Patent Publication Number: US-2021178169-A1

Title: Catheter-based delivery system for delivering a leadless pacemaker and employing a locking hub

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional under 35 U.S.C. § 120 of U.S. patent application Ser. No. 15/942,105, filed Mar. 30, 2018, titled “Catheter-Based Delivery System For Delivering A Leadless Pacemaker And Employing A Locking Hub,” and claims priority under 35 U.S.C. § 119(e) from U.S. Patent Application No. 62/480,087, filed Mar. 31, 2017, titled “Catheter-Based Delivery System For Delivering A Leadless Pacemaker And Employing A Locking Hub,” U.S. Patent Application No. 62/503,888, filed May 9, 2017, titled “Catheter-Based Delivery System For Delivering A Leadless Pacemaker And Employing A Locking Hub” and U.S. Patent Application No. 62/636,063, filed Feb. 27, 2018, titled “Catheter-Based Delivery System For Delivering A Leadless Pacemaker And Employing A Locking Hub,” and the entire contents of those applications are incorporated herein by reference for all purposes. 
    
    
     INCORPORATION BY REFERENCE 
     All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 
     FIELD 
     The present disclosure relates to leadless cardiac pacemakers and related delivery systems and methods. More specifically, the present disclosure relates to devices and methods for delivering a leadless cardiac pacemaker via a catheter-based delivery system. 
     BACKGROUND 
     Cardiac pacing by an artificial pacemaker provides an electrical stimulation of the heart when its own natural pacemaker and/or conduction system fails to provide synchronized atrial and ventricular contractions at rates and intervals sufficient for a patient&#39;s health. Such antibradycardial pacing provides relief from symptoms and even life support for hundreds of thousands of patients. Cardiac pacing may also provide electrical overdrive stimulation to suppress or convert tachyarrhythmias, again supplying relief from symptoms and preventing or terminating arrhythmias that could lead to sudden cardiac death. 
     Cardiac pacing by currently available or conventional pacemakers is usually performed by a pulse generator implanted subcutaneously or sub-muscularly in or near a patient&#39;s pectoral region. Pulse generator parameters are usually interrogated and modified by a programming device outside the body, via a loosely-coupled transformer with one inductor within the body and another outside, or via electromagnetic radiation with one antenna within the body and another outside. The generator usually connects to the proximal end of one or more implanted leads, the distal end of which contains one or more electrodes for positioning adjacent to the inside or outside wall of a cardiac chamber. The leads have an insulated electrical conductor or conductors for connecting the pulse generator to electrodes in the heart. Such electrode leads typically have lengths of 50 to 70 centimeters. 
     Although more than one hundred thousand conventional cardiac pacing systems are implanted annually, various well-known difficulties exist. For example, a pulse generator, when located subcutaneously, presents a bulge in the skin that patients can find unsightly, unpleasant, or irritating, and which patients can subconsciously or obsessively manipulate or “twiddle”. Even without persistent manipulation, subcutaneous pulse generators can exhibit erosion, extrusion, infection, disconnection, insulation damage, or conductor breakage at the wire leads. Although sub-muscular or abdominal placement can address some concerns, such placement involves a more difficult surgical procedure for implantation and adjustment, which can prolong patient recovery. 
     A conventional pulse generator, whether pectoral or abdominal, has an interface for connection to and disconnection from the electrode leads that carry signals to and from the heart. Usually at least one male connector molding has at least one terminal pin at the proximal end of the electrode lead. The male connector mates with a corresponding female connector molding and terminal block within the connector molding at the pulse generator. Usually a setscrew is threaded in at least one terminal block per electrode lead to secure the connection electrically and mechanically. One or more O-rings usually are also supplied to help maintain electrical isolation between the connector moldings. A setscrew cap or slotted cover is typically included to provide electrical insulation of the setscrew. This briefly described complex connection between connectors and leads provides multiple opportunities for malfunction. 
     Other problematic aspects of conventional pacemakers relate to the separately implanted pulse generator and the pacing leads. By way of another example, the pacing leads, in particular, can become a site of infection and morbidity. Many of the issues associated with conventional pacemakers are resolved by the development of a self-contained and self-sustainable pacemaker, or so-called leadless pacemaker, as described in the applications cited below. 
     Similar to active fixation implantable leads used with conventional pulse generators, leadless pacemakers are typically fixed to an intracardial implant site by an actively engaging mechanism such as a screw or helical member that screws into the myocardium. 
     Leadless pacemakers are typically delivered to an intracardial implant site via a delivery system including catheters, sheaths and/or introducers. It is a complicated and delicate task to introduce a leadless pacemaker into the venous system and then navigate the leadless pacemaker through and past delicate tissues and anatomical structures to the implantation site. To achieve this task, the sheaths, catheters and introducers are often manipulated relative to each other, and such manipulation needs to be precise. There is a need in the art for systems and methods that facilitate this precision. 
     SUMMARY OF THE DISCLOSURE 
     Disclosed herein is a delivery system for delivering a leadless pacemaker. In one embodiment the delivery system includes a catheter including a distal end, a proximal end opposite the distal end, a lumen extending between the proximal end and the distal end and shaped to receive an elongate body, and a locking hub operably coupled to the proximal end. The locking hub includes a lumen segment of the lumen. The locking hub biases the lumen segment such that the lumen segment provides a first resistance to movement of the elongate body within the lumen when the elongate body is received by the catheter. Actuating the locking hub causes the lumen segment to provide a second resistance to movement of the elongate body within the lumen when the elongate body is received by the catheter, the second resistance being less than the first resistance. 
     In one implementation, the locking hub includes one or more buttons and actuating the locking hub includes depressing the one or more buttons. The one or more buttons are disposed about a longitudinal axis of the catheter such that depressing the one or more buttons causes the one or more buttons to displace toward the longitudinal axis. 
     In another implementation, the locking hub includes a biasing element for providing the first resistance. The biasing element may include one or more of a helical spring, a leaf spring, a biasing arm, or a resilient elastomeric member. 
     In yet another implementation, the locking hub biases the lumen segment by reducing a diameter of the lumen segment to a first diameter. In such implementations, actuating the locking hub may change the diameter of the lumen segment to a second diameter, the second diameter being greater than the first diameter. 
     In another implementation, the locking hub biases the lumen segment to be out of alignment with a rest of the lumen. In one such implementation, actuating the locking hub places the lumen segment in coaxial alignment with the rest of the lumen. 
     In another embodiment, a delivery system for delivering a leadless pacemaker is provided. The delivery system includes a catheter including a distal end, a proximal end opposite the distal end, a lumen extending between the proximal end and the distal end, and a locking hub operably coupled to the proximal end. The locking hub includes a movable member that further includes a lumen segment of the lumen. The movable member is biased in a first direction transverse relative to a longitudinal axis of the lumen such that the lumen segment is biased out of alignment with a rest of the lumen. Actuating the locking hub translates the movable member in a second transverse direction opposite the first transverse direction to coaxially align the lumen segment with the rest of the lumen. 
     In one implementation, wherein the locking hub includes one or more buttons and actuating the locking hub includes depressing the one or more buttons. In one such implementation, depressing the one or more buttons causes translation of the one or more buttons in a transverse direction relative to the longitudinal axis of the lumen. 
     In another implementation, the locking hub comprises a biasing element configured to bias the movable member in the first direction. Such a biasing element may include, without limitation, one or more of a helical spring, a leaf spring, a biasing arm, or a resilient elastomeric member. 
     In yet another embodiment of the present disclosure, a delivery system of delivering a leadless pacemaker is provided. The delivery system includes a catheter including a distal end, a proximal end opposite the distal end, a lumen extending between the proximal end and the distal end, and a locking hub operably coupled to the proximal end. The locking hub includes an annular member that in turn includes a lumen segment of the lumen. 
     In one implementation, the locking hub includes one or more buttons and actuating the locking hub includes depressing the one or more buttons. In certain implementations depressing the one or more buttons causes translation of the one or more buttons in a transverse direction relative to a longitudinal axis of the catheter. 
     In another implementation, the annular member is compressible such that the diameter of the lumen segment varies in response to a compressive force applied to the annular member. In certain implementations the locking hub further includes a movable member adjacent the annular member, the locking hub biased to reduce the diameter of the lumen segment by biasing the movable member in a first direction to compress the annular member. In such implementations, actuating the locking hub may include translating the movable member in a second direction opposite the first direction, thereby reducing compression of the annular member by the movable member. The locking hub may include a biasing element for biasing the movable member in the first direction, the biasing element including one or more of a helical spring, a leaf spring, a biasing arm, or a resilient elastomeric member. 
     In another embodiment, a delivery system for delivering a leadless pacemaker is provided. The delivery system includes a catheter, which may be a guide catheter. The catheter includes a distal end, a proximal end opposite the distal end, a lumen extending between the distal end and the proximal end, and a locking hub operably coupled to the proximal end. The locking hub includes a lumen segment of the lumen. Self-biasing of the lumen segment places the lumen segment out of alignment with a rest of the lumen. Deflecting the lumen segment against the self-biasing of the lumen segment places the lumen segment in coaxial alignment with the rest of the lumen. 
