# Patent

## Title
Medical device lead including a unifilar coil with improved torque transmission capacity and reduced MRI heating

## Abstract

A conductor assembly for a medical device lead includes a helically coiled conductor including a plurality of turns having a coil pitch and an outer diameter and consisting of one filar having a filar diameter. The coil pitch and outer diameter are selected based on the filar diameter to minimize heating of the helically coiled conductor in the presence of an MRI field. A polymer sheath is formed about the helically coiled conductor such that the coil pitch of the unifilar helically coiled conductor is maintained. The polymer sheath is configured to increase a torque transmitting capacity of the helically coiled conductor.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application claims priority to Provisional Application No. 61/220,658, filed Jun. 26, 2009, which is herein incorporated by reference in its entirety.


TECHNICAL FIELD
The present invention relates to implantable medical devices. More particularly, the present invention relates to medical device lead constructions including a unifilar coil with a polymer coating.
BACKGROUND
Implantable medical devices for treating a variety of medical conditions with electrical stimuli are well known. Implantable medical devices generally include a medical electrical lead for delivering an electrical stimulus to a targeted site within a patient's body such as, for example, a patient's heart or nervous system. Such leads generally have an elongated, flexible insulating body, one or more inner conductors extending through lumens formed in the body and one or more exposed electrodes connected to the distal ends of the conductors.
Leads may be introduced into the patient's vasculature at a venous access site and transvenously guided through veins to the sites where the lead electrodes will be implanted or otherwise contact tissue at the targeted therapy site. A pulse generator attached to the proximal ends of the conductors delivers an electrical stimulus therapy to the targeted site via the one or more conductors.
SUMMARY
The present invention relates to a conductor assembly for a medical device lead. The conductor assembly includes a helically coiled conductor including a plurality of turns having a coil pitch and an outer diameter and consisting of one filar having a filar diameter. The coil pitch is one to about two times the filar diameter, and the outer diameter is at least 4.5 times the coil pitch, to minimize heating of the helically coiled conductor in the presence of an MRI field. A polymer sheath is formed about the helically coiled conductor such that the coil pitch of the unifilar helically coiled conductor is maintained. The polymer sheath is configured to increase a torque transmitting capacity of the helically coiled conductor. In one exemplary embodiment, the medical device lead includes an insulated lead body including at least one electrode, and the helically coiled conductor is electrically coupled to the at least one electrode.
In another embodiment of the present invention the helically coiled conductor, the coil pitch, and outer diameter are selected based on the filar diameter to minimize heating of the helically coiled conductor in the presence of an MRI field. A polymer sheath formed about the helically coiled conductor such that the coil pitch of the unifilar helically coiled conductor is maintained. The polymer sheath is partially ablated in a pattern along a length of the polymer sheath that enhances the torque transmitting capacity of the helically coiled conductor.
While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a cardiac rhythm management system including a pulse generator coupled to a lead deployed in a patient's heart.
FIG. 2A is a perspective view of a conductor assembly including a unifilar conductive coil and a polymer sheath according to an embodiment of the present invention.
FIG. 2B is a cross-sectional view of the conductor assembly shown in FIG. 2A.
FIG. 3A is a cross-sectional view of a conductor assembly including a partially ablated polymer sheath according to another embodiment of the present invention.
FIG. 3B is a plan view of the conductor assembly shown in FIG. 3A.
FIG. 4 is a plan view of a conductor assembly including a polymer sheath ablated with a stent like pattern according to another embodiment of the present invention.



