Patent Publication Number: US-2020289821-A1

Title: Leadless pacing system including sensing extension

Description:
This application is a Continuation of U.S. application Ser. No. 14/694,910 (published as U.S. Publication No. 2016/0015322), filed Apr. 23, 2015 which claims the benefit of the filing date of U.S. Provisional Application Ser. No. 62/025,690, filed Jul. 17, 2014, the content of both of which are incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to cardiac pacing, and more particularly, to cardiac pacing using a leadless pacing device. 
     BACKGROUND 
     An implantable pacemaker may deliver pacing pulses to a patient&#39;s heart and monitor conditions of the patient&#39;s heart. In some examples, the implantable pacemaker comprises a pulse generator and one or more electrical leads. The pulse generator may, for example, be implanted in a small pocket in the patient&#39;s chest. The electrical leads may be coupled to the pulse generator, which may contain circuitry that generates pacing pulses and/or senses cardiac electrical activity. The electrical leads may extend from the pulse generator to a target site (e.g., an atrium and/or a ventricle) such that electrodes at the proximal ends of the electrical leads are positioned at a target site. The pulse generator may provide electrical stimulation to the target site and/or monitor cardiac electrical activity at the target site via the electrodes. 
     A leadless pacing device has also been proposed for sensing electrical activity and/or delivering therapeutic electrical signals to the heart. The leadless pacing device may include one or more electrodes on its outer housing to deliver therapeutic electrical signals and/or sense intrinsic depolarizations of the heart. The leadless pacing device may be positioned within or outside of the heart and, in some examples, may be anchored to a wall of the heart via a fixation mechanism. 
     SUMMARY 
     The disclosure describes a leadless pacing system that includes a leadless pacing device (hereinafter, “LPD”) and a sensing extension extending from a housing of the LPD, where the sensing extension includes one or more electrodes with which the LPD may sense electrical cardiac activity. The sensing extension is electrically coupled to a sensing module of the LPD via a conductive portion of the housing of the LPD. The one or more electrodes of the sensing extension may be carried by a self-supporting body that is configured to passively position the one or more electrodes proximate or within a chamber of the heart other than the chamber in which the LPD is implanted. In some examples, a proximal portion of the sensing extension is configured to reduce interference with the mechanical movement of the heart. 
     The sensing extension facilitates sensing, by the LPD, of electrical activity of a chamber of the heart other than the one in which the LPD is implanted. The LPD is configured to be implanted within a chamber of the heart of the patient and the sensing extension is configured to extend away from the LPD to position an electrode proximate or within another chamber of the heart, e.g., to sense electrical activity of the other chamber. In some examples, the sensing extension includes a feature configured to facilitate control of the sensing extension during implantation of the sensing extension in the patient. The feature may be, for example, an eyelet at a proximal end of the sensing extension, the eyelet being configured to receive a tether that may be used to control the positioning of the proximal end of the sensing extension during implantation of the leadless pacing system in a patient. The tether may also be used to confirm the LPD is fixed to the target tissue site, e.g., to perform a tug test. 
     In one aspect, the disclosure is directed to a system comprising a leadless pacing device comprising a stimulation module configured to generate pacing pulses, a sensing module, a processing module, a housing comprising a conductive portion, wherein the housing is configured to be implanted within a chamber of a heart of a patient and encloses the stimulation module, the sensing module, and the processing module, and a first electrode electrically coupled to the sensing module and the stimulation module. The system further comprises a sensing extension extending from the housing and comprising a self-supporting body extending from the housing and comprising a curved proximal portion, and a second electrode carried by the self-supporting body and electrically connected to the sensing module and the stimulation module via the conductive portion of the housing. The processing module is configured to control the sensing module to sense electrical cardiac activity via the second electrode. 
     In another aspect, the disclosure is directed to a method comprising controlling, by a processor, a stimulation module of a leadless pacing device to deliver a pacing pulse to a patient, the leadless pacing device comprising the stimulation module, a sensing module, the processor, a housing comprising a conductive portion, wherein the housing is configured to be implanted within a chamber of a heart of a patient and encloses the stimulation module, the sensing module, and the processor, and a first electrode electrically coupled to the sensing module and the stimulation module. The method further comprises controlling, by the processor, the sensing module of the leadless pacing device to sense electrical cardiac activity via the first electrode and a second electrode of a sensing extension that extends from the housing, the sensing extension further comprising a self-supporting body extending from the housing and comprising a curved proximal portion, and the second electrode carried by the self-supporting body and electrically connected to the sensing module and the stimulation module via the conductive portion of the housing. 
     In another aspect, the disclosure is directed to a system comprising a leadless pacing device comprising a stimulation module configured to generate pacing pulses, a sensing module, a processing module, a housing comprising a conductive portion, wherein the housing is configured to be implanted within a chamber of a heart of a patient and encloses the stimulation module, the sensing module, and the processing module, and wherein the conductive portion is electrically connected to the sensing module, and a first electrode electrically coupled to the sensing module and the stimulation module. The system further comprises a sensing extension extending from the housing and comprising a self-supporting body mechanically connected to the housing and comprising a conductor electrically connected to the conductive portion of the housing, a second electrode carried by the self-supporting body and electrically connected to the conductor, and an eyelet at a proximal end of the sensing extension. 
     In another aspect, the disclosure is directed to a system comprising a leadless pacing device comprising a stimulation module configured to generate pacing pulses, a sensing module, a processing module, a housing configured to be implanted within a chamber of a heart of a patient, wherein the housing encloses the stimulation module, the sensing module, and the processing module, and a first electrode electrically coupled to the sensing module and the stimulation module. The system further comprises an extension extending from the housing and comprising a body mechanically connected to the housing and comprising a conductor electrically connected to at least one of the sensing module or the stimulation module, a second electrode carried by the body and electrically connected to the conductor, and an eyelet at a proximal end of the extension. 
