Generally speaking, a pacing lead permits an implantable medical device (IMD) such as an implantable pulse generator (IPG) or other IMD to sense the activity of the heart and to provide electrical stimulation to the heart. Pacing leads can be either epicardial (e.g., attached to the exterior surface of the heart), or endocardial (e.g., attached to an interior location of the heart). Endocardial leads are normally placed into contact with the endocardial tissue by passage through the venous system, such as through the subclavian vein or other easily accessible veins located near the implant site of the IMD. Thus a transvenous, endocardial lead refers to a pacing lead that contacts cardiac endocardial tissue through a vein.
An endocardial lead is in electrical communication with the heart via an electrode at its distal end and thereby provides an electrical pathway between the IPG and endocardial tissue. Using this electrical pathway, the IPG is able to both pace the heart via electrical impulses and sense cardiac depolarizations.
In the past, various types of transvenous endocardial leads have been introduced into different chambers of the heart including the right ventricle, right atrial appendage and atrium as well as the coronary sinus. These leads usually are composed of an insulator sleeve that contains a coiled conductor having an electrode tip attached at the distal end. The electrode tip is held in place within the trabeculations of endocardial tissue. The distal ends of many available leads include passive fixation designs that may consist of flexible tines, wedges, or finger-like projections that extend radially outward and usually are molded from and integral with the insulator sleeve of the lead. These tines allow better containment by the trabeculations of endocardial tissue and help prevent dislodgement of the lead tip. Active fixation leads, on the other hand, are designed with lead tips that are lodged into the myocardium and may consist of helical coils, small sharp tips, and barbed tines among others.
Once an endocardial lead is implanted within a chamber of the heart, the body's reaction to its presence furthers its fixation within the heart. Specifically, shortly after implant, a blood clot forms about the lead tip due to enzymes released in response to the irritation of the endocardial tissue caused by the electrode tip which can be any one of a plurality of designs, tined, helical, flanged, among others. Over time, fibrous scar tissue eventually forms over the distal end. This scarring usually occurs within three to six months of implantation. In addition, fibrous scar tissue often forms, in part, over the lead's body or insulative sleeve within the vein through which the lead was passed.
Although the state of the art in pacemaker and lead technology has advanced considerably, endocardial leads nevertheless occasionally fail, due to a variety of reasons, including insulation breaks, breakage of the inner helical coil conductor thereof and an increase in electrode resistance. Also, in some instances, it may be desirable to stimulate different portions of the heart than those being stimulated with leads already in place. Due to these and other factors, therefore, a considerable number of patients may come to eventually have more than one, and sometimes as many as four or five, abandoned, unused leads in their venous system and heart.
Abandoned transvenous leads increase the risk that medical complications will develop. Possible complications associated with leaving unused leads in the heart and venous system include an increased likelihood that such a lead may become a site of infection. Development of an infection may, in turn, lead to septicemia, a possibly fatal complication. Unused leads may also cause endocarditis. Furthermore, unused leads may entangle over time, thereby increasing the likelihood of blood clot formation. Such clots may embolize to the lung and produce severe complications or even death. The presence of unused leads in the venous pathway and inside the heart can also cause considerable difficulty in the positioning and attachment of new endocardial leads in the heart. Moreover, multiple leads within a vein may impede blood flow or occlude the blood vessel, causing swelling of the arm.
As serious as the risks associated with leaving an unused lead in place may be, the risks associated with past methods and devices for lead removal were often greater. One technique used to remove a lead was the application of traction and rotation to the outer free end of the lead. This technique, however, could only be done before the lead tip became fixed in scar tissues within the trabeculations of cardiac tissue, such as at the apex of the right ventricle. Since it is very difficult to determine the formation of a clot, even shortly after lead implantation, there exists the risk a clot has already formed. Removal of a lead at that time may cause various sized emboli to pass to the lungs, thereby producing severe complications.
