"Trapped lung is a type of non-expandable lung, and the term is used when a thick, fibrotic visceral pleural rind prevents lung expansion. This is commonly post-pneumonic, occurring when active pleural inflammation heals with fibrosis. This differs from the situation of actively inflamed and thickened pleura complicating pneumonia which also limits lung expansion and is called lung entrapment."
"In trapped lung, pleural pressure is abnormally low and this causes a pleural transudate. Thoracentesis lowers the intra-pleural pressure further because of the rigidity of the chest wall and the inability of the lung to expand. In lung entrapment however, there is active inflammatory or malignant pleural thickening which is associated with an exudate."