Cardiac pacer lead

In the cardiac pacer lead construction disclosed herein, long life and fatigue resistance is obtained by utilizing as a conductive element a thread of conductive carbon filaments in a resin matrix. The thread is helically wound between a stimulation electrode structure at the distal end of the lead and a terminal at the proximal end, the terminal being adapted for connection to a stimulation pulse generating circuit.

BACKGROUND OF THE INVENTION 
The present invention relates to leads for cardiac pacing and more 
particularly to such a lead having high fatigue resistance and tolerance 
for physical abuse. 
Cardiac pacing by means of an implantable electronic stimulator has come 
into increasing acceptance for treatment of a variety of cardiac ailments 
and arrhythmias. Various types of pacing systems are presently available 
which may stimulate the patient's ventricle, atrium, or both. Various 
pacing modes are employed, including those commonly known as fixed rate, 
standby, and synchronous pacing. Except for the seldom used fixed rate 
mode, most of these forms of pacing are responsive to electrical signals 
occurring spontaneously at the heart as a result of natural cardiac 
activity. 
In virtually all popular pacing systems, the electronic pulse generator 
circuit, together with its associated batteries, is positioned at a site 
somewhat remote from the heart itself and electrical connections to the 
heart are made by means of flexible leads. In the case of epicardial lead 
placement, i.e., where the electrode is placed on the exterior of the 
heart, the electronic pacer itself is typically located in the abdominal 
cavity. In endocardial pacing, the pacemaker circuit is often located in 
the subcutaneous pocket near the patient's shoulder and a lead is 
introduced into the patient's heart through a vein. In each case, the lead 
is subject to continuous flexing due to the beating of the heart and to 
the patient's other natural movements, including breathing. Accordingly, 
substantial efforts have been directed at developing leads which ade 
resonably pliant and highly fatigue resistant. Despite such efforts, 
however, lead failure remains a significant problem with pacing system 
longevity, increasingly so as longer-lived and more reliable battery 
systems are developed. 
One of the more popular types of lead is one in which the body of the lead, 
i.e., the portion extending between the stimulation electrode and the 
pacer connection, comprises a helical coil of Elgiloy wire fitting loosely 
within an insulating tube or sheath of silicone rubber. Elgiloy is an 
alloy developed for its high fatigue resistance and the helical winding 
arrangement minimizes stress concentrations under most circumstances. Such 
a lead is relatively compliant and long-lived, but longer life still is 
desired. In installing an endocardial lead of this general construction, 
the lead is typically stiffened during installation by the insertion of a 
stylet which passes through the center of the helix, the connector at the 
proximal end of the pacer lead being tubular for admitting the stylet. The 
stylet stiffens the lead so that it can be threaded through the vein into 
the patient's heart and appropriately positioned within the patient's 
heart. The stylet is then removed before the lead is connected to the 
stimulation pulse generator circuit. This method of lead introduction is a 
desirable feature of any endocardial lead. Even with this configuration, 
however, life expectancy of the lead is not unlimited, and replacement may 
ultimately be required. 
Among the several objects of the present invention may be noted the 
provision of a lead for cardiac pacing which provides exceptionally long 
life and fatigue resistance; the provision of such a lead which is highly 
reliable and which is resistant to physical abuse; the provision of such a 
lead which is easy to introduce; the provision of such a lead providing 
desirable electrical conduction properties; the provision of such a lead 
which is relatively well-tolerated by the patient's body upon 
implantation; the provision of such a lead which facilitates contact with 
cardiac tissue; and the provision of such a lead which is of simple and 
relatively inexpensive construction. Other objects and features will be in 
part apparent and in part pointed out hereinafter. 
SUMMARY OF THE INVENTION 
Briefly, a cardiac pacing lead in accordance with the present invention 
employs, as its flexible conductive element, one or more threads of 
conductive carbon filaments in a resin matrix, the thread typically 
extending between a stimulation electrode structure at the distal end of 
the lead and a terminal at the proximal end of the lead, the terminal 
being adapted for connection to a cardiac stimulation pulse generator 
circuit. A flexible insulating sheath extends over the thread from the 
electrode to the terminal. 
Carbon in a high modulus graphite filament form is availble from several 
commercial sources, including Union Carbide Corporation, which sells such 
materials under its trademark Thornel. The principal use for these fibers 
is as resin reinforcement replacing fiberglass for very high strength and 
high temperature application. Some forms of these fibers are fairly 
conductive, although this property has by and large been utilized only to 
facilitate electrostatic spray painting of parts in accordance with 
typical automotive practice.

Corresponding reference characters indicate corresponding parts throughout 
the several views of the drawings. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1, the cardiac pacing lead illustrated there 
comprises an elongate flexible body portion 11 having at its distal end a 
metallic electrode tip portion 13 adapted for contacting cardiac tissue 
and having at its proximal end a connector 15 adapted for connection to a 
pulse generating circuit. The connector 15 is essentially conventional, 
including a generally tubular metal terminal portion 17 (FIG. 2) and a 
shaped silicone rubber jacket 19 facilitating a sealed electrical 
connection with the stimulation pulse generating circutry. The terminal 17 
is centrally apertured as indicated so that a stylet can be introduced 
into the lead so as to stiffen it during installation. 