     In one implementation, a portion of an inner circumferential surface of the lumen segment is made of a first material, a second portion of the inner circumferential surface of the lumen segment is made of a second material that has a higher coefficient of friction than the first material, and the second material is compressed against an elongated body extending through the lumen when the lumen segment self-biases out of alignment with the rest of the lumen. The first portion may be semi-cylindrical and the second portion may be semi-cylindrical. The elongated body may include a shaft of a deflectable catheter, the shaft including a distal end and proximal end opposite the distal end. The distal end of the shaft is configured to detachably couple to the leadless pacemaker. The shaft is configured to extend through the lumen of the guide catheter. 
     In one implementation, a portion of an inner circumferential surface of the lumen segment is made of a first material, a second portion of the inner circumferential surface of the lumen segment is made of a second material that is softer than the first material, and the second material is compressed against an elongated body extending through the lumen when the lumen segment self-biases out of alignment with the rest of the lumen. The first portion may be semi-cylindrical and the second portion may be semi-cylindrical. The elongated body may include a shaft of a deflectable catheter, the shaft including a distal end and proximal end opposite the distal end. The distal end of the shaft is configured to detachably couple to the leadless pacemaker. The shaft is configured to extend through the lumen of the guide catheter. 
     In one implementation, the locking hub further includes a button through which the lumen segment extends, a body supporting the button, and a biasing mechanism acting between the button and the body to self-bias the button such that the lumen segment is out of alignment with the rest of the lumen. The biasing mechanism may include at least one of a helical spring, a leaf spring, a biasing arm extending from the button and acting against the body, a biasing arm extending from the body and acting against the button, or a resilient elastomeric member. 
     In one implementation, the button and body both include respective stop limit structures that abut when the button is forced against the self-biasing mechanism to the extent that the lumen segment is in coaxial alignment with the rest of the lumen. 
     In one implementation, a portion of an inner circumferential surface of the lumen segment is made of a first material forming the button. A second portion of the inner circumferential surface of the lumen segment is made of a second material different from the first material and at least one of injected, inserted or molded into a void defined in the first material. The second material is compressed against an elongated body extending through the lumen when the lumen segment self-biases out of alignment with the rest of the lumen. 
     Also disclosed herein is a delivery system for delivering a leadless pacemaker. In one embodiment, the delivery system includes a catheter. The catheter includes a distal end, a proximal end opposite the distal end, a lumen extending between the distal end and the proximal end, and a locking hub operably coupled to the proximal end, the locking hub including a lumen segment of the lumen. The lumen segment includes a first portion of an inner circumferential surface of the lumen segment made of a first material, and a second portion of an inner circumferential surface of the lumen segment made of a second material different from the first material. The lumen segment is displaceable between a first state and a second state, the first state being where the lumen segment is out of alignment with a rest of the lumen and the second state being where the lumen segment is in coaxial alignment with the rest of the lumen. 
     In one implementation, the lumen segment is biased in the first state and needs to be forced into the second state. The first material compresses against a tubular body extending through the lumen when in the first state. The first material may be softer than the second material. The first material may have a higher coefficient of friction than the second material. 
     In another implementation, the locking hub further includes a button through which the lumen segment extends, a body supporting the button, and a biasing mechanism acting between the button and the body to bias the button into the first state. The biasing mechanism may include at least one of a helical spring, a leaf spring, a biasing arm extending from the button and acting against the body, a biasing arm extending from the body and acting against the button, or a resilient elastomeric member. The button and body may both include respective stop limit structures that abut when the button is forced against the biasing mechanism to the extent that the lumen segment is in coaxial alignment with the rest of the lumen. 
     Depending on the embodiment, the first material may be injected, inserted or molded into a void defined in the second material. 
     In yet another embodiment of the present disclosure a delivery system for delivering a leadless pacemaker is provided. The delivery system includes a catheter including a distal end, a proximal end opposite the distal end, a lumen extending between the distal end and the proximal end, and a locking hub operably coupled to the proximal end, the locking hub including a lumen segment of the lumen. Self-biasing of the lumen segment reduces a diameter of the lumen segment to a first diameter, the first diameter less than a diameter of a rest of the lumen. Actuating the locking hub increases the diameter of the lumen segment from the first diameter to a second diameter. 
     In one implementation, the locking hub includes a locking hub body, a compressible seal disposed within the locking hub body and including the lumen segment, the diameter of the lumen segment modifiable by compressing the compressible seal. The locking hub further includes a shuttle movable within the locking hub body. In such implementations, self-biasing of the locking hub includes biasing the shuttle in a first direction to compress the compressible seal, and actuating the locking hub translates the shuttle is in a second direction opposite the first direction, thereby reducing compression of the compressible seal. The locking hub may further include one or more buttons, the locking hub being actuatable to translate the shuttle by depressing the one or more buttons. In such implementations, depressing the one or more buttons may translate the one or more buttons transversely toward a longitudinal axis of the catheter. 
     Each of the one or more buttons may include one or more wedged protrusions and the shuttle may further include angled indentations shaped to receive each of the one or more wedged protrusions when the one or more buttons are depressed. Receipt of the wedged protrusions by the angled indentations in such implementations results in translation of the shuttle in the second direction. 
     The locking hub may further include a biasing element configured to bias the shuttle in the first direction. The biasing element may include, without limitation, at least one of a helical spring, a leaf spring, a biasing arm, a biasing arm, or a resilient elastomeric member. 
     In certain implementations, the compressible seal includes a proximal cylindrical section and a distal tapered section. In such implementations the locking hub body may include a proximal cylindrical inner surface and a distal tapered inner surface such that, when the compressible seal is disposed within the locking hub body, the proximal cylindrical section of the compressible seal is within the proximal cylindrical inner surface and the distal tapered section of the seal is adjacent the distal tapered inner surface. When the shuttle is biased against the compressible seal in such implementations, the distal tapered section of the compressible seal may abuts the distal tapered inner surface of the hub body, thereby reducing the diameter of the lumen segment. 
     The shuttle may include a plurality of ribs disposed within respective channels of the locking hub body. 
     In certain implementations, the locking hub may include a cap coupled to a proximal end of the locking hub. Such coupling may be achieved by one or more of ultrasonic welding, an adhesive, a snap fit, and a pinned coupling. 
     In another embodiment of the present disclosure, a delivery system for a leadless pacemaker is provided. The delivery system includes a catheter including a distal end, a proximal end opposite the distal end, a lumen extending between the distal end and the proximal end, and a locking hub operably coupled to the proximal end, the locking hub comprising a seal element including a lumen segment of the lumen. Self-biasing of the locking hub compresses the seal element, thereby reducing a diameter of the lumen segment. Actuation of the locking hub reduces the compression of the seal element. 
     In one implementation, the locking hub includes a movable shuttle and the self-biasing of the locking hub biases the shuttle in a first direction to compress the seal element, the first direction being along a longitudinal axis of the catheter. In such an implementation, actuation of the locking hub translates the shuttle in a second direction opposite the first direction. The locking hub may further include a biasing element configured to bias the shuttle in the first direction, the biasing element including at least one of a helical spring, a leaf spring, a biasing arm, a biasing arm, or a resilient elastomeric member. 
     In another implementation, actuation of the locking hub comprises depressing one or more buttons of the locking hub such that the one or more buttons translate transversely and inward relative to the longitudinal axis of the catheter. 
     In yet another implementation, the locking hub includes a locking hub body having a tapered inner surface and the seal includes a corresponding tapered outer surface. In such implementations, the self-biasing may apply a longitudinal force to the seal element such that an interface between the tapered inner surface of the locking hub body and the tapered outer surface of the seal element causes transverse compression of the seal element toward a longitudinal axis of the catheter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the invention are set forth with particularity in the claims that follow. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which: 
         FIGS. 1A-1B  are, respectively, side and end views of an example leadless cardiac pacemaker. 
         FIG. 1C  is a diagrammatic medial-lateral cross section of a patient heart illustrating example implantation of leadless pacemakers in the patient heart. 
         FIG. 1D  is one embodiment of a delivery system for delivering a leadless pacemaker. 
         FIG. 1E  illustrates only the guide catheter and introducer sheath of the pacemaker delivery system of  FIG. 1D . 
         FIGS. 2A-2B  are close-up views of a distal portion of the delivery system. 
         FIGS. 3A-3B  are schematic side and cross-sectional views of a pacemaker sheath. 
         FIGS. 4A-4G  are side views of a delivery system attached to a pacemaker. 
         FIGS. 5A-5D  are various views of a catheter handle and tether key. 
         FIGS. 6A-6B  are an alternate embodiment of a delivery system having a single tether. 
         FIG. 7A  is an isometric view of a proximal end of the guide catheter extending through the introducer sheath. 
         FIG. 7B  is a longitudinal cross section of the proximal extent of the guide catheter, including its proximal hub, locking hub and a proximal end of its shaft. 
         FIG. 7C  is a longitudinal cross section of the locking hub in a locked state wherein the button of the locking hub is biased to lock the locking hub on the shaft of the deflectable catheter and prevent displacement between the locking hub and the shaft of the deflectable catheter. 
         FIG. 7D  is the same view as  FIG. 7C , except the compression button has be pressed into the body to cause the locking hub to assume an unlocked state whereby the shaft of the deflectable catheter is free to displace relative to the locking hub. 
         FIG. 7E  is an isometric view of the compression button as viewed from its inward side. 
         FIG. 7F  is an isometric view of the body of the locking hub as viewed from its side. 
