ELECTRODE CABLE FOR ELECTRICAL STIMULATION

Abstract: An electrode cable made of a plurality of helical wires (10, 12, 14, 16) running through the interior of the cable, said cable, which is encased in an insulating outer sheath (6), can preferably serve as an electrical connection between an electrical device connectable to one end of the cable and an electrode connectable to the opposite end of the cable. The cable is a hybrid construction and contains at least a first set (2) of wires, one wire (16) of which devised as a low-resistivity electrical conductor and one wire of which, preferably two or three (10, 12, 14), devised as (a) high-resistivity electrical conductor(s). All the low-resistivity and high-resistivity wires in all the sets of wires are conductor wires with the same external diameter on the order of 0.05-0.20 mm and located side-by-side in strip running parallel to the exterior surface of the cable and helically along the entire cable. Wire connecting means (18 or 20) on both ends of the cable achieve electrical connection between all of the wires (10, 12, 14, 16)in the set (2) of wires and, accordingly, parallel coupling of same in the cable. All the high-resistivity wires (10, 12, 14) in the sets (2) of wires consist of conductor wires made of the same material. The low-resistivity wire (16) in each set of wires consists of a conductor wire with a core (22) made of a low-resistivity material encased in an external sheath (24) made of the same material in the high-resistivity wires (10, 12, 14).

Description: Electrode cable for electrical stimulation Field of the invention The present invention relates to an electrode cable made of a plurality of wires arranged helically along the length of the cable, said cable, which is encased in an outer, tubular sheath made of an electrically insulating material, being mainly intended to serve as an electrical connection between an electrical stimulation device, such as a heart stimulator, defibrillator etc . , which can be connected to the proximal end of the cable, on the one hand, and an electrode connected to the distal end of the cable, on the other hand. The prior art A lead device comprising one or more helices, each consisting of at least one conductor, is previously known from EP Al 0 162 178. Each conductor consists of a plurality of wires arranged in a bundle by intertwining. All the wires in each conductor can be made of the same or of different materials. When different materials are used, the wires can differ in strength and electrical conductivity. This known lead device can be used as an electrical connection between e.g. a heart stimulator and contact electrodes for implantation in the human body. The objective of this known lead device is to achieve a device which, with the smallest possible external diameter for the helix, displays great fatigue resistance, relatively great electrical conductivity and small electrical resistance. Each helical conductor in this known lead device has a core made of a wire and a plurality of wires coiled or wound around that core's longitudinal axis. The wires helically coiled around the core are intertwined into a bundle, making the lead device rather stiff and, therefore, scarcely suitable for advancement through the vascular system tcgether with a stylet unit, inserted into a central channel in sa d lead device, to a desired location in the heart during an implantatio . Summary of the invention One main objective of the present invention is to provide a new type of low- resistivity electrode cable which can be used as an efficient electrical connection between a device for electrical stimulation in the human body and an electrode connected to the device. The device can be e.g. a heart stimulator or defibrillator, and the electrode can be e.g. an lmplantable heart electrode. The electrode cable can also be used for nerve stimulation. The electrode cable must be designed so it has very good electrical conductivity while simultaneously displaying optimal strength, tensile strength, flexural strength and fatigue resistance in particular. In other words, the problem addressed by the invention is to provide an electrode cable with optimal properties in respect to its electrical conductivity, mechanical strength and ability to withstand dynamic stress capable of causing fatigue breakage of the cable, an event which could have fatal consequences for a patient with a heart stimulator. Another objective of the invention is to provide an electrode 'cable design making t possible to minimize the use of expensive materials in manufacturing the wires or conductors in the cable. The problems cited in conjunction with the above objectives have traditionally been tackled to date by the use of either " high-resistivity" electrical conductors (>60Ω for a given reference length) or " low- resistivity" conductors (<15Ω for the same reference length) with a cylindrical spiral (helical) shape, a plurality of conductors wires (also referred to as wires) being helically wound or intertwined with each other. In the use of such high-resistivity conductors, total resistance can obviously be reduced by e.g. increasing the total cross-sectional area of the conductive material in