An infiltrate is the filling of airspaces with fluid (pulmonary oedema), inflammatory exudates (white cells or pus, protein and immunological substances), or cells (malignant cells, red cells or haemorrhage) that fill a region of lung and increase the visual impression of increased soft tissue density.Atelectasis is the collapse of the airspaces which also results in the visual impression of increased soft tissue density.This makes it impossible to distinguish infiltrates from atelectasis on the basis of visual density alone. Atelectasis must have volume loss by definition (collapse of airspaces must reduce lung volume), whereas infiltrate does not.Atelectasis in ICU patients occurs most frequently in the left lower lobe, probably because the heart in the supine position compresses the lower lobe bronchus.The following signs of atelectasis can be seen: ARDS is an acute response to systemic inflammation.It is difficult to distinguish from cardiogenic pulmonary oedema or pneumonia.While cardiogenic pulmonary edema typically begins centrally in the bilateral perihilar areas, ARDS usually causes more uniform opacification.
The distinction from pneumonia is on the basis of non-radiological investigations. The classic Kerley lines are made by the thickening of the interlobular septa that carry the lymphatics (Kerley B lines are short thin lines, 1.5 to 2cm in length, seen in the periphery of the lower lung, extending to the pleura).
Pulmonary oedema is transudate fluid collecting in the lung tissue. As fluid continues to accumulate, it leaks into the pleural space resulting in pleural effusions and into the alveolar spaces causing the classic alveolar pattern of pareynchymal pulmonary edema.
Congestive heart failure is the most common cause of acute pulmonary oedema, and has additional features to the above because of the mechanism of causation that is primarily the increased hydrostatic gradient.
It is often associated with cardiomegaly difficult to determine on an AP film.
Better indications are that the upper lobe blood vessels become more prominent due to vascular congestion (upper lobe blood diversion).
The interstitial edema that results from transudation of fluid also affects the interstitium around the vessels and causes an apparent increase in diameter, causing an elevated arteriole to bronchiole ratio.