Medical electrical lead having improved inductance

A conductor for connecting an electrode near a distal end of a medical electrical lead with an implantable medical device connected with a proximal end of the medical electrical lead includes a multi-filar coil wrapped around a central core. The multi-filar coil has an inductance of approximately 0.5 μH or greater, and the central core is non-conducting and provides reinforcement for the multi-filar coil.

CROSS REFERENCE TO RELATED APPLICATION(S)

The following co-pending application is filed on the same day as this application: “POLYMER REINFORCED COIL CONDUCTOR FOR TORQUE TRANSMISSION” by inventors M. T. Marshall and H. D. Schroder, U.S. application Ser. No. 11/343,884, now abandoned and is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to implantable medical device (IMD) leads for delivering active electrodes to various places in a human body, such as the heart. In particular, the present invention relates to lead conductors that are compatible with radio frequency (RF) fields generated by magnetic resonance imaging (MRI).

Typical leads for use with an IMD, such as an implantable cardioverter defibrillation (ICD) device, deliver multiple conductors to the heart for performing pacing, cardioverting, defibrillating, sensing and monitoring functions. One or more of these conductors typically comprises a multi-filar cable in which nineteen filars are wrapped around a straight central filar. This type of design yields a cable that has good mechanical properties, including flexibility, weldability and high strength. Strength is particularly important for ensuring adequate electrical and mechanical contact between the conductor and an electrode when an electrode is crimped down on the conductor. For example, a good crimp should produce a 2.5 lbf joint. These multi-filar, cables, however, have very low inductance particularly due to the straight central filar. During magnetic resonance imaging, it is necessary to expose the patient and the IMD to a radio-frequency field, which is used to generate the MRI image. Generally, it is desirable for a lead conductor to have increased inductance in order to minimize excitation effects from RF fields generated during magnetic resonance imaging.

BRIEF SUMMARY OF THE INVENTION

The present invention comprises a strength-enhanced conductor for a medical electrical lead. The conductor connects an electrode near a distal end of a medical electrical lead with an implantable medical device connected to a proximal end of the medical electrical lead, and includes a multi-filar coil wrapped around a non-conducting central core. The multi-filar coil includes an inductance of approximately 0.5 μH or greater.

DETAILED DESCRIPTION

FIG. 1shows implantable cardioverter defibrillation (ICD) lead10of the present invention. ICD lead10is used to deliver tip electrode12, ring electrode14, right ventricle (RV) defibrillation coil16and superior vena cava (SVC) defibrillation coil18to a heart for the purposes of providing cardio-therapy.

Tip electrode12, ring electrode14, RV coil16and SVC coil18are connected at distal end20of ICD lead10with various conductors that run to proximal end22of ICD lead10, where the conductors are joined with connector assembly24. Connector assembly24routes the individual conductors to connectors26,28and30for connection with connector sockets of an implantable medical device (IMD).

Tip electrode12and ring electrode14are connected with connector28and with a conductor coil and a conductor cable, respectively, which are electrically isolated within lead10. Tip electrode12and ring electrode14are used to sense cardiac signals and to deliver pacing pulses to the right ventricle of the heart in conjunction with the IMD. RV coil16is joined with connector26, and SVC coil18is joined with connector30through conductor cables, which are electrically isolated from each other within in lead10. RV coil16(which is placed in the right ventricle) and SVC coil18(which is placed in the superior vena cava) can be used as cathode and anode to deliver defibrillation shocks to the heart from the IMD, as a result of a tachycardia or fibrillation condition sensed in the heart by tip electrode12and ring electrode14.

Tip electrode12typically comprises a fixation device, such as a helix or corkscrew, which is used to secure tip electrode12to tissue of the right ventricular apex of the heart. A fixation helix comprises a rigid coil with a sharpened tip that can penetrate into the tissue in order to anchor the position of tip electrode12within the heart. At the proximal end of lead10, a rotational force is applied to a torque coil, which then transmits the torque to its distal end and the fixation helix, whereby it attaches to the heart tissue.

FIG. 2Ashows cross section2-2ofFIG. 1showing the conductors of lead10, including coil conductor34, sense conductor36, RV conductor38and SVC conductor40.FIG. 2Bshows a partially cut away perspective view of cross section2-2ofFIG. 1, in which the features of lead10are illustrated.FIGS. 2A and 2Bare discussed concurrently.

