Abstract:
The present invention provides a header for an implantable medical device that allows it to be used with either a unipolar lead or a bipolar lead. The set screw normally engaging a ring contact on a bipolar lead is stopped from extending far enough into an insulating layer on a unipolar lead to cause damage to the insulating layer. The use of the universal header of the present invention works to significantly reduce the number of different models of implantable medical devices that a manufacturer must have available.

Description:
BACKGROUND OF THE INVENTION 
   I. Field of the Invention 
   This invention relates generally to the mechanical design of a header for an implantable medical device, such as a cardiac pacemaker or cardiac pacemaker/defibrillator, and more particularly to the design of a header having a lead receiving bore with contacts for mating with either a unipolar lead or a bipolar lead. 
   II. Discussion of the Prior Art 
   An implantable medical device typically comprises a metal housing hermetically sealing a battery, a pulse generator circuit and a microprocessor-based controller for the pulse generator such that cardiac stimulating pulses are generated on a timed basis determined by the microprocessor-based controller. Affixed to an exterior wall of the housing is a header for operatively coupling the contacts on the proximal terminals of one or more medical leads to feed-through pins that enter the housing through seals where they connect to circuit inputs and outputs. 
   The medical leads involved may be unipolar or bipolar. A unipolar lead comprises an elongated flexible, plastic lead body having a conductor running the length thereof for connecting an electrode at a distal end thereof to a contact on a terminal on the proximal end thereof. A bipolar lead will have a distal tip electrode and a ring electrode on the surface of the lead body located proximally of, but close to, the tip electrode. Conductors again run the length of the lead, connecting the tip and ring electrodes individually to a conductive pin and a ring contact on the bipolar lead&#39;s proximal terminal. 
   In the past, manufacturers of implantable medical devices generally offer a plurality of models, each with differing functionality and operating modes. Each such model might be compatible with either a unipolar lead or a bipolar lead, but not both. Thus, if a patient happen to have a unipolar lead implanted, it would be necessary to select a stimulating device having a header that would accept a unipolar lead. If, on the other hand, the patient had a bipolar lead implanted, a model having a header with contacts for mating with the proximal terminal of a bipolar lead would have to be selected. 
   It should be obvious, then, that if the device manufacturer offered, say, six models of a cardiac stimulating device exhibiting different functionality, it would be necessary to stock  12  different models to assure that each of the six could operate with either unipolar or bipolar leads. This necessarily increases the cost of inventory, and when it is considered that such devices have a limited shelf life, it will frequently happen that units will have to be scraped because of a lack of demand for particular models. 
   It will be seen, then, that a considerable savings can be realized if a header were designed that could accept either a unipolar lead or a bipolar lead. Unique headers would not have to be made to accommodate the two types of proximal lead terminals in that one such header would work with each type of lead. 
   It is accordingly a principal object of the present invention to provide a header for an implantable medical device that can accept the proximal lead terminal of either a unipolar or a bipolar lead in a single proximal lead terminal receiving bore. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, there is provided a header for an implantable medical device where the header has contacts in a lead terminal receiving bore, the contacts being adapted to mate with a proximal terminal of either a unipolar or a bipolar lead. The proximal terminal of the unipolar lead has a proximal pin contact adjacent to a plastic insulating sleeve. The proximal terminal of the bipolar lead has a proximal pin contact and a distal ring contact. The header comprises a plastic body member that is adapted to be affixed to a housing of an implantable medical device. A longitudinal, lead terminal receiving bore is formed in the plastic body member as are first and second longitudinally spaced apertures that extend transversely to and intersect with the bore formed in the body member. First and second conductive connector blocks are respectively placed in the first and second apertures. Each of the connector blocks has a bore concentric with the bore formed in the plastic body member and a threaded bore that extends transverse to and intersects with the bore of the connector block. The bore of the second connector block has a diameter large enough to receive the pin contact of either a unipolar lead or a bipolar lead and the bore of the first connector block has a diameter large enough to receive the distal ring contact of a bipolar lead or the insulating sleeve of a unipolar lead therein. A first set screw is inserted into the threaded bore in the second connector block and is adapted to lock the pin contact of either a bipolar lead or a unipolar lead in the second connector block. Similarly, a second set screw is inserted into the threaded bore in the first connector block and is adapted to engage either the ring contact of a bipolar lead or the insulating sleeve of a unipolar lead. The threads in the threaded bore of the first connector block are such as to preclude the second set screw from damaging the insulating sleeve of the unipolar lead when the second set screw is fully advanced into the threaded bore in the first connector block. 