the lead, i.e. by increasing the number of wires in the lead and/or increasing the cross-section of each wire in the lead. However, an increase in the number of wires in the lead increases the lead's external diameter which is a disadvantage for a patient in whom such an electrode is to be implanted. The disadvantage is even greater if a plurality of leads must be itrplanted in the patient. However, an increase in the cross-sectional area of a conductor has an adverse impact on e.g. fatigue resistance. When the number of wires in a helical spiral increases, the pitch of each individual conductor wire also increases, thereby impairing the mechanical properties of the lead. When the aforementioned type of low-resistivity conductors are utilized, the conductors employ wires made of either a low-resistivity material or a material which is a combination of a low-resistivity material and a high- resistivity material, the latter devised to carry the low-resistivity material. However, such low-resistivity wires have limiting mechanical properties . But the problems related to aforesaid objectives can, according to the invention, be solved when an electrode cable of the initially described kind displays the features set forth in the characterizing part of claim 1. Preferred embodiments of the electrode cable according to the invention can also display the features set forth in the dependent claims . Thus, a primary distinguishing feature of the electrode cable according to the invention is that the cable is a hybrid containing at least a first set of electrically conductive wires, this first set of wires, resp. each set of wires, including at least one wire devised as a low-resistivity electrical conductor and at least one, preferably at least two or three, wire(s) devised as (a) high-resistivity electrical conductor (s) , and all the low- resistivity and high-resistivity wires in each set of wires are conductors with the same diameter in the 0.05 to 0.20 rrm range and arranged side-by- side in a strip running parallel to the exterior of the cable sheath and extending helically along the cable's length. The following can be noted to clarify the concepts " low-resistivity" and " high-resistivity" wires (conductors) in the present application. These two concepts are designations whose primary purpose is to indicate that resistivities with clearly differing magnitudes are involved. The ratio between them is particularly important in this context. As is well-known, resistivity is an electrical unit measured in Ωm.. The following are examples of magnitudes and ratios applicable to low-resistivity and high-resistivity wires respectively. As an example of a low-resistivity wire , assume we have a 5.8 m long wire with a cross-sectional area of 0.00785 πm 2 in which a low-resistivity material (silver) constitutes 28% of the conductive material and a high- resistivity material ( P35N) constitutes 72%. This results in a wire (conductor wire) with resistivity on the order of 5.4 x 10 "8 Ωm. A high- resistivity wire with the exact same geometry as the low-resistivity wire but made only of the said high-resistivity material (MP35N) will then have a resistivity on the order of about 2.5 x 10 " ' Ωm. The ratio between the resistivity of the high-resistivity wire and the low-resistivity wire according to this example will then amount to about 4.6. With this kind of hybrid helical wire spiral with one or two low-resistivity wires as the primary electrical conductor means, high-resistivity wires serving as secondary electrical conductor means, providing support for the low-resistivity wires and possessing good mechanical properties, a low- resistivity helical wire can be achieved, which also displays good mechanical properties. In one preferred embodiment of the electrode cable according to the invention, wire connecting means are arranged on each cable end and accomplish electrical interconnection of all the wires in the respective wire set, thereby achieving their parallel connection in the cable. In this manner, a low-resistivity helical wire is obtained with good mechanical properties . The wire connecting means on each cable end can e.g. be seme appropriate type of metallic clamping means which achieve surface contact with each low- resistivity wire and one or more of the high-resistance wires. The clamping means can also be made of a pair of interacting parts which grip in/on the wire spiral . Alternately, the wire connecting means can consist of wire- connecting welded or soldered joints on the ends of the cable connecting the wires . The high-resistivity wires in each wire set suitable are conductors made of the same material, and the low-resistivity wire in each wire set suitably is a wire conductor with a core of low-resistivity material (e.g. silver, gold, copper, platinum etc.), said core being encased in an external sheath made of the same kind of material as the material in the high-resistivity wires. The latter material could be e.g. a cobalt alloy such as MP35N or the like. Material in the external sheath of each low-resistivity wire comprises appropriately 65% to 80% of the wire's total volume. It has been shown that particularly good properties are achieved when the