ICD lead10includes multi-lumen lead body42, which includes four lumens42A-42D for conveying each of the four conductors of lead10. Lead body42is typically comprised of extruded silicone rubber, and is covered by sheathing44that protects the components of lead10from the environment of the body in which it is implanted. Sheathing44is also comprised of extruded silicone rubber or another bio-compatible material.

As discussed above, exposure of IMD leads to MRI can result in localized heating of electrodes due to excitation of conductors from RF fields used in obtaining MRI images. When an electrode with a small surface area is vibrated by a conductor, excessive heat can build up in the electrode. High levels of vibration in an electrode are correlated with low inductance of the conductor to which it is connected. Conductors with high inductance are more resistant to excitation in RF fields, and are therefore more RF field compatible. For small electrodes, it is desirable to connect them with the IMD using conductors having a large inductance.

Generally, it is desirable for conductors used in conjunction with tip electrodes to have a total inductance in the range of about 1.0 μH to about 5.0 μH, preferably greater than about 1.5 μH. A large inductance is necessary due to the relative small surface area of tip electrodes, typically about 2.5 mm2(˜0.003875 in2) to about 5 mm2(˜0.00775 in2). For ring electrodes, which have surface areas in the range of about 20 mm2(˜0.0310 in2), the inductance of the conductor can be as low as approximately 0.5 μH, but is preferably higher.

The inductance of a conductor is determined by its geometric properties, particularly if it is wound into a coil or straight. Straight wires have an inductance that approaches zero, and are therefore generally undesirable for small electrodes of leads that have the possibility of exposure to MRI. A conductor that includes straight filars in addition to wound filars also has an inductance that approaches zero.

The inductance of a wound coil is determined by several factors: the diameter of each wire conductor, the pitch of the coil (the distance between turns of the coil), the cross-sectional area occupied by the coil, and the number of filars comprising the coil. These parameters are constrained by the design requirements for each application in which the lead will be used. For example, a typical ICD lead must have an overall diameter less than approximately 6.6 French (˜0.0866″ or ˜0.2198 cm).

RV conductor38comprises a stranded cable conductor in which nineteen wire filars46are wrapped around central wire filar48inside sheathing50. Similarly, SVC conductor40comprises a stranded cable conductor in which nineteen wire filars52are wrapped around central wire filar54inside sheathing56. The inductance of straight, central filars48and52effectively reduces the inductance of conductors38and40to zero. However, because RV conductor38and SVC conductor40are connected with RV coil16and SVC coil18, which have large enough surface areas, excitation heating is not a concern and neither is the inductance of conductors38and40.

Coil conductor34is connected with tip electrode12, which has a relatively small surface area and is thus susceptible to excitation heating. Therefore, it is important for coil conductor34to have a high enough inductance to be RF field compatible. High inductance of coil conductor34must be achieved while also maintaining the torque transmitting capabilities of conductor coil34. Therefore, the inductance of coil conductor34is increased, while maintaining the torque transmitting properties of the coil, utilizing an improved design, the details of which are described in the above referenced co-pending application by Marshall and Schroder. Coil conductor34is comprised of co-radially wound filars68and70, that are enveloped in compression sheathing72. In short, the inductance of coil conductor34is increased by reducing the number of filars in the coil. The pitch of coil conductor34can also be decreased to increase the inductance. In order to maintain the torque transmitting capabilities of coil conductor34, compression sheathing72is extruded around coil conductor34in order to restrict radial expansion of the coil when it is placed under torque, thereby increasing its ability to transmit torque from its proximal to distal ends.

Turning to the present invention, conductor36is connected with ring electrode14, which has a relatively small surface area for electrodes and is thus susceptible to excitation heating. Therefore, it is important for coil conductor36to have a high enough inductance to be RF field compatible. High inductance of conductor36must be achieved, however, while also maintaining a conductor that can produce crimps and welds of suitable strength. Conductor36comprises a multi-filar coil conductor, which is wrapped around a central non-conducting core to form a “coible.” The inductance of sense conductor36is improved by replacing the low-inductance and conducting straight filar of previous designs with the non-conducting core. This eliminates the inductance of the straight wire filar, which essentially reduces the inductance of the entirety of conductor36to zero. Replacing the nineteen wire filars of previous designs is the multi-filar coil, which is wound around the core in a manner that increases the inductance of conductor36.

