A pulmonary-to-systemic shunt is a cardiac shunt which allows, or is designed to cause, blood to flow from the pulmonary circulation to the systemic circulation. This occurs when:
there is a passage between two or more of the great vessels; and,
pulmonic pressure is higher than systemic pressure and/or the shunt has a one-way valvular opening.A pulmonary-to-systemic shunt functions as follows:
right-to-left in the absence of arterioventricular discordance. [Source: Wikipedia 
]
