FIELD OF THE INVENTION 
   The present invention generally relates to an implantable cardiac lead capable of stimulating and sensing in two different chambers of the heart. The present invention is more particularly directed to such a lead implantable in the coronary sinus region of the heart and which has a telescoping configuration to accommodate varying heart sizes and physiology. 
   BACKGROUND OF THE INVENTION 
   Implantable cardiac stimulation devices are well known in the art. Such devices may include, for example, implantable cardiac pacemakers and defibrillators. The devices are generally implanted in a pectoral region of the chest beneath the skin of a patient within what is known as a subcutaneous pocket. The implantable devices generally function in association with one or more electrode carrying leads which are implanted within the heart. The electrodes are usually positioned within the right side of the heart, either within the right ventricle or right atrium, or both, for making electrical contact with their respective heart chamber. Conductors within the leads and a proximal connector carried by the leads couple the electrodes to the device to enable the device to sense cardiac electrical activity and deliver the desired therapy. 
   Traditionally, therapy delivery had been limited to the venous, or right side of the heart. The reason for this is that implanted electrodes can cause blood clot formation in some patients. For example, a blood clot released arterially from the left heart, as for example from the left ventricle, could pass directly to the brain potentially resulting in a paralyzing or fatal stroke. However, a blood clot released from the right heart, as from the right ventricle, would pass into the lungs where the filtering action of the lungs would prevent a fatal or debilitating embolism in the brain. 
   Recently, new lead structures and methods have been proposed and even practiced for delivering cardiac rhythm management therapy to the left heart. These lead structures and methods avoid direct electrode placement within the left atrium and left ventricle of the heart by lead implantation within the coronary sinus region of the heart. As used herein, the phrase “coronary sinus region” refers to the venous vasculature of the left ventricle, including any portions of the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other cardiac vein accessible by the coronary sinus. 
   Studies have shown that patients suffering from congestive heart failure (CHF) may exhibit marked improvement with left heart pacing. This is attributed to improved cardiac output and decreasing mitral valve regurgitation. These improvements decrease mortality rates and provide the patients with a higher quality of life. Left heart pacing can also improve the condition of patients suffering from left ventricular dysfunction. The preferred pacing configuration for these patients is dual chamber pacing wherein either the right or left atrium is paced along with the left ventricular. 
   Left heart pacing via the cardiac sinus region can be difficult as stimulation sites vary from patient to patient due to normal anatomical variations. Also, the location and interior size of the vessels and optimal location of the stimulation sites are complicated by an increase in heart size associated with long term CHF. In addition, many patients suffering from CHF have also undergone myocardial infarct(s), bypass surgery, and/or other types of invasive cardiac surgery. Each of these conditions change the position of the vessels within the heart and the position of the stimulation sites. 
   It is therefore generally thought that left heart pacing requires two separate leads, one lead for placing at least one electrode in either the right atrium or within the coronary sinus adjacent the left atrium and another lead for placing at least one other electrode within the coronary sinus adjacent the left ventricle. However, implanting two such separate leads can be difficult, especially if both leads are to be implanted in the coronary sinus. For example, the implant can be time consuming. Further, there is an inherent risk of interaction between the leads. Moreover, there is an increased risk that one of the leads will become dislodged. 
   SUMMARY 
   The invention provides an implantable cardiac stimulation lead suitable for single-pass placement of electrodes into electrical contact with two chambers of a patient&#39;s heart. The lead includes a first lead body having a first proximal connector, a first electrode located at a distal end thereof, and a first conductor electrically connected between the first electrode and the first proximal connector. The lead further includes a second lead body having a second proximal connector, a second electrode at a distal end of the second lead body, and a second conductor electrically connected between the second electrode and the second proximal connector. The second lead body has an internal lumen dimensioned such that the first lead body slidably fits within the internal lumen and is extendable from the second lead body at a point proximal to the distal end of the second lead body. The sliding of the first lead body relative to the second lead body enables the first electrode to have a varying distance from the second electrode and enables the first lead body to extend into the coronary sinus region of the patient&#39;s heart to place the first electrode into electrical contact with the left ventricle. 
   Each of the first and second lead bodies may include a stylet lumen and a pre-shaped curved portion for stabilizing the lead bodies within the coronary sinus region when stylets are removed from the stylet lumens. The first lead body may exit the second lead body proximally to the pre-shaped curved portion of the second lead body. Alternatively, the first lead body may exit the second lead body at a peak of the pre-shaped curved portion of the second lead body. 
   In accordance with further aspects of the present invention, the second lead body may include a bipolar electrode pair including the second electrode and a bipole electrode and cable conductors connecting the electrodes to the second proximal connector. Similarly, the first lead body may include a bipolar electrode pair including the first electrode and a bipole electrode. Here however, the first lead body may include a co-linearly wound multi-filar coil connecting the first lead body bipolar electrodes to the first proximal connector. 
