Patent Publication Number: US-11654278-B1

Title: Cardiac pacing lead

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/826,007, filed Mar. 20, 2020, which claims the benefit of and priority to U.S. Provisional Application No. 62/821,441, filed Mar. 21, 2019. Each of these applications are hereby fully incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to cardiac pacing. 
     BACKGROUND 
     Typically, pacing leads are deployed to various locations in the heart depending on the nature of the heart condition necessitating the pacing procedure. Conventional ventricular pacing typically involves implanting a lead at the apex of the right ventricle. This placement is still often utilized today even in the face of published evidence of the deleterious effects of bypassing the His/Purkinje system, otherwise known as the cardiac conduction system. 
     Pacemaker lead electrodes have been regularly placed in or on the heart in a position that bypasses the His/Purkinje system since the inception of pacing in 1957. Conventional pacing directly stimulates the myocardium and has been the standard of care even though His bundle pacing has been known and tried occasionally. 
     During and around the 1980s, scientific studies found that over time, ventricular pacing resulted in what was termed, “ventricular remodeling,” which can result in a number of detrimental effects including: myofiber disarray, fatty tissue and fibrotic deposits away from the electrode, impaired endothelium function, acute hemodynamic compromise, redistribution of myocardial fiber strain and blood flow, with hypertrophy away from the electrode, mitral valve regurgitation due to poor papillary muscle timing, cardiac sympathetic activity, decreases in left ventricle (LV) chamber efficiency, slowing of LV isovolumic relaxation, far LV wall contracting against a closed aortic valve, tricuspid valve insufficiency due to lead mechanical disruption, and mitochondrial abnormality away from the electrode. 
     By 2002, large controlled studies found that conventional ventricular pacing also resulted in heart failure hospitalization and mortality, especially when the patient was paced forty percent or more or the time. This iatrogenic problem is referred to as “pacing induced heart failure.” 
     In spite of significant research demonstrating significant mortality reductions for His bundle pacing compared to conventional pacing, the value of His pacing has not been widely recognized or practiced among clinicians responsible for implanting cardiac pacing leads and pacemakers. 
     BRIEF SUMMARY 
     The inventors believe the limited prevalence of His bundle pacing, and when required, pacing the left bundle branch (LBB) of the conduction system, is in part due to lack of effective leads and lead delivery systems. The cardiac conduction system is comprised in part of His bundle which resides between the atrioventricular (AV) node, and the bifurcation of the LBB and right bundle branch (RBB). These anatomic locations are regarded as difficult targets to reach. 
     For example, many patients cannot have LBB block corrected by His bundle pacing but can benefit from LBB pacing. Techniques disclosed herein facilitate both His bundle pacing, generally via the septal wall of the right atrium, and LBB pacing, generally via right ventricle (RV) septal access. The present disclosure describes examples of leads and methods for use including delivering a pacing lead to the LBB, at the septal wall of the right ventricle or the His bundle in the right atrium. 
     Examples of the present disclosure includes a lead with a distal helix to facilitate anchoring to the septal wall of the RV or, alternatively, proximate the His bundle, generally via the septal wall of the right atrium. Such leads may further include a blunt dissection electrode configured for deployment within the septum. The blunt dissection electrode may be advanced to the His bundle or LBB following anchoring the distal end of the lead into the septal wall with the helix. The lead may be implanted via a catheter. 
     Implantation techniques may include selecting a trajectory for the blunt dissention electrode by manipulating the catheter after anchoring the helix to the septal wall. For example, with the distal end of the catheter-lead assembly anchored to the septal wall, the direction of the trajectory of the blunt dissection electrode may be selected by the clinician by bending the catheter through pushing and pulling from a proximal location outside the body of the patient, as well by rotating the catheter from the outside the body of the patient. 
     In one example, this disclosure is directed to a medical lead comprising a lead body, a connector pin proximate to a proximal end of the lead body, a helix extending from a distal end of the lead body, an electrode proximate to the distal end of the lead body, and a cable conductor within the lead body and including an electrode proximate a distal end of the cable conductor, the cable conductor being slideable within the lead body to extend and retract the electrode relative to the distal end of the lead body. 
