Pulmonary effusion is a common name for pleural effusion, which specifically refers to the abnormal increase of fluid in the pleural cavity, mainly due to the following reasons: 1. pleural diseases, most commonly inflammatory diseases of the pleura, such as tuberculous pleurisy, pneumonia-like pleural effusion, etc. 2. cardiac insufficiency, cardiac insufficiency caused by various heart diseases, most commonly causing bilateral pleural effusion. 3. hypoproteinemia. Hypoproteinemia caused by various reasons, such as liver cirrhosis, nephrotic syndrome, glomerulonephritis, etc., can cause pleural effusion due to the decrease in blood colloid osmotic pressure, thus causing an increase in fluid permeability in the pleural cavity. 4, malignant lesions causing pleural effusion, most commonly lung cancer and malignant pleural mesothelioma. 5, various injuries such as trauma or esophageal rupture, thoracic duct rupture, can cause pleural effusion. Injuries such as trauma or rupture of the esophagus or thoracic duct can cause pleural effusion, and hemothorax or hemopneumothorax is more common.