A Systemic-to-Pulmonary artery (SPA) shunt is a connection used to augment pulmonary blood flow in children with congenital heart disease. Current SPA shunts are made from simple synthetic vascular graft, generally made from FDA approved polytetrafluroethylene (PTFE). Clinical difficulties in maintaining appropriate balance between the systemic and pulmonary circulations using the existing shunts necessitate the need for an implantable adjustable shunt. An important indication for an adjustable shunt is the Norwood procedure for hypoplastic left heart syndrome (HLHS). An adjustable SPA shunt is being developed in order to achieve full range of control of blood flow between the systemic and pulmonary circulation. For optimizing the design of the shunt, it is necessary to determine the forces required to constrict the pressurized shunt. In this study, a relation between the forces needed to cause a desired constriction of the shunt at constant internal pressure is determined using an in-vitro set up. The set up consists of an hydraulic system to pressurize the shunt at constant internal pressure, a force gauge attached to the plunger to determine the forces required to produce constriction, and mounting stands to hold the shunt and force gauge and also to measure the downward displacement of the plunger. Effect of different (constant) internal pressures, different plunger designs and different diameter shunts – under different constraining conditions (enclosing the shunt), on the forces required are investigated. Results from this study will be used to determine the torque needed to drive the screw plunger mechanism to cause the desired constriction.