     In another aspect, the disclosure is directed to a method comprising controlling, by a processor, a stimulation module of a leadless pacing device to deliver a pacing pulse to a patient, the leadless pacing device comprising the stimulation module, a sensing module, the processor, a housing configured to be implanted within a chamber of a heart of a patient, wherein the housing encloses the stimulation module, the sensing module, and the processing module, and a first electrode electrically coupled to the sensing module and the stimulation module. The method further comprises controlling, by the processor, the sensing module of the leadless pacing device to sense electrical cardiac activity via the first electrode and a second electrode of a sensing extension that extends from the housing, the sensing extension further comprising a body mechanically connected to the housing and comprising a conductor electrically connected to the sensing module, a second electrode carried by the body and electrically connected to the conductor, and an eyelet at a proximal end of the extension. 
     In another aspect, the disclosure is directed to a computer-readable storage medium comprising computer-readable instructions for execution by a processor. The instructions cause a programmable processor to perform any whole or part of the techniques described herein. The instructions may be, for example, software instructions, such as those used to define a software or computer program. The computer-readable medium may be a computer-readable storage medium such as a storage device (e.g., a disk drive, or an optical drive), memory (e.g., a Flash memory, read only memory (ROM), or random access memory (RAM)) or any other type of volatile or non-volatile memory that stores instructions (e.g., in the form of a computer program or other executable) to cause a programmable processor to perform the techniques described herein. In some examples, the computer-readable medium is an article of manufacture and is non-transitory. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example leadless pacing system that comprises a leadless pacing device and a sensing extension. 
         FIG. 2  is a schematic cross-sectional view of the sensing extension of  FIG. 1 . 
         FIG. 3  is a conceptual illustration of the leadless pacing system of  FIG. 1  implanted in a patient. 
         FIGS. 4A-4C  illustrate example shapes of a proximal end of a sensing extension. 
         FIG. 5  illustrates another example leadless pacing system that comprises a leadless pacing device and a sensing extension. 
         FIG. 6  is a schematic cross-sectional view of the sensing extension of  FIG. 5 . 
         FIG. 7  is a perspective view of another example sensing extension. 
         FIG. 8  is a cross-sectional perspective view of the sensing extension of  FIG. 7 . 
         FIG. 9  is an exploded perspective view of the sensing extension of  FIG. 7 . 
         FIG. 10  is a functional block diagram of an example leadless pacing system. 
         FIG. 11  is a flow diagram of an example technique for delivering therapy and sensing electrical cardiac activity with the leadless pacing system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     A leadless pacing system includes an LPD and a sensing extension that is coupled to the LPD and configured to facilitate sensing of electrical activity of a chamber of the heart other than the one in which the LPD is implanted. The sensing extension includes one or more electrodes and a self-supporting body that extends away from an outer housing of the LPD. In contrast to leaded pacing systems, the leadless pacing systems described herein do not include leads that pass out of the heart. Rather, both the LPD and sensing extension are configured to be entirely implanted in a heart of a patient. In some examples, the sensing extension is sized to be entirely implanted within the same chamber of the heart as the LPD. In other examples, the LPD is configured to be implanted in a first chamber of the heart, and the sensing extension is sized to extend into another chamber. 
     The LPD is configured to be implanted within a first chamber (e.g., a ventricle) of a heart of a patient, and the sensing extension is configured to position one or more electrodes proximate or within a second chamber of the heart, e.g., to sense electrical activity of the second chamber. The sensing extension has a length sufficient to locate one or more electrodes of the sensing extension closer to the second chamber than any electrodes of the LPD. For example, the sensing extension may have a length selected to position the one or more electrodes of the sensing extension adjacent the right atrium or in the right atrium when the LPD is implanted in or near the apex of the right ventricle. The one or more electrodes of the sensing extension may be used to sense intrinsic ventricular electrical activity, as well as detect atrial electrical activity. 
     In some examples described herein, the self-supporting body is configured to passively (i.e., without any active fixation elements, such as tines or a fixation helix) position an electrode extension at a location away from the LPD, e.g., at a location proximate the second chamber of the heart. The self-supporting body may be flexible enough to reduce irritation to the tissue of the heart when the body contacts the tissue, but have sufficient rigidity to permit the sensing extension to extend away from the LPD housing and towards the second chamber, even in the presence of blood in the first chamber of the heart. The stiffness of the self-supporting body is selected to help prevent the body from collapsing in on itself and/or towards the LPD, e.g., in the presence of blood flow. In addition, the stiffness of the self-supporting body may be selected so that the body is configured to support its own weight, e.g., in the presence of gravity. 
     The sensing extension also includes a proximal portion that is configured to help reduce interference with the mechanical movement of the heart. For example, in examples in which the LPD is configured to be implanted within a ventricle of the heart and the sensing extension is configured to extend towards an atrium, the proximal portion of the sensing extension may be shaped and sized to reduce interference with the opening and closing of an atrioventricular valve (e.g., the tricuspid valve or the mitral valve). In addition, the proximal end of the sensing extension is configured to be atraumatic (e.g., blunt) in order to reduce irritation to the heart tissue if the proximal end comes into contact with the heart tissue. As an example of a configuration of a proximal portion that may help reduce interference with the mechanical movement of the heart, the proximal portion may be curved with one or more bends. For example, the proximal portion may define an L-shaped curve, a C-shaped curve, a pigtail, or any other suitable curve. 
     In some examples, a sensing extension also includes a feature configured to facilitate control of the sensing extension during implantation of the sensing extension in the heart. In these examples, the sensing extension may or may not have a self-supporting body. In some examples, the feature includes an eyelet at a proximal end of the sensing extension. A tether may be fed through the eyelet prior to introducing the LPD and the sensing extension in a heart of a patient. During the implantation process, a clinician may pull back on the tether to help control the position of the proximal end of the sensing extension, to confirm that the LPD is adequately fixed to the target tissue site (e.g., a “tug test” that confirms the LPD does not move in response to a pull on the tether). After implantation, the tether may be removed from the eyelet. 
       FIG. 1  is a conceptual illustration of an example leadless pacing system  10  that includes LPD  12  and sensing extension  14 . LPD  12  is configured to be implanted within a chamber of a heart of a patient, e.g., to monitor electrical activity of the heart and/or provide electrical therapy to the heart. In the example shown in  FIG. 1 , LPD  12  includes outer housing  16 , a plurality of fixation tines  18 , and electrode  20 . Sensing extension  14  includes self-supporting body  22 , electrode  24 , and conductor  26 . 
