Patent Publication Number: US-2013253342-A1

Title: Pass-through implantable medical device delivery catheter

Description:
This application claims the benefit of U.S. Provisional Application No. 61/615,611, filed on Mar. 26, 2012, the entire content of which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to delivery and deployment techniques for implantable medical devices. 
     BACKGROUND 
     Various implantable medical devices (IMDs) may be used for therapeutically treating or monitoring one or more physiological conditions of a patient. Such IMDs may be adapted to monitor or treat conditions or functions relating to heart, blood vessels, muscle, nerve, brain, stomach, endocrine organs or other organs and their related functions. Advances in design and manufacture of miniaturized IMDs have resulted in IMDs capable of therapeutic as well as diagnostic functions such as pacemakers, cardioverters, defibrillators, biochemical sensors, pressure sensors, various endovascular IMDs and the like. Such IMDs may have electronic functions and may be associated with electrical leads or may be wireless, with the ability to transmit data electronically either to another IMD implanted in the patient or to another device located externally of the patient, or both. Other IMDs may have purely mechanical and/or pharmaceutical functions, such as stents. 
     Although implantation of some IMDs requires a surgical procedure (e.g., pacemakers, defibrillators, etc.) other IMDs may be small enough to be delivered and placed at an intended deployment site in a relatively noninvasive manner, such as by a delivery catheter introduced percutaneously. Delivery also may be accomplished by advancing a catheter intravascularly through an exposed vasculature during a surgical procedure. 
     SUMMARY 
     In different examples, this disclosure describes techniques for remote deployment of IMDs. 
     In one example, this disclosure is directed to a kit for intravascular implantation of an implantable medical device within a patient, the kit comprising an elongated inner sheath with a distal end, a first coupling module slidably connected to the inner sheath, an elongated outer sheath forming an inner lumen with a distal opening and a proximal opening, the outer sheath sized to traverse a vasculature of the patient. The proximal opening is configured to receive the distal end of the inner sheath. The inner lumen is sized to receive the inner sheath and to contain the implantable medical device. The kit further comprises a mating coupling module secured to a proximal end of the outer sheath. The mating coupling module is configured to connect to the first coupling module such that the inner sheath is axially aligned with the outer sheath. The inner sheath is slidable within the outer sheath while the first coupling module is connected to the mating coupling module. 
     In another example, this disclosure is directed to a method for intravascular implantation of an implantable medical device within a patient comprising positioning a distal end of an elongated outer sheath via a vasculature of the patient proximate to a target site within the patient. The outer sheath forms an inner lumen with a distal opening and a proximal opening. The method further includes connecting a first coupling module that is slidably connected to an elongated inner sheath with a mating coupling module secured to a proximal end of the outer sheath. The mating coupling module is configured to connect to the first coupling module such that the inner sheath is axially aligned with the outer sheath. The inner sheath has a distal end. An implantable medical device is positioned in the inner lumen of the outer sheath. The method further includes pushing the implantable medical device through the inner lumen of the outer sheath and out of the distal opening with the inner sheath to deploy the implantable medical device proximate to the target site within the patient. 
     In a different example, this disclosure is directed to a kit for intravascular implantation of an implantable medical device within a patient, the kit comprising an elongated outer sheath forming an inner lumen with a distal opening and a proximal opening, the outer sheath sized to traverse a vasculature of the patient. The kit further includes an elongated inner sheath with a tapered distal end. The tapered distal end is configured to substantially fill the inner lumen of the outer sheath and close-off the distal opening of the outer sheath. The inner sheath is slidable within the inner lumen of the outer sheath. The inner sheath is selectably removable from the inner lumen of the outer sheath by sliding the inner sheath out of the proximal opening of the outer sheath. The kit further includes an elongated deployment receptacle including a deployment bay at a distal end of the deployment receptacle. The deployment receptacle is slidable within the inner lumen of the outer sheath when the inner sheath is not within the inner lumen of the outer sheath. The deployment bay is configured to carry an implantable medical device through the inner lumen of the outer sheath and facilitate deployment of the implantable medical device out of the distal opening of the outer sheath. 
     In another example, this disclosure is directed to a method for intravascular implantation of an implantable medical device within a patient comprising positioning a distal end of an elongated outer sheath via a vasculature of the patient proximate to a target site within the patient. The outer sheath forms an inner lumen with a distal opening and a proximal opening. The method further includes inserting an elongated deployment receptacle including a deployment bay at a distal end of the deployment receptacle into the proximal opening of the outer sheath. An implantable medical device is positioned within deployment bay, sliding the deployment receptacle through the inner lumen of the outer sheath until the deployment bay is adjacent to the distal opening of the outer sheath, and deploying the implantable medical device from the deployment bay proximate to the target site within the patient. 
     In a different example, this disclosure is directed to a kit for intravascular implantation of an implantable medical device within a patient, the kit comprising an elongated outer sheath forming an inner lumen with a distal opening, the outer sheath sized to traverse a vasculature of the patient. The kit further includes an elongated inner sheath with an inflatable member at a distal portion of the inner sheath. The inflatable member is selectively inflatable from a proximal end of the inner sheath. The inflatable member is configured to substantially fill the inner lumen and close-off the distal opening of the outer sheath when inflated. The inner sheath is slidable within the inner lumen of the outer sheath. The inner sheath further includes a stopper proximally located relative to the inflatable member. The inflatable member is remotely controllable from a proximal end of the inner sheath to retract in a proximal direction towards the stopper. The kit is configured such that the inflatable member can be retracted in a proximal direction towards the stopper and past an implantable medical device positioned within a distal portion of the outer sheath. 
     In another example, this disclosure is directed to a method for intravascular implantation of an implantable medical device within a patient comprising positioning a distal end of an assembly including an elongated outer sheath and an elongated inner sheath via a vasculature of the patient proximate to a target site within the patient. The outer sheath forms an inner lumen with a distal opening. The inner sheath includes an inflatable member at a distal portion of the inner sheath. The inflatable member is selectively inflatable from a proximal end of the inner sheath. The inflatable member is inflated to substantially fill the inner lumen and close-off the distal opening of the outer sheath. The inner sheath further includes a stopper proximally located relative to the inflatable member. The inner sheath is slidable within the inner lumen of the outer sheath. The method further includes deflating the inflatable member, and retracting the inflatable member in a proximal direction towards the stopper and past an implantable medical device that is positioned within a distal portion of the outer sheath. 
     In a different example, this disclosure is directed to a kit for intravascular implantation of an implantable medical device within a patient, the kit comprising an elongated outer sheath forming an inner lumen with a distal opening, the outer sheath sized to traverse a vasculature of the patient. The kit further includes an elongated inner sheath with an enlarged distal portion. The enlarged distal portion is configured to substantially fill the inner lumen and close-off the distal opening of the outer sheath. The enlarged distal portion is slidable relative to the outer sheath. The inner sheath further includes a tether with a helical element that is remotely controllable from a proximal end of the inner sheath to release the implantable medical device from a distal portion of the outer sheath. 
     In another example, this disclosure is directed to a method for intravascular implantation of an implantable medical device within a patient comprising positioning a distal end of an assembly including an elongated outer sheath and an elongated inner sheath via a vasculature of the patient proximate to a target site within the patient. The outer sheath forms an inner lumen with a distal opening. The inner sheath includes enlarged distal portion. The enlarged distal portion substantially fills the inner lumen to close-off the distal opening of the outer sheath. The enlarged distal portion is slidable relative to the outer sheath. The inner sheath further includes a tether with a helical element. The method further includes releasing an implantable medical device from a distal portion of the outer sheath by remotely rotating the helical element such that the helical element releases a looped element of the implantable medical device. 
     In a different example, this disclosure is directed to a kit for intravascular implantation of an implantable medical device within a patient, the kit comprising an elongated outer sheath forming a first inner lumen with a distal opening, the outer sheath sized to traverse a vasculature of the patient. The kit further includes an elongated inner sheath forming a second inner lumen. An outer diameter of the inner sheath is smaller than the diameter of the first inner lumen such that the inner sheath fits within the first inner lumen. The inner sheath is slidable within the first inner lumen. The second inner lumen at a distal end of the inner sheath is configured to carry an implantable medical device. The inner sheath forms a slit at a distal end of the inner sheath to facilitate deployment of the implantable medical device out of the distal opening of the outer sheath. 
     In another example, this disclosure is directed to a method for intravascular implantation of an implantable medical device within a patient comprising positioning a distal end of an assembly including an elongated outer sheath and an elongated inner sheath via a vasculature of the patient proximate to a target site within the patient. The outer sheath forms an inner lumen with a distal opening. The inner sheath forms a second inner lumen. An outer diameter of the inner sheath is smaller than the diameter of the first inner lumen such that the inner sheath fits within the first inner lumen. The inner sheath is slidable within the first inner lumen. Assembly further includes an implantable medical device carried within the second inner lumen at a distal end of the inner sheath. The inner sheath forms a slit at a distal end of the inner sheath to facilitate deployment of the implantable medical device out of the distal opening of the outer sheath. The method further includes sliding the distal end of the inner sheath out of the first inner lumen to expose a portion of the inner sheath and a portion of the implantable medical device out of the distal end of the outer sheath. 
     In a different example, this disclosure is directed to a method for intravascular implantation of an implantable medical device within a patient comprising positioning a distal end of an elongated outer sheath forming an inner lumen with a distal opening adjacent a target site within a vasculature of a patient, partially deploying an implantable medical device from the distal opening. The implantable medical device includes an expandable fixation element expandable from a collapsed position to an expanded position, wherein at least a portion of the expandable fixation element assumes the expanded position when the implantable medical device is partially deployed from the distal opening. The method further comprises advancing the distal end of the outer sheath within the vasculature with the implantable medical device partially deployed from the distal opening, monitoring at least one of the vasculature and the portion of the expandable fixation element for deflection to determine when the size of the portion of the expandable fixation element corresponds to the size of the vasculature. 
     In a different example, this disclosure is directed to a kit for intravascular implantation of an implantable medical device within a patient, the kit comprising an elongated outer sheath forming an inner lumen with a distal opening, the outer sheath sized to traverse a vasculature of the patient. The kit further includes an elongated inner sheath with a stopper configured engage a proximal side of the implantable medical device to preclude the implantable medical device from being located at a more proximal position than the stopper within the inner lumen of the outer sheath. The inner sheath further includes a tether configured to form a loop on a distal side of the stopper, the loop being configured to engage a looped element of the implantable medical device to couple the implantable medical device to the inner sheath. The stopper is slidable relative to the outer sheath between a position that is proximally located relative to the distal opening of the outer sheath and a position in which at least a portion of the stopper is distally located relative to the distal opening of the outer sheath. The tether is configured to release the looped element of the implantable medical device from the inner sheath by opening the tether loop when the at least a portion of the stopper is located distally relative to the distal opening of the outer sheath. 
