Effect of Interfacial Contact Forces and Lay Ratio in Cardiac Lead Outer Insulation due to Internal Cable Motion

The implantable artificial pacemaker enables patients with heart rhythm disorders to enjoy improved quality of life by electrically stimulating the heart to beat at a rate suitable for the activities of daily living. For patients at risk of developing life-threatening ventricular fibrillation or ventricular tachycardia, the implantable cardioverter defibrillator (ICD) delivers levels of current to the heart muscle at levels sufficient to interrupt the abnormal rhythms and restore cardiac function. The implanted hardware for modern pacemaker and ICD systems typically consists of a pulse generator under the skin in the shoulder connected to one or more transvenous conducting leads that enter the vasculature through a vein, typically in the shoulder region, and are anchored inside the heart at locations appropriate for delivering electrical therapy. Leads conduct electrical signals for sensing, pacing, and defibrillation. Typical construction involves extruded tubes of polymer insulation with one or more lumens for conductors. Multiple conductors may be included in a single lead. The outer diameter of a modern lead is typically 2 –3 mm. Insulation materials are limited to those that satisfy a stringent set of criteria: biocompatibility, biostability, and flexibility to navigate tortuous anatomy, low coefficient of friction against biological tissues to ease the implant process, dielectric strength to maintain electrical integrity during operation, and strength to maintain structural integrity during implant, patient activity, and lead extraction. In addition, the insulation must have sufficient interfacial contact forces and lay ratio resistance to avoid a breach or dielectric breakdown due to thickness reduction. Materials found to satisfy these requirements include formulations of silicone, polyurethane, polyurethane-based copolymers, ethylene tetrafluoroethylene (ETFE), and polytetrafluoroethylene  (PTFE). The failure modes associated with insulation interfacial contact forces and lay ratio can be severe. Short circuits between conductors or to body fluids in the pace-sense or defibrillation circuits may lead to loss of therapy, inappropriate shocks, or tissue damage during lead revision. Although the clinical importance of lead insulation interfacial contact forces and lay ratio has been widely recognized, there have been very few published tribological studies of any material used in the construction of cardiac leads. Lead insulation interfacial contact forces and lay ratio is a multifaceted problem. There are several potential tribosystems that may be active. Outer insulation near the pulse generator implant site may interfacial contact forces and lay ratio against the metallic surface of the device. Multiple leads, including leads abandoned due to obsolescence or malfunction, may be in contact with each other along their entire length, resulting in polymer– polymer wear. Biological tissue may be loaded against outer insulation and eventually interfacial contact forces and lay ratio through. Finally, internal lead components may interfacial contact forces and lay ratio against each other, resulting in migration of conductors outside the lead body, also called inside-out abrasion. Insulation interfacial contact forces and lay ratio in implanted cardiac leads may result in serious failure modes, including inappropriate or missing therapy. A Love thin rod mechanics analytical model intended to simulate a range of loading conditions between cardiac lead components has been developed. Analytical model performed in an aqueous environment at loads and velocities anticipated to occur in-vivo shows that a material with decreased toughness exhibited increased interfacial contact forces and lay ratio. Order of magnitude variability within analytical model groups was observed, suggesting that reliability projections should incorporate the statistical nature of the analytical model. Ranking of implantable material interfacial contact forces and lay ratio are possible using this methodology. The inside-out interfacial contact forces and lay ratio of cables against outer insulation material is discussed in this paper.