Central cyanosis mainly refers to patients with complex congenital heart disease. Tetralogy of Fallot is a relatively simple type within complex congenital heart disease. Normally there is no traffic between the two ventricles and a hole between the two ventricles is a congenital developmental malformation. Under normal circumstances the hole is a shunt of blood from the left side to the right side, because the pressure of the right ventricle is lower than the pressure of the left ventricle in normal people, and the blood pressure is normally measured >100 mmHg, and the blood pressure is played from the left ventricle. The right ventricle is the pulmonary artery, the pulmonary artery pressure can not be measured in normal people, the right ventricular pressure and pulmonary artery pressure usually does not exceed 30mmHg, the average is 10-15mmHg, a big difference. The left ventricle is a pressure pump and the right ventricle is a volume pump. Once there is a hole between them, a part of the blood will run to the right ventricle when it contracts, which is called left-to-right shunt, and cyanosis will not occur. Cyanosis is when the oxygenated blood does not exceed 5% and the lips turn purple. In patients with tetralogy of Fallot, the pulmonary artery of the outflow tract of the right ventricle is narrowed, and once the contraction of the right ventricle gradually increases the pressure, which is greater than the left ventricle, very dark purple blood runs to the left ventricle, and once the contraction of the left ventricle hits the whole body, it shows the mouth cyanosis as central cyanosis. If it is peripheral cyanosis such as cold winter to the northeast, it is very cold at that time blood stagnation and poor circulation form peripheral cyanosis, or hand strangulation, artery pinching ischemia for a long time is also peripheral cyanosis. Central cyanosis is when the heart begins to show a right-to-left shunt.