     In another example, this disclosure is directed to a kit for intravascular implantation of an implantable medical device within a patient, the kit comprising an elongated outer sheath forming an inner lumen with a distal opening. The outer sheath sized to traverse a vasculature of the patient. The inner lumen is sized to hold the implantable medical device. The kit further includes an elongated inner sheath with a distal end. The inner sheath is located within the inner lumen of the outer sheath, and a deployment handle located at proximal ends of the outer sheath and the inner sheath. The deployment handle includes a sheath retraction mechanism that facilitates selectively retracting the outer sheath relative to the inner sheath to facilitate remote deployment of the implantable medical device out of the distal opening of the inner lumen of the outer sheath. 
     In another example, this disclosure is directed to a method for intravascular implantation of an implantable medical device within a patient comprising positioning a distal end of an assembly including an elongated outer sheath and an elongated inner sheath via a vasculature of the patient proximate to a target site within the patient. The outer sheath forms an inner lumen with a distal opening. The inner sheath includes a stopper configured to engage a proximal side of the implantable medical device to preclude the implantable medical device from being located at a more proximal position than the stopper within the inner lumen of the outer sheath. The inner sheath further includes a tether forming a loop on a distal side of the stopper, the loop being in engagement with a looped element of the implantable medical device to couple the implantable medical device to the inner sheath within the inner lumen of the outer sheath. The method further comprises retracting the outer sheath relative to the inner sheath such that the implantable medical device exits the inner lumen via the distal opening. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual drawing illustrating an example system that includes an implantable medical device (IMD) coupled to implantable medical leads and a leadless sensor. 
         FIG. 2  is a conceptual drawing illustrating, in greater detail, the example IMD, leads, and sensor of  FIG. 1  in conjunction with a heart. 
         FIG. 3  is a conceptual diagram illustrating an example therapy system comprising a leadless IMD that may be used to monitor one or more physiological parameters of a patient and/or provide therapy to the heart of a patient. 
         FIG. 4  illustrates the leadless IMD of  FIG. 3  in further detail. 
         FIG. 5  illustrates a leadless IMD including an expandable fixation element configured for securing the leadless IMD within a vasculature. 
         FIGS. 6-8E  illustrate an example system for intravascular delivery of an IMD during an implantation procedure. 
         FIGS. 9A-9D  illustrate example techniques for intravascular delivery of a sheath. 
         FIGS. 10A-10D  illustrate example techniques for intravascular delivery of an IMD through a sheath using a deployment receptacle. 
         FIGS. 11A-11D  illustrate example techniques for intravascular delivery of an outer sheath using an inner sheath with a distal inflatable member. 
         FIGS. 12A-12C  illustrate example techniques for intravascular delivery of an IMD using a delivery catheter with a distal inflatable member. 
         FIGS. 13A-13B  illustrate the distal end of an inner sheath with an inflatable member as shown in  FIGS. 12A-12C  in further detail. 
         FIG. 14  illustrates the distal end of an inner sheath with an inflatable member and a distal tapered flexible tip. 
         FIGS. 15A-15F  illustrate example techniques for intravascular delivery of an IMD using a delivery catheter with a slidable inner sheath including an enlarged distal portion and a tether. 
         FIGS. 16A-16B  illustrate example techniques for intravascular delivery of an IMD using a delivery catheter with a slidable inner sheath including an inflatable distal portion and a tether. 
         FIGS. 17A-17E  illustrate example techniques for intravascular delivery of an IMD using an inner sheath being configured to carry an IMD at its distal end, the inner sheath forming a slit at its distal end to facilitate deployment of the IMD. 
         FIGS. 18A-18C  illustrate example techniques for measuring the size of a vasculature using a partially deployed IMD within the deployment receptacle of  FIGS. 10A-10D . 
         FIG. 19  illustrates example techniques for measuring the size of a vasculature using a partially deployed IMD within the inner sheath of  FIGS. 17A-17E . 
         FIG. 20  illustrates example techniques for measuring the size of a vasculature using the delivery catheter with an inner sheath including an inflatable distal portion of  FIGS. 16A-16B . 
         FIG. 21  is a flowchart illustrating example techniques for measuring the size of a vasculature using a partially deployed IMD. 
         FIGS. 22-24C  illustrate example techniques for intravascular delivery of an IMD using a delivery catheter that includes a tether forming a loop to engage a looped element of the IMD. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a conceptual diagram illustrating an example medical system  10  that may be used for sensing of physiological parameters of patient  14  and/or to provide therapy to heart  12  of patient  14 . Medical system  10  includes an IMD  16 , which is coupled to leads  18 ,  20 , and  22 , and programmer  24 . IMD  16  may be, for example, an implantable pacemaker, cardioverter, and/or defibrillator that provides electrical signals to heart  12  via electrodes coupled to one or more of leads  18 ,  20 , and  22 . Patient  14  is ordinarily, but not necessarily, a human patient. 
     IMD  16  may include electronics and other internal components necessary or desirable for executing the functions associated with the device. In one example, IMD  16  includes one or more processors, memory, a signal generator, sensing module and telemetry modules, and a power source. In general, memory of IMD  16  may include computer-readable instructions that, when executed by a processor of the IMD, cause it to perform various functions attributed to the device herein. For example, a processor of IMD  16  may control the signal generator and sensing module according to instructions and/or data stored on memory to deliver therapy to patient  14  and perform other functions related to treating condition(s) of the patient with IMD  16 . 
     The signal generator of IMD  16  may generate electrical stimulation that is delivered to patient  12  via electrode(s) on one or more of leads  18 ,  20 , and  22 , in order to provide, e.g., cardiac sensing, pacing signals, or cardioversion/defibrillation shocks. 
     The sensing module of IMD  16  may monitor electrical signals from electrode(s) on leads  18 ,  20 , and  22  of IMD  16  in order to monitor electrical activity of heart  12 , such as electrocardiogram depolarizations of heart  12 . In one example, the sensing module may include a switch module to select which of the available electrodes on leads  18 ,  20 , and  22  of IMD  16  are used to sense the heart activity. Additionally, the sensing module of IMD  16  may include multiple detection channels, each of which includes an amplifier, as well as an analog-to-digital converter for digitizing the signal received from a sensing channel for, e.g., electrogram signal processing by a processor of the IMD. 
     A telemetry module of IMD  16  may include any suitable hardware, firmware, software or any combination thereof for communicating with another device, such as programmer  24 . Under the control of a processor of IMD  16 , the telemetry module may receive downlink telemetry from and send uplink telemetry to programmer  24  with the aid of an antenna, which may be internal and/or external. 
     The various components of IMD  16  may be coupled to a power source, which may include a rechargeable or non-rechargeable battery. A non-rechargeable battery may be capable of holding a charge for several years, while a rechargeable battery may be inductively charged from an external device, e.g., on a daily or weekly basis. 
     Leads  18 ,  20 ,  22  extend into the heart  12  of patient  14  to facilitate sensing of electrical activity of heart  12  and/or delivery of electrical stimulation to heart  12  by IMD  16 , or to allow other sensors or transducers attached to the leads to make measurements. In the example shown in  FIG. 1 , right ventricular (RV) lead  18  extends through one or more veins (not shown), the superior vena cava (not shown), and right atrium  26 , and into right ventricle  28 . Left ventricular (LV) coronary sinus lead  20  extends through one or more veins, the vena cava, right atrium  26 , and into the coronary sinus  30  to a region adjacent to the free wall of left ventricle  32  of heart  12 . Right atrial (RA) lead  22  extends through one or more veins and the vena cava, and into the right atrium  26  of heart  12 . 
     System  10  also includes IMD  15 , which includes a vascular sensor  38  ( FIG. 5 ). While referred to as including a vascular sensor, IMD  15  could be within a chamber of the heart, or generally within the circulatory system. In the illustrated example, IMD  15  is implanted in pulmonary artery  39 . In one example, IMD  15  is configured to sense blood pressure of patient  14 . For example, IMD  15  may be arranged in pulmonary artery  39  and be configured to sense the pressure of blood flowing from the right ventricle outflow tract (RVOT) from right ventricle  28  through the pulmonary valve to pulmonary artery  39 . IMD  15  may therefore directly measure the pulmonary artery diastolic pressure (PAD) of patient  14 . The PAD value is a pressure value that can be employed in patient monitoring. For example, PAD may be used as a basis for evaluating congestive heart failure in a patient. 
     In other examples, however, IMD  15  may be employed to measure blood pressure values other than PAD. For example, IMD  15  may be arranged in right ventricle  28  of heart  14  to sense RV systolic or diastolic pressure. As shown in  FIG. 1 , IMD  15  is positioned in the main trunk of pulmonary artery  39 . In other examples, a sensor, such as IMD  15  may be either positioned in the right or left pulmonary artery beyond the bifurcation of the pulmonary artery. 
     Moreover, the placement of IMD  15  is not restricted necessarily to the pulmonary side of the circulation. It could potentially be placed in the systemic side of the circulation—e.g., under certain conditions and with appropriate safety measures, it could even be placed in the left atrium, left ventricle, or aorta. Additionally, IMD  15  is not restricted to placement within the cardiovascular system. For example, the sensor might be placed in the renal circulation. IMD  15  placed in the renal circulation may be beneficial, for example, in circumstances in which IMD  16  is configured to treat heart failure based on some estimate of the degree of renal insufficiency in the patient derived from the monitoring of pressure or some other indication of renal circulation by the sensor. In this or other non-cardiovascular examples, the sensor may still communicate with IMD  16 , or one or more sensors on leads  18 ,  20 , or  22 . 
     In some examples, IMD  15  includes a pressure sensor configured to respond to the absolute pressure inside pulmonary artery  39  of patient  14 . IMD  15  may be, in such examples, any of a number of different types of pressure sensors. One form of pressure sensor that may be useful for measuring blood pressure is a capacitive pressure sensor. Another example pressure sensor is an inductive sensor. In some examples, IMD  15  may also comprise a piezoelectric or piezoresistive pressure transducer. In some examples, IMD  15  may comprise a flow sensor. 