     Outer housing  16  has a size and form factor that allows LPD  12  to be entirely implanted within a chamber of a heart of a patient. In some examples, outer housing  16  may have a cylindrical (e.g., pill-shaped) form factor. LPD  12  may include a fixation mechanism configured to fix LPD  12  to cardiac tissue. For example, in the example shown in  FIG. 1 , LPD  12  includes fixation tines  18  extending from housing  16  and configured to engage with cardiac tissue to substantially fix a position of housing  16  within the chamber of the heart. Fixation tines  18  are configured to anchor housing  16  to the cardiac tissue such that LPD  12  moves along with the cardiac tissue during cardiac contractions. Fixation tines  18  may be fabricated from any suitable material, such as a shape memory material (e.g., Nitinol). Although LPD  12  includes a plurality of fixation tines  18  that are configured to anchor LPD  12  to cardiac tissue in a chamber of a heart, in other examples, LPD  12  may be fixed to cardiac tissue using other types of fixation mechanisms, such as, but not limited to, barbs, coils, and the like. 
     Housing  16  houses electronic components of LPD  12 , e.g., a sensing module for sensing cardiac electrical activity via electrodes  20 ,  24 , and an electrical stimulation module for delivering electrical stimulation therapy via electrodes  20 ,  24 . Electronic components may include any discrete and/or integrated electronic circuit components that implement analog and/or digital circuits capable of producing the functions attributed to LPD  12  described herein. In some examples, housing  16  may also house components for sensing other physiological parameters, such as acceleration, pressure, sound, and/or impedance. 
     Additionally, housing  16  may also house a memory that includes instructions that, when executed by one or more processors housed within housing  16 , cause LPD  12  to perform various functions attributed to LPD  12  herein. In some examples, housing  16  may house a communication module that enables LPD  12  to communicate with other electronic devices, such as a medical device programmer. In some examples, housing  16  may house an antenna for wireless communication. Housing  16  may also house a power source, such as a battery. Housing  16  can be hermetically or near-hermetically sealed in order to help prevent fluid ingress into housing  16 . 
     LPD  12  is configured to sense electrical activity of the heart and deliver electrical stimulation to the heart via electrodes  20 ,  24 . LPD  12  comprises electrode  20  and sensing extension  14  comprises electrode  24 . For example, electrode  20  may be mechanically connected to housing  16 . As another example, electrode  20  may be defined by an outer portion of housing  16  that is electrically conductive. Fixation tines  18  may be configured to anchor LPD  12  to cardiac tissue such that electrode  20  maintains contact with the cardiac tissue. 
     Sensing extension  14  is configured to position electrode  24  proximate to or outside the chamber in which LPD  12  is implanted. For example, sensing extension  14  may be configured to position electrode  24  within a chamber other than the one in which LPD  12  resides. In this way, sensing extension  24  may extend the sensing capabilities of system  10 . In the example shown in  FIG. 1 , electrode  24  is carried by self-supporting body  22  of sensing extension  14 , and is located at a proximal end of body  22 . In other examples, however, electrode  24  may have another position relative to body  22 , such mid-way between housing  16  and the proximal end of body  22 , or otherwise away from the proximal end of body  22 . Electrode  24  may have any suitable configuration. For example, electrode  24  may have a ring-shaped configuration, or a partial-ring configuration. Electrode  24  may be formed from any suitable material, such as a titanium nitride coated metal. 
     In other examples, system  10  may include more than two electrodes. For example, LPD  12  and/or sensing extension  14  may have more than one electrode. As an example, one or more additional electrodes having the same polarity as electrode  24  may be carried by sensing extension  14 . The one or more additional electrodes may be electrically connected to the same or a different electrical conductor than sensing extension  14 . The additional electrodes of sensing extension  14  may increase the probability that an electrode of system  10  is positioned to sense electrical activity of a chamber of the heart other than the one in which LPD  12  is implanted. 
     In the example shown in  FIG. 1 , electrode  24  is electrically connected to at least some electronics of LPD  12  (e.g, a sensing module and a stimulation module) via electrical conductor  26  of sensing extension  14  and electrically conductive portion  16 A of housing  16 . Electrical conductor  26  is electrically connected to and extends between conductive portion  16 A of housing  16  and electrode  24 . Conductive portion  16 A is electrically isolated from electrode  20 , but is electrically connected to electrode  24 , such that conductive portion  16 A and electrode  24  have the same polarity and are electrically common. For example, electrode  20  may be carried by second portion  16 B of housing  16 , which is electrically isolated from conductive portion  16 A. Conductive portion  16 A of housing  16  is electrically connected to at least some electronics of LPD  12  (e.g., a sensing module, an electrical stimulation module, or both), such that conductive portion  16 A defines part of an electrically conductive pathway from electrode  24  to the electronics. In some examples, conductive portion  16 A may define at least a part of a power source case of LPD  12 . The power source case may house a power source (e.g., a battery) of LPD  12 . 
     In some examples, conductive portion  16 A is substantially completely electrically insulated (e.g., completely electrically insulated or nearly completely electrically insulated. Substantially completely electrically insulating conductive portion  16 A may help a sensing module of LPD  12  sense electrical cardiac activity with electrode  24  of sensing extension  14 . For example, in examples in which LPD  12  and sensing extension are implanted in a right ventricle, as shown and described with respect to  FIG. 3 , substantially completely electrically insulating conductive portion  16 A may help electrode  24  pick-up a stronger far field P-wave. In other examples, however, at least a part of conductive portion  16 A may be exposed to define one or more electrodes, which have the same polarity as electrode  24 . 
     As shown in  FIG. 2 , which is a schematic cross-sectional view of sensing extension  14  and a part of that conductive portion  16 A of housing  16 , in some examples, conductor  26  may be coiled around conductive portion  16 A to establish an electrical connection between conductor  26  and conductive portion  16 A. In other examples, however, an electrical connection between conductor  26  and conductive portion  16 A may be established using another configuration. For example, conductor  26  may not be coiled within sensing extension  14  and may be crimped or otherwise placed in contact with conductive portion  16 A near distal end  14 A of sensing extension  14 . 