         FIG. 8  is an isometric view of a proximal end of the guide catheter with the shaft of the deflectable catheter extending there through, the locking hub employing aspects of a Tuohy-Borst valve. 
         FIG. 9A  is an isometric view of a proximal end of a guide catheter including a second locking hub. 
         FIG. 9B  is a second isometric view of the proximal end of the guide catheter of  FIG. 9A . 
         FIG. 9C  is an exploded view of the locking hub of  FIGS. 9A-9B . 
         FIG. 9D  is a semi-transparent side view of the guide catheter of  FIG. 9A-9B  including the locking hub. 
         FIG. 10A  is a longitudinal cross section of another guide catheter including a third locking hub, the locking hub in a biased/locked configuration. 
         FIG. 10B  is a longitudinal cross section of the guide catheter and locking hub of  FIG. 10A  in an unlocked configuration. 
         FIG. 11  is an isometric view of a hub body of the locking hub of  FIGS. 10A-10B . 
         FIG. 12  is an isometric view of a seal of the locking hub of  FIGS. 10A-10B . 
         FIG. 13  is an isometric view of a shuttle of the locking hub of  FIGS. 10A-10B . 
         FIG. 14  is an isometric view of a button of the locking hub of  FIGS. 10A-10B . 
         FIG. 15  is an isometric view of a distal cap of the locking hub of  FIGS. 10A-10B . 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is directed to a delivery system and associated methodology for delivering a leadless pacemaker to an implantation site in a patient. The delivery system includes a locking hub of a guide catheter that allows for coarse and fine adjustment of positioning of the guide catheter relative to a deflectable catheter extending through the guide catheter, the leadless pacemaker being supported off of the distal end of the deflectable catheter. 
     As discussed in detail below, in one embodiment, the locking hub includes a compression button with a lumen section extending along its length that is slightly larger in diameter than the diameter of the shaft of the deflectable catheter that extends through the guide catheter and its locking hub. Half of the inner circumferential surface of the lumen section is a hard, low friction material, and the other half of the inner circumferential surface of the lumen section is a soft, high friction material. Unless acted upon by the user, the compression button self-biases such that the lumen section is out of alignment with the rest of the lumen of the guide catheter, thereby causing the soft, high friction material to compress against the shaft of the deflectable catheter and locking the shaft relative to the locking hub. When the user depresses the compression button, the lumen section is caused to align with the rest of the lumen of the guide catheter in a coaxial arrangement, thereby making it possible for the shaft of the deflectable catheter to readily displace through the locking hub and the rest of the guide catheter. 
     In another embodiment, the locking hub includes a compressible seal with a lumen section extending along its length. When the seal is compressed, the diameter of the lumen segment is reduced such that compressible seal engages an outer surface of a shaft or similar elongate body of the catheter extending through the lumen. The seal is biased into compression by a shuttle acted upon by a biasing element, such as a helical spring. When a user depresses compression buttons of the locking hub, the shuttle is translated away from the seal, thereby allowing the seal to decompress. Such decompression causes the diameter of the lumen segment to expand, reducing the engagement between the seal and the elongate body and allowing movement of the elongate body relative to the locking hub. 
     Before beginning a detailed discussion of the locking hub and associated method, a general overview of an example leadless pacemaker and catheter-based delivery system is provided as follows. 
     a. Overview of Leadless Pacemaker and a Catheter-Based Delivery System 
       FIGS. 1A-1B  illustrate an example leadless cardiac pacemaker  102 . The leadless pacemaker  102  can communicate by conducted communication, representing a substantial departure from conventional pacing systems. The leadless pacemaker can perform cardiac pacing that has many of the advantages of conventional cardiac pacemakers while extending performance, functionality, and operating characteristics with one or more of several improvements. 
     In some embodiments of a cardiac pacing system, cardiac pacing is provided without a pulse generator located in the pectoral region or abdomen, without an electrode-lead separate from the pulse generator, without a communication coil or antenna, and without an additional requirement of battery power for transmitted communication. 
       FIG. 1C  illustrates an embodiment of a cardiac pacing system  150  configured to attain these characteristics. The cardiac pacing system  150  includes one or more leadless cardiac pacemakers  102 . Each leadless pacemaker is substantially enclosed in a hermetic housing  151  suitable for placement on or attachment to the inside or outside of a cardiac chamber, such as the right atrium and/or right ventricle of the patient heart  152 , as can be understood from  FIG. 1B . Attachment of a leadless pacemaker to the cardiac tissue can be accomplished via a helical anchor  103  on an anchor mount  155  extending from a distal end of the leadless pacemaker. 
     As can be understood from  FIGS. 1A-1B , the leadless pacemaker  102  can have two or more electrodes  154 ,  156  located within, on, or near the housing  151 , for delivering pacing pulses to muscle of the cardiac chamber and optionally for sensing electrical activity from the muscle, and for bidirectional communication with at least one other device within or outside the body. The housing can contain a primary battery to provide power for pacing, sensing, and communication, for example bidirectional communication. The housing  151  can optionally contain circuits for sensing cardiac activity from the electrodes  154 ,  156 . The housing contains circuits for receiving information from at least one other device via the electrodes and contains circuits for generating pacing pulses for delivery via the electrodes. The housing can optionally contain circuits for transmitting information to at least one other device via the electrodes and can optionally contain circuits for monitoring device health. The housing contains circuits for controlling these operations in a predetermined manner. 
     In some embodiments, a cardiac pacemaker can be adapted for delivery and implantation into tissue in the human body. In a particular embodiment, a leadless cardiac pacemaker can be adapted for implantation adjacent to heart tissue on the inside or outside wall of a cardiac chamber, using two or more electrodes located on or within the housing of the pacemaker, for pacing the cardiac chamber upon receiving a triggering signal from at least one other device within the body. 
     Leadless pacemakers or other leadless biostimulators are typically fixed to an intracardial implant site by an actively engaging mechanism or primary fixation mechanism such as a screw or helical member  103  that screws into the myocardium. Examples of such leadless biostimulators are described in the following publications, the disclosures of which are incorporated by reference: (1) U.S. Pat. No. 8,457,742, issued on Jun. 4, 2013, entitled “Leadless Cardiac Pacemaker System For Usage In Combination With An Implantable Cardioverter-Defibrillator”; (2) U.S. Pat. No. 9,358,400 issued on Jul. 7, 2016, entitled “Leadless Cardiac Pacemaker”; (3) U.S. Pat. No. 9,216,298, issued on Dec. 22, 2015, entitled “Leadless Cardiac Pacemaker System with Conductive Communication”; (4) U.S. Pat. No. 8,352,025 issued on Jan. 8, 2013, entitled “Leadless Cardiac Pacemaker Triggered by Conductive Communication”; (5) U.S. Pat. No. 7,937,148 issued on May 3, 2011, entitled “Rate Responsive Leadless Cardiac Pacemaker”; (6) U.S. Pat. No. 7,945,333 Issued on May 17, 2011, entitled “Programmer for Biostimulator System”; (7) U.S. Pat. No. 8,010,209, issued on Aug. 30, 2011, entitled “Delivery System for Implantable Biostimulator”; and (8) International Application No. PCT/US2006/040564, filed on Oct. 13, 2006, entitled “Leadless Cardiac Pacemaker and System” and published as WO07047681A2 on Apr. 26, 2007. 
     In addition to the primary fixation mechanism, such as a helix, some leadless biostimulators may further include a secondary fixation mechanism to provide another feature for keeping the leadless biostimulator in place within the body. Secondary fixation mechanisms can be either active (e.g., the secondary fixation mechanism can actively engage tissue, either within or outside the heart), or can be passive (e.g., the secondary fixation mechanism is not attached to tissue but rather prevents the leadless biostimulator from moving around in the body in the case of accidental detachment). Further details on secondary fixation mechanisms can be found in U.S. Pat. No. 8,527,068, issued on Sep. 3, 2013. 
     Leadless pacemakers or other leadless biostimulators can be delivered to and retrieved from a patient using any of the delivery systems described herein. In some embodiments, a leadless pacemaker is attached or connected to a delivery system and advanced intravenously into the heart. The delivery system can include features to engage the leadless pacemaker to allow fixation of the leadless pacemaker to tissue. For example, in embodiments where the leadless pacemaker includes an active engaging mechanism, such as a screw or helical member, the delivery system can include a docking cap or key configured to engage the leadless pacemaker and apply torque to screw the active engaging mechanism into the tissue. In other embodiments, the delivery system includes clips designed to match the shape of a feature on the leadless pacemaker and apply torque to screw the active engaging mechanism into the tissue. 
       FIG. 1D  illustrates a pacemaker delivery system  100  configured for delivery of a leadless pacemaker  102  into a patient. The delivery system  100  can include a deflectable catheter  50 , a guide catheter  52 , and an introducer sheath  54 . As can be understood from  FIG. 1D , the deflectable catheter  50  extends through the guide catheter  52  and includes a distal end and a proximal end. The distal end of the deflectable catheter is selectively connectable to the proximal end of the leadless pacemaker  102  and the proximal end of the deflectable catheter includes a handle  108  by which the user may cause the deflectable catheter shaft  106  to distally-proximally displace within the length of the guide catheter and, further, by which the user may actuate the distal end of the deflectable catheter to selectively connect and disconnect from a proximal end of the leadless pacemaker. The deflectable catheter  50  extends from both the distal and proximal ends of the guide catheter  52 . 