     In one example, IMD  15  comprises a leadless pressure sensor including capacitive pressure sensing elements configured to measure blood pressure within pulmonary artery  39 . As illustrated in  FIGS. 1 and 2 , IMD  15  may be in wireless communication with IMD  16  or one or more sensors on leads  18 ,  20 , or  22 , e.g., in order to transmit blood pressure measurements to the IMD. IMD  15  may employ, e.g., radio frequency (RF) or other telemetry techniques for communicating with IMD  16  and other devices, including, e.g., programmer  24 . In another example, IMD  15  may include a tissue conductance communication (TCC) system by which the device employs tissue of patient  14  as an electrical communication medium over which to send and receive information to and from IMD  16  and other devices. 
     In some examples, IMD  15  may be implanted within other body lumens, such as other vasculature of patient  14 . Additionally or alternatively to including a pressure sensor, IMD  15  may also include sensors such as, but not limited to an electrocardiogram sensor, a fluid flow sensor, a tissue oxygen sensor, an accelerometer, a glucose sensor, a potassium sensor, a thermometer and/or other sensors. In some examples, system  10  may include a plurality of sensors  38 , e.g., to provide sensing of one or more physiological conditions of patient  14  at a variety of locations. 
     Referring again to  FIG. 1 , system  10  may, in some examples, additionally or alternatively include one or more leads or lead segments (not shown in  FIG. 1 ) that deploy one or more electrodes within the vena cava or other vein. These electrodes may allow alternative electrical sensing configurations that may provide improved or supplemental sensing in some patients. Furthermore, in some examples, physiological therapy/monitoring system  10  may include temporary or permanent epicardial or subcutaneous leads, instead of or in addition to leads  18 ,  20  and  22 . Such leads may be used for one or more of cardiac sensing, pacing, or cardioversion/defibrillation. Moreover, it is conceivable that some sort of biodegradable fixation element could be used to hold IMD  15  to the epicardium, while a chronic fixation element fixes the IMD  15  permanently in that location. Once fixed permanently, the biodegradable fixation element would dissolve in a controlled fashion, leaving the IMD  15  permanently attached to the epicardium. 
     IMD  16  may sense electrical signals attendant to the depolarization and repolarization of heart  12  via electrodes (not shown in  FIG. 1 ) coupled to at least one of the leads  18 ,  20 ,  22 . In some examples, IMD  16  provides pacing pulses to heart  12  based on the electrical signals sensed within heart  12 . The configurations of electrodes used by IMD  16  for sensing and pacing may be unipolar or bipolar. IMD  16  may detect arrhythmia of heart  12 , such as tachycardia or fibrillation of ventricles  28  and  32 , and may also provide defibrillation therapy and/or cardioversion therapy via electrodes located on at least one of the leads  18 ,  20 ,  22 . In some examples, IMD  16  may be programmed to deliver a progression of therapies, e.g., pulses with increasing energy levels, until a fibrillation of heart  12  is stopped. IMD  16  detects fibrillation employing any of a number of known fibrillation detection techniques. 
     In some examples, IMD  16  may also be solely a monitoring device, attached to various sensors, or even a monitoring device that only communicates with one or more devices  38  in various locations of the heart, or other vasculature, or even other organs. Such a device could be used, for example, to provide an integrated physiologic monitoring system that monitors, e.g., heart failure and one or more of its comorbidities (e.g. diabetes, renal function, etc.). Further, IMD  16  could be a combined monitoring and therapy system with multiple sensor and or “remote” therapy devices,  38 . For example, IMD  16  could control a devices, which may have similar outer housing dimensions, and may be implanted similarly to IMD  15 , but which are configured to act as leadless pacemakers, in the right and left ventricles, (or on the left ventricular epicardium), as a means of providing cardiac resynchronization. IMD  16  could then also communicate with other sensors  38  in other vessels/organs, that serve primarily as sensors of flow, pressure, or other parameters, for the purpose of additional monitoring and control of heart failure. Heart failure is rapidly becoming viewed as a multi-system disease, which may affect the heart, lungs, kidneys, and pancreatic function. 
     Programmer  24  shown in  FIG. 1  may be a handheld computing device, computer workstation, or networked computing device. Programmer  24  may include electronics and other internal components necessary or desirable for executing the functions associated with the device. In one example, programmer  24  includes one or more processors and memory, as well as a user interface, telemetry module, and power source. In general, memory of programmer  24  may include computer-readable instructions that, when executed by a processor of the programmer, cause it to perform various functions attributed to the device herein. Memory, processor(s), telemetry, and power sources of programmer  24  may include similar types of components and capabilities described above with reference to similar components of IMD  16 . The programmer may also be a dedicated wireless system that communicates with IMD  16  remotely, say, from the patient&#39;s bedside table, while the patient sleeps. 
     In one example, programmer  24  includes a user interface that receives input from a user. The user interface may include, for example, a keypad and a display, which may be, for example, a cathode ray tube (CRT) display, a liquid crystal display (LCD) or light emitting diode (LED) display. The keypad may take the form of an alphanumeric keypad or a reduced set of keys associated with particular functions. Programmer  24  can additionally or alternatively include a peripheral pointing device, such as a mouse, via which a user may interact with the user interface. In some examples, a display of programmer  24  may include a touch screen display, and a user may interact with programmer  24  via the display. The user may also interact with programmer  24  remotely via a networked computing device. Or, the “programmer” may be a fully automated monitoring base station for use in the patient&#39;s home, with little or no capability for the user to provide input or programming of the implanted device. A physician could also log into the programmer  24  from a remote location via the internet, cell phone technology, or other satellite-based communication, and program the implanted device(s). 
     A user, such as a physician, technician, surgeon, electrophysiologist, or other clinician, may interact with programmer  24  to communicate with IMD  16 . For example, the user may interact with programmer  24  to retrieve physiological or diagnostic information from IMD  16 . A user may also interact with programmer  24  to program IMD  16 , e.g., select values for operational parameters of the IMD. 
     For example, the user may use programmer  24  to retrieve information from IMD  16  regarding the rhythm of heart  12 , trends therein over time, arrhythmic episodes, or sensor trends). As another example, the user may use programmer  24  to retrieve information from IMD  16  regarding other sensed physiological parameters of patient  14 , such as intracardiac or intravascular pressure, activity, posture, respiration, or thoracic impedance. The sensed physiological parameters may be based on information received from IMD  15 . As another example, the user may use programmer  24  to retrieve information from IMD  16  regarding the performance or integrity of IMD  16  or other components of system  10 , such as leads  18 , and  22 , or a power source of IMD  16 . In some examples, this information may be presented to the user as an alert. 
     The user may use programmer  24  to program a therapy progression, select electrodes used to deliver electrical stimulation to heart  12  (e.g., in the form of pacing pulses or cardioversion or defibrillation shocks), select waveforms for the electrical stimulation, or select or configure a fibrillation detection algorithm for IMD  16 . The user may also use programmer  24  to program aspects of other therapies provided by IMD  16 , such as cardioversion or pacing therapies. In some examples, the user may activate certain features of IMD  16  by entering a single command via programmer  24 , such as depression of a single key or combination of keys of a keypad or a single point-and-select action with a pointing device. 
     IMD  16  and programmer  24  may communicate via wireless communication, e.g. via telemetry modules in each of the devices using any number of known techniques. Examples of communication techniques may include, for example, low frequency or RF telemetry, but other techniques are also contemplated. In some examples, programmer  24  may include a programming head that may be placed proximate to the patient&#39;s body near the IMD  16  implant site in order to improve the quality or security of communication between IMD  16  and programmer  24 . Other example medical systems need not have IMD  16  or provide therapy. For example, a medical system may only include IMD  15 , which may communicate directly with an eternal device, e.g., programmer  24 . 
       FIG. 2  is a conceptual diagram illustrating IMD  16  and leads  18 ,  20  and  22  of medical system  10  in greater detail. Leads  18 ,  20 ,  22  may be electrically coupled to a signal generator, e.g., stimulation generator, and a sensing module of IMD  16  via connector block  34 . In some examples, proximal ends of leads  18 ,  20 ,  22  may include electrical contacts that electrically couple to respective electrical contacts within connector block  34  of IMD  16 . In addition, in some examples, leads  18 ,  20 ,  22  may be mechanically coupled to connector block  34  with the aid of setscrews, connection pins, snap connectors, or another suitable mechanical coupling mechanism. Leads  18 ,  20   22  include electrodes for delivery of stimulation and/or sensing and may additionally include one or more sensors as mentioned above with respect to  FIG. 1 . 
     Each of the leads  18 ,  20 ,  22  includes an elongated insulative lead body, which may carry a number of concentric coiled conductors separated from one another by tubular insulative sheaths. Other lead configurations may also be used. Bipolar electrodes  40  and  42  are located adjacent to a distal end of lead  18  in right ventricle  28 . In addition, bipolar electrodes  44  and  46  are located adjacent to a distal end of lead  20  in coronary sinus  30  and bipolar electrodes  48  and  50  are located adjacent to a distal end of lead  22  in right atrium  26 . In the illustrated example, there are no electrodes located in left atrium  36 . However, other examples may include electrodes in left atrium  36 . 
     Electrodes  40 ,  44  and  48  may take the form of ring electrodes, and electrodes  42 ,  46  and  50  may take the form of extendable helix tip electrodes mounted retractably within insulative electrode heads  52 ,  54  and  56 , respectively. In other embodiments, one or more of electrodes  42 ,  46  and  50  may take the form of small circular electrodes at the tip of a tined lead or other fixation element. Leads  18 ,  20 ,  22  also include elongated electrodes  62 ,  64 ,  66 , respectively, which may take the form of a coil. Each of the electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  62 ,  64  and  66  may be electrically coupled to a respective one of the coiled conductors within the lead body of its associated lead  18 ,  20 ,  22 , and thereby coupled to respective ones of the electrical contacts on the proximal end of leads  18 ,  20  and  22 . 
     In some examples, IMD  16  includes one or more housing electrodes, such as housing electrode  58 , which may be formed integrally with an outer surface of hermetically-sealed housing  60  of IMD  16  or otherwise coupled to housing  60 . In some examples, housing electrode  58  is defined by an uninsulated portion of an outward facing portion of housing  60  of IMD  16 . Other division between insulated and uninsulated portions of housing  60  may be employed to define two or more housing electrodes. In some examples, housing electrode  58  comprises substantially all of housing  60 . Housing  60  may enclose a signal generator that generates therapeutic stimulation, such as cardiac pacing pulses and defibrillation shocks, as well as a sensing module for monitoring the rhythm of heart  12 . 
