Hemothorax is defined as pleural fluid with a hematocrit greater than 50% of the patient’s blood hematocrit. This is distinct from a hemorrhagic pleural effusion which can have an appearance similar to blood with a hematocrit as low as 5%. However, because dilution to less than 50% can occur within a few days, some also include hematocrits of 25–50% that of blood in the definition of hemothorax.
Blood that enters the pleural space is agitated by the motion of the intrathoracic structures which leads to defibrination and incomplete clotting. Within several hours, pleural enzymes begin to lyse the clots. However, if there is incomplete lysis or large amounts of bleeding, clot formation occurs. Early in its development, a fibrin clot that forms on the visceral or parietal pleura has little substance. It forms a thin membrane that is loosely attached. Once the clot has begun to organize, it may adhere to the pleura, making it difficult to remove. By 1 week, there is angioblastic...