Rotatable pin, screw-in pacing and sensing lead having Teflon-coated conductor coil

A rotatable pin, screw-in pacemaker lead assembly has a conductor coil enclosed within a silicone insulating tube. The conductor coil has a distal end secured to a helix electrode and a proximal end including a rotatable connector pin. The outer surface area of the conductor coil is coated with a super thin film of biocompatible Teflon. The Teflon coating substantially reduces the friction between the conductor coil and the inner wall of the insulating tube during lead fixation, thereby substantially reducing the number of turns of the connector pin required to effect fixation of the helix electrode.

FIELD OF THE INVENTION 
The present invention relates generally to implantable medical devices for 
providing stimulating pulses to selected body tissue, and more 
particularly, to the lead assemblies connecting such devices with the 
tissue to be stimulated. 
BACKGROUND OF THE INVENTION 
Although it will become evident to those skilled in the art that the 
present invention is applicable to a variety of implantable medical 
devices utilizing pulse generators to stimulate selected body tissue, the 
invention and its background will be described principally in the context 
of a specific example of such devices, namely, cardiac pacemakers for 
providing precisely controlled stimulation pulses to the heart. However, 
the appended claims are not intended to be limited to any specific example 
or embodiment described herein. 
Pacemaker leads form the electrical connection between the cardiac 
pacemaker pulse generator and the heart tissue which is to be stimulated. 
As is well known, the leads connecting such pacemakers with the heart may 
be used for pacing, or for sensing electrical signals produced by the 
heart, or for both pacing and sensing in which case a single lead serves 
as a bidirectional pulse transmission link between the pacemaker and the 
heart. An endocardial type lead, that is, a lead which is inserted into a 
vein and guided therethrough into a cavity of the heart, includes at its 
distal end an electrode designed to contact the endocardium, the tissue 
lining the inside of the heart. The lead further includes a proximal end 
having a connector pin adapted to be received by a mating socket in the 
pacemaker. A flexible, coiled conductor surrounded by an insulating tube 
or sheath couples the connector pin at the proximal end and the electrode 
at the distal end. 
To prevent displacement or dislodgement of the electrode and to maintain 
the necessary stable electrical contact between the lead tip and the 
endocardial tissue, the electrode must be firmly anchored relative to the 
tissue. To achieve this, the electrode of one known type of lead comprises 
a pointed helix adapted to be screwed into the heart tissue to be 
stimulated. Rotational torque applied to the connector pin at the proximal 
end of the lead is transmitted via the flexible, coiled conductor to the 
helical electrode which is thereby screwed into the heart tissue. In this 
fashion, the position of the electrode tip is mechanically stabilized, 
that is, the tip is positively anchored so as to remain securely in place 
during the lifetime of the implant. Removal of the screw-in electrode from 
the endocardium can be effected by counter rotation of the connector pin. 
Thus, in a rotatable pin, screw-in lead the conductor coil is used not 
only as an electrical conductor coupling for the connector pin and the 
helix electrode, but also as a tool for extending or retracting the helix 
electrode relative to the distal tip of the lead during lead myocardium 
fixation by rotating the connector pin. 
It is desirable to minimize the number of revolutions of the lead connector 
pin required to fully extend or retract the helix electrode during lead 
fixation. The number of connector pin turns is a function of several 
factors: 
(a) conductor coil stiffness, with a stiffer coil requiring fewer connector 
pin turns (a very stiff conductor coil, however, results in a very stiff 
lead that potentially creates problems such as high chronic pacing 
threshold or tip myocardium perforation); 
(b) friction between the helix electrode and seal, where such a seal is 
utilized to prevent ingress of bodily fluids; 
(c) the length of the lead body (a longer lead requires more turns of the 
connector pin to advance or retract the helix electrode a given distance); 
and 
(d) friction between the conductor coil and the surrounding insulating 
sheath or tube. 