   
     DESCRIPTION OF THE DRAWINGS 
     The foregoing features, objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of a preferred embodiment, especially when considered in conjunction with the accompanying drawings in which like numerals in the several views refer to corresponding parts. 
       FIG. 1  is a greatly enlarged, side elevation of the proximal terminal portion of a unipolar lead; 
       FIG. 2  is a greatly enlarged side elevation of the proximal portion of a bipolar lead; 
       FIG. 3  is a horizontal cross-sectional view taken through the header of an implantable medical device where the header is constructed in accordance with the present invention; 
       FIG. 4  is an enlarged detailed view of a portion of  FIG. 3 ; and 
       FIG. 5  is an enlarged detail view of a further portion of the cross-sectional view of  FIG. 3 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , there is shown a greatly enlarged side elevational view of the proximal terminal portion of a unipolar lead. While the present invention does not pertain to the construction of leads per se, a description of a unipolar lead terminal ( FIG. 1 ) and a bipolar lead terminal ( FIG. 2 ) deemed helpful in understanding the problem solved by the present invention. 
   Referring to  FIG. 1 , the unipolar lead includes a conductive metal terminal pin indicated generally by numeral  8  that can be considered as being partitioned into three contiguous zones  10 ,  12  and  14 . In zone  10 , the terminal pin  8  is generally cylindrical of a uniform diameter. In zone  12 , the pin has reduced diameter portions  16  and  18  that define anchor segments  20  and  22 . In zone  14 , the pin is tubular and a bore  24  is formed longitudinally therein. 
   Crimped or otherwise affixed within the lumen of the tubular segment  24  is a core wire  26  that extends the length of the lead and connects to an electrode (not shown) on the distal end of the lead. Wrapped about the outer diameter of the tubular segment  14  is a helically wound wire  28  that also wraps about the core wire  26  where it extends along the length thereof, entering a strain relief member  30 . 
   Covering the terminal pin  8  in zones  12  and  14  and covering the helically wound coil  28  is a layer of elastomeric insulating material, preferably silicon rubber. This sleeve or covering is identified by the dotted lines  32  in  FIG. 1 . Thus, the only portion of the terminal pin  8  having an exposed metal surface is that identified by bracket  10 , the remaining segments thereof being embedded in an insulating sleeve. 
   Referring next to  FIG. 2 , there is shown a greatly enlarged side elevational view of the proximal terminal of a bipolar lead. It is indicated generally by numeral  34  and can be readily partitioned into four discrete zones identified by brackets  36 ,  38 ,  40  and  42 . In the zone indicated by bracket  36 , the metal of the terminal pin is exposed and it is connected internally by a conductor (not shown) to an electrode at the distal tip of the bipolar lead. In zone  38 , this conductor is covered by an insulating plastic, e.g., silicon rubber. Surrounding the silicon rubber layer in the zone indicated by bracket  40  is an annular ring of a conductive metal. A second conductor connects internally to the ring contact  44  and extends the length of the lead where it connects to a ring electrode (not shown) located a predetermined, relatively short distance proximal of the distal tip electrode on the distal end of the lead. The conductors leading to both the terminal pin in zone  36  and the ring contact  44  are insulated from one another and from surface exposure by an insulating sleeve  46  that enters a lumen of a tubular strain relief member  48 . The lead terminal  34  has a generally uniform diameter throughout the zones identified by brackets  38 ,  40  and  42 . The exposed terminal pin in the zone enclosed by bracket  36  is of a slightly reduced diameter. 
