Abstract:
The invention resides in a multi-conductor lead comprising an inner tube containing at least two lumens, the tubing having an outer surface defining an outer diameter thereof. Two coiled conductors are housed within the at least two lumens in the tubing and extend longitudinally therewithin. A third coiled conductor is disposed circumferentially about the outer surface of the tubing. Insulation coating is disposed circumferentially about the outer conductor. Accordingly, the outer coil serves to prevent subclavian crush from causing the inner conductors to short to each other.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 09/050,513, filed Mar. 30, 1998 now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to an implantable stimulation lead for use with a cardiac pacemaker, and more specifically, to an implantable stimulation lead having lead body wherein two or more inner conductors are protected from abrasion by a surrounding outer coil. 
     BACKGROUND OF THE INVENTION 
     It is known that leads used in cardiac stimulation are often implanted transvenously or transthoracically with the result that the lead body can be physically crushed by either bones (i.e. “first rib-clavicle”) or by tissue (costoclavicular ligament complex, subclavius muscle) and by anchoring sleeves which are tied-down so tightly that the lead body can be crushed or damaged. 
     The result of these crushing or constrictive stresses, which are made more pronounced by movements of the patient, can become manifested by severe damage to the conductors, such as by abrasion of the insulation of the conductors within the lead body which, in turn, can result in failed conductors and/or failed insulation. This is because the conductors within the lead are usually disposed in close proximity to one another. For example, in a tripolar pacing lead, at least three conductors would be housed within the lead and must be maintained in a spatially disposed relationship with one another. 
     Conductor mechanical damage including fractures and/or insulation breaks may occur in about 2% to 3% of implanted leads. In patients who are not pacemaker dependent, a failure event is usually not life-threatening, but can require corrective procedures with potential for complications. Mechanical damage can occur as coil deformation, coil fracture, mechanically induced insulation breaches, and insulation wear observed individually or in combination. 
     SUMMARY OF THE INVENTION 
     The invention resides in a multi-conductor lead defined by an inner tube having at least two lumens formed therein. About the inner tube is a first coiled conductor, and within the at least two lumens are disposed a respective second and third conductor extending longitudinally therewith. 
     At the distal end, the first coiled conductor has a portion thereof which is offset wound at a diameter which is smaller than that of the diameter thereof which is coiled about the inner tube so that a connection can be made in-line with the sensor or distal electrode. 
     An advantage of this arrangement is to provide a multi-conductor lead body having a design which resists mechanical damage, in particular abrasion. 
     Another advantage is to provide a lead body of the aforementioned type wherein one of the conductors provides a mechanical barrier against the adverse effects of abrasion relative to the remaining conductors in the lead body. 
     Further, another advantage is to provide a lead body of the aforementioned type which includes a sensor which is capable of connecting to one or more conductors disposed within the tubing, while allowing one conductor to pass through to the distal tip electrode. 
     In the preferred embodiment, the multi-conductor lead has an inner tube extending concentrically along a central axis thereof, and containing at least two lumens therein. The inner tube has an outer surface defining an outer diameter thereof. 
     A first coiled conductor is provided and has a predetermined diameter which is slightly greater than the outer diameter of the tubing such that the first conductor is disposed circumferentially about the outer surface of the tubing. An insulation sheath is provided circumferentially about the first conductor. 
     A second and third conductor are disposed within the at least two lumens in the tubing so as to extend longitudinally therewithin. 
     At the distal end, the first conductor has an offset portion which is directed for connection to another element of the lead (e.g., a ring electrode or a sensor). Ideally, the first conductor&#39;s offset portion is a continual length of the first conductor and ends in a coil portion which has a diameter substantially less than the original diameter of the first conductor length. 
     In one embodiment of the invention, a sensor is disposed axially in line with the inner tube and has a first and second terminal associated with, and connected to, a selected two of the conductors. The remaining one of the conductors may be selected to pass beneath the sensor to the distal end of the lead body. 