     In another example, this disclosure is directed to a method for implanting a medical lead, the method comprising securing a helix of the medical lead to a patient tissue proximate a target site, and extending a cable conductor from a lead body of the medical lead to deploy an electrode within the patient tissue. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional illustration of a human heart depicting the anatomy of the heart and its electrical system. 
         FIG.  2    is a cross-sectional illustration of a human heart wherein an example of a guide catheter is shown advanced to a target site within the central fibrous body, between the tricuspid valve and the aortic valve and corresponding to the His bundle. 
         FIG.  3    is an anatomical illustration of a patient and the manner in which the example shown in  FIG.  2    initially accesses the vasculature prior to advancement into the heart. 
         FIG.  4    is a conceptual illustration of a pacing lead accessing the septum from the RV in accordance with one example of this disclosure. 
         FIGS.  5 A- 5 C  illustrate detailed views of the distal region and tip of the pacing lead while mapping the LBB while attached to and fixed in a patient&#39;s septum. 
         FIG.  6    illustrates the pacing lead with the proximal blunt dissection conductor and a self-stripping connector pin. 
         FIGS.  7 A- 7 C  illustrate an alternative lead design with an exposed helical electrode that facilitates mapping. 
         FIG.  8    is a flowchart illustrating techniques for locating a lead electrode proximate the His bundle. 
         FIG.  9    illustrates an alternative lead design with a helix partially covered by an insulating layer. 
         FIG.  10    illustrates an alternative lead design with a insulated helix fully covered by an insulating layer and an anode ring on the distal end of the lead body. 
     
    
    
     DETAILED DESCRIPTION 
     The prevalence of His bundle pacing, though increasing, is practiced in a small minority of pacing lead implantations both in the United States and worldwide. The His bundle presents a small target and is hard to reach successfully. This increases “fluro time” which is a detriment to both patient and the surgical clinician. However, in one study by the inventors, the mortality rate of one hospital doing conventional pacing was compared with the mortality rate with another hospital doing pacing at the His bundle (for normal physiological ventricular activation). Heart failure and patient mortality was lower at the hospital providing physiological ventricular activation by His bundle pacing. 
     It is generally more difficult to place a cardiac lead electrode at the conduction system for His bundle pacing than it is to place within the ventricle for conventional pacing. However, techniques of the present disclosure mitigate difficulties with locating a lead electrode to capture the His bundle. Disclosed techniques also facilitate locating a lead electrode to capture the LBB, which is important for patients with LBB block in which the capturing the His bundle may not provide effective LBB normalization. 
     In one example, a lead includes a helical anode anchor configured to be anchored on the septum of the right atrium, piercing the endocardial membrane of the right atrium. The cathode is connected via a cable conductor extending from the connector end of the lead. A clinician advances the cable conductor through the coaxial center of the lead, advancing the cathode via the pierced endocardial membrane of the right atrium to a targeted portion of the cardiac conduction system, usually the His bundle within the septum or extending distally to the LBB. 
     The trajectory of the cathode is controlled by the angle of the lead delivery catheter following anchoring of the helical anode anchor. While anchored, the clinician may manipulate the angle of lead delivery catheter. The catheter pivots the helical anode, controlling the trajectory of the cathode. 
     In this manner, the catheter and fixation screw need not be presented at any particular angle (such as perpendicular) to the endocardial surface. The trajectory of the cathode can be manipulated after anode fixation and has no bearing on His pacing threshold. Thus, a variety of lead delivery catheters may be suitable for delivery of leads disclosed herein. 
     Once the clinician is satisfied with the angle of the catheter, the clinician advances the cable, having the cathode electrode attached at the distal tip, is advanced from the connector end, through the lead body and helical anode to the targeted portion of the cardiac conduction system, e.g., via blunt dissection. In other examples, the tissue may be cut with a sharp electrode or RF energy. However, blunt dissection may provide an advantage of mitigating the risk of piercing the septum as the endocardial membrane of the ventricular septum provides a relatively durable and elastic layer resistant to blunt dissection compared to the muscular central portion of the ventricular septum. 