   In accordance with further aspects of the present invention, each of the first and second lead bodies may be formed of polyurethane insulation. Further, the lumen of the second lead body may include a polytetrafluoroethylene (PTFE) lining. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the present invention may be more readily understood by reference to the following description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a simplified diagram illustrating a single-pass cardiac lead embodying the present invention; 
       FIG. 2  is a cross-sectional view taken across lines  2 — 2  of  FIG. 1 ; 
       FIG. 3  is another simplified diagram illustrating a further single-pass cardiac lead embodying the present invention; 
       FIG. 4  shows the lead of  FIG. 1  configured for pacing the right atrium and the left ventricle; and 
       FIG. 5  shows an exit port in the outer lead body from which the inner lead body exits the outer lead body. 
   


   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description is of the best mode presently 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. The scope of the invention should be ascertained with reference to the issued claims. In the description of the invention that follows, like numerals or reference designators will be used to refer to like parts or elements throughout. 
   Referring now to  FIG. 1 , it shows a single-pass cardiac lead  20  embodying the present invention implanted within the coronary veins of the left heart of a heart  10 . More specifically, the lead  20  is implanted within the coronary sinus region  11  of the heart  10 . 
   The lead  20  generally includes a first or inner lead body  22  and a second or outer lead body  24 . As will be seen subsequently, the inner lead body  22  is slidably and telescopically arranged within the outer lead body. This permits the distal end of the inner lead body  22  to extend distally of the distal end of the outer lead body. 
   The distal end of the inner lead body carries a bipolar electrode pair  26  including electrodes  28  and  30 . Similarly, the distal end of the outer lead body  24  carries a bipolar electrode pair  32  including electrodes  34  and  36 . While both lead bodies are shown in the Figures having bipolar electrode pairs, it will be apparent to those skilled in the art that either one or both lead bodies may have unipolar electrodes, as is well known in the art. 
   As will be noted in  FIG. 1 , the inner lead body  22  has been slidingly advanced from the outer lead body  24  within the coronary sinus  11  into the great vein  15  adjacent to and into electrical contact with the left ventricle  18 . The bipolar electrode pair  32  of the outer lead body  24  is within the coronary sinus  13  adjacent to and in electrical contact with the left atrium  16 . With the lead thus configured, both atrial and ventricular pacing and sensing may be carried out from the left side of the heart. Of particular note is the fact that with the inner lead body  22  being slidable within the outer lead body  24 , the spacing between the electrode pairs  26  and  32  is variable and selectable to permit each electrode pair to be positioned within the coronary sinus region  11  for optimum sensing and pacing characteristics notwithstanding variations in heart anatomy. 
   As will be further noted in  FIG. 1 , the outer lead body  24  further includes a pre-formed arcuate or curved portion  38 . The outer lead body  24  assumes the arcuate or curved shape of portion  38  when a stylet (not shown) of the type well known in the art is removed from a stylet lumen within the outer lead body. The stylet lumen will be described subsequently with reference to  FIG. 2 . The pre-shaped arcuate portion  38  serves to stabilize the outer lead body  24  within the coronary sinus  13  of the coronary sinus region  11 . 
   As may further be noted in  FIG. 1 , the inner lead body  22  exits, and hence is extendable from, the outer lead body  24  at a point proximal to the distal end of the outer lead body. It has been found that locating the exit port for the inner lead body at a point proximal to the end of the outer lead body disposes the distal tip of the inner lead body in a direction which is more likely to avoid encountering a vein sidewall when the inner lead body is advanced from the outer lead body. This makes it easier to advance the inner lead body to its desired position within the coronary sinus region. In accordance with this embodiment, the inner lead body  22  exits the outer lead body at a point  37  proximal to the pre-shaped arcuate portion  38  of the outer lead body. 
   The inner lead body  22  also includes a pre-formed arcuate or curved portion  40 . The inner lead body  22  assumes the arcuate or curved shape of portion  40  when a stylet (not shown) is similarly removed from a stylet lumen within the inner lead body. The pre-shaped arcuate or curved portion  40  similarly serves to stabilize the inner lead body  22  within the coronary vein  15  of the coronary sinus region  11 . 
   With further reference to  FIG. 1 , the lead  20  further includes a first proximal connector  42  and a second proximal connector  44 . The first proximal connector  42  is connected to the electrode pair  26  of the inner lead body  22  by a first pair of conductors to be described subsequently with reference to  FIG. 2 . The second proximal connector  44  is connected to the electrode pair  32  of the outer lead body  24  by a second pair of conductors to be described subsequently with reference to  FIG. 2 . 
   The connectors  42  and  44  are accommodated on lead  20  by a junction  46  of the type known in the art. The junction further carries a flush port  48  which communicates with an internal lumen of the outer lead body  24  within which the inner lead body  22  is slidingly arranged. The flush port  48  permits a lubricant, such as saline, to be administered to the internal lumen to lubricate the internal lumen and inner lead body  22 . This permits easier movement of the inner lead body  22  with respect to the outer lead body  24  due to a reduced sliding force. 
