Bipolar electrode lead for medical applications

A bipolar electrode lead for medical applications, such as for electrical stimulation of body tissue, has an elongated conductor extending from a source of electrical stimulation to a tissue location to which the electrical stimulation is to be delivered. The conductor is surrounded by electrical insulation. The lead has an exposed electrode tip at the tissue location. The electrical conductor is in the form of a braided hose. The electrical insulation has a gap, which exposes the braided hose conductor, at a location spaced from the electrode tip. The exposed braided conductor functions as the neutral electrode.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a bipolar electrode lead for medical 
applications, particularly for use with an implantable heart pacemaker to 
deliver electrical stimulation pulses from circuitry within the pacemaker 
housing to a selected heart location. 
2. Description of the Prior Art 
The use of unipolar heart pacemaker electrode lead is known, wherein the 
pacemaker housing serves as the neutral electrode for the stimulation 
system. This requires that the return current path from the electrode at 
the lead tip travel through the intervening body tissue between the lead 
tip and the pacemaker housing. It is known that this can result in 
unwanted muscle stimulations and/or muscle inhibitions in the tissue 
surrounding the pacemaker housing. 
A known solution to this problem is the use of a bipolar electrode lead, in 
which the neutral electrode is formed by an exposed ring of electrically 
conductive material disposed slightly spaced from the active electrode at 
the tip of the lead. If the tip of the lead is disposed, for example, 
inside the heart, the complete electrical current path will also be 
contained inside the heart. In order to keep the current density in the 
neutral electrode low, the ring must have a relatively large surface area. 
The relatively large dimensions of the neutral electrode in such a bipolar 
arrangement result in considerable stiffening of the lead, which is 
otherwise extremely flexible. If, for example, the tip of the electrode 
lead is to be disposed in the left ventricle, the neutral electrode will 
also be in the left ventricle. Given the large number of bends to which 
the electrode lead is exposed, such a stiffening represents a high stress, 
which increases the risk that damage to the insulation, or a rupture of 
the conductor in the proximity of the stiffening, will occur. 
If the bipolar lead is used as an atrial electrode, it can be bent into the 
shape of a J, so that the tip thereof can be applied in the right atrial 
appendage. Under such conditions, the neutral electrode cannot be disposed 
in the region of the bent portion of the lead, because of the 
aforementioned stiffening problem, and also because the neutral electrode 
is relatively heavy, compared to the remainder of the lead, so that an 
unwanted dislocation of the anchoring of the electrode tip can occur. If 
the neutral electrode is disposed at a greater distance from the distal 
end (i.e., the tip) of an atrial lead, it will come to lie in the vena 
cava in most cases. If the neutral electrode is disposed at that location, 
the current pulses associated with the neutral electrode can influence the 
phrenic nerve, which contains sensory fibers leading to parts of the 
pleura and diaphragm. Stimulation of the phrenic nerve can result in 
hiccups and coughing which are unpleasant for the patient. 
A bipolar heart pacemaker electrode lead is described in U.S. Pat. No. 
4,295,270 having an electrical conductor consisting of a braided hose 
which is connected to the neutral electrode. The neutral electrode is 
formed by a stiff ring, so that this electrode lead has the 
above-described disadvantages, despite the use of braided hose. 
To make a bipolar electrode lead more elastic, a plurality of smaller 
neutral electrodes can be applied in succession along a portion of the 
lead. Such a lead is, however, relative complicated to manufacture, and is 
thus expensive. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a bipolar electrode 
lead suitable for use with an implantable heart pacemaker wherein the lead 
is elastic and flexible even in the region of the neutral electrode. 
The above object is achieved in accordance with the principles of the 
present invention in an electrode lead wherein at least one of the 
electrically conductors is formed by a braided hose, and wherein a portion 
of the outer insulation is stripped away, thereby resulting in a gap in 
the insulation through which the braided hose conductor is exposed. The 
exposed portion of the braided conductor functions as the neutral 
electrode. 
The neutral electrode can thus have an arbitrarily large surface area, 
while continuing to remain elastic and flexible, as the remainder of the 
electrode lead, without degrading the electrical properties of the 
electrode lead. 
In a preferred embodiment of the invention, the conductor has an elongated 
core, with the braided hose proceeding over at least a portion of the 
core. This results in the braided hose contracting when a longitudinal 
pulling force acts on the electrode lead, so that the braided hose 
tightens around the core so that the core is not stretched or deformed in 
this region. This effect can be of particular advantage for an electrode 
lead wherein the core consists of a helical electrical conductor. A 
disadvantage of conventional leads using helical conductors is that the 
helix becomes elastically or plastically stretched in an undesirable 
manner when such longitudinal forces act thereon. This disadvantage is 
avoided in the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in the drawing, a bipolar heart pacemaker electrode lead 1 
constructed in accordance with the principles of the present invention 
consists of an electrode tip 3 at a distal end of the lead and an 
electrode connector 4 at a proximate end of the lead, with a lead section 
2 therebetween. The connector 4 is adapted to be received in a known 
manner in a mating connector of a heart pacemaker housing (not shown). The 
lead section 2 consists of an elongated electrical conductor 5 with 
electrical insulation 6. 
The electrode lead 1 has a further electrical conductor in the form of a 
braided hose 7. A section 8 of the braided hose 7 is not covered by 
electrical insulation 6, thereby forming a gap in the electrical 
insulation 6, so that the exposed portion 8 of the braided hose serves as 
the neutral electrode. This exposed portion is just as flexible as the 
remainder of the lead section 2. 
As can also be seen in the drawing, the electrical conductor 5, which is 
surrounded by the exposed portion 8 of the braided hose 7, is helical, and 
is preferably insulated. The braided hose 7 can proceed over the entire 
length of the lead section 2 to the electrode tip 3. As a result, the 
helical conductor 5 is supported, so that it is not significantly deformed 
when a tensile force acts on the electrode lead 1. 
In the exemplary embodiment shown in the drawing, the exposed portion 8 of 
the braided hose 7 is disposed at a sufficient distance from the electrode 
tip 3 to function as the neutral electrode for the electrical stimulation 
system. It will be understood by those skilled in the art, however that 
the exposed portion 8 can be arranged anywhere along the lead section 2 as 
may be useful for other purposes. 
Although further modifications and changes may be suggested by those 
skilled in the art, it is the intention of the inventor to embody within 
the patent warranted hereon all changes and modifications as reasonably 
and properly come within the scope of his contribution to the art.