     Selection of either specific or nonspecific His bundle pacing can be achieved for type two His anatomy because of the cathode is small enough to fit within the His bundle. Type two His anatomy, existing in an estimated 32% of patients, is where the His bundle dives below the central fibrous body and is surrounded by myocardium. Large electrodes, such as helical electrodes of current leads may be too large to exclude the myocardium from activation along with the His bundle (called non-specific His bundle pacing). In contrast, smaller electrodes of leads disclosed herein, such as those with an electrode radius of about 0.5 mm, allow for “specific” His bundle pacing. Such smaller electrodes may also facilitate LBB pacing, in the event that LBB block cannot be corrected at the His bundle due to infra-hisian block, e.g., through trans-septal lead placement. 
     In contrast, a clinician attempting to use a conventional screw-in lead meant for right ventricle or atrial endocardial attachment may try to drill thru the septum—a process that is very tedious, reportedly requiring at times, forty turns, and having the risk of penetration into the lumen of the left ventricle risking embolic stroke. 
       FIG.  1    shows the cardiac anatomy, especially the cardiac conduction system. In a healthy heart, the natural pacemaker, the SA node, activates the high conduction velocity Purkinje fibers within the right and left atria, resulting in coordinated atrial muscle cell contraction. This injects blood collected in the atria, into the powerful left and right ventricles. There is a pause in conduction at the AV node allowing the ventricles to fill. Then, just before blood flows back into the atria, the AV node activates the His bundle and, by high conduction velocity, the left and right bundle branches and the entire Purkinje system. This choreographs ventricular contraction, endocardial myocardium contracting first followed by epicardial muscle contraction. This programmed ventricular muscle activation produces an efficient pumping action that not only squeezes blood out of the ventricles but produces kinetic energy as blood is accelerated from the ventricles. The result of conventional pacing is compromised Hemodynamics due to slow cell-to-cell conduction and an aberrant ventricular activation sequence as the cardiac conduction system is bypassed. The far-left ventricular wall away from the electrode site has been seen contracting against an already closed aortic valve. 
     For contextual understanding, of how examples of the disclosure are intended to function,  FIG.  1    is included to illustrate the structure of a typical human heart  1  with relevant anatomical features shown. As mentioned, one example of the disclosure is directed to a method for deploying an electrical lead to the His bundle  2  at a target site  10  along the septum  3  distal to the atrioventricular (AV) node  4 , but proximal to the left bundle branch (LBB)  5  and the right bundle branch (RBB)  6 . Such a target site  10  for proper deployment of a pacing lead, is depicted in  FIG.  1    at the crest of the ventricular septum  3  on the atrial aspect of the annulus of the tricuspid valve septal leaflet  7  within the right atrium  8 . 
     In some patients, such as those experiencing LBB block, the target site is instead the LBB  5 , potentially be accessed from target site  10 , or from an alternative target site on ventricular septum  3  from within the right ventricle. 
       FIG.  2    is the schematic diagram of  FIG.  1    in which a distal portion or end region  22  of delivery catheter  20  is shown extending into the right atrium  8  of the heart  1 , from the superior vena cava  9  and the left subclavian vein  11 , with the distal tip  24  positioned at the target site  10 . 
     Typically, left pectoral side approach is desired. It involves accessing the heart via the left subclavian vein, the cephalic vein and more rarely the internal or external jugular vein, or femoral vein. However, it is also possible to utilize the less common right pectoral side approach. In either case, for catheter lead placement, a guide wire  50  may be advanced into the heart  1  from the access site. Delivery catheter  20  may be advanced through the vasculature and into the heart  1  over the guidewire; once in position the guidewire is removed. A pacing lead is then advanced through the guiding catheter  1  to be deployed at various regions in the heart. 
