Cyanosis occurs when deoxygenated blood enters the systemic circulation, causing a bluish-purple discolouration of the skin.
Cyanotic heart disease occurs when blood bypasses the pulmonary circulation and the lungs across a right-to-left shunt. A right-to-left shunt occurs when a defect allows blood to flow from the right side of the heart (the deoxygenated blood returning from the body) to the left side of the heart (the blood exiting the heart into the systemic circulation) without travelling through the lungs to get oxygenated.
Heart defects that can cause a right-to-left shunt, and therefore cyanotic heart disease, are:
- Ventricular septal defect (VSD)
- Atrial septal defect (ASD)
- Patent ductus arteriosus (PDA)
- Transposition of the great arteries
Patients with a VSD, ASD or PDA are usually not cyanotic. This is because the pressure in the left side of the heart is much greater than in the right. Blood will flow from the area of higher pressure to the area of lower pressure, preventing a right-to-left shunt. If the pulmonary pressure increases beyond the systemic pressure, blood will flow from right to left across the defect, causing cyanosis. This is called Eisenmenger syndrome.
Patients with transposition of the great arteries will always have cyanosis. The aorta is attached to the right ventricle. Blood drains from the systemic venous system into the right side of the heart, then out of the right ventricle into the aorta and systemic circulation, completely bypassing the lungs.
Last updated November 2024
Now, head over to members.zerotofinals.com and test your knowledge of this content. Testing yourself helps identify what you missed and strengthens your understanding and retention.
