Pacing lead

Abstract: There is provided an endocardial pacing lead having a substantially cylindrical length and an electrode (33) at about the distal end thereof, the lead having a conductor (31) for conducting signals between the proximal and distal ends thereof and an outer tubing (30) enclosing the conduc­ tor (31) for substantially the length of the lead. A tined piece (34) is provided which envelopes the lead tubing just proximal to the distal electrode, the tined piece (34) having a plurality of tines (35), each such tine being formed with a substantially C-shaped cross-section having its inner or backward surface substantially complementary to the outer surface of the tubing. The tined piece (34) also preferably provides riser elements interspaced between respective pairs of the tines for providing a raised lead surface adjacent the tine interangles, thereby increasing the effective tined inter-radius presented to heart wall structures.

Description: This invention relates to pacing leads and more particularly to pacing leads with improved features for positioning and anchoring the lead in the patient's heart. Endocardial pacing leads having fixation structures at or near the distal tip end are well known in the art. A widely used anchoring arrangement is that of the tined lead, where a plurality of pliant tines are disposed near the distal tip to effect good chronic anchoring after the lead tip has been optimally positioned in the heart. While the design has had some problems, the tined anchor has become the most popular passive fixation mechanism for endocardial pacemaker leads. The patent literature contains many disclosures of tined pacing leads. See, for example, U.S. Patents Nos. 3,902,501; 3,939,843; 4,033,357; 4,236,529; 4,301,815; 4,409,994; and 4,582,069. Additionally, the non-patent pacing literature has disclosed and discussed the advantages of the tine-type anchor for chronic fixation, and the acceptance of this type of fixation mechanism in the pacing industry. The fixation mechanism provided by tines is well understood. When heart wall growth, or trabeculae, become lodged within the acute angle formed between the tines and the tube casing, this provides a substantial anchoring effect. The tines must be designed so that they can fold down over the tubing, to reduce the introducer width at time of implanting the lead, and must be able to bend forward (distally) toward the distal tip if it is necessary to dislodge the lead during the acute stage, particularly during initial placement of the lead. This means that the tine characteristics must be chosen carefully to permit easy folding of the tine down onto the casing, and also to provide the right amount of resistance to forward bending of the tine as may be necessary during the acute stage of lead placement. It has been found that one problem that occurs during placement of the tined lead is that the tip becomes entangled in chordae connected to the heart valve, necessitating relative withdrawal of the lead and thus forward bending of the tines. Also, an obvious disadvantage of the tined structure is that when folded back upon the lead during introduction, the tines add to the thickness of the lead, i.e. increasing the introducer size. It is known from US-A- 4,301,815 to provide a mechanism for elevating thinner heart wall elements such as chordae from the lead surface. In this patent, a conical surface provides a ramp which rises radially in the distal direction; the ramp rises from the tube casing to the conical structure that carries tines. While this can be some help in facilitating withdrawal of the tines, it only prevents the chordae from catching behind the transitional cone, and does not provide a solution against the chordae catching behind the tines themselves. Early tine lead designs had relatively large introducer sizes. With the venous introducer technique becoming more popular, tined lead designs were improved so that the lead would fit into smaller introducers. The introducer size of the lead is the minimum tube diameter through which a lead can be pushed with a minimum amount of friction. In practice, it has been determined primarily by: (a) the diameters of the rigid parts in the lead tip and body; (b) the elasticity of the deformable parts of the lead tip and body, since these determine friction forces; and (c) the coefficient of friction between the lead and the introducer, which can be affected by the choice of low friction materials. A classical tined anchor design is a trade-off between rigidity for good anchoring, or holding force, and flexibility for acute removability, e.g., from the chordae of papillary muscles or other unintended positions. The tine bending rigidity is substantially the same in each direction. This rigidity is an advantage when bending the tine forward, since it provides a good holding function, but it is a disadvantage when bending the tine backwards during introduction of the lead. Figures 1A and 1B of this application present a representation of a prior art tined silicone lead design which is representative of the design compromises of the prior