Coil conductor34includes conductor filars68and70, which are wrapped in compression sheathing72, which also acts as an insulator and as a protective barrier. Coil conductor34is connected with tip electrode12at its distal end and with connector28at its proximal end and is used to deliver pacing stimulus to the heart.

Conductor36includes conductor filars60,62and64, which are wound around core58and encased in sheathing66. Filars60,62and64are form a circuit with ring electrode14at their distal end and with connector28at their proximal end, and are used in conjunction with coil conductor34to perform typical sensing and pacing operations. In one embodiment, filars60,62and64are uninsulated from each other and form a single circuit with ring electrode14and connector28. In other embodiments, more or less filars are used for conductor36. For example, in one embodiment, only two conductor filars are used to further increase the inductance for leads used with tip electrodes, where the electrode surface area is small.

Conductor36has an inner diameter ID, which approximately matches the outer diameter of core58. Filars60,62and64of conductor36are wound to have pitch p. The pitch p of coil conductor36is selected to produce a high enough inductance in coil conductor36to be RF field compatible, given the number of filars chosen for the particular design. In one embodiment, pitch p remains constant from near the proximal end to near the distal end of conductor36. In the three-filar embodiment shown inFIG. 3, filars60,62and64are comprised of 0.0018″ (˜0.0457 mm) diameter cobalt based sheath, silver core wire such as MP35N®, wound over a 0.007″ (˜0.1778 mm) diameter core and having a pitch of approximately 0.007″ (˜0.1778 mm). This configuration yields a conductor with an inductance of approximately 1.0 μH, which is suitable for use with ring electrodes having a surface area of about 20.0 mm2(˜0.0310 in2). In other embodiments, similar wire materials can be used, such as Tantalum sheathings, or silver or gold cores.

In another three-filar embodiment, conductor36is comprised of 0.0012″ (˜0.0305 mm) diameter MP35N wire wound over a 0.005″ (˜0.127 mm) diameter core at a pitch of 0.006″ (˜0.1524 mm). This configuration also yields a conductor with an inductance of approximately 1.0 μH, which is also suitable for use with ring electrodes.

In another embodiment, a two-filar design includes 0.004″ (˜0.1016 mm) diameter MP35N® wire wound over an approximately 0.018″ (˜0.4572 mm) diameter core at a pitch of approximately 0.010″ (˜0.254 mm). This configuration yields a conductor with an inductance of approximately 2.5 μH, which is suitable for use with electrodes having small surface areas, such as tip electrodes with a 2.5 mm2(˜0.003875 in2) or greater surface area.

In another embodiment, a four-filar design includes 0.001″ (˜0.0 mm) diameter filars wound over a 0.0055″ core at a pitch of 0.006″ (˜0.0 mm). This yields a conductor with an inductance of approximately 0.5 mH, which is more suitable for use with electrodes having larger surface areas, such as ring electrodes.

Inner diameter ID approximately matches the outer diameter of non-conducting core58since filars60,62and64are wrapped directly around core58. In one embodiment, filars60,62and64are wrapped tightly around core58, but not so tight as to constrict or compress core58or to significantly reduce the flexibility of core58. Core58is selected to be of a material having good mechanical properties and is non-conducting. Core58must be non-conducting so that it does not affect the inductance of conductor36. Core58must have good strength so that ring electrode14can be properly crimped with conductors60,62and64, such that a sound electrical and mechanical connection is formed. Core58also provides tensile strength to conductor36when electrodes are connected with it. Also, core58must be able to withstand elevated temperatures produced during heat treatment of conductor36. Core58must also have suitable flexibility for implantation and utilization of medical electrical lead10.

Core58is comprised of a twisted multi-strand fiber, such as a liquid crystal polymer. In another embodiment, core58is comprised of expanded Teflon® (ePTFE). In other embodiments, core58is comprised of other materials that achieve the above mentioned properties and can have various constructions, such as solid, stranded or particle.

Conductor36is wrapped in sheathing66, which is comprised of silicone rubber or another bio-compatible material, such as Ethylene Tetrafluoroethylene (ETFE). The thickness of the jacket is determined by the overall diameter of lead10and in one embodiment is 0.00115″ (˜0.0 mm) thick. Sheathing66serves as an insulating and protective barrier around conductor36.