Implantable devices for continuously monitoring vascular lumen dimensions, in particular in the inferior vena cava (IVC) for determining heart failure status of a patient. Related therapy systems as well as monitoring and therapy methods are also disclosed. Devices include active or passive marker elements placed in contact with, adhered to or injected into the vessel wall to generate or reflect signals from which lumen diameter may be determined. Disclosed devices may be fully implantable and self-contained including capabilities for wirelessly communication monitored parameters.