The lead body 11 includes a flexible tubular body compatible cover 21 such 
as silicone rubber which is fitted loosely over a helically wound 
conducting element 23 and bonded to the terminal jacket 19 and the tip 
jacket 35. As shown in greater detail in FIG. 3, the conducting element 23 
has a generally flat ribbon-like cross-sectional configuration and 
includes a conductive core portion 25 of parallel cabon filaments in a 
resin matrix together with a resinous sheath 27. A suitable form of 
conductive carbon filament material is available from Union Carbide 
Corporation, Carbon Products Division, under its product designation 
"THORNEL" 300, WYP 15 I/O. This material is available from the 
manufacturer with a 10% Teflon (tetrafluoroethylene) impregnation. 
The conductive element 23 is formed by flattening the impregnated thread 
material obtained from the manufacturer and then extruding, over the 
thread, the Teflon sheath 27 in a cross-head extruder of conventional 
construction. A second method is to extrude the Teflon sheath separately 
and pull the thread through later. The material is then wound on a mandrel 
to obtain the desired helical configuration and is then heated to set the 
material in that shape, the heating procedure being chosen in relation to 
the particular resin materials employed. The helically coiled conductive 
element can then be inserted through the silicone rubber tube 21. 
To establish the electrical connection at each end of the conductive 
element, the carbon filaments themselves are first bared and are then 
bonded with a conductive, e.g., silver-or platinum-loaded, epoxy adhesive 
to the metal electrode and terminal components, i.e. as indicated at 26 
and 28 respectively. The individual carbon filaments may be bared by 
heating since the carbon will withstand much higher temperatures than the 
resin matrix. The bared filaments are preferably then washed in a solvent 
to remove any residue left by the resins upon decomposition. 
A relatively hard resin, such as methylmethacrylate, is preferably molded 
around the last turn or so of the conductive element adjacent each of the 
connections, as indicated at 31 and 33, to act as a strain relief and as a 
moisture barrier for the epoxy. The jacket 19 is then molded around the 
terminal 17, extending to and bonded with the tube 21 which surrounds the 
conductive element 23. Similarly, a conically shaped jacket 35 of silicone 
rubber is molded at the electrode end of the lead body, joining the tube 
21 to the stimulation electrode tip 13. 
As compared with the winding of Elgiloy wire in conventional leads, the 
winding of the carbon filament conductive element is preferably done at a 
relatively wide draft angle so that a wide strand or multiple strands can 
be used, and so that the length of each thread is relatively short as 
compared with the total length of wire normally required to make up a 
conventional Elgiloy lead. For comparison, it may be noted that a standard 
Elgiloy lead 62 cm. long uses 555 cm. of Elgiloy wire, whereas a lead of 
the same length constructed in the manner illustrated employs a carbon 
filament strand of 130 cm. in length. The greater flexibility and fatigue 
resistance of the carbon filament material allows this broader winding 
angle and yet still yields much higher fatigue resistance. The broader 
winding angle in turn allows a broader or multiple parallel threads to be 
used, which are individually of shorter length, and further are 
electrically in parallel. Thus, though carbon filaments are relatively 
poor conductors as compared with metal and accordingly do not suggest 
themselves as wire substitutes, in fact a lower total resistance can be 
obtained from terminal to tip using the construction illustrated as 
compared with the comparable Elgiloy wire construction. For example, a 
conventional Elgiloy lead of 61 cm. length will exhibit a resistance of 89 
ohms, whereas a lead of the same length constructed as described above 
with one carbon thread will exhibit a resistance of about 128 ohms and 
with two carbon threads about 43 ohms, owing to the broader winding angle. 
Where elastic elongation of the overall lead is not deemed necessary, it 
is possible, with the carbon material, to have the conductive element 
extend fully parallel to the lead axis producing an even lower resistance. 
While a conventional electrode may be used for contacting the cardiac 
tissue, as described above, a presently preferred form of terminating an 
endocardial lead according to the present invention is to carry the carbon 
filaments themselves through the electrode structure and to have them 
project from the lead tip in a brush-like electrode structure. An 
alternative embodiment employing this construction is illustrated in FIGS. 
4-7. 
