left-to-right shunt
A shunt is an abnormal passage that lets blood take a shortcut between the two sides of the heart or great vessels. In a left-to-right shunt, blood escapes from the high-pressure left (systemic) side into the lower-pressure right (pulmonary) side, like water leaking from a pressurized pipe into a lower-pressure one next to it.
Because the leaking blood is already oxygenated, the body is not starved of oxygen, so this kind of shunt is acyanotic. The problem is volume: the same blood is pumped through the lungs again and again, overloading the pulmonary circulation and the chambers that handle it. Common causes are atrial septal defects, ventricular septal defects, and a patent ductus arteriosus.
Over time a large left-to-right shunt can stretch the right heart, congest the lungs, and slowly drive up pulmonary blood pressure. If that pressure climbs high enough, the shunt can reverse direction, transforming a harmless pink condition into a cyanotic one.
A child with a moderate ventricular septal defect has a left-to-right shunt that recirculates extra blood through the lungs, leading to fast breathing and slow weight gain.
Left-to-right shunts overload the lungs with recirculated blood.
Clinicians sometimes describe the burden of a shunt as the ratio of lung blood flow to body blood flow (Qp/Qs); the further this rises above one, the larger the left-to-right shunt.