art. As can be seen in Figure 1A, and as is conventional with tined lead construction, the normal position of the tine is to slope backward proximally from the distal tip, at an acute angle with respect to the lead casing. However, as seen in Figure 1B, during introduction the tines must be folded backward (proximally) upon the casing, causing a substantial increase in the overall width at that point, limiting the introducer size. Another typical problem with the prior art tined lead design is that the lead casing, either silicone tubing or PUR tubing, is not extended all the way forward to the tip. In the design of Figures 1A and 1B, the distal end of the coil is attached to the tip element by crimping, the crimping sleeve necessarily requiring a greater radius than that of the coil. The tined anchor piece is placed over the crimping sleeve and the silicone tubing, causing a bulge at the point of overlap with the silicone tubing. Note that this overlap is necessary in order to provide good sealing, and to prevent flow of body fluids into the coil area. Thus, the conventional crimp arrangement provides an additional limitation to reduction of the introducer size. US-A- 4,301,815 provides several illustrations of a pacing lead wherein the electrode structure has a proximally extending inner portion which makes contact with the conductor. While the tube casing comes up to the proximal end of that inner structure, it does not extend to the electrode surface at the distal tip; only the casing portion the carries the tines extends to the electrode surface. Accordingly, the tine member must be relied on by itself at the distal end to provide sealing and to prevent flow of body fluids into the inner electrode and coil area. In practice, the tined lead as exemplified by Figure 1A and 1B is introduced with the use of a stylet, the distal end of which presses up against the inner core piece of the tip. A problem that exists in practice is that the introduction of the stylet and the maneuvering of the lead and stylet into the heart can result in the distal end of the stylet poking through the coil near the distal tip end, which can be ruinous to the lead. It is thus advantageous to include, in any lead design, means for preventing the piercing of the distal lead end by the stylet while it is being used. The crimping sleeve solves this problem, but at the expense of introducer size. Another design feature of importance is the inner radius R (as seen in Figure 1A) between the tine and the longitudinal surface of the lead. In practice, this radius is a compromise. A small radius gives the best flexibility to the tine, i.e., reduces its stiffness to bending in either direction. If the design anticipates the placement of the lead will involve undesired catching of heart wall structures, e.g. chordae, a larger radius is desired to better permit the heart structure to bend the tine forward, for extraction of the lead. However, a larger inner radius R will negatively affect the bending capability of the tine, and will increase the introducer size. The tined lead of Figs. 1A and 1B, made commercially by the assignee of this application, fits into a 13 F introducer with its tines folded backward. Further tine flattening permits reduction to an 11 F size. Since the body size is 5.5 F, the introducer size is seen to be twice as great. By contrast, the objective of the lead of this design is an 8F introducer size for a silicone lead, and a 6F size for a lead made with PUR 55D. There thus have existed a number of problems associated with lead tip design, which the design of the instant invention reduces. The problem of introducer thickness is achieved by an improvement in the cross-sectional design of the tine and by extending the lead tubing all the way to the forward portion of the tip element. Instead of crimping the end electrode piece to the coil, the inside of the electrode is spot welded to the distal end of the coil, enabling extending the lead tubing all the way to the tip end. The problem of optimally selecting the inner radius R is alleviated by the introduction of riser elements between respective pairs of tines. Further, the problem of ensuring against piercing the lead during use of the stylet is reduced by providing a sheathing around the coil for a distance proximal to where the coil is attached to the tip element. The objects of this invention are to overcome the above-recited prior art problems, and to provide design solutions with advantages not heretofore available. According to the invention these objects are solved by a pacing lead as described in claim 1. Preferred embodiments of the pacing lead are described in claims 2 to 11. Figure 1A is a cross-sectional representation of a prior art tined lead design, with the tines in a normal position forming an acute angle with the lead length; Figure 1B is a cross-sectional schematic representation of the same lead as illustrated in Fig. 1A, showing the tine folded backward for introduction; Figure 2A is a perspective view of the distal portion of the lead of this invention, illustrating the tines and the interspaced risers; Figure 