While the invention is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION
FIG. 1 is a schematic view of a cardiac rhythm management system 10 including an implantable medical device (IMD) 12 with a lead 14 having a proximal end 16 and a distal end 18. In one embodiment, the IMD 12 includes a pulse generator such as a pacemaker or a defibrillator. The IMD 12 can be implanted subcutaneously within the body, typically at a location such as in the patient's chest or abdomen, although other implantation locations are possible. The proximal end 16 of the lead 14 can be coupled to or formed integrally with the IMD 12. The distal end 18 of the lead 14, in turn, can be implanted at a desired location in or near the heart 20.
As shown in FIG. 1, a distal portion of the lead 14 is disposed in a patient's heart 20, which includes a right atrium 22, a right ventricle 24, a left atrium 26, and a left ventricle 28. In the embodiment illustrated in FIG. 1, the distal end 18 of the lead 14 is transvenously guided through the right atrium 22, through the coronary sinus ostium 29, and into a branch of the coronary sinus 31 or the great cardiac vein 33. The illustrated position of the lead 14 can be used for sensing or for delivering pacing and/or defibrillation energy to the left side of the heart 20, or to treat arrhythmias or other cardiac disorders requiring therapy delivered to the left side of the heart 20. Additionally, it will be appreciated that the lead 14 can also be used to provide treatment in other regions of the heart 20 (e.g., the right ventricle 24).
Although the illustrative embodiment depicts only a single implanted lead 14, it should be understood that multiple leads can be utilized so as to electrically stimulate other areas of the heart 20. In some embodiments, for example, the distal end of a second lead (not shown) may be implanted in the right atrium 22, and/or the distal end of a third lead (not shown) may be implanted in the right ventricle 24. Other types of leads such as epicardial leads may also be utilized in addition to, or in lieu of, the lead 14 depicted in FIG. 1.
During operation, the lead 14 can be configured to convey electrical signals between the IMD 12 and the heart 20. For example, in those embodiments where the IMD 12 is a pacemaker, the lead 14 can be utilized to deliver electrical stimuli for pacing the heart 20. In those embodiments where the IMD 12 is an implantable cardiac defibrillator, the lead 14 can be utilized to deliver electric shocks to the heart 20 in response to an event such as a heart attack or arrhythmia. In some embodiments, the IMD 12 includes both pacing and defibrillation capabilities.
The electrical signals are carried between the IMD 12 and electrodes at the distal end 18 by one or more conductors extending through the lead 14. The one or more conductors are electrically coupled to a connector suitable for interfacing with the IMD 12 at the proximal end 16 of the lead 14, and to one or more electrodes at the distal end 18. According to the present invention, the one or more conductors are helically coiled including a plurality of turns having a coil pitch and an outer diameter and consisting of one filar having a filar diameter. The coil pitch and outer diameter are selected based on the filar diameter to minimize effects of magnetic resonance imaging (MRI) scans on the functionality and operation of the lead 14. A polymer sheath is formed about the helically coiled conductor such that the coil pitch of the unifilar helically coiled conductor is maintained. The polymer sheath is also configured to increase a torque transmitting capacity of the helically coiled conductor.
FIG. 2A is a perspective view, and FIG. 2B is a cross-sectional view, of a conductor assembly 50 according to the present invention. The conductor assembly 50 extends through the interior of the lead 14 and includes a coil 52 and a polymer sheath 54. The coil 52 is coupled to the IMD 12 via a connector at the proximal end 16 of the lead 14 and to one or more electrodes at the distal end 18 of the lead 14. While a single coil 52 is shown in FIG. 2A and FIG. 2B, the conductor assembly 50 can be configured to included multiple coils 52 each capable of delivering signals between the IMD 12 and the electrodes at the distal end 18.
The coil 52 includes a single filar 56 that is helically wound around a longitudinal axis of the conductor assembly 50. The filar 56 has a diameter d. A lumen 58 extends through the center of the coil 52 and is suitable for receiving a tool to deliver the lead 14, such as a guidewire or stent. The coil 52 includes a plurality of turns having an outer diameter OD and an inner diameter ID. The coil 52 also has a coil pitch p that extends from the center of a turn of the coil 52 to the center of an adjacent turn of the coil 52.
Exposure of the lead 14 to magnetic resonance imaging (MRI) fields can result in localized heating of the electrodes at the distal end 18 due to excitation of the lead conductors (e.g., coil 52). Conductors with high inductance (>1 μH) are more resistant to excitation in MRI fields. The inductance of the conductor is determined by its geometric properties, including whether the conductor is straight or coiled. For a coiled or wound conductor, such as the coil 52, several parameters influence its inductance, including the coil pitch p, the outer diameter OD, the cross-sectional area of the coil 52, and the number of filars comprising the coil. Thus, the dimensions of the coil 52 may be selected to minimize the effects of magnetic resonance imaging (MRI) fields on the performance and response of the lead 14. For example, for a conductor assembly 50 as shown including a single, unifilar coil 52, a coil pitch p in the range of one to about two times the filar diameter d, and an outer diameter OD at least about 4.5 times the coil pitch p increases the inductance of the coil sufficiently to minimize the energy picked up by the coil 52.
Table 1 below provides example dimensions for the coil 52 to minimize electrode heating caused by MRI fields. The listed dimensions are for a coil 52 having a length (extending from the connector to the distal end 18) in the range of about 450 mm to about 600 mm.