       FIG. 2  also illustrates an example electrical connection between electrode  24  and conductor  26 . In particular,  FIG. 2  illustrates an example in which a proximal portion of conductor  26  is welded to a distal portion of electrode  24 , the distal portion including distal end  24 A. In other examples, electrode  24  and conductor  26  may be electrically connected using another configuration. As shown in  FIG. 2 , electrode  24  may be substantially closed at a proximal end in some examples, which may help prevent fluids from entering an inner portion (e.g., where conductor  26  is positioned) of sensing extension  14 . 
     In the example shown in  FIGS. 1 and 2 , self-supporting body  22  of sensing extension  14  extends between housing  16  and electrode  24 . Self-supporting body  22  has a stiffness that permits body  22  to substantially maintain (e.g., completely maintain or nearly maintain) its position relative to LPD  12 , or at least the position of electrode  24  relative to LPD  12 , even in the presence of gravity and in the presence of blood flow in the heart. For example, self-supporting body  22  may have a bending stiffness of about 0.8 e −6  N-m 2  to about 4.8 e −6  N-m 2  (about 0.8×10 −6  to about 4.8×10 −6  N-m 2 ), such as about 1.6 Newtons-square meter (N-m 2 ). In other examples, self-supporting bodies having other bending stiffness values may also be used. 
     Self-supporting body  22  is configured to passively position electrode  24  at a location away from LPD  12 , e.g., proximate or within a chamber of the heart other than the one in which LPD  12  is implanted. For example, self-supporting body  22  may have sufficient rigidity (e.g., stiffness) to permit sensing extension  14  to extend away from housing  16 , even as the sensing extension moves within blood in the chamber of the heart. In addition, self-supporting body  22  may be flexible enough to minimize irritation to the tissue of the heart, should body  22  contact the tissue. 
     In some examples, a bending stiffness of self-supporting body is substantially the same throughout the length of self-supporting body  22  (e.g., the same or nearly from a distal end to a proximal end of body  22 ). In other examples, self-supporting body  22  may have a variable stiffness along its length. For example, self-supporting body may decrease in stiffness from a distal end (closest to housing  16  LPD  12 ) to a proximal end, such that a distal portion of body  22  closest to housing  16  may have a higher stiffness than a proximal portion of body  22  closest to electrode  24  and including the proximal end. For example, the distal portion may be configured to have the highest stiffness and the proximal portion may be configured to have the lowest stiffness. A lower stiffness at the proximal portion of body  22  may help further minimize irritation to the tissue of the heart, should the proximal end of body  22  contact tissue, while the stiffer distal portion may permit body  22  to position electrode  24  at a location away from LPD  12 . 
     In the example shown in  FIGS. 1 and 2 , electrical conductor  26  is covered by an electrically conductive material, such as a polymer (e.g., polyurethane) or silicone. For example, conductor  26  may be housed within a polyurethane or silicone sleeve  28 , as shown in  FIGS. 1 and 2 . In some cases, coiled conductor  26  may not provide sufficient stiffness to sensing extension  14  to enable self-supporting body  22  to substantially maintain its position relative to LPD  12  in the presence of blood flow in the heart. Thus, in some examples, sensing extension  14  may also include a stiffness member  30 , which has a higher stiffness than coiled conductor  26  (when coiled). In the example shown in  FIGS. 1 and 2 , self-supporting body  22  of sensing extension  14  is defined by conductor  26 , sleeve  28 , and stiffness member  30 . 
     Stiffness member  30  has a stiffness that helps prevent self-supporting body  22  from collapsing in on itself and/or towards LPD  12 , e.g., in the presence of blood flow. For example, in examples in which conductor  26  is coiled and is enclosed in a polyurethane or silicone sleeve, stiffness member  30  may have a stiffness that results in self-supporting body  22  having a stiffness of about 0.8 e −6  N-m 2  to about 4.8 e −6  N-m 2  (about 0.8×10 −6  to about 4.8×10 −6  N-m 2 ). The stiffness, however, for stiffness member  30  that may be suitable for providing the desired stiffness characteristics to self-supporting body  22  may depend on various factors, such as length of self-supporting body  22  and the diameter (or other cross-sectional dimensions in examples in which self-supporting body  22  has a non-circular cross-sectional shape when the cross-section is taken substantially perpendicular to a longitudinal axis) of self-supporting body  22 . Stiffness member  30  may be more stiff as the length of self-supporting body  22  increases, and as the diameter of self-supporting body increases. A bigger diameter may cause the blood flow to push self-supporting body  22  around more within the heart. As with self-supporting body  22 , in some examples, stiffness member  30  may also have a variable stiffness along its length or may have substantially the same stiffness along its length. 
     Stiffness member  30  may be formed from any suitable material non-metallic or metallic material, such as a nickel-cobalt-chromium-molybdenum alloy (e.g., MP35N, such as a 7×7 MP35N cable). 
     In addition, stiffness member  30  may limit the amount sensing extension  14  stretches in response to a pulling force applied to the proximal end of sensing extension  14  (the end furthest from LPD  12 ) during a tug test performed to confirm that LPD  12  is secured to a target tissue site, e.g., that tines  18  are securely engaged with tissue of the heart of the patient. In some examples, such as examples in which conductor  26  is coiled, conductor  26  may stretch (e.g., elongate) in response to the pulling force. However, stiffness member  30  may be configured to stretch less than conductor  26  in some examples, and, as a result, when a clinician applies a pulling force to the proximal end of sensing extension  14  (the end furthest from LPD  12 ) during a tug test to confirm that LPD  12  is secured to a target tissue site, stiffness member  30  may limit the amount sensing extension  14  stretches in response to the pulling relative to examples in which sensing extension  14  does not include stiffness member  30 . 
     As shown in  FIGS. 1 and 2 , in some examples, stiffness member  30  extends through a center of coiled conductor  26  (e.g., conductor  26  may be coiled around member  30 ) and is coaxial with a longitudinal axis of sensing extension  14 . In other examples however, stiffness member  30  may have another position within sensing extension  14 . 
     In other examples, such as examples in which conductor  26  is not coiled, sensing extension  14  may not include stiffness member  30 . For example, the material of sleeve  28 , in combination with the conductor  26 , may provide body  22  with sufficient stiffness to permit body  22  to maintain its position relative to LPD  12 , even in the presence of gravity and in the presence of blood flow in the heart. 
