Cyanosis is a term used to describe the purple-blue colour of the skin due to reduced oxygen supply to the tissues. The blue colour is caused by high levels of deoxygenated haemoglobin in the blood and less oxygen carried by the blood cells. A congenital heart defect that results in central cyanosis is the cyanotic heart defect. It is characterized by structural defects of the heart that cause deoxygenated blood to enter the systemic circulation, without passing through the lungs first.