What pathophysiological mechanism causes cyanosis in congenital heart defects?

Cyanosis in congenital heart defects primarily occurs due to right-to-left shunting of blood. In this scenario, deoxygenated blood from the right side of the heart is allowed to bypass the lungs and enter the systemic circulation directly. This results in a mixture of oxygen-poor blood with oxygen-rich blood, leading to a decrease in the overall oxygen saturation of the blood circulating throughout the body. As a consequence, tissues and organs do not receive sufficient oxygen, which manifests as cyanosis—characterized by a bluish discoloration of the skin and mucous membranes.

This right-to-left shunt can occur in conditions such as tetralogy of Fallot or transposition of the great arteries, where structural anomalies in the heart or great vessels facilitate this abnormal flow pattern. The degree of cyanosis may vary based on the severity of the shunt and the oxygenation levels of the mixed blood.

Understanding this mechanism is crucial when evaluating and managing congenital heart defects that could lead to cyanosis, as it indicates a potentially critical lack of oxygen delivery that may require urgent intervention.