pulmonary-to-systemic shunt pulmonary-to-systemic shunt 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. pulmonary-to-systemic shuntspecialtyvascular medicine a pulmonary-to-systemic shunt functions as follows: right-to-left in the absence of arterioventricular discordance. left-to-right if the great vessels are transposed.