     According to one method, a clinician positions guide wire  50  into the heart  1 , for example via a “sub-clavian stick” or central venous access procedure such as is illustrated in  FIG.  3   . Accordingly, the catheter  20  is passed over the guide wire and advanced into the superior vena cava  9  from the left subclavian vein  11  and into the right atrium  8  such as is in the manner shown in  FIG.  2   . 
       FIG.  4    illustrates a medical electrical lead  30  in accordance with one example of this disclosure. Medical electrical lead  30  includes a central cable conductor  33  terminating at cathode electrode  34  and extending within a central lumen of lead  30  about a length of lead body  38  for coupling to connector pin  44  of proximal connector  40 . 
     Cable conductor  33  includes one or more conductive elements forming an electrical connection between cathode electrode  34  and connector pin  44 , once connector pin  44  is connected to the conductive elements of cable conductor  33 . In various examples, cable conductor  33  may include a solid wire conductor, a stranded wire, or a coil conductor. In a particular example, cable conductor  33  may include a fiber core coil with one or more electrically conductive wires coiled on a fiber core. The fiber core may provide tensile strength for cable conductor  33  and mitigate stretching of the coiled conductors during retraction of cable conductor  33 . 
     In the same or different examples, cable conductor  33  may be an insulated cable conductor including outer insulating layer, leaving the distal tip exposed for cathode electrode  34 . The insulating layer may include silicone rubber, polyurethane parylene, polymide and/or ethylene tetrafluoroethylene (ETFE) cable insulation. In some examples, connector pin  44  may be a self-stripping connector pin  44  to allow contact with the conductive elements of cable conductor  33 . Alternatively, connector pin  44  may make electrical contact with conductive elements of cable conductor  33  upon tightening of a setscrew of the connector of a pulse generator or other device connected to proximal connector  40 . Central cable conductor  33  is slideable the central lumen of lead body  38  to extend and retract cathode electrode  34  relative to the distal end of lead body  38 . 
     In the same or different examples, cathode electrode  34  may be a unitary component with the conductive element(s) of cable conductor  33 , or may be a separate component physically and electrically coupled to the distal end of the conductive element(s) of cable conductor  33 , for example, by solder or welding, such as laser welding. In some examples, the cathode electrode  34  is a 0.5 to 2.0 mm diameter, such as 0.7 to 1.0 mm diameter hemispherical electrode, such as a half sphere with a diameter of about 0.87 mm, at the end of an insulated cathode conductor of the same diameter in order to provide blunt dissection. 
     Medical electrical lead  30  further includes a second conductor within lead body  38  extending between ring terminal  42  and helix anode electrode  32 . In some examples, the second conductor is a coil conductor surrounding the central cable conductor  33  within the lead body  38 . 
     Helix anode electrode  32  may be made from a wire. The number of turns and length of the helix may be adapted for a particular application. For example, helix anode electrode  32  may have 1 to 8 turns, such as 2 to 4 turns to support adequate fixation within patient tissue. A septal thickness can be anywhere from 0.9 cm to 1.2 cm in normal individuals. A risk of perforation will likely go up if the helix is too long and the entire helix penetrates the septum. Accordingly, the dimensions of the helix should be selected to allow fixation but mitigate a risk of perforation. In the present example, a helix length of 1.0 to 8.0 mm may be appropriate to mitigate a risk of piecing the septum, such as a helix length of 1.5 to 4 mm, such as about 1.8 mm. As used herein, the term about means within a range of tolerances of manufacturing techniques used to produce the referenced element. Moreover, the length of helix anode electrode  32  should be selected to provide a suitable distance between helix anode electrode  32  and cathode electrode  34  to support stimulation with helix anode electrode  32  and cathode electrode  34 . 
     In one particular example of lead  33  the following dimensions may be used. Lead body  38  diameter 3 to 6 French, such as about 4.1 French, cable conductor  33  diameter, 0.02 to 0.05 inches, such as about 0.028 inches, helix  32  length 1 to 4 mm, such as about 1.8 mm, helix  32  pitch, 0.5 to 2 mm, such as about 1 mm, helix  32  wire diameter 0.006 to 0.03 mm, such as about 0.012 mm. In the same or different examples, the following materials may be utilized for the helix anode electrode  32  wire: Pt 80%/Ir 20% or Pt 90%/Ir 10% for a thinner wire. 