     IMD  16  may sense electrical signals attendant to the depolarization and repolarization of heart  12  via electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  62 ,  64  and  66 . The electrical signals are conducted to IMD  16  from the electrodes via the respective leads  18 ,  20 ,  22 . IMD  16  may sense such electrical signals via any bipolar combination of electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  62 ,  64  and  66 . Furthermore, any of the electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  62 ,  64  and  66  may be used for unipolar sensing in combination with housing electrode  58 . The sensed electrical signals may be processed as a cardiac electrogram (EGM) signal by IMD  16 . 
     Any combination of electrodes  40 ,  42 ,  44 ,  46 ,  48 ,  50 ,  58 ,  62 ,  64  and  66  may be considered a sensing configuration that has one or more electrodes. In some examples, a sensing configuration may be a bipolar electrode combination on the same lead, such as electrodes  40  and  42  of lead  18 . In any sensing configuration, the polarity of each electrode in the sensing configuration may be configured as appropriate for the application of the sensing configuration. 
     In some examples, IMD  16  delivers pacing pulses via bipolar combinations of electrodes  40 ,  42 ,  44 ,  46 ,  48  and  50  to cause depolarization of cardiac tissue of heart  12 . In some examples, IMD  16  delivers pacing pulses via any of electrodes  40 ,  42 ,  44 ,  46 ,  48  and  50  in combination with housing electrode  58  in a unipolar configuration. Furthermore, IMD  16  may deliver cardioversion or defibrillation pulses to heart  12  via any combination of elongated electrodes  62 ,  64 ,  66 , and housing electrode  58 . Electrodes  58 ,  62 ,  64 ,  66  may also be used to deliver cardioversion pulses, e.g., a responsive therapeutic shock, to heart  12 . Electrodes  62 ,  64 ,  66  may be fabricated from any suitable electrically conductive material, such as, but not limited to, platinum, platinum alloy or other materials known to be usable in implantable defibrillation electrodes. 
     The configuration of medical system  10  illustrated in  FIGS. 1 and 2  is merely one example. In other examples, a therapy system may include epicardial leads and/or patch electrodes instead of or in addition to the transvenous leads  18 ,  20 ,  22  illustrated in  FIG. 1 . Further, IMD  16  need not be implanted within patient  14 . In examples in which IMD  16  is not implanted in patient  14 , IMD  16  may deliver defibrillation pulses and other therapies to heart  12  via percutaneous leads that extend through the skin of patient  14  to a variety of positions within or outside of heart  12 . 
     In addition, in other examples, a therapy system may include any suitable number of leads coupled to IMD  16 , and each of the leads may extend to any location within or proximate to heart  12 . For example, other examples of therapy systems may include three transvenous leads located as illustrated in  FIGS. 1 and 2 , and an additional lead located within or proximate to left atrium  36 . As another example, other examples of therapy systems may include a single lead that extends from IMD  16  into right atrium  26  or right ventricle  28 , or two leads that extend into a respective one of the right ventricle  26  and right atrium  26 . 
       FIG. 3  is a conceptual diagram illustrating an example medical system  11  that may be used to monitor one or more physiological parameters of patient  14  and/or to provide therapy to heart  12  of patient  14 . Medical system  11  includes IMD  17 , which is coupled to programmer  24 . IMD  17  may be an implantable leadless pacemaker that provides electrical signals to heart  12  via one or more electrodes (not shown in  FIG. 3 ) on its outer housing. Additionally or alternatively, IMD  17  may sense electrical signals attendant to the depolarization and repolarization of heart  12  via electrodes on its outer housing. In some examples, IMD  17  provides pacing pulses to heart  12  based on the electrical signals sensed within heart  12 . 
     IMD  17  includes a set of active fixation tines to secure IMD  17  to a patient tissue. In other examples, IMD  17  may be secured with other techniques such as a helical screw or with an expandable fixation element. In the example of  FIG. 3 , IMD  17  is positioned wholly within heart  12  proximate to an inner wall of right ventricle  28  to provide right ventricular (RV) pacing. Although IMD  17  is shown within heart  12  and proximate to an inner wall of right ventricle  28  in the example of  FIG. 3 , IMD  17  may be positioned at any other location outside or within heart  12 . For example, IMD  17  may be positioned outside or within right atrium  26 , left atrium  36 , and/or left ventricle  32 , e.g., to provide right atrial, left atrial, and left ventricular pacing, respectively. 
     Depending on the location of implant, IMD  17  may include other stimulation functionalities. For example, IMD  17  may provide atrioventricular nodal stimulation, fat pad stimulation, vagal stimulation, or other types of neurostimulation. In other examples, IMD  17  may be a monitor that senses one or more parameters of heart  12  and may not provide any stimulation functionality. In some examples, medical system  11  may include a plurality of leadless IMDs  17 , e.g., to provide stimulation and/or sensing at a variety of locations. 
     As mentioned above, IMD  17  includes a set of active fixation tines. The active fixation tines in the set are deployable from a spring-loaded position in which distal ends of the active fixation tines point away from the IMD to a hooked position in which the active fixation tines bend back towards the IMD. The active fixation tines allow IMD  17  to be removed from a patient tissue followed by redeployment, e.g., to adjust the position of IMD  17  relative to the patient tissue. For example, a clinician implanting IMD  17  may reposition IMD  17  during an implantation procedure if the original deployment of the active fixation tines provides an insufficient holding force to reliably secure IMD  17  to the patient tissue. As another example, the clinician may reposition IMD  17  during an implantation procedure if testing of IMD  17  indicates an unacceptably high capture threshold, which may be caused by, e.g., the specific location of IMD  17  or a poor electrode-tissue connection. 
       FIG. 3  further depicts programmer  24  in wireless communication with IMD  17 . In some examples, programmer  24  comprises a handheld computing device, computer workstation, or networked computing device. Programmer  24  includes a user interface that presents information to and receives input from a user. The user may also interact with programmer  24  remotely via a networked computing device. 
     A user, such as a physician, technician, surgeon, electrophysiologist, other clinician, or patient, interacts with programmer  24  to communicate with IMD  17 . For example, the user may interact with programmer  24  to retrieve physiological or diagnostic information from IMD  17 . A user may also interact with programmer  24  to program IMD  17 , e.g., select values for operational parameters of the IMD  17 . For example, the user may use programmer  24  to retrieve information from IMD  17  regarding the rhythm of heart  12 , trends therein over time, or arrhythmic episodes. 
     As an example, the user may use programmer  24  to retrieve information from IMD  17  regarding other sensed physiological parameters of patient  14  or information derived from sensed physiological parameters, such as intracardiac or intravascular pressure, intracardiac or intravascular fluid flow, activity, posture, tissue oxygen levels, respiration, tissue perfusion, heart sounds, cardiac electrogram (EGM), intracardiac impedance, or thoracic impedance. In some examples, the user may use programmer  24  to retrieve information from IMD  17  regarding the performance or integrity of IMD  17  or other components of system  17 , or a power source of IMD  17 . As another example, the user may interact with programmer  24  to program, e.g., select parameters for, therapies provided by IMD  17 , such as pacing and, optionally, neurostimulation. 
     IMD  17  and programmer  24  may communicate via wireless communication using any techniques known in the art. Examples of communication techniques may include, for example, low frequency or radiofrequency (RF) telemetry, but other techniques are also contemplated. In some examples, programmer  24  may include a programming head that may be placed proximate to the patient&#39;s body near the IMD  17  implant site in order to improve the quality or security of communication between IMD  17  and programmer  24 . 
       FIG. 4  illustrates leadless IMD  17  of  FIG. 3  in further detail. In the example of  FIG. 4 , leadless IMD  17  includes tine fixation subassembly  100  and electronic subassembly  150 . Tine fixation subassembly  100  includes active fixation tines  103  and is configured to deploy anchor leadless IMD  17  to a patient tissue, such as a wall of heart  12 . 
     Electronic subassembly  150  includes control electronics  152 , which controls the sensing and/or therapy functions of IMD  17 , and battery  160 , which powers control electronics  152 . As one example, control electronics  152  may include sensing circuitry, a stimulation generator and a telemetry module. As one example, battery  160  may comprise features of the batteries disclosed in U.S. patent application Ser. No. 12/696,890, titled IMPLANTABLE MEDICAL DEVICE BATTERY and filed Jan. 29, 2010, the entire contents of which are incorporated by reference herein. 
     The housings of control electronics  152  and battery  160  are formed from a biocompatible material, such as a stainless steel or titanium alloy. In some examples, the housings of control electronics  152  and battery  160  may include a parylene coating. Electronic subassembly  150  further includes anode  162 , which may include a titanium nitride coating. The entirety of the housings of control electronics  152  and battery  160  are electrically connected to one another, but only anode  162  is uninsulated. Alternatively, anode  162  may be electrically isolated from the other portions of the housings of control electronics  152  and battery  160 . In other examples, the entirety of the housing of battery  160  or the entirety of the housing of electronic subassembly  150  may function as an anode instead of providing a localized anode such as anode  162 . 
     Delivery tool interface  158  is located at the proximal end of electronic subassembly  150 . Delivery tool interface  158  is configured to connect to a delivery device, such as a catheter used to position IMD  17  during an implantation procedure. For example, delivery tool interface  158  represents a looped element of IMD  17  and may be engaged by a catheter during delivery as discussed herein with respect to a variety of different examples. 
     Active fixation tines  103  are deployable from a spring-loaded position in which distal ends  109  of active fixation tines  103  point away from electronic subassembly  150  to a hooked position in which active fixation tines  103  bend back towards electronic subassembly  150 . For example, active fixation tines  103  are shown in a hooked position in  FIG. 4 . Active fixation tines  103  may be fabricated of a shape memory material, which allows active fixation tines  103  to bend elastically from the hooked position to the spring-loaded position. As an example, the shape memory material may be shape memory alloy such as Nitinol. 
     In some examples, all or a portion of tine fixation subassembly  100 , such as active fixation tines  103 , may include one or more coatings. For example, tine fixation subassembly  100  may include a radiopaque coating to provide visibility during fluoroscopy. In one such example, active fixation tines  103  may include one or more radiopaque markers. As another example, active fixation tines  103  may be coated with a tissue growth promoter or a tissue growth inhibitor. A tissue growth promoter may be useful to increase the holding force of active fixation tines  103 , whereas a tissue growth inhibitor may be useful to facilitate removal of IMD  17  during an explantation procedure, which may occur many years after the implantation of IMD  17 . 