latter percentage amounts to about 72%. With the invention, it also becomes possible to minimize the use (i.e. the required quantity) of a corrosible material (e.g. silver, ccpper etc.) in the electrode cable. The low-resistivity material risks becoming corroded or forming a cathode, the surrounding material becoming anodic, which may cause corrosion. Thanks to the invention, the amount of mixed material can be reduced, thereby increasing the percentage of material such as MP35N, thus also improving the conditions for obtaining an effective welded joint. An electrode cable according to the invention can consist of a unipolar cable, but the electrode cable can alternately contain additional electrical conductors (poles) , the cable thus becoming bipolar or multipolar. Claim 7 provides an example of an electrode cable devised as a bipolar cable according to the invention. Here, the cable also contains a second set of helical wires forming the cable's second pole. Claim 8 sets forth an embodiment of a bipolar electrode cable which is hollow throughout its length by having a longitudinal channel into which a stylet unit can be inserted. The stylet unit is temporarily inserted into the electrode cable during cable implantation to facilitate advancement of the cable through the vascular system to the desired location in the heart. Description of drawings The invention will now be further described and explained, referring to an exemplifying embodiment, schematically depicted in the attached drawing, showing the way in which a bipolar electrode cable according to the invention could be devised. However, the drawing is only intended to elucidate the electrode cable's structure and does not claim to show the invention with accurate dimensions nor to scale. The drawing in the FIGURE only shows a short section of the bipolar electrode cable as a highly schematic longitudinal section. The preferred embodiment The drawing shows a short section of a bipolar electrode cable according to the invention. This cable contains a pair of coaxially arranged, mutually insulated electrical conductor means forming the cable's two poles at either end of the cable. Each of these two conductor means, generally designated 2 and 4 respectively, consists of four helically and cylindrically arranged wires, placed side-by-side, each wire of which constituting an electrical conductor. The outer conductor means 2, as seen in the radial direction of the cable, consist in this instance of a first set of electrically conductive wires 10, 12, 14 and 16. Of these four wires, the wire designated 16 is devised as a low-resistivity electrical conductor wire, whereas the wires 10, 12, 14 are all devised as high-resistivity electrical conductor wires providing lateral support for the low-resistivity wire 16 in addition to serving as conductors. As the FIG. also shows, the cable is externally provided with a tubular sheath 6 made of an electrically insulating material, such a silicon rubber or polyurethane. A tubular sheath 8 made of an electrically insulating material is also arranged between the cable's two poles, i.e. between the first, outer set of wires, generally designated 2, and the inner set of wires, generally designated 4. As the FIG. shows, the electrode cable is hollow throughout its length because the inner, second set of wires, generally designated 4, with its four helical wires, forms a longitudinal channel inside the cable. Thanks to this longitudinal channel, a stylet unit for cable maneuvering can be inserted into the interior of the electrode cable. The ability to maneuver the electrode cable is needed, in practice, when the cable is to serve as an electrical connection between a heart stimulator connectable to the proximal end of the cable, on the one hand, and an electrode on the distal end of the cable, which is to be advanced into a cavity of the heart and affixed to the cavity wall, on the other hand. Wire connecting means are arranged on both cable ends (the proximal end and distal end respectively) . Their task is to accomplish mutual electrical connection between all the wires in the respective wire sets so as to achieve parallel coupling of the wires in the respective wire set. These wire connecting means on either end of t-he cable can be devised and arranged in different ways. In one conceivable embodiment, the wire connecting means consist of a U-shaped metallic clamping means 18 with two parallel jaws which achieve direct surface contact with the wires in a set of wires. A clamping device of this kind is schematically shown in a side view of the set 2 of wires and in a axial longitudinal section of the inner set of wires 4 (at the right end of the inner set 4 of wires) . An alternative type of wire connection means at each end of the cable can consist of welded or soldered joints 20 between the wires in the set of wires. One such embodiment of the wire connecting means is shown to the left m FIGURE at the end of the outer set 2 of wires. As described above, the first, outer set 2 of wires consists of the low- resistivity wire 16 and three high-resistivity wires 10, 12, 14 All four of the wires in this wire set consist of conductor wires with the same diameter in the 0.05 to 