       FIG. 1E  illustrates only the guide catheter  52  and introducer sheath  54  of the pacemaker delivery system  100  of  FIG. 1D . As can be understood from  FIGS. 1D and 1E , the guide catheter  52  extends through the introducer sheath  51  and includes a distal end and a proximal end. The distal end of the guide catheter  52  includes a protective pacemaker sheath  104  discussed in greater detail below. The proximal end of the guide catheter includes a flush port  114   b  extending from a proximal hub  125  and a locking hub  130  proximally extending from the proximal hub  125 . While the locking hub  130  is shown as located adjacent the proximal hub  125 , in other embodiments the locking hub  130  may be located at other locations on the guide catheter. The locking hub  130  is discussed in detail below. 
     As shown in  FIGS. 1D and 1E , the guide catheter  52  extends from both the distal and proximal ends of the introducer sheath  54 . As shown in  FIG. 1E , the shaft  111  of the guide catheter  52  includes a distal soft durometer section  122  and a proximal high durometer section  123  that is longer than the distal soft durometer section. 
     As depicted in  FIGS. 1D and 1E , introducer sheath  54  includes a distal end  126  and a proximal end. The proximal end of the introducer includes a flush port  114   a  and a hub  127 . 
     As can be understood from  FIGS. 1D and 1E  and for purposes of discussion, the pacemaker delivery system  100  may be considered to include the various components of the deflectable catheter  50 , the guide catheter  52  and the introducer  54 . For example, the pacemaker delivery system  100  may be considered to include the pacemaker sheath  104 , guide catheter shaft  111 , pacemaker introducer sheath  107 , handle  108 , deflection knob  110 , tether shuttle  112 , and flush ports  114   a ,  114   b , and  114   c  The deflection knob  110  can be used to steer and guide the catheter during implantation and/or removal of the pacemaker. The flush ports  114   a ,  114   b , and  114   c  can be used to flush saline or other fluids through the catheter. Sheath  107  can be advanced distally over catheter shaft  111  to provide additional steering and support for the delivery catheter during implantation and to surround the pacemaker as it is introduced through a trocar or introducer into the patient. 
       FIG. 2A  is a close-up view of a distal portion of delivery system  200  and pacemaker  202 . The pacemaker of  FIG. 2A  can include a helix  203  for attachment of the pacemaker to tissue. In  FIG. 2A , the pacemaker is attached to docking cap  218  of catheter shaft  206 . Pacemaker sheath  204  is shown pulled back proximally along catheter shaft  206  and guide catheter shaft  211  to expose the pacemaker  202  and helix  203 . In  FIG. 2B , pacemaker sheath  204  is extended distally along guide catheter shaft  211  to cover the catheter shaft  206 , pacemaker  202 , and helix to protect the tissue from the sharp edges of the helix during implantation. When the pacemaker sheath is pulled back proximally, as shown in  FIG. 2A , the pacemaker  202  is in an exposed, delivery configuration. When the pacemaker sheath is advanced distally to protect the pacemaker and helix, as shown in  FIG. 2B , the pacemaker  202  is in a protected, advancement configuration. 
       FIGS. 3A-3B  are close-up and cross sectional views of pacemaker sheath  304  of delivery system  300 . As shown, pacemaker sheath  304  can include crease or fold  320  along the length of the sheath. During initial insertion of the delivery system into a patient, a physician can gain access to the patient&#39;s venous system with an introducer sheath using the Seldinger technique (not shown). The delivery system, including the leadless pacemaker and catheter shaft, can then be advanced through the introducer sheath into the patient&#39;s venous system to facilitate delivery of the pacemaker into the heart. Reducing the diameter of the pacemaker, the delivery system, and thus the introducer sheath, provides for easier and less intrusive access to a patient&#39;s venous system. 
     By designing pacemaker sheath  304  with a fold  320  that runs longitudinally along the sheath, the cross sectional diameter of the pacemaker sheath can be reduced by folding the sheath over itself. Thus, during initial implantation of the pacemaker through a introducer sheath into the patient, the pacemaker sheath can be positioned just proximally to the pacemaker, and folded along fold  320  so as to have a cross sectional diameter close to or equal to the same diameter as the pacemaker. This allows a smaller diameter introducer sheath to be used than would normally be necessary, since those delivery systems must incorporate a larger introducer sheath to allow passage of a full sized pacemaker sheath. After the delivery system is inserted through the introducer sheath into the patient, the sheath can be advanced distally over the leadless pacemaker. Advancing the pacemaker sheath distally causes fold  320  to unfold, thereby increasing the diameter of the pacemaker sheath so that it can slide over and cover the pacemaker and fixation helix.  FIG. 3B  is a cross sectional view of the pacemaker helix  304  and fold  320 , giving another view on how the cross sectional diameter of the pacemaker sheath can increase and decrease. 
       FIG. 4A  illustrates delivery system  400 , including pacemaker  402  comprising helix  403  and attachment feature  424 , and the delivery catheter comprising pacemaker sheath  404 , catheter shaft  406 , docking cap  418 , and tethers  422   a  and  422   b . The tethers can comprise wires, shafts, tubes, cords, ropes, strings, or other similar structures that can extend throughout the catheter shaft. In some embodiments, the tethers comprise a shape memory material, such as nitinol. In other embodiments, the tethers comprise stainless steel wires or braids. In  FIG. 4A , the pacemaker  402  is not attached to docking cap  418  of the delivery catheter. The process of connecting the pacemaker to the delivery catheter will now be described. 
     Referring to  FIG. 4B , tethers  422   a  and  422   b  can include distal features  426   a  and  426   b . The distal features can be, for example, features on the tethers that protrude radially from the tether, such as bumps, spheres, cylinders, rectangles, or other similar shapes extending outwards from the tethers. In some embodiments, the distal features can be expandable, such as balloons or expandable mechanical structures. Generally, the distal features have a cross sectional diameter larger than the cross sectional diameter of the tethers. As shown, in one embodiment, distal feature  422   a  can be advanced further from the catheter than distal feature  422   b , so that when the tethers are pushed together, distal feature  422   b  rests against tether  422   a . This causes the combined cross sectional diameter of both distal features and tethers to be less than if the distal features were lined up side by side. By way of comparison, in  FIG. 4C  the distal features  426   a  and  426   b  are lined up side by side and therefore have a greater combined cross sectional diameter when pressed together than is shown in  FIG. 4B . 
     The length of tethers  422   a  and  422   b , and thus the position of distal features  426   a  and  426   b , can be adjusted so that distal features  426   a  and  426   b  are not aligned in a side by side configuration (e.g., the un-aligned configuration shown in  FIGS. 4A-4B ). When the tethers and distal features are in this un-aligned configuration, the cross sectional diameter of the distal features is reduced since the distal features are not positioned side by side. The tether distal features  426   a  and  426   b  can then be advanced in this un-aligned configuration through hole  428  of attachment feature  424 , as shown in  FIGS. 4D-4F . In this embodiment, the diameter of hole  428  should be sufficiently large enough to allow the distal features  426   a  and  426   b  of tethers  422   a  and  422   b  to pass when in the un-aligned configuration. Upon passing the distal features through the hole  428 , the length of the tethers can then be adjusted to align the distal features in the side by side configuration (e.g., as shown in  FIGS. 4C and 4E ). When the distal features are positioned side by side, the combined cross sectional diameter of the distal features becomes larger than the diameter of hole  428 , which essentially locks the tethers and distal features in the attachment feature  424  be preventing the distal features from being able to pass proximally through the hole  428 . 
     Still referring to  FIGS. 4C and 4D , the docking cap  418  of the delivery catheter can include a torque slot  430  (shown in  FIG. 4C ) sized and configured to mate with a torque key  432  (shown in  FIG. 4D ) disposed on a proximal end of the pacemaker. The torque slot  430  can be coupled to a torque shaft  431 , which runs the length of the delivery catheter extending into the handle (not shown). In  FIGS. 4C and 4D , torque key  430  is shown as a “male” key and torque slot  430  is shown as a “female” key, but it should be understood that in other embodiments, the “male” key can be located on the attachment feature  418 , and the “female” key can be disposed on the pacemaker. It should also be appreciated that key  432  and slot  430  can comprise any number of shapes, including, without limitation, square, rectangle, triangle, pentagon, hexagon, cross, or “X”, so long as key  432  fits within and can apply rotational torque to slot  430 . Once the tethers are locked within the attachment feature, the tethers can be pulled proximally to pull attachment feature  424  and the pacemaker towards the catheter and to attach the pacemaker to the delivery catheter, thereby engaging torque slot  430  with torque key  432  (as shown in  FIG. 4G ). 
       FIGS. 5A-5D  are close-up views of handle  508  of delivery system  500 . In  FIG. 5A , handle  508  includes deflection knob  510 , tether knob  512 , tether adjustment feature  514 , and flush ports  516 . As described above, deflection knob  510  provides for steering and guidance of the catheter during implantation and/or removal of the pacemaker. The flush ports  516  can be used to flush saline or other fluids through the catheter. Referring now to  FIGS. 5B and 5C , tether adjustment feature  514  can be configured to adjust then length of tethers  522   a  and  522   b  that extends distally outwards from the delivery catheter, causing the distal features (not shown) to be in either a side by side “locked” configuration or an un-aligned “unlocked” configuration. 