     As one example, IMD  17  and active fixation tines  103  may comprise features of the active fixation tines disclosed in U.S. Provisional Pat. App. No. 61/428,067, titled, “IMPLANTABLE MEDICAL DEVICE FIXATION” and filed Dec. 29, 2010, the entire contents of which are incorporated by reference herein. 
       FIG. 5  illustrates leadless IMD  15 , which includes sensor element  38  and expandable fixation element  41 . Expandable fixation element  41  is configured for securing leadless IMD  15  within a vasculature. 
     Expandable fixation element  41  is configured such that the outer diameter of expandable fixation element  41  is expandable to provide an interference fit with the inner diameter of pulmonary artery  39 , or other body lumen. In some examples, expandable fixation element  41  may be partially deployable. As an example, the distal end of expandable fixation element  41  may be deployed from a catheter and expanded to provide an interference fit with the body lumen while the proximal end of expandable fixation element  41  may remain in a collapsed position within the distal end of the catheter. 
     Expandable fixation element  41  allows IMD  15  to be retracted before fully deploying IMD  15 , e.g., to adjust the position of IMD  15  with a vasculature to a location in the vasculature providing a tighter (or looser) interference fit. For example, a clinician implanting IMD  15  may reposition IMD  15  during an implantation procedure if partial deployment of expandable fixation element  41  provides an insufficient holding force indicating that full deployment of expandable fixation element  41  may not reliably secure IMD  15  within the vasculature. As another example, a clinician may select an expandable fixation element with a size better suited for the vasculature than expandable fixation element  41  that provided an insufficient holding force. 
     Sensor element  38  includes control electronics that control the sensing and/or therapy functions of IMD  15  and a battery that powers the control electronics. As one example, the control electronics may include sensing circuitry and a telemetry module. Moreover, the battery may comprise features of the batteries disclosed in U.S. patent application Ser. No. 12/696,890, titled IMPLANTABLE MEDICAL DEVICE BATTERY and filed Jan. 29, 2010, the contents of which were previously incorporated by reference herein. The housing of sensor element  38  may be formed from a biocompatible material, such as stainless steel and/or titanium alloys. 
     Expandable fixation element  41  may be fabricated of a shape memory material that allows expandable fixation element  41  to bend elastically from the collapsed position to the expanded position. As an example, the shape memory material may be shape memory alloy such as Nitinol. As an example, expandable fixation element  41  may store less potential energy in the expanded position and thus be naturally biased to assume the expanded position when in the collapsed position. In this manner, expandable fixation element  41  may assume an expanded position when no longer constrained by a catheter or other delivery device. 
     In some examples, expandable fixation element  41  may resemble a stent. Techniques for a partially deployable stents that may be applied to expandable fixation element  41  are disclosed in U.S. Pat. Pub. No. 2007/0043424, titled, “RECAPTURABLE STENT WITH MINIMUM CROSSING PROFILE” and dated Feb. 22, 2007, the entire contents of which are incorporated by reference herein, as well as U.S. Pat. Pub. No. 2009/0192585, titled, “DELIVERY SYSTEMS AND METHODS OF IMPLANTATION FOR PROSTETIC HEART VALVES” and dated Jul. 30, 2009, the entire contents of which are also incorporated by reference herein. 
     In some examples, all or a portion of expandable fixation element  41 , may include one or more coatings. For example, expandable fixation element  41  may include a radiopaque coating to provide visibility during fluoroscopy. As another example, expandable fixation element  41  may be coated with a tissue growth promoter or a tissue growth inhibitor. 
       FIGS. 6-8E  illustrate a system for intravascular delivery of an IMD during an implantation procedure. As referred to herein, intravascular IMD delivery not only includes delivering IMD through a vasculature to a target site within a vasculature, but also includes delivering IMD through a vasculature to other target sites such as target sites within the heart and other transvascular IMD deliveries. The kit of the IMD delivery system includes assembly  201  and assembly  221 , shown in  FIGS. 6 and 7  respectively. As shown in  FIG. 6 , assembly  201  includes elongated inner sheath  202  and coupling module  206 , which is slidably connected to inner sheath  202 . As shown in  FIG. 7 , assembly  221  includes elongated outer sheath  234  and coupling module  226 . 
     Outer sheath  234  of assembly  221  forms an inner lumen  227  ( FIG. 8A ) with proximal opening  233  ( FIG. 8A ) and distal opening  235  ( FIG. 7 ). Outer sheath  234  is sized to traverse a vasculature of a patient during a surgical procedure to facilitate positioning distal opening  235  proximate a target site within the patient. In different examples, outer sheath  234  may be steerable or be configured to traverse a guidewire to be directed to the target site from the access point of the vasculature. In any event, outer sheath  234  includes sufficient longitudinal stiffness to facilitate manipulation from its proximal end, but also include sufficient radial flexibility to facilitate following the patient&#39;s vasculature from an access point, such as a femoral artery, to a position proximate the target site within the patient. 
     In one example, outer sheath  234  may have an inner diameter of about 0.15 inch and an outer diameter of about 0.18 inch. It may be extruded from polyether block amide copolymer (PEBA) having 55 shore D durometer or, alternatively, may be formed as a reinforced tube having an inner polytetrafluoroethylene (PTFE) liner, an intermediate reinforcing layer of braided stainless steel and an outer jacket of 55 D durometer PEBA. In other examples, other flexible polymers may be used such as nylons and polyethylenes. The distal end of outer sheath  234  preferably includes a radiopaque ring that may be formed by incorporating barium sulfate or other suitable radiopaque material such as tungsten into or at the end of outer sheath  234 . 
     During an implantation procedure, the distal end of outer sheath  234  is positioned proximate a target site for implantation of the IMD. Inner lumen  227  is configured to receive the distal end of inner sheath  202 , as well as IMD  214 , and inner sheath  202  is used to push the IMD through the entirety outer sheath  234  to the target site. In this manner, an IMD is passed through the entirety of the inner lumen  227  before exiting distal opening  235  of outer sheath  234  during the implantation procedure. 
     In assembly  221 , coupling module  226  is secured to the proximal end of outer sheath  234 . Coupling module  226  includes valve  230 , which is configured to prevent bodily fluids from passing through inner lumen  227  and leaking out of proximal opening  233 . Coupling module  226  further includes Luer fitting  232  that facilitates flushing outer sheath  234 . 
     Coupling module  226  is configured to connect to coupling module  206  of assembly  201  such that inner sheath  202  is axially aligned with outer sheath  234 . For example, coupling modules  206 ,  226  include quick connect features for mating coupling module  226  with coupling module  206  such that inner sheath  202  is in coaxial alignment with outer sheath  234 . In the example, shown in  FIG. 6 , the quick connect features of coupling module  206  are grooves  208 , which are configured to receive protrusions  228  of coupling module  226  in a rotating snap-fit configuration. It should be noted, however, that the particular techniques used for mating coupling module  206  with coupling module  226  are not germane to this disclosure, and any suitable connecting features, such as snap-fit or threaded features may be used in other examples. 
     As previously mentioned, elongated inner sheath  202  is slidably connected to inner sheath  202  as part of assembly  201 . This allows inner sheath  202  to enter proximal opening  233  of inner lumen  227  of outer sheath  234  once coupling module  206  is mated to coupling module  226  such that inner sheath  202  is in coaxial alignment with outer sheath  234 . In one example, outer sheath  234  may have an inner diameter of about 0.15 inches and the distal end of inner sheath may have an outer diameter of about 0.12-0.14 inches at its distal end. In some examples, inner sheath  202  may have a smaller profile along its length than at its distal end, e.g., a tighter fitting distal cap to enable a good pushing surface with lower proximal friction due to the smaller profile along the length of inner sheath  202 . In addition or alternately, inner sheath  202  may be shaped with a lower contact friction design such as a triangular to star like profile to minimize drag friction with outer sheath  234 . In different examples, inner sheath may have a solid profile, a hollow tubular profile or a combination thereof. 
     In some examples, inner sheath may be formed from 70 D durometer PEBA. In other examples, other flexible polymers may be used such as nylons and polyethylenes. 
     Inner sheath  202  includes finger grip  204 , which allow a clinician to slidably move sheath  202  relative to coupling module  206  and outer sheath  234  during an implantation procedure. In some examples, inner lumen  227  of outer sheath  234  and/or the outer surface of inner sheath  202  may include a friction-reducing coating to reduce the force required to move inner sheath within inner lumen  227  of outer sheath  234 . Coupling module  206  further includes seal  210  ( FIGS. 8A-8C ), which creates a seal between coupling module  206  and inner sheath  202  while allowing inner sheath  202  to slide in a longitudinal direction. 
       FIGS. 8A-8E  illustrate techniques for intravascular implantation of IMD  214  using a kit including assemblies  201 ,  221 .  FIGS. 8A-8E  illustrate assemblies  201 ,  221  as well as IMD  214 . IMD  214  includes expandable fixation element  215 , which is deployable from a collapsed position to an expanded position secure the IMD  214  proximate a target site within a patient. 
     While  FIGS. 8A-8E  illustrate implantation techniques using IMD  214 , in different examples, IMD  214  may be substantially similar to IMD  17  ( FIG. 4 ) or IMD  15  ( FIG. 5 ). As one example, the intravascular IMD delivery system of  FIGS. 6-8E  may be used to deliver IMD  17  to a position within a heart of a patient, such as a position proximate to an inner wall of the right ventricle, within the right atrium, the left atrium, and/or left ventricle. As another example, the intravascular IMD delivery system of  FIGS. 6-8E  may be used to deliver IMD  15  to an intravascular position such as a pulmonary artery or other vasculature of the patient. 
     As shown in  FIGS. 8A-8E , coupling module  206  of assembly  201  is configured to mate to coupling module  226  of assembly  221  to facilitate intravascular implantation of IMD  214  within a patient. During an implantation procedure, the clinician would position the distal end of outer sheath  234  proximate to a target site within the patient via a vasculature accessed during a surgical procedure. For example, outer sheath  234  may be advanced into an entry vessel, such as the femoral artery, and then manipulated and navigated through the patient&#39;s vasculature until the distal end of outer sheath  234  proximate to a target site within the patient. The clinician may use imaging techniques, such as fluoroscopy, to monitor the position of outer sheath  234 , inner sheath  202 , and IMD  214  throughout the implantation procedure. Assemblies  201 ,  221  and/or IMD  214  may include radiopaque portions or markers to facilitate visualization. 