     In some examples, in addition to, or instead of, electrically connecting electrode  24  to electronics of LPD  12  via electrical conductor  26 , stiffening member  30  may be electrically conductive and may electrically connect electrode  24  to electronics of LPD  12 . For example, a proximal portion of stiffening member  30  may be welded or otherwise electrically connected to a distal portion of electrode  24 . Thus, in some examples, sensing extension  14  does not include electrical conductor  26  and stiffening member  30  may both electrically connect electrode  24  to electronics of LPD  12  and increases the stiffness of sensing extension  14 , e.g., to help prevent self-supporting body  22  from collapsing in on itself and/or towards LPD  12 . Stiffness member  30  may have a higher stiffness than, for example, sleeve  28 . In examples in which both electrical conductor  26  and stiffening member  30  electrically connect electrode  24  to electronics of LPD  12 , sensing extension  14  may provide redundant electrical pathways for electrically connecting electrode  24  to electronics of LPD  12 . 
     In the example shown in  FIGS. 1 and 2 , system  10  includes retrieval member  31 , which is positioned at or near the distal end of sensing extension  14 , which is mechanically connected to outer housing  16  of LPD  12 . Retrieval member  31  can be, for example, a bump, protrusion, or any other suitable feature that can be used to grasp system  10 , e.g., when removing or implanting system  10  in a patient. For example, retrieval member  31  can be a bump configured to be grabbed by a snare. In some examples, retrieval member  31  is incorporated in a molded part used for insulating sensing extension  14 , or may be formed integrally with outer housing  16 . In other examples, retrieval member  31  can be separate from and attached to sensing extension  14 , outer housing  16 , or both. 
     As discussed above, sensing extension  14  is configured to position electrode  24  proximate to or within a chamber of a heart other than the one in which LPD  12  is implanted.  FIG. 3  illustrates system  10  implanted in right ventricle  32  of heart  34  of patient  36 . In the example shown in  FIG. 3 , sensing extension  14  is configured to extend away from LPD  12  and towards right atrium  38  when LPD  12  is implanted in an apex of right ventricle  32 . In some examples, sensing extension  14  may have a length that permits sensing extension  14  to remain in right ventricle  32  with LPD  12 , as shown in  FIG. 3 . For example, sensing extension  14  may have a length of about 40 millimeters (mm) to about 150 mm, such as about 60 millimeters (as measured from the distal end connected to LPD  12  and a proximal end of electrode  24 ). A single chamber system  10  may provide the advantages of sensing electrical activity of two chambers (e.g., right ventricle  32  and right atrium  38  in the example shown in  FIG. 3 ) without the burden of placing extension  14  in right atrium  38 . 
     In examples in which extension  14  remains in the same chamber as LPD  12 , a proximal portion of sensing extension  14  may be configured to help reduce interference with the mechanical movement of the heart, such as, in the example shown in  FIG. 3 , movement of the tricuspid valve. For example, electrode  24  at the proximal end of extension  14  may define an L-shaped curve, a C-shaped curve, a pigtail, or any other suitable curve, as shown with respect to electrodes  25 A,  25 B, and  25 C in  FIGS. 4A, 4B, and 4C , respectively. The L-shaped curve is also shown in  FIG. 3 . 
     The L-shaped curve, the C-shaped curve, and the pigtail shaped curve shown in  FIGS. 4A, 4B, and 4C  may define curved or relatively flat surfaces (e.g., surfaces  27 A- 27 C) against which the tricuspid valve (or other valve in the case of other implantation sites for LPD  12 ) may still substantially close, which may help prevent blood from back flowing into another chamber of heart  34 , e.g., right atrium  38 . In some examples, the shape of proximal portion of sensing extension  14  may be selected based on the implant location for system  10 . Different shapes may help reduce interference with different valves and different implantation sites for LPD  12  and sensing extension  14 . 
     In other examples, a portion of sensing extension  14  in addition to, or other than, electrode  24  may define the shapes shown in  FIGS. 4A-4C . For example, sleeve  28  and stiffness member  30  may be configured to define the proximal portion shapes shown in  FIGS. 4A-4C  and electrode  24  may be positioned on an outer surface of sleeve  28 . 
     In other examples, sensing extension  14  may have a length that enables at least electrode  24  to extend into right atrium  38  when LPD  12  is implanted in an apex of right ventricle  32 . In examples in which sensing extension  14  extends into right atrium  38 , sensing extension  14  may be relatively small and flexible enough to permit the tricuspid valve to sufficiently close around the sensing extension  14  to prevent backflow into right atrium  38  from right ventricle  32 . For example, sensing extension  14  may be about 4 French (i.e., about 1.33 millimeters in diameter. 
     LPD  12  may sense electrical activity of right atrium  38  or right ventricle  32  with electrodes  20 ,  24 . As shown in  FIG. 3  sensing extension  14  is passive and extends away from LPD  12 , which enables electrode  24  to be positioned relatively close to right atrium  38 . The distance between electrode  24  and right atrium  38  may be less than the distance between electrode  20  of LPD  12  and right atrium  38 . As a result, electrode  24  may be positioned to pick up higher amplitude P-waves than electrode  20 . In this way, sensing extension  14  may facilitate atrial sensing when LPD  12  is implanted in right ventricle  32 . 
     Rather than being affixed to cardiac tissue such that electrode  24  is in direct contact with heart  34 , a proximal portion of sensing extension  14  is passive, such that sensing extension  14  may move within right ventricle  32 . However, due at least in part to the self-supporting configuration of body  22  ( FIGS. 1 and 2 ), sensing extension  14  is configured to continue to extend away from LPD  12  and towards right atrium  38 , even in the presence of blood flow from right atrium  38  to right ventricle  32 . Providing self-supporting member  22  of sensing extension  14  with some flexibility may enable sensing extension  14  to minimize interference with blood flow in right ventricle  32  (or another chamber if LPD  12  is implanted in another chamber). 
     Also shown in  FIG. 3  is medical device programmer  40 , which is configured to program LPD  12  and retrieve data from LPD  12 . Programmer  40  may be a handheld computing device, desktop computing device, a networked computing device, etc. Programmer  40  may include a computer-readable storage medium having instructions that cause a processor of programmer  40  to provide the functions attributed to programmer  40  in the present disclosure. LPD  12  may wirelessly communicate with programmer  40 . For example, LPD  12  may transfer data to programmer  40  and may receive data from programmer  40 . Programmer  40  may also wirelessly program and/or wirelessly charge LPD  12 . 
