Pulmonary plethora. Chest X-ray showing pulmonary plethora - enlargement of blood vessels in the lungs. Enlarged blood vessels show clearly in both lungs. The heart (centre right, white) is also enlarged. This condition is caused by a hole (septal defect) between the two sides of the heart. The hole causes abnormal flow of blood (shunt) from the left side of the heart, where the pressure is higher, to the right. As a result blood flow through the lungs is excessive. The condition is usually caused by abnormal heart development in the foetus and can be repaired by heart surgery if complications develop.