     The tether adjustment feature can comprise an Allen wrench or any other suitable key, and can be configured to mate with and engage proximal keys  534   a  and  534   b  of tethers  522   a  and  522   b , respectively, which are disposed within shuttle  512 . In another embodiment, the tether adjustment feature can comprise knobs or dials on the handle itself, and a user can simply turn the knobs or dials to adjust the length of the tethers. The shuttle can be inserted into handle  508 , as shown in  FIG. 5D . The proximal keys  534   a  and  534   b  of tethers  522   a  and  522   b  are shown without shuttle  536  in  FIG. 5C  for ease of illustration. Rotation of tether adjustment feature  514  causes proximal keys  534   a  and/or  534   b  to move distally or proximally within shuttle  512 , which therefore changes the length of tethers  522   a  and/or  522   b  extending distally from the delivery catheter. Thus, the tether key can be used to either align the distal features of the tethers in a side by side (e.g., locked) configuration, or alternatively, to place the distal features of the tethers in an un-aligned (e.g., unlocked configuration), permitting docking and locking of the pacemaker to the delivery catheter. 
     Referring back to  FIGS. 4D-4G and 5A , it can now be understood how the pacemakers described herein can be delivered and attached to tissue, and then released from the delivery system. In  FIGS. 4D-4F , tethers  422   a  and  422   b  can be inserted in an “unlocked” or un-aligned configuration into hole  428  of attachment feature  424 . The distal features of the tethers can then be aligned so as to lock the distal features in the attachment feature. Referring to  FIG. 5A , tether shuttle  512  can then be pulled proximally to cause the tethers to move proximally, thereby docking the pacemaker against the delivery catheter (as shown in  FIG. 4G ). When the pacemaker is docked against the delivery catheter, torque key  432  of the pacemaker (shown in  FIG. 4D ) fits within and is mated to torque slot  420  of the delivery catheter (shown in  FIG. 4C ). 
     Referring to  FIG. 5A , tether shuttle  512  of handle  508  can then be rotated, which rotates torque shaft  431  (shown in  FIG. 4C ) within the delivery catheter and applies torque to torque slot  430 , and thus to torque key  432  on the pacemaker. By rotating the shuttle, and thus the torque shaft, the delivery catheter applies torque to the pacemaker to screw the fixation helix of the pacemaker into tissue. Once the fixation helix is fully inserted into tissue, the tethers can be placed into an un-aligned or “unlocked” configuration with tether adjustment feature  514 , allowing the tethers and distal features to be removed from the attachment feature of the pacemaker. Once the delivery catheter is disengaged from the pacemaker, the catheter can be removed from the patient, leaving the pacemaker in place at the target tissue. 
       FIGS. 6A and 6B  illustrate an alternate embodiment for attaching a delivery catheter to a pacemaker. The embodiment shown in  FIGS. 6A and 6B  employs a similar concept to that described above. However, instead of using two tethers, as described above, the embodiment of  FIGS. 6A and 6B  utilizes a single tether  622 , having both a distal feature  626   a  and a proximal feature  626   b . In the embodiment of  FIGS. 6A and 6B , the tether  622  can comprise a shape memory alloy, such as nitinol, and can include a pre-bent or pre-biased shape. This pre-biased shape can allow the distal feature  626   a  of the tether to naturally bias outwards, as shown in  FIG. 6A . 
     To attach the pacemaker  602  to the delivery catheter, as shown in  FIG. 6A , the distal feature  626   a  of tether  622  can be threaded through attachment feature  624  of pacemaker  602 . Once the tether is threaded through the attachment feature, the tether can be folded back against itself, so that distal feature  626   a  is adjacent to, but not directly beside proximal feature  626   b . The distal and proximal features should be aligned in an un-aligned or “unlocked” configuration, as described above in the two-tether embodiments. This configuration allows the distal and proximal features to be inserted into hole  628  of docking cap  618 , as shown in  FIG. 6B . Once the distal and proximal features are advanced past the hole  628 , an interior chamber (not shown) in the catheter opens up to a diameter larger than the diameter of the hole  628 . This interior chamber has a diameter large enough to accommodate both the distal and proximal features in a side by side or “locked” configuration. Thus, the length of the tether can be adjusted to align the distal and proximal features in the side by side configuration, causing the combined cross sectional diameter of the distal and proximal features to be larger than the diameter of hole  628 . The result is the locking of tether  622  within the delivery catheter. 
     Other features of the embodiment of  FIGS. 6A-6B  can be the same as described above, such as the torque keys, slots, and shafts that allow the delivery catheter to apply rotational torque to the pacemaker to screw it into tissue. 
     For additional detail regarding the catheter-based delivery systems described above with respect to  FIGS. 1D-6B , see U.S. Pat. Nos. 8,615,310, 8,958,892, and 9,205,225. Other catheter-based delivery systems, such as those disclosed in U.S. Patent Applications 62/408,494 and 62/434,537, may also be employed to deliver a leadless pacemaker. Any of these catheter-based delivery systems and associated leadless pacemakers are readily capable of being coupled together in the catheterization laboratory via the loading tool and associated methods discussed in the following section of the present disclosure. 
     b. Locking Hub with Movable Lumen Segment and Associated Method of Use 
       FIG. 7A  is an isometric view of a proximal end of the guide catheter  52  extending through the introducer sheath  54 . As can be understood from  FIG. 7A , the locking hub  730  proximally extends from the proximal hub  725  of the guide catheter  52 . The locking hub  730  includes a body  760  and a compression button  762  projecting from a lateral side surface  764  of the body. The body also includes a distal end  766  and a proximal end  768  opposite the distal end, the distal end abutting against, and connecting to, the proximal end of the proximal hub  725  of the guide catheter  52 . 
     As discussed in detail below, the compression button  762  has a round or eccentric lumen section  770 E through its length that is slightly larger in diameter than the diameter of the shaft  106  of the deflectable catheter  50  that extends through the guide catheter  52  and its locking hub  130 , as can be understood from  FIG. 1D . On the top side of the lumen section  770 E is a hard, low friction material  781 , and on the bottom side  782  of the lumen section  770 E is a soft, high friction material. When the user is not pushing on the compression button  762 , the high friction material on the compression button is in contact with the deflectable shaft, allowing the hub to lock onto the shaft. When the user pushes the compression button into the hub body  760 , the locking hub  730  is no longer locked onto the deflectable shaft. 
       FIG. 7B  is a longitudinal cross section of the proximal extent of the guide catheter  52 , including its proximal hub  725 , locking hub  730  and a proximal end of its shaft  711 . As can be understood from  FIG. 7B , a male-female interference fit arrangement  769  is formed between a female coupling structure of the distal end  766  that receives a male coupling structure of a proximal end of the proximal hub  725 . The interference fit may be sufficient to maintain the locking hub attached to the proximal hub, or the interference fit may be supplemented by being welded or via application of an adhesive. Alternatively, the locking hub  730  to could attached to the proximal hub  725  or another portion of the guide catheter via molding, ultrasonic or other types of welding, or adhesive bonding. 
     As shown in  FIG. 7B , a lumen  770  extends as lumen segments  770 A,  770 B,  770 C,  770 D,  770 E and  770 F through the shaft  711 , the proximal hub  725  and components of the locking hub  730 , respectively. One component of the locking hub includes an elastomeric seal  771  sandwiched between the opposed surfaces of the male and female coupling structures forming the male-female interference fit arrangement  769 . The elastomeric seal  771  includes an opening  770 C, which forms one of the lumen segments and is coaxially aligned with the lumen segments  770 A,  770 B,  770 D and  770 F. In one embodiment, the elastomeric seal  771  is formed of, without limitation, one or more of silicone rubber, silicone polyurethane copolymer, or other rubber-like polymers and substances. 
     The opening  770 C of the elastomeric seal  771  defines a distal opening into the rest of the lumen  770  extending proximally through the locking hub  730  to proximally daylight at a proximal opening  770 F in the body  760 . This proximal opening  770 F is also coaxially aligned with the lumen segments  770 A,  770 B,  770 C and  770 D and defines a proximal opening into the rest of the lumen  770  extending distally through the locking hub  730  and the rest of the guide catheter  52 . Immediately proximally adjacent the opening  770 C in the elastomeric seal  771  is the distal opening  770 D in the body  760 , which is coaxially aligned with the lumen segments  770 A,  770 B,  770 C and  770 F. 
     As depicted in  FIG. 7B , the compression button  762  occupies a void in the body  760  and includes a lumen segment  770 D extending the longitudinal length of the button. Springs  772  act between the button  762  and the body  760  such that the button projects from the side  764  of the body and the lumen segment  770 D of the button is out of coaxial alignment with the rest of the lumen segments  770 A,  770 B,  770 C and  770 E. This out of alignment condition is also reflected in  FIG. 7C , which is a longitudinal cross section of the locking hub  730  in the locked state wherein the button of the locking hub is biased to lock the locking hub on the shaft of the deflectable catheter and prevent displacement between the locking hub and the shaft of the deflectable catheter. 