     Ideally, the first outer conductor is a drawn-filled conductor. The drawn-filled conductor is preferably a silver core MP35N conductor used to reduce the resistance inherent to a large diameter conductor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a perspective view of a lead body of the present invention; 
     FIG. 2 shows a sectional view along line  2 — 2  of FIG. 1; 
     FIG. 3 shows a partially cross-sectional side view of the lead body; and 
     FIG. 4 shows a partially cross-sectional top view of the lead body. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 depicts a multilumen pacing lead  20  according to the present invention. While a three electrode lead is shown in FIG. 1, it will be abundantly apparent that the principles of the present invention can be adapted to any lead requiring three or more conductors, such as a lead incorporating a sensor with one or more pacing electrodes, a quadrapolar (i.e., 4 electrode) system, a single-pass lead having electrodes in the atrium, or a multi-site pacing electrode for use in the coronary sinus. Suffice it to say that these applications are all within the spirit of the invention, and that one of skill in the art can readily apply the principles taught herein to satisfy these applications. 
     As shown in FIG. 1, the multilumen pacing lead  20  has an elongated lead body  22 , extending between a proximal end  26  and a distal end  25 , which includes electrical conductors extending through lumens within a multilumen tubing (not shown). At a proximal end  26  of the pacing lead  20  is a connector assembly  28 , which is provided with sealing rings  30  and electrical connectors  32 ,  34  and  36  for connection to a pulse generator. 
     The insulating portions of the connector assembly  28  which space apart the connectors  32 ,  34  and  36  may be fabricated from segments of multilumen tubing of silicone, rubber, polyurethane, or other suitable plastic. The electrical connectors  32 ,  34  and  36  are preferably fabricated of stainless steel or other suitable conductive material. 
     At the distal end  25  of the pacing lead  20  is an electrode assembly  42  which may include multiple electrodes or sensors, and which is intended to be implanted into the heart. A tip electrode  51  is located at the distal end  25  of the electrode assembly  42 . Two ring electrodes  52  and  58  are shown proximal to the tip electrode  51 . The ring electrode  52  may be used, for example, as a anode in a bipolar pacing system. 
     Alternatively, the electrodes  52  and  58  can be used as sensor electrodes to determine various parameters of cardiac activity, such as, atrial electrical activity, ventricular electrical activity, or to sense impedance changes to determine stroke volume, pre-ejection fraction, and respiratory rate. Monitoring of these parameters is beneficial for advanced pacing systems to allow the pacemaker to more effectively control cardiac activity. 
     As shown in FIG. 2, the pacing lead  20  includes a multilumen tubing  24  which has an outer diameter D defined by an outer surface  18 . A first coiled conductor  48  is circumferentially disposed about the multilumen tubing  24  (i.e., the inner diameter of the first coiled conductor is slightly greater than the outer diameter of the tubing). The first coiled conductor  48  is, in turn, covered by an outer sheathing  21 . 
     The first conductor  48  is formed as a coiled conductor, but has an inner diameter D 1  substantially larger than that of the second and third conductors,  44  and  46 , yet is only slightly smaller in size than the inner diameter of the outer sheathing  21  as defined by the cylindrical surface  18 . The inner diameter D 1  of the first conductor  48  is such that it is coiled about the outer surface  18  of the multilumen tubing  24 . Thus, in the event that abrasion occurs through the outer sheathing  21  of the multilumen inner tube, the first conductor  48  would act to stop such abrasion without jeopardizing the second and third conductors,  44  and  46 , disposed internally thereof. The first conductor  48  is preferably a (DFT) drawn-filled inner tube type conductor. 
     Both the multilumen tubing  24  and the outer sheathing  21  are preferably fabricated of silicon, rubber, polyurethane, or another suitable plastic material having the properties of biocompatibility, biostability and flexibility. 
     As illustrated in FIG. 2, the multilumen inner tube  24  is generally circular in transverse cross-section and is concentric about the central axis CA of the lead body  22 . A first lumen  44 ′ is disposed, for example, on one side of the central axis CA while a second lumen  46 ′ is disposed, for example, on the other side of the central axis. 
     The multilumen tubing  24  includes a second conductor  44  and a third conductor  46  disposed within the lumens  44 ′ and  46 ′, respectively. The conductors  44  and  46  are helically wound electrical conductors, each conductor being separated by the interposed insulation material of the multilumen tubing  24 . 
