THE ENCAPSULATION OF POLYURETHANE-INSULATED TRANSVENOUS CARDIAC-PACEMAKER LEADS

When cardiac pacemakers are implanted, the tranvenous route is typical ly preferred. For dual chamber pacemakers, an atrial and a ventricular lead are required. Based on postmortem examination of 101 canines wit h polyurethane insulated leads implanted from 10 days through 13 years , encapsulation of these leads is initiated by thrombus secondary to e ndothelial damage and/or blood flow perturbations. Organization of thr ombus results in a vascularized collagenous capsule. With continued bl ood flow perturbation, more thrombi can form and reorganize to cause t he collagenous capsules to grow with implant time. Under certain condi tions, the encapsulating sheath can differentiate into cartilage, mine ralized tissue, and even bone. The least commonly encapsulated area is in or just superior to the annulus of the valve. The most common loca tions for capsule formation are within the upper right atrium (where t wo leads are often bound together by the tissue) and the right ventric le. The ventricular sheaths are often adhered firmly to tricuspid valv e structures. The presence of relatively large, friable, partially org anized thrombi on chronic leads is not unusual, even after more than 1 0 years' implantation. It is recommended that chronic leads be imaged prior to attempts to remove them to detect the presence and location o f embolizable structures.