     Once the distal end of outer sheath  234  is proximate to a target site within the patient, as shown in  FIG. 8A , a clinician positions assembly  201  adjacent to assembly  221  such that coupling module  206  faces coupling module  221 . Then, as shown in  FIG. 8B , coupling module  206  is mated to coupling module  226  such that inner sheath  202  is in coaxial alignment with outer sheath  234 . 
     Coupling module  206  forms inner lumen  207 , which is configured to hold IMD  214  when coupling module  206  is not connected coupling module  226 . When coupling module  206  is mated to coupling module  226 , coupling module  206  presses open the leaflets of valve  230  such that inner lumen  207  of coupling module  206  opens to inner lumen  227  of outer sheath  234 . 
     In this manner, valve  230  is configured to open to allow inner sheath  202  to enter inner lumen  227  of outer sheath  234 . In addition, coupling module  206  forms a seal with coupling module  226  when coupling module  206  is connected to coupling module  226 . Even though valve  230  is open when coupling module  206  is connected to coupling module  226 , the seal between coupling module  206  and coupling module  226  and seal  210  between inner sheath  202  coupling module  206  combine to prevent bodily fluids from continuously exiting the patient through inner lumen  227  of outer sheath  234 . 
     As shown in  FIG. 8C , a clinician uses inner sheath  202  to push IMD  214  into proximal opening  233  of inner lumen  227  of outer sheath  234 . For example, IMD  214  may be preloaded within inner lumen  207  of coupling module  206  in assembly  201  before coupling module  206  is mated to coupling module  226 . In the example in which IMD  214  includes a pressure sensor, such as a pressure transducer, preloading IMD  214  within inner lumen  207  by the manufacturer may serve to protect the transducer from damage, such as damage caused by handling. By pushing on finger grip  204  ( FIG. 6 ), the clinician may slide inner sheath  202  in a longitudinal direction to push IMD  214  out of inner lumen  207  of coupling module  206  and into inner lumen  227  of outer sheath  234 . 
     As shown in  FIG. 8D , the clinician may continue push IMD  214  through inner lumen  227  of outer sheath  234  advancing IMD  214  through the patient&#39;s vasculature as navigated by outer sheath  234 .  FIG. 8D  illustrates inner sheath  202  pushing IMD  214  up to distal opening  235  of outer sheath  234 .  FIG. 8E  illustrates inner sheath  202  pushing IMD  214  through distal opening  235  of outer sheath  234 . As shown in  FIG. 8E , expandable fixation element  215  of IMD  214  is expanded from a collapsed position to an expanded position as IMD  214  through distal opening  235 . In the expanded position, expandable fixation element  215  will secure IMD  214  within the patient, e.g., as described with respect to IMD  17  ( FIG. 4 ) or IMD  15  ( FIG. 5 ). 
     The IMD delivery system of  FIGS. 6-8E  may provide one or more advantages. As one example, intravascular delivery of larger implants may often necessitate large bore delivery sheaths to be tracked through complex anatomy, which can require a clinician to use a variety of specialized tools. As compared to an intravascular delivery system in which an IMD is delivered to a target site simultaneously with the delivery system, e.g., the IMD is contained in the distal end of the delivery system as the delivery system is routed to the target site, the IMD delivery system of  FIGS. 6-8E  may simplify routing of the delivery system to the target site. For example, with the IMD delivery system of  FIGS. 6-8E , the clinician first routes outer sheath  234  to the target site. As clinicians often have experience routing intravascular sheaths, the process and instruments used to route outer sheath  234  may be familiar to the clinician. Furthermore, with the IMD delivery system of  FIGS. 6-8E , only after first routing outer sheath  234  to the target site, does the clinician then introduce the IMD. This may reduce the chance for damage to the IMD as compared to a delivery system in which the IMD is transported proximate to the target site within the distal portion of the delivery system. 
       FIGS. 9A-9D  illustrate example techniques for intravascular delivery of a sheath. As one example, the techniques illustrated in  FIGS. 9A-9D  may be used for intravascular delivery of outer sheath  234 . In such an example, outer sheath  320  of  FIGS. 9A-9D  may be considered to be substantially similar to outer sheath  234 . For example, outer sheath  320  may be included in an assembly with coupling module  206 , and outer sheath  320  may be used to deliver an IMD as described with respect to  FIGS. 8A-8E . However, the techniques illustrated in  FIGS. 9A-9D  are not the only manner in which outer sheath  234  may be delivered within a patient and any technique known to those in the art for intravascular delivery of a sheath may be used to position outer sheath  234 . 
     As represented by  FIG. 9A , guidewire  310  is first routed from an access point through a vasculature of the patient until distal end  312  of guidewire  310  is positioned proximate to a target site within the patient. In different examples, the target site may be within a pulmonary artery of the patient, within another vasculature of the patient, or a position within a heart of a patient, such as a position proximate to an inner wall of the right ventricle, right atrium, left atrium, and/or left ventricle. Guidewire  310  may be routed using any techniques known to those in the art. For example, clinician may use imaging techniques, such as fluoroscopy, to monitor the position of guidewire  310 . 
     As represented by  FIG. 9B , after distal end  312  of guidewire  310  is positioned proximate to a target site within the patient, the clinician routes an assembly including outer sheath  320  and inner sheath  330  over the proximal end guidewire  310  and pushes the assembly along guidewire  310  until distal opening  322  of outer sheath  320  is proximate the target site within the patient. 
     Elongated outer sheath  320  is sized to traverse the vasculature of the patient. Outer sheath  320  forms inner lumen  324 , which has distal opening  322 . In some examples, inner lumen  324  may extend the length of outer sheath  320  and also provide a proximal opening. 
     Elongated inner sheath  330  includes tapered distal end  332 . In one example, tapered distal end  332  may have a conical shape. Tapered distal end  332  is configured to substantially fill inner lumen  324  of outer sheath  320  to close-off distal opening  322  of outer sheath  320 . Inner sheath  330  includes guidewire lumen  334 , which may extend throughout the length of inner sheath  330 . The diameter of guidewire lumen  334  at distal tip  332  corresponds to the diameter of guidewire  310 . In some examples, guidewire lumen  334  may be greater at other portions of inner sheath  330  than at distal tip  332 , and such a configuration may limit friction between guidewire  310  and inner sheath  330 . In other examples, guidewire lumen  334  may have a consistent diameter throughout the length of inner sheath  330 . In one example, distal tip  332  may be formed from 35 D durometer PEBA or a blend of 40 D durometer PEBA and barium sulfate, bismuth compounds (trioxide, oxychloride), tungsten and/or polymer fillers. In other examples, other flexible polymers may be used such as nylons and polyethylenes. In any case, these and other polymer fillers may be selected to provide desirable material properties, such as stiffness, flexibility, reduce friction and/or increase radiopacity. 
     In the assembly of outer sheath  320  and inner sheath  330 , tapered distal end  332  extends beyond distal opening  322  of outer sheath  320 ; however, inner sheath  330  may be advanced and retracted relative to outer sheath  320  by the clinician during the implantation procedure, if desired, as inner sheath  330  is slidable within inner lumen  324  of outer sheath  320 . 
     After the assembly of outer sheath  320  and inner sheath  330  is advanced along guidewire  310  until distal opening  322  of outer sheath  320  is proximate the target site within the patient ( FIG. 9C ), guidewire  310  and inner sheath  330  are withdrawn from outer sheath  320  ( FIG. 9D ). For example, a clinician may simply pull on guidewire  310  and inner sheath  330  from a proximal end of outer sheath  320  to slide guidewire  310  and inner sheath  330  out of the proximal opening of outer sheath  320 . In another example, a clinician may remove inner sheath  330  and leave the guidewire  312  in place to enable the tracking of ancillary devices. 
       FIGS. 10A-10D  illustrate example techniques for intravascular delivery of IMD  380  through outer sheath  320  using deployment receptacle  340 . Specifically,  FIGS. 10A-10D  illustrate distal portions of outer sheath  320  and elongated deployment receptacle  340 . As previously mentioned, outer sheath  320  may be considered to be substantially similar to outer sheath  234 . For example, outer sheath  320  may be included in an assembly with coupling module  206 . In such an example, deployment receptacle  340  may be slidably coupled to coupling module  226  in a mating assembly, and the distal end of deployment receptacle  340  may be positioned proximate a target site within a patient in a similar manner that inner sheath  202  is positioned proximate a target site within a patient as described with respect to  FIGS. 8A-8E . 
     Deployment receptacle  340  includes deployment bay  342  at a distal end of deployment receptacle  340 . Deployment bay  342  is configured to carry IMD  380  through inner lumen  324  of outer sheath  320 . Deployment receptacle  340  is slidable within inner lumen  324  of outer sheath  320  when inner lumen  324  is open, e.g., when inner sheath  330  is not within inner lumen  324  of outer sheath  320 . 
     IMD  380  includes expandable fixation element  381 , which is deployable from a collapsed position to an expanded position secure the IMD  380  proximate a target site within a patient. While  FIGS. 10A-10D  illustrate implantation techniques using IMD  380 , in different examples, IMD  380  may be substantially similar to IMD  17  ( FIG. 4 ) or IMD  15  ( FIG. 5 ). As one example, the techniques of  FIGS. 10A-10D  may be used to deliver IMD  17  to a position within a heart of a patient, such as a position proximate to an inner wall of the right ventricle, within the right atrium, the left atrium, and/or left ventricle. As another example, the techniques of  FIGS. 10A-10D  may be used to deliver IMD  15  to an intravascular position such as a pulmonary artery or other vasculature of the patient. 
     Deployment receptacle  340  facilitates deployment of IMD  380  out of distal opening  322  of outer sheath  320 . In particular, deployment receptacle  340  includes tether  350 , which has helical element  352  on its distal end. Tether  350  is remotely controllable from a proximal end of deployment receptacle  340  to release IMD  380  from deployment bay  342 . Tether  350  is stiff enough to facilitate pushing IMD  380  out of deployment bay  342  as well as pushing IMD  380  into deployment bay  342 . 
