Pleural effusion. Coloured frontal X-ray of the lungs of a 70-year-old patient with a pleural effusion of the lungs. The lungs are the dark spaces either side of the chest, which are smaller than usual due to an accumulation of fluid (lower left and right) in the chest cavity. The liquid compresses the lung, which leads to breathing difficulties. This case of effusion may have been caused by congestive heart failure, as the patient is wearing electrode pads (white wires running from bottom to upper left and right) to monitor their heart. Other causes of pleural effusion include pleurisy (inflammation of the lungs' pleural cavity), infection and injury. Treatment is of the underlying cause and by draining the fluid.