With respect to factor (d), to minimize frictional resistance between the 
"torque transfer" conductor coil and the surrounding insulation tubing, 
polyurethane tubing has been preferred over silicone tubing because the 
coefficient of friction between conductor coils (such as the multifilar 
MP35N conductor coil utilized in various screw-in pacemaker leads 
manufactured by Siemens Pacesetter, Sylmar, Calif., U.S.A.) and 
polyurethane tubing is less than that between such coils and silicone 
tubing. However, because the use of polyurethane tubing has several 
disadvantages, such as stiffness and limited long term biostability, it 
would be desirable to use silicone tubing instead. 
Accordingly, it is an overall object of the present invention to provide a 
screw-in lead assembly using silicone insulation tubing but in which the 
friction between the conductor coil and tubing, and hence the torque and 
number of connector pin turns required to extend or retract the helix 
electrode, is substantially reduced. 
SUMMARY OF THE INVENTION 
In accordance with the broader aspects of the present invention, the 
foregoing object is attained in a screw-in lead assembly employing 
silicone insulation tubing by coating the outer surface area of the 
conductor coil with a super thin biocompatible Teflon 
(polytetrafluoroethylene). It has been found that such a coating 
significantly reduces the torque required to extend or retract the helix 
electrode thereby making the use of silicone insulation tubing in screw-in 
leads substantially more advantageous than polyurethane tubing. Indeed, 
comparison tests between uncoated and Teflon-coated conductor coils within 
silicone tubing have demonstrated that the coated conductor coil decreases 
the torque required to rotate the coil by approximately 67%.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following description presents several preferred embodiments 
representing the best mode contemplated for practicing the invention. This 
description is not to be taken in a limiting sense but is made merely for 
the purpose of describing the general principles of the invention whose 
scope is defined by the appended claims. 
Referring to FIG. 1, there is shown a unipolar screw-in pacing lead 10 
having a proximal end 12 and a distal end 14. The proximal end 12 is 
adapted to be plugged into a receptacle in a cardiac pacemaker (not shown) 
and includes a tubular housing 16 made of an insulating biocompatible 
material such as silicone. The tubular housing 16 includes annular ribs 17 
for sealing the pacemaker receptacle against the entry of bodily fluids. 
The tubular housing 16 encloses a generally cylindrical, rotatable 
connector pin 18 having a portion 20 projecting from the proximal end of 
the housing 16. The pin portion 20 is adapted to be received by a 
pacemaker socket coupled to the pulse generating and pulse sensing 
circuits within the pacemaker. 
The distal end 14 of the lead 10 includes a metallic, tubular housing 22 
including a distal tip 23 having an opening 24. Except for the ring-shaped 
tip 23 the tubular housing 22 is enclosed within an insulating sheath 26. 
The distal end 14 of the lead 10 further includes a rotatable, 
extendable/retractable helix electrode 28 shown in FIG. 1 fully retracted 
within the tubular housing 22. As is well known, the helix electrode 28 
serves both as a fixation means to securely anchor the distal end of the 
lead relative to the tissue to be stimulated and as an electrically 
conductive contact element for transmitting electrical stimulation and 
sensed pulses. The helix electrode 28, which may be made of a 
platinum-iridium alloy, for example, has a sharp end 30 adapted to pierce 
the endocardial tissue. 
The helix electrode 28 is carried by a shaft 32 welded or otherwise secured 
to the proximal end of the electrode 28. The shaft 32, in turn, is carried 
by a fluid-tight seal 34 mounted within the tubular housing 22, the shaft 
being rotatably and axially movable relative to the seal 34. Projecting 
inwardly from the inner wall of the tubular member 22 proximate the tip 23 
is a post 36 interposed between adjacent turns of the helix electrode 28. 
In this fashion, rotation in one direction or the other of the helix 
electrode 28 within the tubular housing 22 will cause the helix electrode 
to be extended or retracted relative to the tip 23. 
The shaft 32 and connector pin 18 are electrically and mechanically coupled 
by means of a conductor coil 38 having an outer surface area 40 along the 
length of the conductor coil. The conductor coil 38 is housed within an 
insulating tube 42 interconnecting the tubular housings 16 and 22 and 
having an inner wall 44. In accordance with an aspect of the invention, 
the tube 42 is made of silicone. 
