thus, the lesions appear mainly in the dependent lung regions (dishomogeneous lung). During inspiration, at plateau pressure, the pulmonary units reopen and, if the PEEP applied is adequate, they stay open during the following expiration. Adequate PEEP is equal to or higher than the hydrostatic forces compressing that unit. Prone position is another manoeuvre which allows previously collapsed lung regions to reopen and, conversely, compresses previously aerated regions, reversing the distribution of gravitational forces. During late ARDS, there is less compression atelectasis and the lung undergoes structural changes, due to the reduced amount of oedema. This is usually associated with CO2 retention and the development of emphysema-like lesions. In conclusion, computed tomography is not only a research tool, but a useful technique which allows a better understanding of the progressive change in strategy needed to ventilate the adult respiratory distress syndrome lung at different stages of the disease.