The encapsulation of polyurethane-insulated transvenous cardiac pacemaker leads.

When cardiac pacemakers are implanted, the tranvenous route is typically preferred. For dual chamber pacemakers, an atrial and a ventricular lead are required. Based on postmortem examination of 101 canines with polyurethane insulated leads implanted from 10 days through 13 years, encapsulation of these leads is initiated by thrombus secondary to endothelial damage and/or blood flow perturbations. Organization of thrombus results in a vascularized collagenous capsule. With continued blood flow perturbation, more thrombi can form and reorganize to cause the collagenous capsules to grow with implant time. Under certain conditions, the encapsulating sheath can differentiate into cartilage, mineralized tissue, and even bone. The least commonly encapsulated area is in or just superior to the annulus of the valve. The most common locations for capsule formation are within the upper right atrium (where two leads are often bound together by the tissue) and the right ventricle. The ventricular sheaths are often adhered firmly to tricuspid valve structures. The presence of relatively large, friable, partially organized thrombi on chronic leads is not unusual, even after more than 10 years' implantation. It is recommended that chronic leads be imaged prior to attempts to remove them to detect the presence and location of embolizable structures.