A plurality of carbon filament strands 42 are bonded to a pulse generator 
connector (not shown), e.g. by means of conductive adhesive, and are then 
wound helically around a core tube 41. In a preferred construction of this 
form of lead, four such strands or threads were used, each thread being 
itself of about 0.4 mm diameter and comprising approximately 3000 
individual carbon filaments, each filament being on the order of 7 microns 
diameter. The filaments comprising each strand are thoroughly impregnated 
with a resin matrix which prevents the individual filaments from breaking 
and destructing each other as the lead flexes and which bonds the 
filaments into a cohesive mass. A suitable resin matrix is Teflon such as 
the type emulsion 30-B manufactured by the DuPont de Nemours, E.I. & Co. 
of Wilmington, Del. A particularly thorough impregnation of the filaments 
may be obtained by submersing the strand in a 10% Teflon emulsion 
dispersed in 1% polyurethane binder followed by heat treatment. Other 
means of working the resin into the fibers may also be satisfactorily 
employed. 
The tip portion of this lead includes a highly radiopaque tungsten stylet 
stop and holder 51. Stop 51 engages the end of the style when it is 
inserted for stiffening the lead during installation and prevents the 
stylet from piercing the end of the lead as well as acting as a radiopaque 
marker for x-ray visualization during placement. The core tube 41 is 
bonded to the stylet stop 51 as indicated and the carbon filament strands 
pass around the stop 51, the stop being provided with four peripheral 
channels 53 as may be seen in FIG. 8. 
In constructing the lead, the conductors 42, core tube 41, and stylet stop 
51 are preferably assembled on a suitable mandrel. The carbon filament 
strands 42 are cemented to the pacer connector and then wound over the 
core tube 41 and past the stylet stop 51. A tubular jacket or sleeve 57 is 
then slipped over the body 11 of the lead. Over the entire assembly is 
then cast a jacket such as silicone rubber forming a shell 55 and lateral 
projections 59 around the tip and the jacket around the connector. After 
the Silastic jackets are formed, the portion of the carbon filament 
strands projecting beyond the stop 51 are cut off to an appropriate 
length. The filaments themselves having previously been bared, e.g. by 
burning away of the resin matrix as described earlier, so that the 
filaments themselves serve to establish electrical contact with the 
cardiac tissue. As the individual filaments are body compatible and quite 
compliant, the contact portion of the tip has very little force acting on 
it to cause fibrotic build-up. 
The portion of the jacket adjacent the electrode tip is formed with two 
sets of lateral projections 59 which are useful in retaining the tip 
within the cardiac ventricle. The first set of projections next to the end 
stabilizes the tip with the heart movement. The second set of projections 
take the brunt of the forces set up by the lead movement with respect to 
the heart movement. The space between the projections is very flexible to 
prevent the transmission of the forces. The projections become enmeshed in 
the trabeculae at the bottom of the ventricle after the tip has been in 
place a few days. The projections 35 are generally flat and blade-like, 
but are somewhat curved in transverse section as illustrated. This 
curvature somewhat stiffens these projections, i.e., in the manner of a 
tape measure, making flexing easier when the lead is being pushed in than 
when it is being pulled out. The portion of the jacket over the body is 
preferably somewhat thinner adjacent the electrode tip portion, i.e., as 
indicated at 61, to provide somewhat greater compliance at this point 
where there is continuous flexing due to the regular beating of the heart. 
As indicated previously, a lead constructed as described above is at least 
electrically equal to a conventional lead with a coiled metal wire, while 
being essentially longer-lived in terms of fatigue resistance. A further 
advantage of this construction struction is that it is relatively 
resistant to physical abuse. The entire lead is relatively flexible and 
the conductive elements are not liable to be damaged if the lead is 
crushed, e.g. by a hemostat. Likewise, the outer sheath is less likely to 
be cut since the inner core tube and strand winding is relatively 
compliant as compared with the lateral compliance of a helical metal coil. 
Further, although the body compatible Silastic materials may allow body 
salts to penetrate the lead, it is of essentially no effect since the 
carbon filaments are not degraded or corroded by salts as are less noble 
metal conductors. 
The embodiment illustrated in FIG. 9 is arranged for epicardial use, and 
utilizes an electrode structure 65 which is adapted to be sutured to the 
outside of the patient's heart. Electrode structure 65 includes a flat 
plastic tab portion 67 which is adapted to be sutured to the patient's 
heart. A helical Elgiloy or platinum electrode 69 projects from one face 
of the tab-like portion 67 so as to imbed itself in the cardiac tissue. 
The body of the lead 71 is similar to the lead body illustrated in FIGS. 
1-3, but the carbon filament strand 73 is connected to the epicardial form 
of electrode structure illustrated, rather than to the endocardial form of 
electrode tip. 
While each of the embodiments illustrated uses but a single electrode and a 
single longitudinal conducting path, it should be understood that multiple 
conducting paths might also be provided by separating plural conductive 
threads with insulating material and that such multiple conductors could 
be used with plural stimulating electrodes, e.g. bipolar electrodes, in 
various arrangements known in the art. 
In view of the foregoing, it may be seen that several objects of the 
present invention are achieved and other advantageous results have been 
attained. 
As various changes could be made in the above constructions without 
departing from the scope of the invention, it should be understood that 
all matter contained in the above description or shown in the accompanying 
drawings shall be interpreted as illustrative and not in a limiting sense.