     For example, as reflected in  FIG. 7C , the springs  772  bias the button  762  outward such that the longitudinal axis  775  of the button lumen  770 E is not coaxial (i.e., is out of alignment) with the longitudinal axis  776  of the rest of the lumen segments  770 D and  770 F (and by extension and as can be understood from  FIG. 7B , the lumen segments  770 A,  770 B and  770 C). In such a locked state, a catheter shaft, such as, for example, the shaft  106  of the deflectable catheter  50 , is pinched, clamped or compressed by the lumen section  770 E of the button being out of alignment with the rest of the lumen sections  770 A,  770 B,  770 C,  770 D and  770 F forming the overall lumen  770  with the button lumen section  770 E. As a result, the locking hub is locked on the catheter shaft  106  and relative displacement between the shaft and locking hub is prevented. 
       FIG. 7D  is the same view as  FIG. 7C , except the compression button has be pressed into the body to cause the locking hub to assume an unlocked state whereby the shaft of the deflectable catheter is free to displace relative to the locking hub. Specifically, as illustrated in  FIG. 7D , when the button  762  is depressed inwardly against the biasing force of the springs  772  a sufficient distance that an inward limit structure  777  of the button  762  abuts against an inward limit structure  778  of the body  760  (also see  FIG. 7C  for the limit structures), the longitudinal axes  775 ,  776  of the lumen segments  770 D,  770 E and  770 F are placed in coaxial alignment. Thus, a catheter shaft  106  extending through the overall lumen  770  is no longer pinched, clamped or compressed by the lumen section  770 E of the button and is thereby free to displace through the overall lumen  770 . 
     The limit structures  777  of the compression button  762  may be in the form of the most inward extents or edges  777  of the cylindrical openings  779  that serve as receptacles for the springs  772 , as can be understood from  FIGS. 7C and 7D . The nature of these cylindrical openings  779  and the inward extents  777  of the button  762  are readily apparent in  FIG. 7E , which is an isometric view of the compression button as viewed from its inward side. Specifically, each cylindrical openings  779  and respective inward extent  777  defines a cylindrical protrusion  788 , as reflected in  FIG. 7E . 
     Similar cylindrical openings in the interior of the body  760  similarly provide receptacles for the other ends of the springs  772 , as can be understood from  FIGS. 7C and 7D . Accordingly, for each spring  772 , one end of the spring is received in the cylindrical opening  779  of the button, and the opposite end of the spring, plus the surrounding cylindrical protrusion  788  that defines the cylindrical opening  779  of the button, is received in the confines of the respective cylindrical opening of the interior of the body  760  when the button  762  is fully displaced inwardly in the body  760  such that the most inward extent  777  of the protrusion  788  ends up abutting against the inward limit structure  778  of the body, the inward limit structure of the body defining the floor  778  of the surrounding cylindrical opening in the interior of the body. 
     In other embodiments, the limit structures of the button and body may be other respective structures that abut at a point in the inward displacement of the button such that the longitudinal axes  775 ,  776  coaxially align as depicted in  FIG. 7D  and further inward displacement of the button is prevented by the abutment of the respective limit structures. Also, the springs  772  may be retained in position between the button and body via other structures defined in or on the body and button. 
     The springs  772  may be helical compression springs. In other embodiments, the helical springs may be replaced with other types of springs that act between the button and body, such as, for example, leaf springs. In other embodiments, the springs  772  may be replaced with another type of biasing member that acts between the button and the body, such as, for example, a resilient elastomeric body. The biasing force may also be a biasing arm or other member extending from the button as a part of the unitary construction of the button to act against the body. Of course, such an arrangement could be reversed such that the biasing arm or other member extends from the body as part of the unitary construction of the body to act against the button. 
     Regardless of what type of biasing mechanism is employed to bias the button outwardly relative to the body and, thereby, cause the lumen axes  775 ,  776  to be out of alignment as depicted in  FIG. 7C , the biasing mechanism can be sized appropriately to provide the proper locking force of the locking hub onto the shaft  106  of the deflectable catheter  50 , as can be understood from  FIG. 1D . 
     As indicated in  FIGS. 7C-7E , the cylindrical wall  780  of the lumen section  770 E of the compression button  762  may be in the form of an upper semi-cylindrical portion  781  and a lower semi-cylindrical portion  782 . The upper portion  780  is adjacent an outer actuation or depression surface  782  of the button  762  that is acted upon by the user when depressing the button. The lower portion  782  is opposite the upper portion  781  and adjacent the inward extents  777  of the button  762 . The upper portion  781  may be part of the material of the overall button  762 , which, in one embodiment, may be formed of various polymers. Other materials such as ceramic or metal would not be ideal but would not be outside the scope of this invention. In one embodiment, the overall button  762  may be injection molded as a single piece or as two or more pieces and ultrasonically welded, snap fit, or bonded together. 
     The lower portion  782  may be of another material  789  that is different from the material of the rest of the button  762  and have a surface texture that is more likely to adhere or grip a tubular body extending through the lumen section  770 E. In other words, the lower portion  782  may have a higher coefficient of friction than the upper portion  781 . In one embodiment, the lower portion  782  may be formed of silicone rubber or a low durometer polymer. The lower portion  782  may be molded, injected, inserted or otherwise provided within the confines of the rest of the button  762  to define the lumen segment  770 E in combination with the upper portion  781 . 
     The embodiment depicted in  FIG. 7B  shows the elastomeric seal  771  and the elastomeric semi-circular lumen portion  782  as being separate elements. However, in other embodiments, the elastomeric seal  771  may be part of the same unitary construction as the elastomeric semi-circular lumen portion  782 , the seal  771  and lumen portion  782  being joined together by a flexible elastomeric extension continuously extending uninterrupted between the seal and lumen portion, the lumen portion  782 , seal  771  and extension all forming together a unitary body. In such an embodiment, the flexible elastomeric extension is sufficiently flexible to allow for displacement of the button  762  between its non-aligned and aligned states respectively depicted in  FIGS. 7C and 7D . 
     As indicated in  FIG. 7E , the exterior of the compression button  762  includes lateral sidewalls  783  and end sidewalls  784 . As can be understood from a comparison of  FIG. 7E  to  FIG. 7F , which is an isometric view of the body  760  of the locking hub as viewed from its side, these sidewalls  783 ,  784  of the button may have contours that respectively match the sides  785  and ends  786  of the opening  787  that is occupied by the button  762 . As a result of the sliding interface formed between the button sidewalls  783 ,  784  and the sides  785  and ends  786  of the opening  787  of the body  760 , the button  762  can displace in a guided and restricted manner only inward and outward within the opening  787 , as can be understood from  FIGS. 7C and 7D . 
     As can be understood from  FIGS. 7A-7E , in one embodiment, the button  762  includes a contoured surface that provides ergonomic contact for user digit contact. This contoured button surface may be textured or not, and may or may not have a resilient or soft-touch surface to improve grip. This soft-touch surface may be over-molded over the rest of the button. 
     Similarly, as can be understood from  FIGS. 7A-7F , the bottom of the hub body  760  may be ergonomically contoured to help the user maintain grip on the locking hub while sliding the guide catheter  52  relative to the shaft  106  of the deflectable catheter  50 , as can be understood from  FIG. 1D . This protrusion could have features added to it to or be made of a soft-touch surface to further improve grip. This contoured surface of the body may be textured or not, and may or may not have a resilient or soft-touch surface to improve grip. This soft-touch surface may be over-molded over the rest of the body. 
     In one embodiment, the body  760  is formed of Arkema Pebax 7233 SA01 or a similar material. In one embodiment, the body may be injection molded as a single piece or as two or more pieces and ultrasonically welded together. 
     In an example procedure for implanting a leadless pacemaker  102  via the delivery system  100  disclosed herein, the guide catheter  52 , with its integrated protective sleeve  104 , is advanced and retracted multiple times along the deflectable catheter  50 . Depending on the procedural sequence of steps, the guide catheter may be advancing or retracting several centimeters or millimeters. 
     For example, during introduction of the leadless pacemaker and delivery system into the patient, the guide catheter is fully retracted (e.g., approximately 5 cm) along the deflectable catheter, exposing the leadless pacemaker on the distal end of the deflectable catheter and reducing the overall diameter that must be passed into the patient via the percutaneous access. Once in the femoral vein, the user fully advances the guide catheter over the deflectable catheter to cause the integrated protective sleeve guide catheter to surround the leadless pacemaker to protect the surrounding tissue from trauma. As the system is navigated across the tricuspid valve, the user may find improved performance by subtlety retracting the protective sleeve (e.g., retracting the guide catheter millimeters along the deflectable catheter). When approaching sensitive tissue structures (e.g., the right ventricle apical region or right atrium, or any other friable tissue substrates) of the final implant location, subtle advancements or retractions (e.g., advancing/retracting the guide catheter millimeters along the deflectable catheter) may enhance the device safety and improve implant control. 
     To facilitate the precisely controlled displacement of the guide catheter  52  relative to deflectable catheter  50  about which the guide catheter extends, the locking hub  130  of the guide catheter may be employed. Specifically, when the button  762  of the locking hub  730  is not actuated to align the longitudinal axis  775  of its lumen section  770 E with the longitudinal axis  776  of the rest of the overall lumen  770 , the locking hub  730  locks on the shaft  106  of the deflectable catheter  50  as described above, thereby allowing for the locking hub to be grasped to move both the guide catheter  52  and the deflectable catheter  50  together as one unit. On the other hand, when the button  762  of the locking hub  730  is actuated to align the longitudinal axis  775  of its lumen section  770 E with the longitudinal axis  776  of the rest of the overall lumen  770 , the locking hub  730  no longer locks on the shaft  106  of the deflectable catheter  50 , and the locking hub can be grasped to move the guide catheter  52  independent and relative to the deflectable catheter  50 , thereby making it possible to cause the leadless pacemaker to recess within or extend from the integrated protective sleeve  104  of the guide catheter  50 . 
