The arterial blood gas of patients with pulmonary embolism often shows hypoxemia and hypocapnia, and the difference of oxygen partial pressure of alveolar arterial blood is increased, and the decrease of oxygen partial pressure can be seen when the pulmonary vascular bed is blocked by 15%-20%, and the incidence is 88% when the oxygen partial pressure is lower than 80mmHg. However, the most frightening thing of the pulmonary embolism is that 70% of the patients do not have any clinical manifestations or symptoms, so the blood gas of some of them can be normal, and the clinical gold standard for confirming the diagnosis of pulmonary embolism is the pulmonary artery CTA. The gold standard for the diagnosis of pulmonary embolism is pulmonary artery CTA, and patients admitted to the hospital in an emergency are often judged according to the D-dimer in plasma, because the sensitivity of D-dimer for the diagnosis of acute pulmonary embolism and deep vein thrombosis is as high as 90%-100%, but its specificity is only 40%-43%, therefore, the use of D-dimer to judge acute pulmonary embolism has a greater exclusion rate in the clinic. Therefore, in clinical practice, D-dimer can be used to diagnose acute pulmonary embolism, which has a greater diagnostic value of exclusion, and the possibility of acute pulmonary embolism can be basically ruled out when its level is lower than 500 μg/L. Therefore, CTA of the pulmonary artery is often used in clinical practice as the gold standard for the diagnosis of acute pulmonary embolism.