Polycythemia: Chronic low oxygen levels stimulate increased red blood cell production, leading to thicker blood and increased risk of clots.

Here are five complications of cyanotic congenital heart disease:

1. Polycythemia: Chronic low oxygen levels stimulate increased red blood cell production, leading to thicker blood and increased risk of clots.
2. Clubbing of digits: A physical sign characterized by the enlargement of the fingertips and toes, and changes in the nails, due to chronic hypoxemia.
3. Brain abscess/Stroke: Due to paradoxical emboli (clots bypassing the lungs and going directly to the systemic circulation) or increased blood viscosity from polycythemia.
4. Growth retardation/Failure to thrive: Inadequate oxygen delivery and increased metabolic demands can impair normal growth and development.
5. Pulmonary hypertension: If there is a significant left-to-right shunt that later reverses (Eisenmenger syndrome), leading to increased pressure in the pulmonary arteries.