     Specifically, a clinician, from the proximal end of deployment receptacle  340 , may remotely push tether  350  distally relative to deployment bay  342  to push IMD  380  out distal opening  343  of deployment bay  342 . This maintains the position of IMD  380  within the patient during deployment, which facilitates precise positioning of IMD  380 . In one example, clinician actually retracts outer sheath  320  proximally to push tether  350  distally relative to deployment bay  342  to push IMD  380  out distal opening  343  of deployment bay  342 . Then the clinician may, again from the proximal end of deployment receptacle  340 , remotely rotate tether  350  such that helical element  352  releases a looped element of IMD  380  to deploy IMD  380 . Specifically, in the example illustrated in  FIG. 10C , as expandable fixation element  381  is the looped element of IMD  380 , and rotating helical element  352  releases expandable fixation element  381  from deployment receptacle  340 . Skeptical   
     During an implantation procedure, the clinician would position the distal end of outer sheath  320  proximate to a target site within the patient via a vasculature accessed during a surgical procedure. For example, outer sheath  320  may be advanced into an entry vessel, such as the femoral artery, and then manipulated and navigated through the patient&#39;s vasculature until the distal end of outer sheath  320  proximate to a target site within the patient. The clinician may use imaging techniques, such as fluoroscopy, to monitor the position of outer sheath  320 , deployment receptacle  340 , and IMD  380  throughout the implantation procedure. In some examples, may be outer sheath  320  routed to the target site using the techniques described with respect to  FIGS. 9A-9D ; however, any technique known to those in the art for intravascular delivery of a sheath may be position outer sheath  234  such that the distal end of outer sheath  320  is proximate the target site within the patient. 
     Once the distal end of outer sheath  320  proximate to a target site within the patient, as represented by  FIG. 10A , a clinician delivers IMD  380  to the target site by pushing deployment receptacle  340  through inner lumen  324  of outer sheath  320 . In one example, the clinician may align distal opening  343  of deployment receptacle  340  with the proximal opening of inner lumen  324  of outer sheath  320 . As an example, IMD  380  may be preloaded within deployment bay  342  before deployment receptacle  340  is inserted into outer sheath  320 . In the example in which IMD  380  includes a pressure sensor, such as a pressure transducer, preloading IMD  380  within deployment bay  342  by the manufacturer may serve to protect the transducer from damage, such as damage caused by handling. The clinician continues to push deployment receptacle  340  through inner lumen  324  of outer sheath  320  at least until distal opening  343  of deployment receptacle  340  reaches distal opening  322  of outer sheath  320 . 
     As represented by  FIG. 10B , once deployment bay  342  is positioned proximate the target site, the clinician deploys IMD  380  from deployment receptacle  340 . Specifically, a clinician, from the proximal end of deployment receptacle  340 , may remotely push tether  350  distally relative to deployment bay  342  to push IMD  380  out distal opening  343  of deployment bay  342 . As shown in  FIG. 10B , a portion of expandable fixation element  381  of IMD  380  is expanded from a collapsed position to an expanded position as IMD  380  passes out of distal opening  343  of deployment bay  342 . In the expanded position, expandable fixation element  381  will secure IMD  380  within the patient, e.g., as described with respect to IMD  17  ( FIG. 4 ) or IMD  15  ( FIG. 5 ). 
     As represented by  FIG. 10C , once IMD  380  is fully removed from deployment bay  342 , expandable fixation element  381  of IMD  380  is expanded assumes the expanded position. In order to fully deploy IMD  380  from deployment receptacle  340 , the clinician remotely rotates tether  350  such that helical element  352  releases expandable fixation element  381 , as represented by  FIG. 10D . At this point, IMD  380  is fully deployed proximate to the target site, e.g., within a vasculature of the patient. IMD  380  is engaged to the vasculature of the patient because the expandable fixation element  381  elastically compressed within deployment receptacle  340  and expands to engage vasculature of the patient once released from deployment receptacle  340 . 
     The clinician may optionally recapture IMD  380  by first grabbing a looped element of IMD  380 , e.g., expandable fixation element  381 , with helical element  352 , and then using tether  350  to pull IMD  380  into deployment bay  342 . In one example, tether  350  is held in a fixed location while outer sheath  320  is advanced distally to pull IMD  380  into deployment bay  342 . In this manner, deployment receptacle  340  may be used to adjust the position of IMD  380  after full deployment, or to remove IMD  380  from the patient after full deployment. As one example, the clinician may decide to remove IMD  380  from the patient after full deployment if electronic testing of IMD  380  produces unsatisfactory results. As another example, the clinician may decide to remove IMD  380  from the patient after full deployment if the clinician determines that expandable fixation element  381  is improperly sized to locate IMD  380  at the target site. In such an example, IMD  380  may be replaced with an IMD including an expandable fixation element with a proper size. As another example, a clinician may use deployment receptacle  340  to remove IMD  380  during a subsequent surgical procedure, e.g., once IMD  380  has met or exceeded its projected lifespan. During such a subsequent surgical procedure, IMD  380  could be replaced with a new IMD using the same outer sheath used during the removal of IMD  380 . 
       FIGS. 11A-11D  illustrate example techniques for intravascular delivery of sheath  420  using inner sheath  430 , which includes a distal portion with inflatable member  432 . As one example, the techniques illustrated in  FIGS. 11A-11D  may be used for intravascular delivery of outer sheath  234 . In such an example, outer sheath  420  of  FIGS. 11A-11D  may be considered to be substantially similar to outer sheath  234 . For example, outer sheath  420  may be included in an assembly with coupling module  206 , and outer sheath  420  may be used to deliver an IMD as described with respect to  FIGS. 8A-8E . 
     As represented by  FIG. 11A , guidewire  410  is first routed from an access point through a vasculature of the patient until distal end  412  of guidewire  410  is positioned proximate to a target site within the patient. In different examples, the target site may be within a pulmonary artery of the patient, within another vasculature of the patient, or a position within a heart of a patient, such as a position proximate to an inner wall of the right ventricle, right atrium, left atrium, and/or left ventricle. Guidewire  410  may be routed using any techniques known to those in the art. For example, clinician may use imaging techniques, such as fluoroscopy, to monitor the position of guidewire  410 . 
     As represented by  FIG. 11B , after distal end  412  of guidewire  410  is positioned proximate to a target site within the patient, the clinician routes an assembly including outer sheath  420  and inner sheath  430  over the proximal end guidewire  410  and pushes the assembly along guidewire  410  until distal opening  422  of outer sheath  420  is proximate the target site within the patient. 
     Elongated outer sheath  420  is sized to traverse the vasculature of the patient. Outer sheath  420  forms inner lumen  424 , which has distal opening  422 . In some examples, inner lumen  424  may extend the length of outer sheath  420  and also provide a proximal opening. 
     Elongated inner sheath  430  includes inflatable member  432 . Inflatable member  432  is selectively inflatable from a proximal end of inner sheath  430 . When inflated inflatable member  432  is configured to substantially fill inner lumen  424  of outer sheath  420  and close-off distal opening  422  of outer sheath  420 . 
     Inner sheath  430  includes guidewire lumen  434 , which may extend throughout the length of inner sheath  430 . The diameter of guidewire lumen  434  at the distal portion of inner sheath  430  corresponds to the diameter of guidewire  410 . In some examples, guidewire lumen  434  may be greater at other portions of inner sheath  430  than at the distal portion of inner sheath  430 . Such a configuration may limit friction between guidewire  410  and inner sheath  430 . In other examples, guidewire lumen  434  may have a consistent diameter throughout the length of inner sheath  430 . 
     In the assembly of outer sheath  420  and inner sheath  430 , inflatable member  432  extends beyond distal opening  422  of outer sheath  420 ; however, inner sheath  430  may be advanced and retracted relative to outer sheath  420  by the clinician during the implantation procedure, if desired, as inner sheath  430  is slidable within inner lumen  424  of outer sheath  420 . For example, inflatable member  432  may be remotely deflated by the clinician. Once inflatable member  432  is deflated, the clinician may pull into inflatable member  432  into inner lumen  424  of outer sheath  420  by pulling on the proximal end of inner sheath  430 . 
     After the assembly of outer sheath  420  and inner sheath  430  is advanced along guidewire  410  until distal opening  422  of outer sheath  420  is proximate the target site within the patient ( FIG. 11B ), the clinician remotely deflates inflatable member  432  retracts inner sheath  430  and guidewire  410  into distal opening  422  of outer sheath  420  ( FIG. 11C ). Then guidewire  410  and inner sheath  430  are withdrawn from outer sheath  420  ( FIG. 11D ). For example, a clinician may simply pull on guidewire  410  and inner sheath  430  from a proximal end of outer sheath  420  to slide guidewire  410  and inner sheath  430  out of the proximal opening of outer sheath  420 . 
     Inflatable member  432  serves to improve deliverability by protecting the distal edge of outer sheath  420 . In addition, inflatable member  432  may enhance trackability by providing a distal force input on the assembly of outer sheath  420  and inner sheath  430 . For example, inflatable member  432  can be inflated in the blood stream to allow blood flow to carry the assembly of outer sheath  420  and inner sheath  430  through the patient anatomy and ultimately to the target implant site. In addition, vessel sizing can be done by occluding a vasculature proximate to the target site and applying a localized contrast injection in combination with fluoroscopy. 
       FIGS. 12A-12C  illustrate example techniques for intravascular delivery of IMD  380  using delivery catheter  400 . Delivery catheter  400  includes elongated outer sheath  460 , which forms inner lumen  464  with distal opening  462 . Delivery catheter  400  further includes inner sheath  440  with inflatable member  432  at its distal end. Delivery catheter  400  and outer sheath  460  is sized to traverse a vasculature of the patient, and delivery catheter  400  is configured to carry IMD  380  within a distal portion of inner lumen  464  of outer sheath  460  while traversing the vasculature of the patient. Inner sheath  440  is slidable within inner lumen  464  of outer sheath  460 . 
     Inflatable member  432  may be constructed of a compliant polymer material or be constructed of less-compliant polymers, if so desired. The polymer material may have a low-pressure rating, as high-pressure capability is not required. The diameter of inflatable member  432  may controlled by inflation media volume. For example, inner sheath  440  may include an inflation lumen extending a length of inner sheath  440 . The distal end of the inflation lumen terminates at inflatable member  432 , whereas the proximal end of the inflation lumen terminates at an inflation control mechanism, like a syringe. The inflation media is normally, but not necessarily, a liquid, such as a saline solution; in other examples the inflation media may be a gas, such as air. 
     IMD  380  includes expandable fixation element  381 , which is deployable from a collapsed position to an expanded position secure the IMD  380  proximate a target site within a patient. While  FIGS. 12A-12C  illustrate implantation techniques using IMD  380 , in different examples, IMD  380  may be substantially similar to IMD  17  ( FIG. 4 ) or IMD  15  ( FIG. 5 ). As one example, the techniques of  FIGS. 12A-12C  may be used to deliver IMD  17  to a position within a heart of a patient, such as a position proximate to an inner wall of the right ventricle, within the right atrium, the left atrium, and/or left ventricle. As another example, the techniques of  FIGS. 12A-12C  may be used to deliver IMD  15  to an intravascular position such as a pulmonary artery or other vasculature of the patient. 