     While the locking hub disclosed herein is discussed in the context of allowing a shaft of a deflectable catheter to selectively displace through the locking hub and its guide catheter, in other embodiments, the shaft extending through the locking hub may be another type of elongated body, including for example a guidewire, stylet or another type of catheter or sheath. Also, the locking hub may be employed on other types of delivery systems whether in the context cardiology or elsewhere. 
     While the selectable locking concepts discussed herein are given in the context of the above described dual textured lumen section  770 E extending through a biased actuation button, similar control and benefits may be obtained by locking hubs employing other locking mechanism such as, for example and without limitation, one or more of mechanical clips similar to clothes pins, hydraulic or electronically actuated clamps, and screw mechanisms. For example, as illustrated in  FIG. 8 , which is an isometric view of a proximal end of the guide catheter  52  with the shaft  806  of the deflectable catheter  50  extending there through, in one embodiment the locking hub  830  employs aspects of a Tuohy-Borst valve. Specifically, sealing actuation is via clockwise or counter-clockwise torque to a twist hub  890 . 
     One version of the embodiment depicted in  FIGS. 7A-7F  may be configured for hydraulic or electronic actuation of the lumen section  770 E as opposed to displacement caused by user digit pressure. 
     b. Locking Hub with Longitudinal Biasing Element and Associated Method of Use 
     The previously discussed implementation of  FIGS. 7A-7F  generally included a locking hub including a segment of a catheter lumen. The locking hub including biasing elements, such as springs, that biased the lumen segment into misalignment with the rest of the lumen. By doing so, a shaft or similar elongate body disposed within the lumen segment is similarly misaligned or pinched, thereby preventing movement of the elongate body relative to the catheter. By depressing a button (or similar feature) the lumen segment could be brought into coaxial alignment with the rest of the lumen. Accordingly, the previously disclosed implementation generally relied on misalignment or displacement of the lumen segment relative to the rest of the lumen in order to provide pinching or locking functionality for preventing movement of an elongate body disposed within the catheter. 
     In other implementations, the diameter of the lumen segment may instead be varied to selectively restrict movement of the elongate body within the catheter. As described below in more detail, one such implementation includes a locking hub having a compressible seal element shaped to be disposed about an elongate body extending through the catheter. The locking hub is biased such that a force is applied to the seal element, compressing the seal element and reducing the diameter of the lumen segment. Such reduction in the diameter of the lumen segment causes the seal to frictionally engage the elongate body, thereby resisting or preventing movement of the elongate body relative to the catheter. In other words, the locking hub is biased to lock or otherwise prevent movement of the elongate body by compressing the seal element about the elongate body resulting in frictional engagement between the seal element and the elongate body. 
     The locking hub may be actuated, such as by depressing one or more buttons of the locking hub, to overcome the bias and to allow the seal element to decompress. Such decompression generally expands the seal element, increasing the diameter of the lumen segment and reducing the frictional engagement between the seal element and the elongate body. As a result, the elongate body is allowed to move relative to the locking hub with no or reduced resistance as compared to when the locking hub is in the non-actuated/biased state. 
     The foregoing implementation may be used, for example, to lock a protective sleeve or sheath about an implantable medical device, such as a leadless pacemaker, during delivery of the implantable medical device into a patient. When delivered, the physician may actuate the locking hub to enable retraction of the protective sleeve, thereby exposing the implantable medical device for implantation. For example, implantable medical devices may include fixation mechanisms (such as a fixation helix) or other features that may become inadvertently caught or otherwise cause damage to patient tissue during their delivery. Accordingly, implementations of the present disclosure ensure that such features of the implantable medical devices are protected during delivery while enabling their ready exposure when implantation is to occur. 
     Another advantage of implementations disclosed herein is that the locking hub may be used to allow for correct positioning of a protective sleeve or sheath relative to the implantable medical device prior to insertion into the patient. More specifically, a physician may select a sheath for use with a delivery catheter system and, using the locking hub, may adjust the position of the sheath relative to an implantable medical device coupled to a distal end of the delivery catheter system. Once properly adjusted, the sheath may be locked in place. As a result, a single length of sheath may be used in multiple applications and with implantable medical devices of varying dimensions. 
       FIG. 9A  is an isometric view of a proximal end of a guide catheter  52  extending through an introducer sheath  54  including an alternative implementation of a locking hub  904 .  FIG. 9B  is a more detailed isometric view of the locking hub  904 . As illustrated in  FIGS. 9A-9B , an elongate member  990 , such as a catheter shaft, may extend through the guide catheter  52  and the locking hub  904 . The locking hub  904  includes a body  906  and a pair of compression buttons  908 ,  910  projecting from opposite lateral side surfaces  912 ,  914  of the body  906 . The body  906  also includes a distal end  916  and a proximal end  920  opposite the distal end  916  and may further include a cap  918  coupled to the proximal end  920 . As illustrated, in certain implementations the locking hub  904  may include one or more flush ports, such as flush port  950 . 
       FIG. 9C  is an exploded view of the locking hub  904  and  FIG. 9D  is a semi-transparent side elevation view of the locking hub  904 , each of which are intended to illustrate the internal components and assembly of the locking hub  904 . 
     Referring first to  FIG. 9C , the locking hub  904  includes a hub body  906  within which a seal  922 , a shuttle  924 , and a biasing element  926  are disposed. In the illustrated implementation, the shuttle  924  is disposed proximal the seal  922  and distal the biasing element  926 . Each of the seal  922 , the shuttle  924 , the biasing element  926 , and the cap  918  define respective through-bores, thereby allowing insertion of an elongate body  990 , such as a deflectable catheter shaft, through the locking hub  904 , as illustrated in  FIG. 9C . 
     As illustrated in  FIG. 9C , the shuttle  924  may include a plurality of ribs, such as ribs  934 ,  936 , shaped to be received within corresponding channels  938 ,  940  (channel  940  being shown in  FIG. 9D ) formed within the hub body  906 . When assembled, the ribs  934 ,  936  are disposed within the channels  938 ,  940 , respectively, such that the shuttle  922  is allowed to translate longitudinally within the hub body  906 . 
     As previously noted, the locking hub  904  further includes buttons  908 ,  910  that may be used to actuate the locking hub  904 , as discussed in more detail below in the context of  FIGS. 10A-10B . The buttons  908 ,  910  may be disposed on lateral sides of the locking hub  904  such that depression of the buttons  908 ,  910  causes the buttons  908 ,  910  to translate inwardly toward a longitudinal axis  915  of the guide catheter  52 . In certain implementations, each button  908 ,  910  may include flexible side tabs, such as side tabs  911 ,  913  of the button  910  (shown in  FIG. 9D ), that are configured to flex inwardly toward each other to allow insertion of the buttons into the hub body  906 . Once inserted the side tabs expand such that they engage the hub body  906 , thereby retaining the buttons  908 ,  910  within the hub body  906 . 
     The biasing element  926  is generally adapted to provide a longitudinal force that biases the shuttle  924  against the seal  922 . As illustrated in  FIGS. 9C and 9D , for example, the biasing element  926  is a helical spring. In other implementations, however, the helical spring may be replaced or used in conjunction with other similar elements including, without limitation, one or more of a leaf spring, a biasing arm, or a resilient elastomeric member similarly configured to bias the shuttle  924  against the seal  922 . 
       FIGS. 10A-10B  are longitudinal cross-sectional views of a catheter  1000  including a guide catheter  1002  having a locking hub  1004 . The locking hub  1004  includes a hub body  1006  and a pair of compression buttons  1008 ,  1010  projecting from opposite lateral side surfaces  1012 ,  1014  of the hub body  1006 . The hub body  1006  also includes a cap  1018 . The locking hub  1004  further includes a seal  1022 , a shuttle  1024 , and a biasing element  1026 , each of which is disposed within the hub body  1006 . 
     The guide catheter  1002  and the locking hub  1004  collectively define a lumen  1060  within which an elongate body  1001 , such as a shaft, may be disposed. The lumen  1060  may be divided into multiple lumen segments. For example, in the catheter  1000  illustrated in  FIGS. 10A-10B , the lumen  1060  may include each of lumen segments  1062 A- 1062 F with lumen segment  1062 A being defined by the guide catheter  1002 , lumen segment  1062 B being defined by a section of the hub body  1006  between the guide catheter  1002  and the seal  1022 , lumen segment  1062 C defined by the seal  1022 , lumen segment  1062 D defined by the shuttle  1024 , lumen segment  1062 E defined by the biasing element  1026 , and lumen segment  1062 F defined by and extending through the cap  1018 . 
     Each of the lumen segments  1062 A- 1062 F is sized and shaped to receive respective portions of the elongate body  1001  such that the elongate body  1001  extends through the locking hub  1004 . The lumen segment  1062 C of the seal  1022 , however, is further adapted to have a variable diameter. In particular, the seal  1022  is formed of a compressible material such that when the seal  1022  is compressed and a portion of the elongate body  1001  is disposed within the lumen segment  1062 C, a diameter  1064  of the lumen segment  1062 C is reduced, resulting in frictional engagement of the seal  1022  with the elongate body  1001 . 