In cases where the lead tip has become attached by fibrous scar tissue to the heart wall, removal of the lead has presented further major problems and risks. Porous lead tips may have an ingrowth of fibrous scar tissue attaching them to the heart wall. Sufficient traction on such leads in a removal attempt could cause disruption of the wall prior to release of the affixed lead tip, causing fatality. Moreover, a sheath of fibrotic scar tissue may further prevent lead removal and endothelium surrounding the outer surface of the lead body and specifically the insulator sleeve, as mentioned above, at least partway along the venous pathway. Such “channel scar” tissue prevents withdrawal because of encasement of the lead. Continual strong pulling or twisting of the proximal free end of the lead could cause rupturing of the right atrial wall or right ventricular wall. Encasement by fibrous scar tissue in the venous pathway and in the trabeculations of cardiac tissue typically occurs within three to six months after the initial placement of the lead.
In the context of implantable leads, and particularly in the context of implantable cardiac leads, there is often a need to remove a lead after it has been implanted in a patient's body for some period of time. In conjunction with lead removal, it is often necessary to apply traction to the lead, in order to pull it free from tissue adhering thereto. It has therefore been recognized for some time that a reinforcement of some type, extending along the lead body would be beneficial, in order to prevent breakage or partial disassembly of the lead during removal. For example, in U.S. Pat. No. 5,231,996 issued to Bardy et al., a variety of reinforcement mechanisms are disclosed, including cords, filaments, braids, and the like.
More recently, in the context of implantable cardiac leads, the use of cabled or stranded conductors in place of the previously more commonly employed coiled conductors has become more popular. These cabled or stranded conductors, such as disclosed in U.S. Pat. No. 5,584,873 issued to Shoberg et al., U.S. Pat. No. 5,760,341 issued to Laske et al. and U.S. Pat. No. 5,246,014 issued to Williams et al. inherently provide an increased tensile strength lead, at least along the segment between the point at which the stranded or cabled conductor is coupled to an electrode and the point at which the conductor is coupled to an electrical connector at the proximal end of the lead. While this new conductor inherently provides a lead of enhanced tensile strength, in most endocardial cardiac pacing leads employing cabled or stranded conductors, the conductor that extends to the distal-most portion of the lead is still a coiled conductor in order to permit passage of a stylet. This distal-most portion of the lead is the portion of the lead that is most likely to be firmly embedded in fibrous tissue. It is therefore desirable that this portion of the lead in particular should be capable of withstanding high tensile forces without breakage.
As the designs of pacing leads have progressed over the years, there has been a general trend toward reduction in the diameter of the bodies of such leads, that is, those that are ≦4 French in diameter. However, as the diameter of the lead body is reduced, the production of a pacing lead that has an adequate tensile strength to allow relatively easy extraction via traction and rotation has become more difficult.
One approach to providing a small diameter lead with a high tensile strength is to fabricate the lead using an inextendible conductor, for example, a stranded conductor as disclosed in U.S. Pat. No. 5,246,014 issued to Williams et al., a cabled conductor as disclosed in U.S. Pat. No. 5,584,873 issued to Shoberg et al., or a tinsel-wire conductor as disclosed in U.S. Pat. No. 3,844,292 issued to Bolduc all incorporated herein by reference in their entireties. Another approach to increasing the tensile strength of a lead including a coiled, normally extensible conductor is to provide a reinforcing fiber or core within the lead, as disclosed in the previously mentioned U.S. Pat. No. 5,231,996 issued to Bardy et al., and U.S. Pat. No. 5,056,516, both also incorporated herein by reference in their entireties.
The previously referenced U.S. patent application Ser. No. 09/559,161, entitled, “Medical Electrical Leads with Fiber Core,” which discloses how to fabricate a high strength, small diameter pacing lead. This lead that can be as small as 2 to 4 French in diameter utilizes a fiber core fabricated from a single length of fiber that is d-oubled and twisted together, around which a multi-filar coil is wound. The composite conductor coil/core structure is coupled to a connector assembly at its proximal end and, preferably, to a helical electrode at its distal end. Having such a pacing lead available, there still remains the issue of how to extract such a lead in those patients who require such a procedure.