TABLE 1





Filar Diameter (d)
Coil Pitch (p)
Coil Outer Diameter (OD)


(inch)
(inch)
(inch)

















0.0005
0.0005-0.0008
0.002


0.001
0.001-0.002
0.004


0.002
0.002-0.003
0.009


0.003
0.003-0.004
0.013


0.004
0.004-0.005
0.020


0.005
0.005-0.007
0.022


0.006
0.006-0.008
0.027


0.007
0.007-0.009
0.031


0.008
0.008-0.010
0.036


0.009
0.009-0.011
0.040


0.010
0.010-0.012
0.045


0.011
0.011-0.013
0.049









These dimensions are suitable for a conductor assembly 50 including a single, unifilar coil 52. The listed dimensions for the filar diameter d, coil pitch p, and coil outer diameter OD are only by way of example, and other dimensions that reduce electrode heating due to MRI fields to suitable levels are also contemplated. In addition, for embodiments of the conductor assembly 50 including multiple coaxial unifilar coils, these dimensions may change to account for the interaction of the coils with each other in the presence of an MRI field.

The coil 52 with a small diameter OD and having a small pitch p may be prone to damage during construction and use. For example, in active fixation leads, the coil 52 is intended to rotate relative to the lead body and drive torque to extend the fixation helix into tissue of the heart 20. Unifilar coils, such as coil 52, often do not conduct torque well, and the forces typically encountered by the lead 14 can cause the coil 52 to experience stress concentrations in portions of the coil 52, which can lead to premature fatigue of the coil 52. In order to improve the torque transmitting capacity of the coil 52, as well as to maintain the integrity of the coil pitch p, the polymer sheath 54 is formed around the coil 52 such that the polymer sheath 54 covers or envelops the coil 52.
The polymer sheath 54 may be formed over the coil 52 such that portions of the polymer sheath 54 extend between turns of the coil 52 to maintain proper spacing of the coil turns with respect to each other. In some embodiments, the polymer sheath 54 is a sleeve that is pulled over the coil 52 during manufacture. In other embodiments, the polymer sheath 54 is extruded over, molded around, adhered to, or heat shrunk over the coil 52. The polymer sheath 54 may be formed over a coil 52 with an open lumen 58. Alternatively, the coil 52 may be coiled around a tube or cylinder of insulative material, and the polymer sheath 54 subsequently formed around the coil 52.
The polymer sheath 54 is thick enough and is comprised of a material that is stiff enough to increase the torque transmission capacity and maintain the coil pitch p of the coil 52, while still allowing the conductor assembly 50 to sufficiently flex during use. In some embodiments, the thickness t of the polymer sheath 54 is less than about 0.002 inch and is made of a material selected from the group consisting of expanded polytetrafluoroethylene (ePTFE), layered ePTFE, polytetrafluoroethylene (PTFE), ethylene/tetrafluoroethylene copolymer (ETFE), fluorinated ethylene propylene (FEP), silicone, polyurethane, silicone-polyurethane copolymer, and a porous polymer. It will be appreciated that other materials and other thicknesses t are also possible.
The polymer sheath 54 may be adhered to portions of the coil 52 to prevent the polymer sheath 54 from delaminating from the coil 52. Often, this may be accomplished by applying an adhesive material to the coil 52 prior to forming the polymer sheath 54 thereon. However, in some cases, the material used for the polymer sheath 54 does not adhere well to the material used for the coil 52. In order to assure good adhesion, the coil 52 may be coated with a material that bonds well with the polymer sheath 54. For example, the filar 56 may be coated in a suitable polymer prior to coiling the filar 56 into coil 52. Alternatively, the coil 52 may be etched, such as via laser etching, with a pattern that allows for good bonding with the polymer sheath 54.
In some cases, portions of the polymer sheath 54 may be modified to increase the flexibility of the polymer sheath 54. For example, in a J-shaped lead, the lead has a sharp bend at the location of the J-shaped portion. The interaction between certain materials for the polymer sheath 54 and the conductive coil 52 at this sharp bend may cause the conductor assembly 50 to remain in the J-shape, which may prevent manipulation of the J-shaped portion during implantation.
FIG. 3A is a cross-sectional view and FIG. 3B is a plan view of a conductor assembly 60 according to another embodiment of the present invention that includes features that improve the flexibility of the conductor assembly 60. The conductor assembly 60 includes a coil 62 and a polymer sheath 64 having material and dimension characteristics substantially similar to the coil 52 and polymer sheath 54, respectively, discussed above with regard to FIGS. 2A and 2B. In this embodiment, the polymer sheath 64 is partially ablated with an ablation pattern 70 along a length of at least a portion of the conductor assembly 60. The polymer sheath 64 is partially ablated in that the polymer sheath 64 is not completely removed (i.e., the coil 52 is not exposed) at the ablated portions of the polymer sheath 64. For example, in a J-shaped lead, the polymer sheath 64 may be partially ablated along the inner radius of the sharp bend at the J-shaped portion. While improving the flexibility of the conductor assembly 60, the remaining thickness of the polymer sheath 64 at the partially ablated locations serve to maintain the pitch p of the coil 62 and improve the torque transmission capacity of the coil 62.
In the embodiment shown, the ablation pattern 70 is a spiral pattern that winds around polymer sheath 64, and about 20% of the polymer sheath 64 is partially ablated. However, it will be appreciated that the ablation pattern 70 may comprise any form, and any percentage of the polymer sheath 64 may be partially ablated, to improve the flexibility of the conductor assembly 60 while maintaining the coil pitch p and improving the torque transmission capacity of the conductor assembly 60.
The polymer sheath 64 may be partially ablated with the ablation pattern 70 using a variety of techniques. In some embodiments, an unmodified polymer sheath 64 is formed over the coil 62 and subsequently modified into the desired pattern. The ablation pattern 70 may be formed by, for example, laser ablating the ablation pattern 70 into the polymer sheath 64. The ablation pattern 70 may alternatively be formed by etching or grinding the ablation pattern 70 into the polymer sheath 64.
FIG. 4 is a plan view of a conductor assembly 80 according to another embodiment of the present invention. The conductor assembly 80 includes a coil 82 and a polymer sheath 84 having material and dimension characteristics substantially similar to the coil 52 and polymer sheath 54, respectively, discussed above with regard to FIGS. 2A and 2B. In this embodiment, the polymer sheath 84 is ablated with a stent-like pattern 90 along a length of at least a portion of the conductor assembly 80. The stent-like pattern 90 includes substantially diamond-shaped ablated portions 92 and unablated portions 94. In some embodiments, the ablated portions 92 are partial ablations that do not extend completely through the thickness t of the polymer sheath 84. In other embodiments, the ablated portions 92 are ablated completely through the thickness t of the polymer sheath 84.
In summary, the present invention relates to a conductor assembly for a medical device lead that includes a helically coiled conductor including a plurality of turns having a coil pitch and an outer diameter and consisting of one filar having a filar diameter. The coil pitch and outer diameter are selected based on the filar diameter to minimize heating of the helically coiled conductor in the presence of an MRI field. In some embodiments, the coil pitch is one to about two times the filar diameter, and the outer diameter is at least 4.5 times the coil pitch. A polymer sheath is formed about the helically coiled conductor such that the coil pitch of the unifilar helically coiled conductor is maintained. The polymer sheath is configured to increase a torque transmitting capacity of the helically coiled conductor. In some embodiments, the polymer sheath is partially ablated in a pattern along a length of the polymer sheath that enhances the torque transmitting capacity of the helically coiled conductor. In one exemplary embodiment, the medical device lead includes an insulated lead body including at least one electrode, and the helically coiled conductor is electrically coupled to the at least one electrode.
Various modifications and additions can be made to the exemplary embodiments discussed without departing from the scope of the present invention. For example, while the embodiments described above refer to particular features, the scope of this invention also includes embodiments having different combinations of features and embodiments that do not include all of the described features. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the scope of the claims, together with all equivalents thereof.