     As illustrated in  FIG. 10A , the locking hub  1004  is configured to be biased into a locked configuration by disposing the biasing element  1026  proximal the shuttle  1024  such that the shuttle  1024  applies a distal longitudinal force onto the seal  1022 , thereby compressing the seal  1022  distally into or against the hub body  1006 . In particular, the seal  1022  includes a distal tapered section  1023  that interfaces with a corresponding internal distal tapered surface  1027  of the hub body  1006  such that the longitudinal force applied by the shuttle  1024  is at least partially redirected toward a longitudinal axis  1015  of the catheter  1000 . By doing so, the diameter  1064  of the lumen segment  1062 C is similarly reduced, thereby causing engagement of the seal  1022  with the elongate body  1001 . As a result of this engagement, the elongate body  1001  is locked in place or otherwise prevented from movement relative to the locking hub  1004 . 
     As illustrated in  FIG. 10B , the buttons  1008 ,  1010  may be depressed to transition the locking hub  1004  from the biased/locked configuration into an unlocked configuration in which the elongate body  1001  may be moved relative to the locking hub  1004 . More specifically, sufficiently depressing the buttons  1008 ,  1010  overcomes the longitudinal force provided by the biasing element  1026 , causing compression of the biasing element  1026  by proximal translation of the shuttle  1024 . As the shuttle  1024  proximally translates, the longitudinal force applied to the seal  1022  by the shuttle  1024  is relieved, allowing the seal  1022  to decompress. Such decompression results in expansion of the diameter  1064  of the lumen segment  1062 C such that the seal  1022  no longer frictionally engages the elongate member  1001 . 
     In the implementation illustrated in  FIG. 10B , translation of the shuttle  1024  is achieved by interaction between the buttons  1008 ,  1010  and the shuttle  1024 . In particular, each of the buttons  1008 ,  1010  includes a plurality of angled protrusions, such as protrusions  1050 ,  1051 ,  1052 , and  1053  that are distally slanted. The shuttle  1024  includes corresponding indentations, such as indentations  1054 ,  1055 ,  1056 ,  1057 , shaped to receive each of the angled protrusions when the buttons  1008 ,  1010  are depressed. As the buttons  1008 ,  1010  are depressed inwardly, the protrusions push against the indentations and, by virtue of their respective slanted faces, translates the transverse force applied to the buttons  1008 ,  1010  into a longitudinal force that proximally translates the shuttle  1024 . 
       FIG. 11  is an isometric view of the hub body  1006  of  FIGS. 10A-10B . The hub body  1006  may include a distal end  1007  and a proximal end  1009  opposite the distal end  1007 . The distal end  1007  of the hub body  1006  may include a sideport  1011  to facilitate flushing of the hub body  1006 . The distal end  1007  includes a distal opening  1013  adapted to receive and be coupled to a proximal end of the guide catheter  1002  (shown in  FIGS. 10A-10B ). For example, in certain implementations the hub body  1006  is molded or adhesively bonded on the guide catheter  1002 . 
       FIG. 12  is an isometric view of the seal  1022  of  FIGS. 10A-10B . As previously discussed, the seal  1022  defines a lumen segment  1062 C having an inner diameter  1064 . The seal  1022  is sized and shaped such that the diameter  1064  is slightly larger than an outer diameter of the elongate body  1001  (shown in  FIGS. 10A-10B ) with which the locking hub  1004  is to be used. Accordingly, when in an uncompressed state, the seal  1022  allows free or relatively low friction movement of the elongate body  1001  within the lumen segment  1062 C. 
     As illustrated in  FIG. 12 , the seal  1022  may include a distal tapered section  1023  and a proximal cylindrical section  1025 . As previously noted, the distal tapered section  1023  may be sized and shaped to be received by a corresponding internal tapered surface  1027  of the hub body  1006  (shown in  FIGS. 10A-10B ) such that when a longitudinal force is applied to the seal  1022 , the interface between the distal tapered section  1023  and the internal tapered surface  1027  redirects a portion of the longitudinal force inwardly toward a longitudinal axis  1015  of the catheter  1000  (each illustrated in  FIGS. 10A-10B ). Such inward force causes reduction of the inner diameter  1064  such that the seal  1022  engages the elongate body  1001 . The proximal cylinder section  1025 , in contrast, may be sized to be received within a corresponding section of the hub body  1006  proximal the internal tapered surface  1027 . In certain implementations, the proximal cylinder section  1025  may be sized to have an outer diameter greater than the inner diameter of the hub body  1006  such that the seal  1022  is maintained within the hub body  1006  by an interference fit. 
       FIG. 13  is an isometric view of the shuttle  1024  of  FIGS. 10A-10B . As illustrated, the shuttle  1024  includes indentations disposed on opposite sides of the shuttle  1022 . For example, the shuttle  1024  includes indentations  1054 ,  1055  (indentations  1056 ,  1057  being disposed on an opposite side of the shuttle  1024 , as shown in  FIGS. 10A-10B ) that receive corresponding angled extensions  1050 - 1053  of the buttons  1008 ,  1010 .  FIG. 14  is an isometric view of the button  1008 , for example, further illustrating angled extensions  1050 ,  1051 . As previously discussed in the context of  FIGS. 10A-10B , the angled protrusions  1050 - 1053  of the buttons  1008 ,  1010  interface with the indentations  1054 - 1056  of the shuttle  1022  such that when the buttons  1008 ,  1010  are depressed, the angled protrusions  1050 - 1053  push against the indentations  1054 - 1056  to proximally translate the shuttle  1024  within the housing body  1006  and away from the seal  1022 . In certain implementations, one or both of the angled protrusions  1050 - 1053  and the indentations  1054 - 1056  may be formed from a low friction material or may be coated with a low-friction coating to reduce drag therebetween. The shuttle  1024  is also illustrated as including ribs  1034 ,  1036  that may be receive by corresponding channels defined by the hub body  1006  to guide the shuttle  1024  within the hub body  1006 . The shuttle  1024  may further include a proximal recess  1040  shaped to receive the biasing element  1026 . 
     As previously noted,  FIG. 14  is an isometric view of the button  1008  of  FIGS. 10A-10B . As illustrated, the button  1008  may include flexible sidewalls  1042 ,  1044  including lips or similar features shaped to engage the hub body  1006 . For example, during assembly the flexible sidewalls  1042 ,  1044  may be depressed toward each other to allow insertion into the hub body  1006 . Once inserted the flexible sidewalls  1042 ,  1044  may expand such that the lips prevent or resist removal of the button  1008  once inserted into the hub body  1006 . 
       FIG. 15  is an isometric view of the cap  1018  of  FIGS. 10A-10B . As previously discussed, the cap  1018  is inserted into and coupled to the proximal end of the hub body  1006 . By doing so, the cap  1018  contains the biasing element  1026  and provides a rigid structure against which the biasing element  1026  is supported. In certain implementations, the cap  1018  is coupled to the hub body  1006  by, without limitation, one or more of ultrasonic welding, a snap fit, and an adhesive. Coupling of the cap  1018  to the hub body  1006  may also be reinforced by inserting pins or similar fastening elements through the cap  1018  and into the hub body  1006 . For example, the cap  1018  includes holes  1070 ,  1072  that may be aligned with corresponding holes of the hub body  1006  to receive coupling pins or similar fasteners. 
     The implementations of the present disclosure discussed in  FIGS. 9A-10B  include configurations in which the internal components of the locking hub are arranged such that the seal is distally disposed relative to the shuttle and biasing element. In such implementations, the biasing element is configured to bias the shuttle in a distal direction to compress the seal. In other implementations, however, the seal may instead be proximal the shuttle and the biasing element may be distal the shuttle such that the shuttle is biased in a proximal direction against the seal. In such implementations, the seal may be arranged such that it tapers in a proximal direction and the cap of the locking hub may include a tapered internal surface, similar to the internal surface  1027  of the hub body  1006 , adapted to cause inward compression of the seal when a longitudinal force is applied to the seal. 
     Any of the above mentioned embodiments may also include, but are not limited to, electronic indicators on the system (e.g., LEDS or screens) or on adjunct support-screens to communicate status. Finally, the above mentioned embodiments may also include shaft position indicators via, for example, detents located on the shaft of the deflectable catheter and complementary features for interacting on the detents, the complementary features being located on the guide catheter or even the locking hub. Of course the opposite arrangement is also possible. The position indicator aspects can be used to notify the user of the extent to which the protective sleeve covers the leadless pacemaker. 
     As for additional details pertinent to the present invention, materials and manufacturing techniques may be employed as within the level of those with skill in the relevant art. The same may hold true with respect to method-based aspects of the invention in terms of additional acts commonly or logically employed. Also, it is contemplated that any optional feature of the inventive variations described may be set forth and claimed independently, or in combination with any one or more of the features described herein. Likewise, reference to a singular item, includes the possibility that there are plural of the same items present. More specifically, as used herein and in the appended claims, the singular forms “a,” “and,” “said,” and “the” include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation. Unless defined otherwise herein, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The breadth of the present invention is not to be limited by the subject specification, but rather only by the plain meaning of the claim terms employed.