It will thus be seen that given the helical sense of the electrode 28 
illustrated in FIG. 1, rotation of the connector pin 18 in a clockwise 
direction (as viewed from the proximal end of the lead) will cause 
advancement of the helix electrode and its extension from the opening 24 
in the tip 23 of the tubular housing 22 to a fully extended position, 
while rotation of the connector pin in a counterclockwise direction will 
result in retraction of the electrode 28 to its fully retracted position 
shown in FIG. 1. As already explained, the number of turns of the 
connector pin required to fully extend or fully retract the helix 
electrode is a function of several factors among which is the friction 
between the outer surface area 40 of the conductor coil 38 and the inner 
wall 44 of the insulation tube 42. It has been found that such friction, 
and therefore the number of turns required for full extension or 
retraction of the helix electrode, can be substantially reduced by coating 
the outer surface area 40 of the conductor coil with a super thin film of 
biocompatible Teflon. Accordingly, once the situs of lead fixation has 
been determined, fixation is effected both expeditiously and without 
displacement of the tip relative to the fixation location which might 
otherwise occur if friction levels were higher. 
The retracting spring force imposed by the conductor coil 38 on the helix 
electrode maintains the helix electrode 28 in electrical contact with the 
metallic tubular housing 22 via the conductive post 36. In this fashion, 
the ring tip 23 permits threshold mapping of the heart tissue prior to 
lead fixation. 
FIG. 2 shows a fixture for performing comparative torque tests on rotatable 
pin, screw-in type lead assemblies. The fixture includes a base 50 having 
mounted adjacent one end thereof a low friction bearing block 52. A shaft 
54, rotatably carried by the bearing block 52, has an outer end portion 56 
and an inner end portion 58. Wrapped around the outer end portion 56 of 
the shaft is a thread 60 having a vertical run coupled to a device 62 for 
pulling the thread upwardly at a predetermined velocity and for measuring 
the force required to rotate the shaft 54. 
The inner end portion 58 of the shaft 54 carries a clamp 64 to which is 
secured one end of a conductor coil 66 of a selected length of test lead 
68. The test lead 68, which includes outer insulative tubing 70, is held 
in place by anchoring blocks 72 and 74 mounted on the base 50. The free 
end of the lead 68 projects from the block 74. The portion of the lead 68 
between the blocks 72 and 74 is wound around a pair of space apart 
cylinders 76 mounted on the base 50 so as to introduce resistance to the 
rotation of the conductor coil 66 within the insulative tubing 70. By way 
of example, each cylinder may have a diameter of 2.0 inches and the 
spacing between the axes of the cylinders is 3 inches. It will thus be 
seen that for a given diameter of the shaft 54, the force required to pull 
the thread is a measure of the torque required to rotate the conductor 
coil 66 within the insulative tubing 70. (It will be understood that the 
friction force introduced by the bearing block 52 can be separately 
measured and subtracted out.) 
FIG. 3 is a graph of load (lbs) v. displacement (inches) showing typical 
results of torque tests using the test fixture of FIG. 2 performed on (1) 
a test lead comprising silicone insulative tubing with a Teflon-coated 
conductor coil and (2) a test lead (of the same length as the first test 
lead) comprising silicone insulative tubing with an uncoated conductor 
coil. Each test lead had a length of 21 inches and the thread was advanced 
at a rate of 40 inches per minute. The diameter of shaft 54 was 0.125 
inch. In the examples reported in FIG. 3, two test runs were performed on 
the lead incorporating the noncoated coil while three test runs were 
performed on the test lead having the Teflon-coated coil. In both cases, 
the conductor coil 66 was a standard MP35N multifilar coil. 
It will be seen that the results of the comparative tests illustrated in 
FIG. 3 show a dramatic reduction in the force required to rotate the 
conductor coil 66 when coated with Teflon. The average peak force required 
to rotate the noncoated coil is approximately 0.023 lb. while the average 
peak force required to rotate the coated coil is 0.0075 lb., a reduction 
of about 67%. 
Although the present invention has been described in terms of unipolar 
pacing lead assemblies it will be understood that the invention is 
applicable as well to bipolar pacing leads having two separate conductors, 
and to multipolar pacing leads employing multiple conductor leads.