   An implantable medical device is indicated generally by numeral  50  in  FIG. 3 .  FIGS. 4 and 5  are detailed views of the portions of  FIG. 3  contained within the dotted and dashed lines. It is a top view sectioned horizontally through the device header  52 . The implantable medical device  50  includes a housing  54  formed from a suitable metal and preferably titanium. Typically contained within the housing  54  is a battery source of electrical power, a pulse generator circuit and a microprocessor-based controller that controls the time at which cardiac stimulating pulses are generated by the pulse generator. 
   The header  52  comprises a block of a suitable medical grade plastic  56  having a longitudinal bore  58  formed therein from a front face  60  of the header inward. Also formed in the header block  56  are first and second apertures  62  and  64  that are longitudinally spaced from one another along the length of the bore  58  and which intersect with the bore  58 . Inserted into the aperture  62  is a first connector block  66  that is formed from a conductive material. A bore  68  is formed longitudinally through this connector block and it is concentric with bore  58 . The connector block  66  further includes a threaded bore  70  that is adapted to receive the threaded shank of a set screw  72  that is insertable into the aperture  62 . 
   The aperture  64  likewise has a second connector block  74  inserted therein. It also has a longitudinal bore  76  formed through it. A threaded bore  78  extends transversely to the bore  76  and intersects the bore  76 . A set screw  80  is adapted to be inserted into the aperture  64  and its threaded shank mates with the threaded bore  78 . 
   In order for the header to accept both a bipolar lead and a unipolar lead, it is necessary that the bore in the second connector block  74  be sufficiently large to receive the O.D. of a proximal pin contact of either type of lead and that the set screw can be advanced sufficiently far so that the bottom end of the set screw presses against the proximal pin contact of the lead with sufficient force to press the proximal pin contact against the wall of the bore  64  in the second connector block to effectively lock the two together. The bore  68  in the first connector block must be sufficiently large to receive the distal ring contact of a bipolar lead and the set screw  72  must be able to be advanced until the bottom of the set screw  72  presses against the ring contact with a force sufficient to assure an intimate contact between the distal ring and the bore wall. However, since the insulating sleeve of a unipolar lead also falls within the bore of the first connector block, the set screw  72  must not penetrate so far into the bore  68  that it can cut into the insulating sleeve to thereby compromise its role as an insulator for the proximal pin connector that extends through the insulating sleeve. 
   In accordance with the present invention, to prevent the set screw used in the first connector block from damaging the insulating sleeve of a unipolar lead connector, a set screw stop is built into the first connector block to limit the extent to which the set screw used in the first connector block may be advanced. While it can be advanced sufficiently far to engage a distal ring contact on a bipolar lead terminal, the stop prevents it from being advanced to the point where it can damage the insulating sleeve on the unipolar lead. The set screw stop is established by the depth to which the bore  70  in the first connector block is tapped with due consideration being given to worse case tolerances of the bipolar lead ring outside diameter and the diameter of the bore  68  formed longitudinally through the first connector block  66 . 
   I have empirically determined that if a connector block serving as the first connector block  66  in the present invention has a bore diameter of 0.0880±0.0015 inch and with a distal ring contact of a diameter of 0.0820±0.0015 inch, an interference fit between the set screw and the distal ring contact of 0.002 inch provides an adequate force to insure a good electrical connection. However, it is important that there be no more than 0.008 inch of interference between the set screw and the silicone insulating sleeve on the unipolar lead when the set screw is turned down all the way if no damage is to occur to the insulation when a unipolar lead is stripped free of the header. This dictates that for a connector block having the center of its longitudinal bore 0.141±0.003 inch from the top edge of the block, the screw stop should be 0.013 inch from the top edge of the block. 
   It can be seen, then, that by proper attention to dimensional tolerances, a header can be constructed having a lead bore to mate with both a bipolar and a unipolar lead. Accordingly, it is unnecessary to provide unique headers on implantable medical devices to accommodate the two types of lead connectors. 
   This invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment and operating procedures, can be accomplished without departing from the scope of the invention itself.