Implantable multi-pole coaxial lead
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an implantable multipole coaxial lead or cable of the type suitable for use in an electrode arrangement for stimulating intracardiac tissue in a pacer system. 
     2. Description of the Prior Art 
     The demands on a multi-pole coaxial lead for use as part of an implanted electrode arrangement in systems such as heart pacemakers with a long life time are extremely high. A lead of this type must be body compatible, of low resistance to maintain energy consumption at a low level thereby increasing life time of pacemaker batteries, exhibit a high fatigue strength under repeated bending stresses, pliant in order to cause minimum disruptions to surrounding organs, as well as stiff enough when pushed forward to enable an easy insertion by the physician. 
     Moreover, these demands are at least in part contradicting as is obviously the case for pliant and low resistance requirements. In order to be pliant the conductors of the lead should essentially have a small diameter and in order to be of low resistance the conductors should have a larger diameter. 
     The attempts to overcome these difficulties in the past are manifold. A coaxial lead is disclosed in German OS No. 3 031 752 wherein the conductors are individually insulated and wound into a single multi-pole helix. Although the diameter of this lead is reduced in comparison to a lead with an inner and an outer helix of conductors separated from another by an insulating sheath and the conductors also being surrounded at the exterior of the lead by a further insulating sheath, the multi-pole helix necessarily has a steeper pitch and thus exhibits reduced flexibility. Further, the single insulating material requires a compromise between the electric properties and body compatibility for that material. 
     In European Application No. 92 798 a lead is disclosed wherein the outer helix is a metal tape or tape-shaped cord in order to reduce diameter and resistance while increasing flexibility. 
     A multi-pole lead for catheters and electrical measurements in the medical field is exemplified in German OS No. 2 408 707 in which a multi-pole helix of axially separated conductors with a first insulating material is further insulated by disposing the helix in the wall of a tube-formed second insulating material. The interior of the tube is a fluid conduit. Although this arrangement where the conductors and the first insulating material are embedded in the second insulating material achieves having a reduced diameter as well as good electrical insulating and body compatibility properties, the explicitly stated materials for the conductor, copper, and for the first insulating material, insulating paint, have properties which make this lead unsuitable for implantation, especially intracardial implantation. The materials in a lead for intracardial implantation must withstand around 100,000 flexes a day, and consequently the lead must exhibit an extremely high fatigue strength, which copper does not satisfy. Further, if a stylet were used at insertion, insulating paint is unsuitable because the walls of insulating material in the central opening of the lead could easily be penetrated. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an implantable multi-pole coaxial lead presenting an optimal compromise to the diverging requirements for stiffness, flexibility, fatigue strength and resistance discussed in the preceding paragraphs. 
     The above and other objects are achieved in accordance with the principles of the present invention in a multi-pole lead having a helix of individually insulated conductors embedded in the wall of the lead tubing, with the inner opening of the tubing receiving a helically wound stylet guide coil. The lead exhibits reduced diameter, good electrical and body compatibility properties in combination with a stylet guide member protective against penetration. 
     Preferably, the stylet guide coil consists of individually insulated conductors in a multi-pole arrangement to increase the versatility of the lead. 
     Preferably, to increase flexibility of the lead, at least one conductor embedded in the lead tubing is a wire consisting of helically wound multiple filaments. Additionally or alternatively, if the guide coil consists of a plurality of conductors, at least one of those conductors may consist of helically wound multiple filaments. 
     Preferably, in order to further improve lead properties at insertion, all helices in the lead are wound in the same (left or right hand) direction. This preferable embodiment corresponds to the habit of surgeons to insert the lead so that the forward direction of the lead includes helical movement of the surgeon&#39;s hand in one direction, and withdrawal of the lead includes a movement of the surgeon&#39;s hand in the opposite direction. As it is preferable that the lead is flexible at withdrawal from the fixation tissue and stiff when approaching the fixation tissue, and several attempts may be required before an optimal electrode fixation is achieved, it is of great importance that this is an intrinsic property of the lead irrespective of stylet function. This is achieved by arranging all helices (i.e., the helix embedded in the tubing, the guide coil stylet helix, and the multiple filament helices) in the same direction, viz., for right handed surgeons the helices should be wound in the left hand direction and for left handed surgeons the helices should be wound in the right hand direction. 
     Other embodiments of the invention are directed to combinations of conductor and insulating materials. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an axial sectional view of a multi-pole coaxial lead constructed in accordance with the present invention illustrating a four-pole conductor arrangement embedded in an insulating tube of silicone rubber, with a four-pole stylet guide coil arrangement. 
     FIG. 2 is a radial sectional view of a helically wound multi-filament conductor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the figures, an embodiment of the invention includes a multi-pole lead 1 having outer helically wound conductors 2 in a four-pole arrangement 2 1 , 2 2 , 2 3 , 2 4 . The conductors 2 consist of multi-filament (e.g., refined steel) wire exhibiting high fatigue strength under repeated flexing stress. The filaments are all wound in the same direction. The conductors are surrounded by a first insulating material 3 with good electric insulating properties, such as FEP (fluorinated ethylene propylene) or PTFE (polytetrafluoroethylene). The conductors are embedded in a second insulating material 4 with good body compatibility properties, e.g., silicone rubber such as silastic Q7-4840 produced by Dow Corning Company. The insulating material 4 is in the form of a tube having a central opening in which a stylet guide coil 5 arranged as a multi-pole conductor arrangement 5 1 , 5 2 , 5 3 , 5 4 . The guide coil conductors may consist of MP 35 N, possibly in a multi-filament arrangement. 
     The conductors 2 and 5 may each be individually identified by colored insulation material, possibly combined with an X-ray contrast medium. By way of example, the guide coil 5 may have an inner diameter of about 0.5 mm and the lead may have an outer diameter of about 2.0 mm. The outer conductor helix is wound with a pitch of 2 to 4 mm, and the distance between the insulated conductors is around 0.2 mm. The tube-formed second insulating material 4 is produced in two steps. The first step is extrusion of the inner part of the tube, and the second step is extrusion of the outer part of the tube over the insulated outer conductors 2 wound on the extruded inner part of the tube, so that the outer conductor helix is embedded in the thus-formed integrated tube wall. 
     The described embodiment could be modified to accomodate various other pole arrangements of the conductors in the outer and in the guide coil helices. 
     Although modifications and changes may be suggested by those skilled in the art it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.