## Claims

1. A medical device lead comprising:
at least one electrode;
a helically coiled conductor electrically coupled to the at least one electrode, the helically coiled conductor including a plurality of turns helically wound around a longitudinal axis of the conductor the helically coiled conductor having a coil pitch and an outer diameter, the helically coiled conductor consisting of one filar having a filar diameter, wherein the coil pitch is one to about two times the filar diameter and the outer diameter is at least 4.5 times the coil pitch to minimize heating of the helically coiled conductor in the presence of an MRI field; and
a polymer sheath formed over the helically coiled conductor such that the coil pitch of the unifilar helically coiled conductor is maintained, wherein the polymer sheath is configured to increase a torque transmitting capacity of the helically coiled conductor.

2. The medical device lead of claim 1, wherein the polymer sheath is partially ablated in a pattern along a length of the polymer sheath to increase the flexibility of the conductor assembly around the partial ablation.

3. The medical device lead of claim 1, wherein the helically coiled conductor is adapted to promote adhesion with the polymer sheath.

4. The medical device lead of claim 3, wherein the helically coiled conductor is coated in a material that promotes adhesion between the polymer sheath and the helically coiled conductor.

5. The medical device lead of claim 3, wherein a surface of the helically coiled conductor is etched to promote adhesion with the polymer sheath.