     As used herein, the terms anode and cathode merely represent example uses of particular lead electrodes. For example, anode electrode  32  and cathode electrode  34  are electrically isolated within medical lead  30  such that such that anode electrode  32  and cathode electrode  34  may form an electrode pair to deliver stimulation. However, the polarity of the stimulation is controlled by a pulse generator and not inherent to the structure of electrical lead  30  itself. Thus, the pulse generator could reverse the polarity of anode electrode  32  and cathode electrode  34 , or even use one or both of anode electrode  32  and cathode electrode  34  in combination with the pulse generator housing. For example, one or both of anode electrode  32  and cathode electrode  34  could be configured as a cathode while the pulse generator housing serves as the cathode. 
     In  FIG.  4   , a close-up view of the distal tip  24  of the catheter  20  is shown following advancement of medical electrical lead  30  though a lumen of the catheter  20  to the target site  10 . In this example, the target site is the LBB, although the His bundle can also be targeted. 
     The lead  30  is extended distally from distal tip  24  and anchored into the septum  3  by clockwise rotation of the lead body  38  targeting the LBB, so that helical anode electrode  32  screws through the endocardial membrane and into the septal wall. The cathode electrode  34  is extended into the septum  3  to provide pacing to the heart  1  via the LBB. In some examples, cathode cable conductor  33  and cathode electrode  34  may extend through the hole in the endocardial membrane created by helical anode electrode  32 . In other examples, RF energy may be applied to cathode cable conductor  33  to cross the RV endocardium, then detaching the RF connection to cathode cable conductor  33  and advance it to the LV endocardium. The preferred method of advancement is via blunt dissection. 
       FIGS.  5 A- 5 C  illustrate detailed views of the distal region and tip of the pacing lead at a target site in a patient&#39;s septum. Specifically,  FIG.  5 A  shows the trajectory of the cathode direction for mapping of the LBB during catheter introduction of the lead  30 , while  FIG.  5 B  shows a closeup of the lead delivery catheter and lead tip. The helix anode electrode  32  for lead attachment is shown anchored and is pivoted by the lead delivery catheter. 
       FIG.  5 C  shows the LBB mapping process and possible range of cathode location. Mapping for lead location is accomplished by sensing the LBB potential and/or pacing the LBB to produce a narrow QRS on the surface ECG, typical of physiologically normal ventricular activation. The trajectory of the cathode advancement is controlled by manipulation of the lead delivery catheter. Resistance to advancement of cable conductor  33  is felt when the electrode  34  impinges on the tough left ventricular endocardial membrane. See the tenting effect of the opposite endocardial membrane in  FIG.  5 C . 
       FIG.  6    illustrates lead  30  including a proximal connector  40  with a self-stripping connector pin  44 . After selecting a proper lead position in the heart, cathode cable conductor  33  is cut flush with the connector pin  44 . The pulse generator&#39;s connector setscrew is tightened to make electrical contact, fracturing the cable insulation and completing the circuit to cathode electrode  34 . Also illustrated is proximal ring terminal  42 , located adjacent and distal to connector pin  44 . Ring terminal  42  provides an electrical connection to anode electrode  32 . 
     The example proximal connector  40  illustrated in  FIG.  6    conforms to the IS-1 standard. In various examples, connector  40  may conform to a standard pulse generator connector, such as an IS-1, IS-4, DF-1, DF-4 or other industry standard connector. 
       FIG.  7 A- 7 C  illustrate the distal end of a medical electrical lead  130 , which provides an alternative lead design as compared to medical electrical lead  30 . With this example, an anode helix  132  is attached over the lead delivery catheter tip  124 . This exposes the anode  132  for mapping purposes. As shown in  FIG.  7 A , anode helix  132  extends through slot  122  which extends along a length of delivery catheter  120  at lead delivery catheter tip  124 . The pointed distal tip of anode helix  132  is closely fitted to the catheter outer diameter to prevent snagging on inter-vascular tissue during venous passage of the catheter lead assembly. Deployment of anode helix  132 , e.g., by extending in direction  141 , then turning lead  130  relative to the patient tissue in direction  142  (corresponding to the curvature of anode helix  132 ) anchors the distal tip of anode helix  132  in the patient tissue. 