     Data retrieved from LPD  12  using programmer  40  may include cardiac EGMs stored by LPD  12  that indicate electrical activity of heart  34  and marker channel data that indicates the occurrence and timing of sensing, diagnosis, and therapy events associated with LPD  12 . Data transferred to LPD  12  using programmer  40  may include, for example, operational programs for LPD  12  that causes LPD  12  to operate as described herein. 
     Leadless pacing system  10  may be implanted in right ventricle  32 , or another chamber of heart  34 , using any suitable technique. In some cases, sensing extension  14  may include a feature that helps control a position of proximal end of sensing extension  14  during implantation of system  10  in patient  36 . The feature may also be used to facilitate relatively easy capture of a proximal end of sensing extension  14  by a retrieval device, e.g., during explanation of system  10  from patient  36 .  FIGS. 5 and 6  illustrate an example of such a feature. 
       FIGS. 5 and 6  illustrate an example leadless pacing system  50 , which is similar to system  10  of  FIG. 1 , but further includes eyelet  52  at a proximal end of sensing extension  14 . In other examples of the system  50 , however, the sensing extension may be any suitable extension, e.g., may not include a self-supporting body, as described above with respect to  FIGS. 1 and 2 , may include one or more additional electrodes, which may be used for sensing or electrical stimulation, or any combination thereof. 
     Eyelet  52  defines an opening  54  configured to receive, e.g., a tether or another tool used during implantation, during explanation, or both implantation and explanation. A tether may, for example, a suture thread or another material that is relatively thin and flexible, compared to sensing extension  14 . The tether may be looped through opening  54  prior to inserting system  10  in right ventricle  32 , and, after sensing extension  14  is implanted in heart  34  ( FIG. 3 ), a clinician may pull back on the tether in order to pull back on proximal end  14 B of sensing extension  14 , to move proximal end  14 B of sensing extension  14 , or to otherwise control the position of proximal end  14 B. In addition, eyelet  52  may be configured to facilitate capture of system  10  by a retrieval device, e.g. during explanation of system  10  from the patient or to move LPD  12  to another location after tines  18  have fixed to a particular location. 
     Although shown to have a circular cross-section in  FIGS. 5 and 6 , eyelet  52  may have any suitable cross-sectional shape configured to receive a tether or other tool. In addition, although shown in  FIGS. 5 and 6  to define an opening that has a center axis  53  that is transverse and substantially orthogonal (e.g., orthogonal or nearly orthogonal) to longitudinal axis  15  of sensing extension  14 , in other examples, center axis  53  may have another orientation relative to longitudinal axis  15 . For example, the opening defined by eyelet  52  may be oriented such that center axis  53  is substantially parallel (e.g., parallel or nearly parallel) or oriented at an angle less than 90 degrees relative to longitudinal axis  15 . Thus, in some examples, the opening defined by eyelet  52  may be oriented such that center axis  53  is 90 degrees or less relative to longitudinal axis  15 . 
     In addition, in some examples, center axis  53  may not be aligned with longitudinal axis  53 , but, rather, eyelet  52  may extend away from a side surface of extension  14 . In  FIGS. 5 and 6 , center axis  53  is aligned with longitudinal axis  53 . However, if, for example, sensing extension  14  defines a curved proximal portion (e.g., as shown in  FIGS. 4A-4C ), center axis  53  may not be aligned with longitudinal axis  53 . 
     Eyelet  52  may be mechanically connected to sensing extension  14  using any suitable technique. In the example shown in  FIGS. 5 and 6 , eyelet  52  includes base portion  56  that is received in cavity  58  defined by electrode  24 . Electrode  24  and base portion  56  may be attached using any suitable technique, such as by crimping electrode  24  around base portion  56 , via an adhesive, welding, or another suitable technique. The attachment between sensing extension  14  and eyelet  52  is strong enough to maintain the mechanical connection between eyelet  52  and sensing extension  14 , even in the presence of forces (e.g., from a tether or other retrieval tool) pulling the eyelet  52  in a direction away from sensing extension  14 . Likewise, the attachment between LPD  12  and sensing extension  14  is strong enough to maintain the mechanical connection between LPD  12  and sensing extension  14 , even in the presence of forces pulling the sensing extension  14  and LPD  12  away from each other. 
     In some examples, end  58 A of cavity  58  may be closed (i.e., cavity  58  may be a blind hole), which may help prevent environmental contaminants from being introduced into the portion of sensing extension  14  including conductor  26 . 
     Eyelet  52  may be formed from any suitable material. In some examples, eyelet  52  is formed from an electrically nonconductive material. In other examples, eyelet  52  is formed from an electrically conductive material. In some examples in which eyelet  52  is formed from an electrically conductive material, eyelet is configured to function as an extension of electrode  24 . Thus, LPD  12  may sense electrical cardiac signals and deliver electrical stimulation with the aid of eyelet  52 . Eyelet  52  may be electrically connected to electrode  24  by virtue of being in contact with electrode  24 . In other examples in which eyelet  52  is formed from an electrically conductive material, the conductivity of eyelet  52  may be relatively low when compared to the conductive of electrode  24  for eyelet  52  to function as an extension of electrode  24 . For example, eyelet  52  may be formed from stainless steel. In addition, eyelet  52  is configured to not be in contact with cardiac tissue when system  10  is implanted in a patient, e.g., sensing extension is configured to position eyelet not in contact with cardiac tissue, such that eyelet  52  may not function as a stimulation electrode. 
     Base portion  56  of eyelet  52  is shown in  FIGS. 5 and 6  as being coaxial with a longitudinal axis of sensing extension  14 , in some examples, base portion  56  may have another arrangement relative to the longitudinal axis of sensing extension  14 . For example, in examples in which a proximal portion of extension  14  defines a curve (e.g., as shown in  FIGS. 4A-4C ), when eyelet  52  is positioned at a proximal end of sensing extension  14 , base portion  56  may curve with the proximal portion. As another example, base portion  56  may be curvilinear or otherwise nonlinear (e.g., may define a 90 degree angle) and attached to sensing extension  14  such that base portion  56  extends away from electrode  24 . Other configurations of eyelet  52  may also be used. 