2B is a cross-sectional representative view of the distal portion of the lead of this invention, with the tines in a normal working position; Figure 2C is a front end view of the tip of the lead of this invention, illustrating the relationship of the tines and the riser elements; Figure 2D is a cross sectional view of a tine of this invention; Figure 2E is a cross-sectional representative view of the distal portion of the same lead as Fig. 2B, but with the tines folded backward into the introducer position; Figure 2F is a front view of the lead of Fig. 2E; and Figure 3A is a diagrammatic sketch showing the relation of the riser elements to the tines; Figure 3B is a perspective sketch showing the elevating function of the riser elements; and Figure 3C is a diagrammatic sketch showing the action of the riser elements as seen from a position proximal to the tines. Referring now to Fig. 2A, there is shown a perspective view of the distal end portion of the lead of this invention. The tip electrode surface is shown at 33. It is noted that the electrode surface as such is not a feature of the invention, the design permitting any electrode surface which is consistent with the reduced diameter of the lead. Of course, a second electrode, for bipolar systems, may be used within the scope of the invention. A tined piece 34 is shown, made of silicone rubber or preferably from a rigid material having good frictional properties. Examples of acceptable materials include polyurethanes such as those sold under trademark designations PUR 55D, PUR 75D, where the numbers indicate Shore D hardness. PE and PTFE may also be used. The piece 34 has a cylindrical tube portion which has at its proximal end a plurality of tines 35 spaced circumferentially from each other, and interspaced risers 36. In accordance with this invention each tine has a C-shaped cross-section, as seen in Fig. 2D, which provides good stiffness against bending forward, i.e. in the distal direction, but which buckles easily in the backwards or proximal direction. The C-shaped cross-section provides the requisite forward stiffness with a thinner tine wall, such that the thickness of the tines can be reduced over the prior art. The C-shaped cross-section is complementary to and closely follows the outer cylindrical periphery of the casing 30, such that it adds very little to the introducer size of the lead. The thickness of the tube portion of the tined piece 34, distal to the tines, is likewise reduced in this design. For example, the actual tine thickness may be only 0.15 mm, as indicated in Figure 2D. Referring to Figs. 2B and 2C, there are shown additional Views of the distal end portion of the lead of this invention. For a lead design using PUR 55D tubing, the coil diameter is suitably 0.8 mm, and the outer diameter of the tubing, or casing 30, is 1.22 mm. Note that the casing 30 extends through to the tip end of the exposed surface portion of electrode 33, to provide continual sealing of the lead, regardless of whether the tines have degraded. The thin-walled tined piece 34 is glued around the end of the tubing, providing a smooth contiguous surface from the exposed electrode tip backwards. The tines are suitably displaced at about a 45° angle. The risers 36 are illustrated in Fig. 2C as being spaced between adjacent pairs of tines 34. The risers provide the function of elevating thinner heart wall elements such as chordae from the lead surface without effectively increasing the introducer size, as is described more fully below in connection with Figs. 3A, 3B and 3C. Referring now to Figs. 2E and 2F, the lead of this invention is illustrated in its introducer position, i.e. with the tines flattened down against the casing 30. For the tine as shown, the overall introducer size in the folded back position is about 2.0 mm, a considerable advantage over existing lead designs. It is noted that the design does not include a crimping bush. Instead, the coils are laser spot welded to the electrode core 38. The absence of the conventional crimping bush introduces the risk of stylet perforation just behind, or proximal to the electrode core element. In one embodiment of this invention, the adjoining coil windings at the free distal end of the coil are laser spot-welded to one another, to provide a sealed cylinder through which the stylet cannot penetrate. In another embodiment, a very thin-walled casing of PTFE shrink tubing, illustrated at 40, is placed around the free end of the coil, both stiffening and encapsulating the coil. When the shrink tubing is shrunk onto the coil, it penetrates into the areas between the coil loops, providing the desired continuous encasing. The lead of this invention may, of course, be made with other materials. A silicone lead in accordance with the design of this invention has an outer casing diameter of 1.73 mm, and a distal tip diameter at the electrode and the tined piece of 2.2 mm. The overall introducer size, with the tines folded back, is about 2.6 mm. The invention permits a lead of about 8F using silicone tubing and about 6F using 55 Shore D polyurethane (PUR 55D) tubing. Referring