1.  Field of the Invention
The present invention relates to connector assemblies for cardiac pacing leads, and more particularly to a connector assembly which readily connects to the proximal end of an in-line multielectrode pacing lead.  The connector assembly is designed to be expandable to accommodate a plurality of electrical contacts for making contact with ring electrodes on the proximal end of the pacing lead and is mounted in a soft, pliable, so-called pacer neck, which fits closely about the pacing lead to prevent fluid flow into the connector assembly and between the electrical contacts and includes a cap which fits tightly over the neck so as to squeeze the contacts against the ring electrodes on the pacing lead. 
2.  Description of the Prior Art
Pacing systems include a pacer, which comprises a metal container or case which contains electronic circuitry and a power supply, and a pacing lead.  The proximal end of the lead is connected to output terminals of the pacer in a component of the pacer which is commonly referred to as the neck of the pacer mounted to a top side of the case. 
Multiple electrode cardiac pacing leads are well known and have been utilized for pacing both the atrial and ventricular chambers of the heart.  Such a pacing lead includes a multipolar electrode assembly at the distal end and terminal ring electrodes or bands at the proximal end.  The multipolar electrodes at the distal end of the lead usually comprise a tip electrode and one or more ring electrodes along the length of the lead.  Additionally, the lead may contain one or more sensors along its length which measure and monitor physiological parmeters, such as the partial pressure of oxygen or carbon dioxide within the chambers of the heart. 
In a pacing system with one electrode on the pacing lead, the distal end of the lead contains the electrode and is usually placed in the ventricle of the heart.  The electrode is connected to the lead by an insulated helically coiled wire conductor.  At the proximal end of the lead, a terminal pin is affixed to the lead.  This pin is inserted into the pacer neck where it makes contact with a pacer lead connector socket which is, in turn, connected to cardiac pacer circuitry and a power source in the pacer case. 
With one electrode on the distal end of the lead, referred to as a unipolar system, one pacer lead connector or contact is needed in the neck of the pacer to serve as a cathode connector, with the case or can of the pacer normally serving as an anode.  In a bipolar system, where two electrodes are on the distal end of the lead, a pacing lead terminal pin and a terminal ring electrode or band are provided on the proximal end of the lead and two pacer lead connectors or contacts are necessary in the neck of the pacer. 
Currently, cardiac pacing systems focus upon (a) the sensing of electrical signals generated by the myocardium or middle layer of the atrial and/or ventricular chambers of the heart and (b) the stimulation of one or both of these chambers in the absence of spontaneous electrical activity.  In such a dual system, either two different leads are used or the lead used measures ventricular activity at its tip and atrial activity along its length and therefore, more than one connector is needed in the pacer neck.  In dual chamber unipolar systems, two pacer lead connectors are required in the neck of the pacer; one for the atrial lead terminal ring electrode and one for the ventricular lead terminal pin.  In a bipolar dual chamber system, four pacer lead connectors are required in the pacer neck for connection to the pacing lead proximal terminal pin and terminal ring electrodes for monitoring both chambers of the heart. 
Future pacing systems will include physiological sensors, either as part of the pacing lead or separate from it.  These physiological sensors will measure parameters such as oxygen and carbon dioxide levels, pH or any combination thereof, to name a few.  These parameters will be transmitted to the pacer circuitry for use in setting various pacer outputs.  Additionally, each of these sensors will require an electrical contact or connector within the pacer neck in addition to the contacts or connectors described above for connection to the electrodes for sensing electrical activity. 
Multielectrode pacing leads most suitable for this type of electrode and sensor system include a lead with multiple ring electrodes and sensors on the distal end and multiple terminal ring electrodes on the proximal end of the lead in an in-line arrangement.  Such a lead allows the monitoring of several different parameters while only causing one insertion to be made into the heart which reduces trauma.  An example of such a lead is disclosed in U.S. Pat.  No.  4,469,104 which discloses a lead assembly for a body implantable tissue stimilator which contains a connector system of a terminal electrode assembly on the proximal end of the lead and a connector assembly in the pacer neck which relies on garter springs or conductive elastic O-rings to contact each, in-line ring electrode on the proximal end of the lead.  Although such a connector system is effective, the terminal assembly on the proximal end of the lead electrode is hard to manufacture, is sometimes difficult to insert into the pacer neck and is too large to accommodate a large number of terminal ring electrodes within the minute confines of the pacer neck. 
Heretofore various connectors have been utilized for connecting the proximal end of a single or multielectrode pacing lead to the electrical output terminals in the neck of a pacer.  The most commmon type of connector system employs a terminal pin on the proximal end of the pacing lead which is secured inside the the neck to a connector of the pacer by a set screw.  This arrangement is not completely desirable since it usually requires the surgeon to tighten the screw after the terminal pin of the lead is in place inside the neck of the pacer during implantation in a body.  Such a procedure is complicated due to the small size of the screw and the conditions of the operating room. 
Additionally, the number of contacts for a multielectrode lead is restricted if a different set screw is needed for each lead terminal ring electrode received within the connector assembly.  Present connectors which employ such a set screw are limited to a maximum of four connectors due to the size limitations of the pacer neck. 
Finally, the set screws encounter problems with body fluids over an extended period of time which causes deterioration of the screw and entry of fluid into the electrical contact area causing damage and malfunction.  In some instances, a cap has been used to cover each screw head but such a design has not been entirely effective and has further complicated the installation procedure. 
Therefore, the need exists for a multiple contact connector assembly in a pacer neck which is small in size, easy to manufacture, readily accommodates a multielectrode lead, is impervious to body fluid and can be adapted to receive and make contact with a number of ring electrodes on the lead, the number being variable and dependent upon the number of distal electodes and sensors required for a particular patient. 
As will be described in greater detail hereinafter, the assembly of the present invention provides a device which is capable of readily accommodating an in-line multielectrode lead without any complex installation procedures and which is completed sealed from the body it is implanted in. 
Moreover, the connector assembly of the present invention differs from the previously proposed connector assemblies and pacer neck constructions by providing a connector assembly and pacer neck construction which is small in size, contains strain relief for the lead, is easy to use, can accommodate a large number of electrode contacts, provides for pressure against the contacts engaging ring electrodes on a pacing lead, is easy to manufacture, and maintains high reliability of electrical contact throughout the life of the pacing system.