Widening of the pulmonary artery trunk is mainly seen in the following causes: 1. Congenital heart disease, most commonly atrial septal defect, ventricular septal defect and arteriovenous ductus insufficiency. Structural heart disease tends to cause blood from the left atrium and left ventricle to enter the right ventricle and right atrium through this particular defect or abnormal duct, resulting in increased blood flow to the lungs, gradually forming pulmonary hypertension and causing widening of the main pulmonary artery. 2. Chronic obstructive pulmonary disease, most commonly chronic bronchitis and emphysema, tends to form hypoxemia causing vascular proliferation at the end of the pulmonary artery, resulting in luminal narrowing and increased resistance to the pulmonary circulation. The cause of idiopathic pulmonary hypertension is unknown and may be related to genetic factors and autoimmune factors, which also lead to widening of the pulmonary artery trunk.