An implant assembly is implanted in vivo within a vascular system in which a vessel divides at a furcation into two sub-vessels, each sub-vessel having a diameter smaller than the diameter of said vessel. An implant assembly is released into a vessel such as a pulmonary arterial vessel of a patient. The implant assembly has a diameter smaller than or substantially equal to the inner diameter of the vessel and larger than the inner diameter of each of the sub-vessels. The implant assembly is configured so that it moves downstream within the vessel along with the flow of blood. When the implant assembly reaches a furcation where the vessel divides, the implant assembly is too large and not sufficiently compliant to fit through either of the smaller branch vessels. The implant assembly thus lodges at the furcation, prevented from moving downstream by being too large and stiff to fit into the branch vessels, and prevented from moving upstream by the flow of blood through the arteries. Alternatively, the implant assembly has a diameter greater than a vessel and compliance characteristics such that, upon release, the implant assembly travels down a narrowing vessel until an interference fit is created between the anchor structure and the vessel wall, thereby preventing further distal movement.