0.20 range. As the FIGURE shows, the wires 10, 12, 14 and 16 are located side-by-side in a strip, running " parallel" to the cable's external sheath 6 and extending helically along the length of the cable, located on the surface of an imaginary circular cylinder coaxial to the electrode cable's midlme. The three high-resistivity wires 10, 12, 14 in the set 2 of wires consist of conductor wires made of the same material, e g MP35N, and the only low- resistivity wire 16 in the set 2 of wires is a conductor wire with a core 22 made of a low-resistivity material, such as silver, as shown in a highly enlarged cross-section of the wire 16 at the bott n of the FIGURE. Here, the core 22 has a sheath 24 made of the same material as the material in the high-resistivity wires 10, 12, 14, i.e. MP35N m this instance. The above-described construction according to the invention achieves the advantage that only one of the four wires in the set 2 of wires, viz. the low-resistivity wire 16, is an expensive type with a core 22 of silver encased in a sheath 24 surrounding the core, made of the much cheaper cobalt alloy. All the other three of wires 10, 12, 14 m the set 2 of wires are also made of the cheaper cobalt alloy, so the entire electrode cable is cheaper than if all the wires were of the same expensive type as the wire 16. All told, an electrode cable is achieved which is cheaper to make and m which expensive material is only used to optimize the cable's electrical conductivity, and cheaper material being used for the wires whose primary task is to give the cable the desired strength. The cheaper-to-make high- resistivity wires 10, 12, 14 have the double task of providing lateral support for the expensive low-resistivity wire 16 employed as the primary conductor, and serving as reserve (secondary) cable conductors if the low- resistivity wire or conductor 16 should break, e.g. due to a fatigue stress. This reserve function for the wires 10, 12, 14 is also the reason why they are made of an electrically conductive material, even though the material displays poorer conductivity than the material in the wire's 16 low- resistlvity core 22. A hybrid type of helically wound electrode cable devised according to the invention and containing three high-resistivity wires and one low- resistivity wire can be expected to be 3-5 times more conductive than a geometrically identical, conventional, helically wound electrode cable. As the above shows, the cable in the FIG., which is a bipolar cable, also has an inner, second set 4 of helical wires forming the cable's second pole, in addition to the first set 4 of wires serving as one of the cable's poles. The four wires in the second set 4 of wires are designated 26, 28, 30 and 32. These four wires can also be devised as a hybrid, i.e. only one of the wires in the inner set of wires 4 can be devised as a low-resistivity conductor containing an expensive material, whereas the other wires can be made of a material which gives the set of wires the desired strength. However, there is no inherent reason why all the wires in the inner set 4 of wires could not be made from the same material. As the above shows, certain other properties of an electrode cable according to the invention are also important, in addition to their electrical properties, and therefore merit special attention. As regards the cable's fatigue resistance and flexibility, the ratios s/D and d/D (where s = pitch, D = external diameter and d = the diameter of wire in the wire helix) are especially important parameters. Some examples of typical ratios in this context and strength values are given below. Fatigue For a spiral conductor to survive millions of bendings, as an electrode cable must do in vivo, s/D and d/D must be as small as possible. The value s/D = 1.0 is a large value, whereas s/D = 0.1 is a small value. The value d/D = 0.5 is very large, whereas d/D<0.1 is small. Example l A spiral conductor, made of the material MP35N, with four wires has the ratio d/D = 0.0875 and s/D = 0.4. If this spiral conductor is bent with a radius of 6 mm, an effective stress (von Mise's) amounting to about 320 MPa is obtained. Example 2 A spiral conductor (MP35N) with d/D = 0.0875 and s/D = 0.1, bent with a radius of 6 πrn, gives an effective stress of about 75 MPa. Example 3 A spiral conductor (MP35N) with d/D = 0.218 and s/D = 1.0, bent with a radius of 6 mm, gives an effective stress of about 1050 MPa. The material MP35N has a breaking point at about 1850 MPa and a yield point of about 1250 MPa at a 0.2% offset. Flexibility Both the ratio s/D and d/D must be as small as possible if the spiral is to be very flexible. This cannot be achieved in practice with traditional conductors made of MP35N without resistance rising to an excessively high level . Tensile strength Tensile strength obviously declines with a declining cross-sectional area for all the wires in a spiral conductor. When a wire has a filled core (e.g. a silver filling) , tensile strength declines in the spiral, which is undesirable. When a hybrid spiral according to the invention with only one silver-filled wire is used, strength and resistance are optimized compared to the use of three or four wires filled with silver.

