right-to-left shunt

A right-to-left shunt is an abnormal passage that lets oxygen-poor blood from the right (pulmonary) side cross over into the left (systemic) side and out to the body, skipping the lungs entirely. It is the reverse of the more common left-to-right pattern, like a detour that dumps untreated water straight into the clean supply.

Because deoxygenated blood now reaches the body directly, oxygen levels fall and the patient turns blue; right-to-left shunts are therefore a key mechanism of cyanotic heart disease. They occur when an obstruction or high pressure on the right side pushes blood through a hole, as in tetralogy of Fallot, or when the great vessels are misconnected.

Aside from low oxygen, a right-to-left shunt also lets blood clots or debris from the veins pass directly into the arterial circulation, bypassing the lung's natural filter, which can raise the risk of stroke. Treatment targets the underlying defect rather than the shunt alone.