Cardiac output (CO) is the total amount of blood ejected from one side of the ventricle per minute, also known as cardiac output per minute, or simply cardiac output. Decreased cardiac output can lead to shock. So how is this caused? Incomplete relief of right ventricular outflow tract obstruction in Tetralogy of Fallot or incomplete correction of pulmonary artery malformations is an important cause of low postoperative cardiac output. In addition to the imbalance of blood volume, insufficient diastolic volume can affect the cardiac output, the main reason is due to intracardiac operation, need to block the cardiac circulation, ischemia, hypoxia can cause damage to the myocardium, resulting in myocardial contraction insufficiency. In addition, if there is insufficient oxygen exchange, hypoxia or acidemia after the operation, it can aggravate myocardial contraction insufficiency. Tachycardia or bradycardia affects atrial diastolic insufficiency. Arrhythmias such as tertiary conduction block caused by hypoxic or surgical trauma are also often the cause of postoperative hypoexcretion. In addition, cardiac compression affecting ventricular filling, such as pericardial tamponade or tight bundling after pericardial suturing, is also a cause of postoperative hypoexcretion. Inadequate coronary blood supply and myocardial infarction due to coronary air embolism are occasional causes. Children with poor preoperative cardiac function are in poor general condition, with varying degrees of cardiac, pulmonary, hepatic, and renal dysfunction, and are prone to low cardiac output syndrome. Patients with large left-to-right cardiac shunt flow are prone to pulmonary arterial hypertension, and patients with severe pulmonary arterial hypertension have hardening of the walls of small pulmonary arteries, wall thickening and lumen narrowing, often accompanied by thickening of alveolar and intercapillary tissue, interstitial edema, increasing pulmonary vascular resistance, right ventricular hypertrophy and enlargement, and the balance between supply and demand for myocardial oxygen is in a compensatory state before the operation, and improper intraoperative treatment can also be a contributing factor to the development of low cardiac output syndrome.