     Eyelet  52  provides a feature for controlling a positioning of extension  52 , as well as a feature that facilitates retrieval of system  50  from an implant site. These features may be useful with other type of extensions that are connected to electronics of LPD  12  (e.g., a stimulation module, a sensing module, or both). Thus, in some examples, system  50  may include an extension having a configuration different than sensing extension  14 , the extension including eyelet  52  at a proximal end. For example, in  FIGS. 5 and 6 , rather than being connected to sensing extension  14  including electrode  24  electrically connected to conductive portion  16 A of housing  16  of LPD  12 , LPD  12  may be mechanically connected to an extension that includes multiple electrodes electrically connected to conductive portion  16 A of housing  16 , and the extension may extend away from housing  16  of LPD  12  and include eyelet  52  at a proximal end (similar to the position shown in  FIG. 5 ). 
     As another example, LPD  12  may be mechanically connected to an extension that includes one or more electrodes that are not electrically connected to conductive portion  16 A of housing  16 , but, rather, connected to electronics (e.g., a sensing module and a stimulation module) of LPD  12  using another conductive path, such as a conductive feedthrough that extends through housing  16 ; in this example, eyelet  52  may be positioned at a proximal end of the extension, which may also extend away from housing  16 . As yet another example, LPD  12  may be mechanically connected to an extension that is not self-supporting and/or includes one or more fixation elements. In these examples, eyelet  52  may be positioned at a proximal end of the extension. Other configurations of extensions including eyelet  52  are also contemplated. 
     In other examples of system  50 , sense electrode  24  and eyelet  52  may be integrated into a common, integral component.  FIGS. 7-9  illustrate an example of such a sensing extension.  FIG. 7  is a perspective view of example sensing extension  60 , which may be similar to sensing extension  14  of  FIGS. 5 and 6 , but includes sense electrode  62  defining electrode portion  64  and eyelet portion  66  instead of sense electrode  24  and eyelet  52 .  FIG. 8  is a cross-sectional perspective view of sensing extension  60  and illustrates self-supporting body  22 , electrical conductor  26 , stiffness member  30 , and sense electrode  62 .  FIG. 9  is an exploded perspective view of sensing extension  60 . 
     As shown in  FIGS. 7-9 , electrode portion  64  and eyelet portion  66  are continuous and are portions of a common body of sense electrode  62 , rather than being separate components that are attached together. In contrast, sense electrode  24  and eyelet  52  shown in  FIGS. 5 and 6  are separate components. Eyelet portion  66  is configured similarly to eyelet  52  and defines an opening  68  configured to receive, e.g., a tether or another tool used during implantation, during explanation, or both implantation and explanation. 
     Electrode  62  including integral electrode portion  64  and eyelet portion  66  may minimize the number of openings through which a fluid may enter an inner portion (e.g., where conductor  26  is positioned) of sensing extension  60 . 
     Electrode  62  may be formed using any suitable technique. In some examples, electrode  62  may be produced with a cold-heading operation that defines a metal or other suitable electrically conductive material into the shape of electrode  62 . In some examples, after forming the shape of electrode  62 , eyelet portion  66  may be polished. All or only a part of electrode  62  may be electrically conductive. For example, in some examples, both electrode portion  64  and eyelet portion  66  are electrically conductive, though they may have different impedances, while in other examples, eyelet portion  66  is not electrically conductive and electrode portion  64  is electrically conductive. In some examples, to form electrode  62  including eyelet portion  66  that is not electrically conductive, eyelet portion  66  may be masked during the coating of electrode portion  62  with an electrically conductive material, such as titanium nitride (TiN). 
     As with electrode  24 , sense electrode  62  may be electrically connected to electrical conductor  26 , stiffness member  30 , or both stiffness member  30  and electrical conductor  26  using any suitable technique, such as the ones described above with respect to electrode  24 . For example, a proximal portion of conductor  26  or stiffness member  30  may be welded or crimped to a distal portion of electrode  62 . 
     Electrode  62  may define a distal portion  62 A that is configured to be received in self-supporting body  22 . In addition, in some examples, as shown in  FIG. 9 , distal portion  62 A may define an opening configured to receive stiffness member  30 , such that stiffness member  30  and electrode  60  are partially co-extensive, e.g., overlap in a longitudinal direction. In other examples, however, stiffness member  30  and electrode  60  may not be co-extensive. Electrode  60  may, for example, provide sufficient stiffness to the proximal end of sensing extension  60  without stiffness member  30 . 
     Electrode  62  may be mechanically connected to self-supporting body  22  using any suitable technique, such as by a friction fit achieved when distal portion  62 A of electrode  62  is received in proximal end  22 B of self-supporting body  22 , by ultrasonic welding, by an adhesive, or any other suitable technique or combinations of techniques. The mechanical connection may define a relatively fluid tight seal between electrode  62  and self-supporting body  22  to help prevent the ingress of fluids into self-supporting body  22 . 
     In each of the examples described herein, stiffness member  30  may comprise one or more elements. For example, in the example shown in  FIG. 9 , stiffness member  30  includes three members that are substantially co-axial. Using two or more elements to form stiffness member  30  may provide design freedom for achieving the desired stiffness of stiffness member  30  than, for example, one element. 
       FIG. 10  is a functional block diagram of an example LPD  12 . LPD  12  includes a processing module  70 , memory  72 , stimulation module  74 , electrical sensing module  76 , communication module  78 , sensor  80 , and power source  82 . Power source  82  may include a battery, e.g., a rechargeable or non-rechargeable battery. 
     Modules included in LPD  12  represent functionality that may be included in LPD  12  of the present disclosure. Modules of the present disclosure may include any discrete and/or integrated electronic circuit components that implement analog and/or digital circuits capable of producing the functions attributed to the modules herein. For example, the modules may include analog circuits, e.g., amplification circuits, filtering circuits, and/or other signal conditioning circuits. The modules may also include digital circuits, e.g., combinational or sequential logic circuits, memory devices, and the like. The functions attributed to the modules herein may be embodied as one or more processors, hardware, firmware, software, or any combination thereof. Depiction of different features as modules is intended to highlight different functional aspects, and does not necessarily imply that such modules must be realized by separate hardware or software components. Rather, functionality associated with one or more modules may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. 