     FIG. 2  shows a cross section of the outer lead body  24  with the inner lead body  22  slidingly arranged therein. The outer lead body  24  and the inner lead body  22  are preferably formed of polyurethane insulation such as Pellethane 2363-55D. This particular insulation material allows for smaller lead cross-sections, better strength, and better surface lubriciousness. However, other insulation materials known in the art may also be used to form the outer and inner lead bodies. 
   The inner lead body  22  is slidingly received within the internal lumen  50  of the outer lead body  24 . The flush port  48  ( FIG. 1 ) communicates with the space  51  between the inner lead body  22  and the inner wall of the internal lumen  50  to permit the sliding arrangement of the inner and outer lead bodies to be lubricated. 
   To accommodate the previously mentioned outer lead body stylet, the outer lead body  24  includes a stylet lumen  52 . The stylet lumen  52  is preferably lined with a tube liner  54  of polytetrafluoroethylene (PTFE) or other suitable material such as ethyltetrafluoroethylene (ETFE). 
   As previously mentioned, a pair of conductors connect the electrode pair  32  of the outer lead body to the second proximal connector  44 . In accordance with this preferred embodiment, those conductors take the form of cable conductors  56  and  58 . Other forms of conductors as are known in the art may also be employed. 
   To accommodate the other previously mentioned stylet for the inner lead body, the inner lead body includes a stylet lumen  64  formed by the inner surface of multi-filar coil wires  60  and  62 . The multi-filar coil wires  60  and  62  form the aforementioned first pair of conductors which couple the electrode pair  26  to the first proximal connector  42 . In accordance with this preferred embodiment, the wires  60  and  62  include an insulation  61  and  63  respectively which may be, for example, PTFE or ETFE. 
     FIG. 3  shows another single-pass lead  70  embodying the present invention. The lead  70  is identical to the lead  20  of  FIG. 1  except that the outer lead body  74  is arranged to permit the inner lead body  22  to exit at the peak  39  of the pre-shaped arcuate portion  38 . This configuration may serve to assist the extension of the inner lead body  22  into the great vein  15  in the heart of some patients. 
     FIG. 4  shows the lead  20  of  FIG. 1  configured for pacing the right atrium and the left ventricle. Here it may be seen that the pre-shaped arcuate portion  38  resides just in or distally to the coronary sinus ostium  14 . This places the bipolar electrode pair  32  in electrical contact with the right atrium to permit right atrial pacing. The pre-shaped arcuate portion  38  stabilizes the outer lead body  24  within the coronary sinus  13 . 
   The inner lead body  22  has been extended into the great vein  15  of the coronary sinus region. Here it may also be seen that the pre-shaped arcuate portion  40  is more pronounced for stabilizing the inner lead body  22  within the great vein  15  (or other appropriate coronary vein). The bipolar electrode pair  26  is placed toward the apex of the heart in electrical contact with the left ventricle to permit left ventricular pacing. 
   With respect to each of the embodiments illustrated in  FIGS. 1 ,  3  and  4 , the distal electrode  36  of the bipolar electrode pair  32  is located at or immediately adjacent the distal end of the outer lead body  24 . The bipole or proximal electrode  34  of the bipolar electrode pair  32  may be spaced from the distal electrode  36  by a distance between 1 and 30 millimeters and preferably 5 millimeters. As shown in  FIG. 5 , the inner lead body  22  exits the outer lead body  24  proximal of electrode  34 , and the exit port  29  is about 1 to 10 centimeters, and preferably about 4 to 5 centimeters, from the distal end of lead body  24 . In an alternate embodiment, the exit port  29  is located between electrode  34  and electrode  36 . In any event, the exit port  29  is positioned so that it is relatively close to the distal end of outer lead body  24  to permit both outer lead body  24  and inner lead body  22  to be placed in the coronary sinus. In this manner, the outer lead body  24  may be sufficiently advanced through the coronary sinus so that the exit port  29  is disposed within the coronary sinus. Once the exit port is located in the coronary sinus, the inner lead body  22  may be deployed from the exit port  29  and into the coronary sinus, for further advancement within the coronary sinus. 
   While the invention has been described by means of specific embodiments and applications thereof, it is understood the numerous modifications and variations could be made thereto by those skilled in the art without departing from the spirit and scope of the invention. For instance, the disclosed features, either singularly or in groups, could be used with other leads to advantageous results. It is therefore to be understood that within the scope of the claims, the invention may be practices otherwise than as specifically described herein.
An implantable single-pass cardiac stimulation lead provides for placement of electrodes into electrical contact with two chambers of a patient's heart. The lead includes an inner lead body having at least one electrode at its distal end and an outer lead body having at least one electrode at its distal end. The outer lead body has an internal lumen that slidingly receives the inner lead body. The inner lead body is extendable from the outer lead body at a point proximal to the distal end of the outer lead body. The sliding of the inner lead body relative to the outer lead body enables the inner lead body distal electrode to have a varying distance from the outer lead body distal electrode and enables the inner lead body to extend into the coronary sinus region of the heart to place the inner lead body electrode into electrical contact with the left ventricle.