Recently I have been watching khan academy videos on acyanotic and cyanotic diseases. I know that acyanotic means not blue and cyanotic blue. Because I still can not understand the difference between the two, what is it? Can you please explain in a clear way?

  • 2
    Please add a little more about what it is that you specifically do not understand, including a little background on what the definitions are based on what you understand. That will help answerers to tailor the answer, otherwise there are entire textbooks on the subject
    – DoctorWhom
    Feb 7, 2019 at 19:14

1 Answer 1


Cyanosis refers to a blue discolouration of the skin caused by hypoxia (inadequate oxygenation of the arterial blood supply). It is the presence or absence of this hypoxia that is actually the distinction between these two types of heart disease.

Firstly, some heart anatomy. The right side of the heart receives blood from the head and body (via the vena cava) and pumps it to the lungs to be oxygenated. The left side of the heart receives oxygenated blood from the lungs and returns it to the body (via the aorta). The left side is at a higher pressure than the right.

Heart anatomy

There are various types of congenital heart defects. Put simply, any that cause reduced blood flow to the lungs will reduce oxygenation of the blood and cause hypoxia.

An example of this is transposition of the great arteries, which tends to cause cyanosis within the first 24 hours of life. In this condition, the aorta and pulmonary arteries are swapped. This means the circulation to the lungs and the circulation to the body are separate, running in parallel, so insufficient oxygen reaches the body tissues. The infant will only survive if there is a means of the blood mixing, such as the embryological connection between the aorta and pulmonary artery (the ductus arteriosus) remaining open. In simple terms, the ductus serves to bypass the lungs before birth as they are not needed.

Transposition of the great arteries

Other types of congenital heart disease do not necessarily cause hypoxia. For example, a ventricular septal defect is a hole in the septum between the two ventricles. Because the pressure is greater on the left side, blood is shunted over to the right side, increasing blood flow to the lungs. However, over time this can lead to the complication of Eisenmenger's syndrome, in which the shunting reverses due to lung damage and this results in hypoxia and cyanosis.

Some examples of the types of congenital heart disease. Adapted from this source.

Acyanotic congenital heart disease

With a left to right shunt - acyanotic shunting.

  • Atrial septal defects
  • Ventricular septal defects
  • Patent ductus arteriosus

With no shunt - acyanotic obstructive.

  • Coarctation of the aorta
  • Pulmonary stenosis
  • Congenital aortic stenosis
  • Bicuspid aortic valve
  • Dextrocardia

Cyanotic congenital heart disease

These are due to a right to left shunt (where there is a communication between the ventricles of the heart) or reduced pulmonary (lung) circulation.

  • Fallot's tetralogy
  • Pulmonary stenosis
  • Eisenmenger's syndrome
  • Transposition of the great arteries
  • Total anomalous pulmonary venous drainage
  • Tricuspid atresia
  • Pulmonary atresia

See also this answer of mine about Eisenmenger syndrome; a complication of several heart defects that results in hypoxia and cyanosis.

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