     Similar to lead  30 , lead  130  includes a blunt dissection electrode  134 , the trajectory of which is selectable by a clinician by manipulating the catheter lead assembly after anchoring helical anode  132  within a tissue of the patient, such as the septal wall. This design of catheter  120  and lead  130  may increase the percutaneous introduction size, such as by 2 French as compared to catheter  20  and lead  30 . Following the selection of the trajectory, the clinician may deploy electrode  134  within the patient tissue along direction  143 , representing the selected trajectory, by pushing cable conductor  133  relative to anode helix  132  of lead  130 . 
       FIG.  8    is a flowchart illustrating techniques for locating a lead electrode to pace the cardiac conduction system, either the LBB or proximate the His bundle. For clarity, the techniques of  FIG.  8    are described with respect to catheter  20  and medical electrical lead  30 , although the techniques may likewise be applied to medical electrical lead  130  and to variations of the example leads disclosed herein. 
     First, a clinician positions the distal tip  24  of catheter  20  at the target site  10  on a patient&#39;s septum or proximate the His bundle ( 202 ). In some example, a guidewire may be used to direct the catheter to target site  10 . Once, the distal tip  24  of catheter  20  is positioned at the target site  10 , the clinician removes guidewire (if any) and introduces lead  30  via the central lumen of the catheter  20 . The distal end of lead  30  is delivered to the target site  10  in the septum via catheter  20  ( 204 ). In other examples, lead  30  may be introduced with a stylet, after temporarily extracting the central cable conductor  33  from the central lumen of lead  30 . The relatively stiff stylet may also be used by, blunt dissection, to clear a pathway through tough tissue such as the central fibrous body or along the left ventricular septum paralleling the LBB, especially when a relatively flexible cable conductor  33  is desirable. 
     The clinician anchors the catheter lead assembly to the target site  10  in the septum by rotating the lead  30  to engage the septum with the helical electrode  32  of lead  30  ( 206 ). 
     The clinician may then manipulate the catheter  20  to set a desired trajectory for blunt dissection of the septum with the blunt dissection cathode electrode  34  ( 208 ). For example, the clinician may select a trajectory for the blunt dissention electrode by manipulating the catheter after anchoring the helical electrode to the septal wall by bending the catheter through pushing and pulling from a proximal location outside the body of the patient, as well by rotating the catheter from the outside the body of the patient. Once helix anode electrode  32  is fixed to the septum, the cathode can be advanced, by blunt dissection, at least 1.8 cm from the base of helix anode electrode  32 , toward the left bundle branch, just inside left ventricular septum. 
     For mapping, the clinician may optionally withdraw the blunt dissection electrode  34  ( 214 ), set a new desired trajectory, and redeploy the blunt dissection electrode  34 . Generally, however, a clinician will only want to reset the position of blunt dissection electrode  34  if the capture threshold is undesirable ( 212 ). If mapping finds adjustment is necessary, cathode electrode  34  is extracted to helix anode electrode  32  and helix anode electrode  32  can be pivoted to a desired new trajectory for advancement of cable conductor  33 . 
     For example, with His bundle pacing, pacing threshold voltage is generally greater than that of conventional pacing, but current threshold is generally less than that of the electrode of conventional pacing leads having higher electrode surface area, thus lower pacing impedance, so that the battery drain is comparable or better than with to conventional pacing. Cathode pacing impedance may be on the order of 1,000 ohms. 
