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.

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'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.

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 D1 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 D1 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 D2 substantially less
 than the inner diameter D1 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.