Claims: Claims 1. An electrode cable made of a plurality of wires arranged helically along the length of the cable, said cable, which is encased in an outer, tubular sheath (6) made of an electrically insulating material, preferably being able to serve as an electrical connection between an electrical stimulation device, such as a heart stimulator, defibrillator etc., which can be connected to the proximal end of the cable, on the one hand, and an electrode connected to the distal end of the cable, on the other hand, characterized in that the cable is a hybrid type comprising at least a first set (2) of electrically conductive wires, this first set of wires, resp. each set of wires, containing at least one wire devised as a low-resistivity conductor (16) and at least one wire, preferably two or three wires, devised as (a) high-resistivity conductor(s) (10, 12, 14), all the low-resistivity and high-resistivity wires in all the respective sets of wires being conductor wires having the same external diameter and arranged side-by-side in a strip (10, 12, 14, 16) running parallel to the cable's exterior surface (6) and extending helically along the length of the cable, all the high- resistivity wires (10, 12, 14) m the sets (2) of wires consist of conductor wires made of the same material, and the low-resistivity wire (16) m each set of wires consists of a conductor wire with a core (22) made of a low- resistlvity material encased m an external sheath (24) made of the same material as the material m the high-resistivity wires (10, 12, 14) . 2. The electrode cable according to claim 1 , characterized in that wire connecting means (18; 20) are arranged on each end of the cable and establish electrical connection between all the wires (10, 12, 14, 16) in the respective sets (2) of wires, thereby achieving parallel coupling of these wires in the cable. 3. The electrode cable according to claim 1 or claim 2 , characterized in that the high-resistivity wires (10, 12, 14) are made of a cobalt alloy such as MP35N, and in that the low-resistivity wire (16) comprises a core (22) made of silver, gold, aluminum, copper, platinum. 4 The electrode cable according to claim 3, characterized in that material m the outer sheath (24) of each low-resistivity wire (16) constitutes 65% to 80% of the wire's total volume. 5 The electrode cable of any of the preceding claims, characterized in that the wire connecting means on each cable end consist of metallic clamping means (18) arranged m direct surface contact w th at least each low- resistivity wire (16) and at least one of the high-resistivity wires (10, 12, 14) . 6 The electrode cable according to any of claims 1 4, characterized in that the wire connecting means on each cable end consist of welded or soldered joints (20) connecting the wires (10, 12, 14, 16) 7 The electrode cable according to any of the preceding claims, characterized in that the cable is hollow throughout its length. 8 The electrode cable according to any of the preceding claims, characterized in that the cable is a bipolar cable which, in addition to the first set (2) of wires forming one of the cable's poles, contains a second set (4) of helical wires (26, 28, 30, 32) which forms the cable's other pole, the two sets (2, 4) of helical wires being coaxially arranged, the second (4) inside the first (2) , and separated by a tubular sheath (8) made of an electrically insulating material, such as silicon rubber or polyurethane, between them 9. The electrode cable according to claim 8 , characterized in that the second set (4) of wires is devised at least mainly in the same way as the first set (2) of wires and forms, with its helically wound wires (26, 28, 30, 32) , a longitudinal channel inside the cable, said channel being intended to admit a stylet unit insertable therein. 10. The electrode cable according to any one of the preceding claims, characterized in that said wires in the respective set has a diameter with a size in the range 0.05-0.20 mm

Inventors: BARSNE MAANS, BARSNE, MAANS
Applicants: BARSNE MAANS, PACESETTER AB, BARSNE, MAANS
Application year: 1997
Main classification: A61N1
IPC codes: A61N1, H01B7