     In some examples, the cathode electrode  34  is a 0.7 to 1 mm diameter hemispherical electrode at the end of an insulated conductor of the same diameter in order to provide blunt dissection. This relatively small surface area (of about one square mm) for the exposed electrode may provide one or more advantages. For example, the smaller area may facilitate a reduced battery drain. Micro-dislodgement issues with other small pacing electrodes should be limited due to the embedded myocardial electrode placement of cathode electrode  34  as opposed to placement on the endocardial surface as is the case for conventional tined leads. Thus, the disclosed techniques may mitigate instances of micro-dislodgement as can occur with electrodes positioned on the surface of a patient tissue. 
     In the same or different examples, the hemispherical tip electrode  34  of cathode cable conductor  33  may be coated with a steroid to mitigate scar tissue and its negative effects on capture threshold over time. 
     Occasionally, His bundle pacing (at the tip of the ventricular septum and within the right atrium) cannot correct LBB block. LBB block cannot be corrected by His bundle pacing in as many as 30-40% of patients due to infra-hisian block. When that happens, cable conductor  33  may be retracted and the trajectory of cable conductor  33  adjusted to target the LBB. Specifically, the clinician may target the LBB from with the right atrium without repositioning the anode helix, rather than from the RV as described previously. In such examples, the previously anchored anode helix can be pivoted about ninety degrees to align with the ventricular septum. When cable conductor  33  is advanced, cathode electrode  34  slides along the endocardial membrane of the left ventricle, targeting the LBB, thereby bypassing the infra-hisian block. 
     When used for His pacing, particularly when the left bundle branch block cannot be corrected at the His bundle due to distal block, cathode electrode  34  is retracted to the anode helix. The helix is rotated to a cathode trajectory that aims at the left bundle branch. The cathode is then advanced along the left ventricular endocardial membrane to pace the left bundle branch. 
     Once the position of the blunt dissection electrode  34  is satisfactory, the clinician may withdraw catheter  20 , cut the cable conductor  33  flush with connector pin  44 , insert proximal connector  40  into the pulse generator&#39;s connector, and tighten the set-screw to cut through the insulation and complete the circuit ( 216 ). In examples in which the pulse generator is an implantable pacemaker, the clinician then inserts the implantable pacemaker in a pocket under the skin in the patient&#39;s chest and is ready for sensing and/or pacing via the lead  30 . 
       FIG.  9    illustrates medical electrical lead  330 . Lead  330  is substantially similar to lead  30  except that lead  330  includes an insulating layer  331  over the distal portion of helix anode  332 , partially covering helix anode  332 . The insulating layer  331  limits the exposed anodal surface area  335 , increasing field density adjacent anode  332  to allow for bifocal stimulation. In all other aspects, lead  330  is the same as lead  30 . For brevity, details discussed with respect to lead  30  are discussed in limited or no detail with respect to lead  330 . 
     Medical electrical lead  330  includes a central cable conductor  333  for a cathode  334 , as well as a second conductor for anode  332 . In some examples, the anode conductor is a coil conductor surrounding the cathode conductor within the lead body  338 . In the same or different examples, the cathode conductor may be an insulated conductor, such as a solid wire, a stranded wire, or a coil conductor. 
     In some examples, the cathode  334  is a 0.5 to 2 mm diameter, such as 0.7 to 1 mm diameter hemispherical electrode, such as a half sphere with a diameter of about 0.87 mm, at the end of an insulated cathode conductor of the same diameter in order to provide blunt dissection. In the same or different examples, the following materials may be utilized for the helix anode  332  wire: Pt 80%/Ir 20% or Pt 90%/Ir 10% for a thinner wire. In the same or different examples, the insulating layer  331  over anode helix  332  may be any suitable dielectric material, such as a polymer material, such as silicone rubber, polyurethane, parylene, polymide and/or ETFE or other non-conductive material. 
     The configuration of lead  330  provides reduced anodal pacing threshold compared to lead  30 . Such a configuration may be particularly useful when right septal bifocal stimulation is desired for example. Such pacing may be useful to support LV/RV synchronization. The configuration of lead  330  is also suitable for His bundle pacing near the endocardial surface by prevention of cathode shorting to the helical anode as may occur with lead  30 . 
