In addition to cardiac causes, cyanosis can also be caused by various congenital or acquired pulmonary diseases (such as abnormal lung development or inflammation of the lungs) that result in hypoventilation of the lung tissue and lack of oxygen. In healthy children with red lips, cyanosis is easily detected when the body is deprived of oxygen (i.e., insufficient oxygen saturation in the arterial blood or in the capillary blood flow) and is revealed in the mouth and lips. In children with cyanotic preconditioning (a congenital defect that causes venous blood to mix with arterial blood), in addition to cyanosis of the lips and mouth, cyanosis can also be seen on the tongue and the ends of the fingers and toes, and in severe cases, the ends of the fingers and toes can be mallet-shaped, called pestle-like fingers (toes), which is the most common manifestation of severe heart disease. In addition to cardiac causes, blue lips can also be caused by various congenital or acquired lung diseases (such as abnormal lung development or inflammation of the lungs), resulting in insufficient ventilation of the lung tissue and lack of oxygen. In addition, when there is an increase in abnormal hemoglobin (such as methemoglobin or hemoglobin) in the blood that cannot carry oxygen, the lips may be blue, and in severe cases, the whole body may be cyanotic.