An enlarged heart shadow is a diagnostic imaging finding of an enlarged heart. An enlarged heart shadow is an image of the heart that shows a larger than normal range on x-ray. The cardiothoracic ratio is measured on X-ray by adding the distance from the left heart edge to the midline of the body to the distance from the right heart edge to the midline of the body, i.e., the transverse diameter of the heart, and then comparing the transverse diameter of the heart to the transverse diameter of the thorax to arrive at the cardiothoracic ratio, which is a commonly used cardiac measurement today. The upper limit of normal cardiothoracic ratio in adults is 0.5. A cardiothoracic ratio range of 0.51-0.55 is considered mild cardiomegaly; a cardiothoracic ratio range of 0.56-0.60 is considered moderate cardiomegaly; and a cardiothoracic ratio range of 0.60 or more is considered severe cardiomegaly. Therefore, if there is an enlarged heart shadow, it is recommended to cooperate with the doctor and take into account one’s own medical history, symptoms and signs, and auxiliary examinations to clarify the disease, and then treat the disease as soon as possible after discovery.