     Each conductor  44  and  46  is itself defined by a spiral winding resulting in a hollow central area  54 , which allows the lead body  22  to remain quite flexible. Also, the hollow central area  54  accommodates insertion of a guide wire or stylet (not shown) which is relatively stiff and which allows the doctor to guide and control the implantation of the pacing lead  20 . 
     In addition, it should be understood that each conductor  44  and  46  may preferably be made up of a plurality of filars contained in a bundle to provide redundancy while also retaining flexibility by reducing the cross-sectional thickness which would be required for a single conductor. The conductors  44  and  46  are standard MP35N conductors. 
     In the preferred embodiment, the diameter of the lead body is in the range of between about 1.50 mm and 3.50 mm and preferably about 2.5 mm. It is to be understood that the conductors appear solid in cross-section, which results from a tightly wound helix having many turns per inch. obviously, for helixes with fewer turns per inch, the cross-section would show portions of adjacent windings. 
     FIGS. 3 and 4 illustrate two cross-sectional views of a sensor embodiment. It is recognized that ring electrodes, as shown in FIG. 1, could be substituted for the sensor terminals, and one of skill in the art could readily modify the present invention to achieve such configuration. 
     As seen in FIGS. 3 and 4, the lead body  20  includes a sensor device  60  which may be part of the electrode assembly  42  and is axially defined by a first end wall  57  and a second end wall  59 . The sensor device  60  is axially connected at end wall  59  to one end of the multilumen tubing  24  as described below. 
     As illustrated in FIG. 4, the sensor  60  includes a first terminal  62  and a second terminal  64 , each respectively associated with a selected one of the conductors  44 ,  46  or  48 . As presently shown, the first terminal  62  is coupled to the second conductor  44  and the second terminal  64  is coupled to the first conductor  48 . 
     The first terminal  62  is located on the second end wall  59  of the sensor  60  in line with the first lumen  44 ′. The second terminal  64  is also located on the second end wall  59  and is located offset from the central axis CA to effect connection with the first conductor  48  in a manner which will become apparent. 
     Each terminal  62  and  64  connects to associated electronic components within the sensor  60  in order to transmit the sensed physical conditions within the body to electronic signals sent to the pulse generator located remotely thereof. Each conductor to sensor terminal connection is made using a standard connection, such as, by a weld or by crimping. 
     As best seen in FIGS. 2 and 4, the first conductor  48  has a connection portion  70  extending axially thereof and is defined by a coiled connection section  72  which extends generally parallel to the central axis CA. The coiled connection section  72  is of an outer diameter D 2  substantially less than the inner diameter D 1  of the remainder of the first conductor  48 . The coiled connection section  72  connects to the second terminal  64  and includes a bridging portion  76  which is a continuous length of the first conductor  48  and is directed in a manner so as to locate the coiled connection section  72  offset from the central axis CA and the remainder of the first conductor  48 . 
     As seen in FIG. 3, each of the first and second end walls  57  and  59  of the sensor device  60  has a through opening  78  and  80  formed therein. Each such opening is generally circular in form and has a diameter which is only slightly larger in size than the outer diameter of the second conductor  46 . In this way, the second conductor  46  being of a length longer than that of the conductors  44  and  48 , is caused to pass through the sensor device via openings  78  and  80 . As illustrated, an insulative sheathing  82  is provided around the second conductor  46  in the region where it passes through the sensor device  60  so as to protect against shorting with the terminals  62  and  64 , and/or with the electronic components within the sensor device  60 . The second conductor  46  is of a length sufficient to extend beyond the first end wall  57  and ultimately connects to the tip electrode  51  where it delivers a pulse for pulse control and/or defibrillation control. 
     By the foregoing, a multi-conductor lead body has been described by way of the illustrative sensor embodiment. However, numerous modifications and substitutions may be made to the invention without departing from the spirit of the invention. For example, while disclosed in the preferred embodiment as a three conductor lead, it is entirely within the purview of the invention to use additional conductors within additional lumens provided in the multilumen tubing  24 . In addition, the invention may be used in a unipolar or bipolar mode as well as with a stimulation coil and/or pacing lead. 
     Accordingly, the invention has been described by way of the illustrated embodiment.