now to Figs. 3A, 3B and 3C, there is illustrated the manner by which the risers aid in the object of preventing small structures such as chordae from getting caught too close to the base of the tines. The holding force of the tine against backward withdrawal of the lead is greatest with respect to small diameter structures, and lessens as the diameter increases. Figure 3A shows the moment arms, indicated as arrows, for different heart wall structures. A relatively larger heart wall structure, such as the trabecula illustrated at 42, does not seat so far down into the intersection of the tine and the casing, and presents a reasonable good lever arm to bend the tine backward. On the other hand, smaller structures, such as chordae, one of which is illustrated at 43, can be positioned down close to the base of the tine, and present virtually no lever arm for pulling the tine backward. Thus, in practice with conventional tined leads, the lead can be withdrawn for replacement relatively easily if the trabeculae are caught in the tine intersection, but with much greater difficulty if the lead is caught by chordae. Figures 3B and 3C further illustrate the positioning of the chordae with respect to the tube casing for the lead of this invention, showing that the chordae lie somewhat tangentially over a pair of risers, which risers are elevated away from the tube casing so that the chordae 44 has a greater mechanical advantage. The risers are flexible elements, preferably made integrally with the tines, and formed to normally project radially from the cylindrical surface of the lead. The risers are formed so that their point of peak elevation is proximal, i.e., back from the tine base and lateral to each tine angle. By so placing the risers, the chordae are held at an elevated position relative to the tine base, which is the equivalent of giving the tines an effective greater inner radius. Of course, the tines can be made with a conveniently smaller inner radius, so that the detrimental effect of a small inner radius is reduced without comprising the flexibility of the tines. As seen in the view of Fig. 3C, looking toward the distal tip from a point proximal to the tines, the risers provide bulges that project radially outward and lift chordae away from the casing 30. The risers must have sufficient height to keep the chordae elevated with respect to the backside of the tine, but clearly cannot rise as high as the tines. Further, they cannot be too large or they will increase the introducer size. By making the risers about the same thickness as the tines, they don't add to the introducer size, as seen in Fig. 2F.

Claims: A pacing lead having proximal and distal ends, a substantially cylindrical length between said ends, and an electrode (33) having an exposed surface portion at about the distal end thereof, said length having a conductor (31) for conducting signals between said proximal end and said electrode and an outer tubing (30) enclosing said conductor, and a tined piece (34) enveloping said tubing just proximal to said electrode (33), said piece having a plurality of tines (35) extending radially and proximally from said tubing, chracterized in that each such tine is formed with a C-shaped cross-section having a concave inner surface substantially complementary to the adjacent surface of said tubing, said tubing (30) extending distally to said electrode surface portion. The pacing lead of Claim 1, wherein said piece (34) comprises a casing portion which envelopes said tubing for a predetermined distance proximal from said electrode surface portion, and said tines extend from said casing portion, said tines normally extending at an acute angle from the axis of said tubing. The pacing lead of Claim 2, wherein said tined piece has a wall thickness of less than about 0.25 mm. The pacing lead of Claim 3, wherein said tined piece (34) is made from a relatively rigid polyurethane material. The pacing lead of Claim 2, wherein said tined piece comprises shallow riser members (36) extending longitudinally from said casing portion and positioned between respective pairs of tines. The pacing lead of Claims 5, wherein said riser members extend radially outward a distance no greater than that of said tines when they are folded against the tubing surface, whereby they do not increase the introducer size of said lead. The pacing lead of Claim 2, wherein said conductor is a coil (31), and said electrode is spot welded to the inside of the distal end of said coil. The pacing lead of Claim 1, wherein said tubing is made of a silicone rubber. The pacing lead of Claim 1, wherein said tubing is made of a polyurethane. The pacing lead of Claim 1, further having riser members located between pairs of said tines. The pacing lead of Claim 10, wherein each of said riser members is flexible and normally presents an effective raised lead surface just back from the tine positions between which it is located.

Inventors: VAN KRIEKEN FRITS MAARTEN, VAN KRIEKEN, FRITS MAARTEN
Applicants: VITATRON MEDICAL BV, VITATRON MEDICAL B.V.
Application year: 1990
Main classification: A61N1
IPC codes: A61N1