     A ratio of anode to cathode surface area should be selected to support bifocal pacing. In some examples, the anode to cathode area ratio should be in 1:1 to 4:1, such as 1.5:1 to 3:1 such as about 2.3:1. In one particular example of lead  330  the following dimensions may be used. Lead  330  diameter 4.1F, cable conductor  333  diameter 0.028 inches, helix  332  length 1.8 mm, helix  332  pitch 1 mm, helix  332  wire diameter 0.012 mm, cathode  334  surface area 1.2 mm 2 , exposed anode  335  surface area 2.8 mm 2    
       FIG.  10    illustrates medical electrical lead  430 . Medical electrical lead  430  includes an insulating layer  431  fully covering portion of helix  432 . Medical electrical lead  430  further includes an anode ring electrode  439  on the distal end of lead body  438 . In other examples, ring electrode  439  may be partial ring electrode. In the same or different examples, helix  432  is laser welded to ring electrode  439 . In the same or different examples, ring electrode  439  may be on a distal portion of lead body  438 , but not necessarily on the distal tip of lead body  438 . Lead  430  is otherwise substantially similar to lead  30 . 
     In the configuration of medical electrical lead  430 , ring electrode  439  serves as the anode, thereby increasing field density adjacent anode  432  to allow for bifocal stimulation. In all other aspects, lead  430  is the same as lead  30 . For brevity, details discussed with respect to lead  30  are discussed in limited or no detail with respect to lead  430 . 
     Medical electrical lead  430  includes a central cable conductor  433  for a cathode  434 , as well as a second conductor for anode  432 . In some examples, the anode conductor is a coil conductor surrounding the cathode conductor within the lead body  438 . In the same or different examples, the cathode conductor may be an insulated conductor, such as a solid wire, a stranded wire, or a coil conductor. 
     In some examples, the cathode  434  is a 0.5 to 2 mm diameter, such as 0.7 to 1 mm diameter hemispherical electrode, such as a half sphere with a diameter of about 0.87 mm, at the end of an insulated cathode conductor of the same diameter in order to provide blunt dissection. In the same or different examples, the following materials may be utilized for the helix anode  432  wire: Pt 80%/Ir 20% or Pt 90%/Ir 10% for a thinner wire. In the same or different examples, the helix coating  431  may be any suitable dielectric material, such as a polymer material, such as parylene, polymide or other non-conductive material 
     Like lead  330 , the configuration of lead  430  provides reduced anodal pacing threshold compared to lead  30 . Such a configuration may be particularly useful when right septal bifocal stimulation is desired for example. A ratio of anode (ring electrode  439 ) to cathode surface area should be selected to support bifocal pacing. In some examples, the anode to cathode area ratio should be in 1:1 to 4:1, such as 1.5:1 to 3:1 such as about 2.3:1. 
     Such pacing may be useful to support LV/RV synchronization. For example, if the anode and cathode are designed to have a similar pacing capture voltage, the left bundle branch can be stimulated at the same time as the right septal myocardial (or right bundle if it were viable and in range of the anode) to promote LV/RV synchrony. The configuration of lead  430  is also suitable for His bundle pacing near the endocardial surface by prevention of cathode shorting to the helical anode as may occur with lead  30 . Specifically, the location of anode ring  439  keeps the anode separated from the distal cathode  434  by the endocardial membrane. Such a configuration prevents short circuiting of the electrodes even with a shallow cathode placement. Such placement may be particularly useful when the His bundle presents near the endocardium as a shallow cathode placement would position the cathode adjacent the His bundle. 
     A number of modifications to the techniques described herein are within the spirit of this disclosure. For example, while the disclosed techniques are described with respect to selecting the trajectory of an electrode for His bundle pacing and LBB pacing, the leads and other techniques disclosed herein may also be used for different target sites, for cardiac pacing and otherwise. 
     As another example, the disclosed techniques could be used with any pulse generator, whether it is in the pectoral pocket or in the right ventricle or in conjunction with a leadless pacemaker. In this manner, the transseptal pacing techniques are suitable with any implantable pulse generator. 
     Various examples of this disclosure have been described. These and other examples are within the scope of the following claims.