     Processing module  70  may include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or integrated logic circuitry. In some examples, processing module  70  may include multiple components, such as any combination of one or more microprocessors, one or more controllers, one or more DSPs, one or more ASICs, or one or more FPGAs, as well as other discrete or integrated logic circuitry. 
     Processing module  70  may communicate with memory  72 . Memory  72  may include computer-readable instructions that, when executed by processing module  70 , cause processing module  70  to perform the various functions attributed to processing module  70  herein. Memory  72  may include any volatile, non-volatile, magnetic, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), Flash memory, or any other memory device. Furthermore, memory  72  may include instructions that, when executed by one or more processors, cause the modules to perform various functions attributed to the modules herein. For example, memory  72  may include pacing instructions and values. The pacing instructions and values may be updated by programmer  40  ( FIG. 3 ). 
     Stimulation module  74  and electrical sensing module  76  are electrically coupled to electrodes  20 ,  24 . Processing module  70  is configured to control stimulation module  74  to generate and deliver electrical stimulation to heart  34  (e.g., right ventricle  32  in the example shown in  FIG. 3 ) via electrodes  20 ,  24 . Electrical stimulation may include, for example, pacing pulses, or any other suitable electrical stimulation. Processing module  70  may control stimulation module  74  to deliver electrical stimulation therapy via electrodes  20 ,  24  according to one or more therapy programs including pacing instructions that define a ventricular pacing rate, which may be stored in memory  72 . 
     In addition, processing module  70  is configured to control electrical sensing module  76  monitor signals from electrodes  20 ,  24  in order to monitor electrical activity of heart  34 . Electrical sensing module  76  may include circuits that acquire electrical signals. Electrical signals acquired by electrical sensing module  76  may include intrinsic cardiac electrical activity, such as intrinsic atrial depolarization and/or intrinsic ventricular depolarization. Electrical sensing module  76  may filter, amplify, and digitize the acquired electrical signals to generate raw digital data. Processing module  70  may receive the digitized data generated by electrical sensing module  76 . In some examples, processing module  70  may perform various digital signal processing operations on the raw data, such as digital filtering. 
     Processing module  70  may sense cardiac events based on the data received from electrical sensing module  76 . For example, processing module  70  may sense atrial electrical activity based on the data received from electrical sensing module  76 . For example, in examples in which LPD  12  and sensing extension  14  are implanted in right ventricle  32 , processing module  70  may detect far field P-waves indicative of atrial activation events based on the data received from electrical sensing module  76 . In some examples, processing module  70  may also sense ventricular electrical activity based on the data received from electrical sensing module  76 . For example, processing module  70  may detect R-waves indicative of ventricular activation events based on the data received from electrical sensing module  76 . In examples in which processor  70  uses both electrodes  20  and  24  for both R-wave and P-wave sensing, processor  70  may detect the R-waves and P-waves from the same sensed signal, and the sensing vector can be between electrodes  20 ,  24 . 
     In some examples, in addition to electrical sensing module  76 , LPD  12  includes sensor  80 , which may comprise at least one of a variety of different sensors. For example, sensor  80  may comprise at least one of a pressure sensor and an accelerometer. Sensor  80  may generate signals that indicate at least one of parameter of patient  12 , such as, but not limited to, at least one of: an activity level of patient  36 , a hemodynamic pressure, and heart sounds. 
     Communication module  78  may include any suitable hardware (e.g., an antenna), firmware, software, or any combination thereof for communicating with another device, such as programmer  40  ( FIG. 3 ) or a patient monitor. Under the control of processing module  70 , communication module  78  may receive downlink telemetry from and send uplink telemetry to other devices, such as programmer  40  or a patient monitor, with the aid of an antenna included in communication module  78 . 
       FIG. 11  is a flow diagram of an example technique performed by leadless pacing system  10 . While  FIG. 11  is described as primarily being performed by processing module  70  of LPD  12 , in other examples, another processor (e.g., a processor of programmer  40 ), alone or with the aid of processing module  70 , may perform any part of the technique shown in  FIG. 11 . In addition, while the technique is described with reference to an example in which LPD  12  is implanted in right ventricle  32  ( FIG. 3 ), the technique shown in  FIG. 11  may also be used with other examples. 
     In accordance with the example shown in  FIG. 11 , processing module  70  controls stimulation module  74  to generate and deliver pacing pulses to right ventricle  32  via electrodes  20 ,  24  ( 90 ). For example, electrode  20  may be selected as a source electrode and electrode  24  may be selected as the sink electrode. Processing module  70  also controls electrical sensing module  76  ( FIG. 10 ) to sense electrical cardiac activity with electrodes  20 ,  24  ( 92 ). The electrical cardiac activity can be, for example, any combination of the following: intrinsic ventricular depolarization, intrinsic atrial depolarization, other ventricular activation events (e.g., paced events), or other atrial activation events (e.g., paced events). Processing module  70  may receive sensed electrical cardiac signals from sensing module  76  and detect atrial depolarization by at least detecting a far field P-wave. In some examples, processing module  70  controls electrical sensing module  76  ( FIG. 10 ) to sense electrical cardiac activity with electrodes  20 ,  24  during a refractory period of heart  34 . 
     The techniques described in this disclosure, including those attributed to image IMD  16 , programmer  24 , or various constituent components, may be implemented, at least in part, in hardware, software, firmware or any combination thereof. For example, various aspects of the techniques may be implemented within one or more processors, including one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components, embodied in programmers, such as physician or patient programmers, stimulators, image processing devices or other devices. The term “processor” or “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry. 
     Such hardware, software, firmware may be implemented within the same device or within separate devices to support the various operations and functions described in this disclosure. In addition, any of the described units, modules or components may be implemented together or separately as discrete but interoperable logic devices. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. 
     When implemented in software, the functionality ascribed to the systems, devices and techniques described in this disclosure may be embodied as instructions on a computer-readable medium such as RAM, ROM, NVRAM, EEPROM, FLASH memory, magnetic data storage media, optical data storage media, or the like. The instructions may be executed to support one or more aspects of the functionality described in this disclosure. 
     Various examples have been described. These and other examples are within the scope of the following claims.