6. The medical device lead of claim 1, wherein the polymer sheath is comprised of a material selected from the group consisting of expanded polytetrafluoroethylene (ePTFE), layered ePTFE, polytetrafluoroethylene (PTFE), ethylene/tetrafluoroethylene copolymer (ETFE), fluorinated ethylene propylene (FEP), silicone, polyurethane, silicone-polyurethane copolymer, and a porous polymer.

7. The medical device lead of claim 1, wherein the filar diameter is less than about 0.005 inch (0.127 mm) and the outer diameter is less than about 0.035 inch (0.889 mm).

8. The medical device lead of claim 1, wherein a thickness of the polymer sheath is less than about 0.002 inch (0.051 mm).

9. A medical device lead comprising:
at least one electrode;
a helically coiled conductor electrically coupled to the at least one electrode, the helically coiled conductor including a plurality of turns helically wound around a longitudinal axis of the conductor the helically coiled conductor having a coil pitch and an outer diameter and consisting of one filar having a filar diameter, wherein the coil pitch and outer diameter are selected based on the filar diameter to minimize heating of the helically coiled conductor in the presence of an MRI field; and
a polymer sheath formed over the helically coiled conductor such that the coil pitch of the unifilar helically coiled conductor is maintained, wherein the polymer sheath is partially ablated in a pattern along a length of the polymer sheath such that a thickness of the polymer sheath along the partially ablated pattern is less than a thickness of non-ablated portions of the polymer sheath.

10. The medical device lead of claim 9, wherein the coil pitch is one to about two times the filar diameter.

11. The medical device lead of claim 9, wherein the outer diameter is at least 4.5 times the coil pitch.

12. The medical device lead of claim 9, wherein the helically coiled conductor is adapted to promote adhesion with the polymer sheath.

13. The medical device lead of claim 12, wherein the helically coiled conductor is coated in a material that promotes adhesion between the polymer sheath and the helically coiled conductor.

14. The medical device lead of claim 12, wherein a surface of the helically coiled conductor is etched to promote adhesion with the polymer sheath.

15. The medical device lead of claim 9, wherein the polymer sheath is comprised of a material selected from the group consisting of expanded polytetrafluoroethylene (ePTFE), layered ePTFE, polytetrafluoroethylene (PTFE), ethylene/tetrafluoroethylene copolymer (ETFE), fluorinated ethylene propylene (FEP), silicone, polyurethane, silicone-polyurethane copolymer, and a porous polymer.

16. The medical device lead of claim 9, wherein the filar diameter is less than about 0.005 inch (0.127 mm) and the outer diameter is less than about 0.035 inch (0.889 mm).

17. A conductor assembly for a medical device lead, the conductor assembly comprising:
a helically coiled conductor configured to be coupled to a pulse generator at a proximal end and at least one electrode at a distal end, the helically coiled conductor including a plurality of turns helically wound around a longitudinal axis of the conductor assembly, the helically coiled conductor having a coil pitch and an outer diameter, the helically coiled conductor consisting of one filar having a filar diameter, wherein the coil pitch is one to about two times the filar diameter and the outer diameter is at least 4.5 times the coil pitch to minimize heating of the helically coiled conductor in the presence of an MRI field; and
a polymer sheath formed over the helically coiled conductor such that the coil pitch of the unifilar helically coiled conductor is maintained, wherein the polymer sheath is configured to increase a torque transmitting capacity of the helically coiled conductor.

18. The conductor assembly of claim 17, wherein the polymer sheath is partially ablated in a pattern along a length of the polymer sheath to increase the flexibility of the conductor assembly around the partial ablation.

19. The conductor assembly of claim 17, wherein the helically coiled conductor is adapted to promote adhesion with the polymer sheath.

20. The conductor assembly of claim 19, wherein the helically coiled conductor is coated in a material that promotes adhesion between the polymer sheath and the helically coiled conductor.

21. The conductor assembly of claim 19, wherein a surface of the helically coiled conductor is etched to promote adhesion with the polymer sheath.

22. The conductor assembly of claim 17, wherein the polymer sheath is comprised of a material selected from the group consisting of expanded polytetrafluoroethylene (ePTFE), layered ePTFE, polytetrafluoroethylene (PTFE), ethylene/tetrafluoroethylene copolymer (ETFE), fluorinated ethylene propylene (FEP), silicone, polyurethane, silicone-polyurethane copolymer, and a porous polymer.

23. The conductor assembly of claim 17, wherein the filar diameter is less than about 0.005 inch (0.127 mm) and the outer diameter is less than about 0.035 inch (0.889 mm).