     During an implantation procedure, a clinician first positions delivery catheter  400  such that the distal end of outer sheath  460  is proximate to a target site within the patient via a vasculature accessed during a surgical procedure, as represented by  FIG. 12A . For example, delivery catheter  400  may be advanced into an entry vessel, such as the femoral artery, and then manipulated and navigated through the patient&#39;s vasculature until the distal end of outer sheath  460  proximate to a target site within the patient. In different examples, delivery catheter  400  may be steerable or be configured to traverse a guidewire to be directed to the target site from the access point of the vasculature. The clinician may use imaging techniques, such as fluoroscopy, to monitor the position of outer sheath  460 , inner sheath  440 , and IMD  380  throughout the implantation procedure. In some examples, delivery catheter  400  may have an internal lumen for contrast injections. 
     Delivery catheter  400  further includes stopper  441 , which is proximally located relative to inflatable member  432 . Inflatable member  432  is remotely controllable from a proximal end of delivery catheter  400  to retract in a proximal direction towards inner sheath  440 . Once the distal end of outer sheath  460  is proximate to a target site within the patient, the clinician may deflate inflatable member  432  and draw inflatable member  432  back towards stopper  441  prior to deployment of IMD  380 . As represented by  FIG. 12B , inflatable member  432  is retractable to a position within inner lumen  464  of outer sheath  460  that is proximal to IMD  380 . Retracting inflatable member  432  to a position that is proximal to IMD  380  prior to deployment of IMD  380  prevents the opportunity for post-deployment interaction between inflatable member  432  and IMD  380 . For example, if inflatable member  432  were not refracted to a position that is proximal to IMD  380  prior to deployment of IMD  380 , inflatable member might catch on IMD  380  after IMD  380  were deployed, which could move or even dislodge IMD  380  from the target site within the patient. 
     Stopper  441  includes an enlarged distal end that facilitates deployment of IMD  380  out of distal opening  462  of outer sheath  460 . Enlarged distal end  441  may include a recess to receive a deflated inflatable member  432 . In any event, inner sheath  440  is remotely controllable from a proximal end of inner sheath  430  to release IMD  380  from the distal end of outer sheath  460 . Once inflatable member  432  retracted to a position within inner lumen  464  of outer sheath  460  that is proximal to IMD  380 , the clinician deploys IMD  380  from outer sheath  460 . Specifically, a clinician, from the proximal end of inner sheath  440 , may remotely move inner sheath  440  distally relative to deployment bay  442  to push IMD  380  out distal opening  462  of outer sheath  460 . As shown in  FIG. 12C , expandable fixation element  381  of IMD  380  is expanded from a collapsed position to an expanded position as IMD  380  passes out distal opening  462  of outer sheath  460 . In one example, inner sheath  440  is held in a fixed location while outer sheath  460  is retracted proximally to release IMD  380  from distal opening  462  and allow the expansion of IMD  380  at the target location. In the expanded position, expandable fixation element  381  will secure IMD  380  within the patient, e.g., as described with respect to IMD  17  ( FIG. 4 ) or IMD  15  ( FIG. 5 ). 
     Delivery catheter  400  may provide one or more advantages. For example, inflatable member  432  may provide improved deliverability of delivery catheter  400  in the inflated state in that inflatable member  432  may cross tricuspid and pulmonary valves without issue or risk of damaging leaflets, e.g., to reach a target site within a pulmonary artery. Inflatable member  432  may also allow delivery catheter  400  to negotiate the chordae in the right ventricle without hanging up on the chordae. Furthermore, inflatable member  432  may be used to measure the size of a vasculature, which may be useful to find a target site having a vessel size corresponding to the size of expandable fixation element  381 . As one example, a clinician may find a target site by applying a localized contrast injection while viewing delivery catheter  400  under fluoroscopy. Once a vasculature is fully occluded by inflatable member  432 , the clinician would then know the size of the vasculature corresponds to the diameter of inflatable member  432 . In some examples, the clinician may selectively inflatable member  432  to a size associated with a desired size of the vasculature and advance delivery catheter  400  within the vessel until a vasculature is fully occluded. 
     Inflatable member  432  is shown in further detail in  FIGS. 13A-13B . Specifically,  FIG. 13A  is a cross-sectional illustration of inflatable member  432  in a deflated configuration, whereas  FIG. 13B  is a cross-sectional illustration of inflatable member  432  in an inflated configuration. 
     As shown in  FIGS. 13A-13B , inner sheath  430  includes two coaxial lumens. Tube  452  provides central lumen  434  may serve as a guidewire lumen, and may also be suitable for contrast injections. Tube  450  surrounds tube  452  and provides annular inflation lumen  433 . The distal end of annular inflation lumen  433  terminates at inflatable member  432 , whereas the proximal end of annular inflation lumen  433  terminates at an inflation control mechanism, like a syringe. Inflatable member  432  is secured to the outside of tube  450  at the distal end of tube  450 . Tube  450  includes apertures  435 , which allow the inflation media to pass from within inflation lumen  433  to inflatable member  432 . In other examples, tube  450  may include a single aperture in place of apertures  435 . The area between the distal ends of tubes  450 ,  452  is sealed to direct the inflation media inflatable member  432 . As mentioned previously, the inflation media is normally, but not necessarily, a liquid, such as a saline solution. 
       FIG. 14  illustrates the distal end of inner sheath  470 , which provides an alternative design as compared to inner sheath  430 . Specifically, inner sheath  470  includes tapered flexible tip  480 , which is located distally relative to inflatable member  432 . Tapered flexible tip  480  is mounted to the distal end of tube  452 , distally relative to inflatable member  432 . Central lumen  434  extends through tube  452  and through tapered flexible tip  480 . The other components and features of the distal end of inner sheath  470  are substantially similar to those of inner sheath  430 . For brevity, these components and features are not discussed with respect to inner sheath  470 . 
     Tapered flexible tip  480  is formed from a compliant biocompatible material, such as silicon. Tapered flexible tip  480  may serve to help a delivery catheter, such as delivery catheter  400 , navigate a guidewire to negotiate the vasculature of a patient. For example, tapered flexible tip  480  may lead inflatable member  432  around bends, vascular branches and through valves such as tricuspid and pulmonary valves, the chordae in the right ventricle and other obstacles during positioning of a delivery catheter. Thus, tapered flexible tip  480  may improve the deliverability of delivery catheter by preventing hang-ups during insertion of the delivery catheter. In some examples, the material of tapered flexible tip  480  may be doped with radiopaque materials (such as barium sulfate) to aid a clinician during implant. 
       FIGS. 15A-15F  illustrate exemplary techniques for intravascular delivery of IMD  380  using delivery catheter  500 . Delivery catheter  500  includes elongated outer sheath  520  and elongated inner sheath  540 . Delivery catheter  500  and outer sheath  520  are sized to traverse a vasculature of the patient, and delivery catheter  500  is configured to carry IMD  380  within a distal portion of inner lumen  524  of outer sheath  520  while traversing the vasculature of the patient. Inner sheath  540  is slidable within outer sheath  520  and includes enlarged distal portion  532  and tether  550 . Enlarged distal portion  532  provides a tapered distal end. Alternatively, an enlarged distal portion may be selected from the examples shown previously with respect to  FIGS. 13-14 . In some examples, inner sheath  540  and enlarged distal portion  532  may include a lumen (not shown) configured to receive a guidewire and/or deliver contrast injections during an implantation procedure. 
     IMD  380  includes expandable fixation element  381 , which is deployable from a collapsed position to an expanded position secure the IMD  380  proximate a target site within a patient. While  FIGS. 15A-15F  illustrate implantation techniques using IMD  380 , in different examples, IMD  380  may be substantially similar to IMD  17  ( FIG. 5 ) or IMD  15  ( FIG. 5 ). As one example, the techniques of  FIGS. 15A-15F  may be used to deliver IMD  17  to a position within a heart of a patient, such as a position proximate to an inner wall of the right ventricle, within the right atrium, the left atrium, and/or left ventricle. As another example, the techniques of  FIGS. 15A-15F  may be used to deliver IMD  15  to an intravascular position such as a pulmonary artery or other vasculature of the patient. 
     During an implantation procedure, a clinician first positions delivery catheter  500  such that the distal end of outer sheath  520  is proximate to a target site within the patient via a vasculature accessed during a surgical procedure, as represented by  FIG. 15A . For example, delivery catheter  500  may be advanced into an entry vessel, such as the femoral artery, and then manipulated and navigated through the patient&#39;s vasculature until the distal end of outer sheath  520  proximate to a target site within the patient. In different examples, delivery catheter  500  may be steerable or be configured to traverse a guidewire to be directed to the target site from the access point of the vasculature. The clinician may use imaging techniques, such as fluoroscopy, to monitor the position of outer sheath  520 , inner sheath  540 , and IMD  380  throughout the implantation procedure. In some examples, delivery catheter  500  may have an internal lumen for contrast injections. Enlarged distal portion  532  is configured to substantially fill inner lumen  524  of outer sheath  520  and close-off distal opening  522  of outer sheath  520  while delivery catheter  500  is advanced to a location proximate a target site within a patient. As one example, enlarged distal portion  532  may provide a tapered distal end with a profile larger than a cross section of inner lumen  524  of outer sheath  520  such that the tapered distal end cannot pass through inner lumen  524 . 
     After positioning the distal end of outer sheath  520  is proximate to a target site within the patient, the clinician moves enlarged distal portion  532  distally relative to distal opening  522  of outer sheath  520  to allow room for IMD  380  to deploy from distal opening  522  of outer sheath  520  ( FIG. 15B ). In some examples, the clinician may retract sheath  520  proximally to expose and allow fixation of IMD  380  to seat in a vessel wall. Thereafter, tip  532  is retracted while helix  552  insures that IMD  380  is not dislodged from the vessel wall while tip  532  is retracted past the implant. 
     Inner sheath  540  facilitates deployment of IMD  380  out of distal opening  522  of outer sheath  520 . In particular, inner sheath  540  includes tether  550 , which has helical element  552  on its distal end. Tether  550  is remotely controllable from a proximal end of inner sheath  540  to release IMD  380  from the distal end of outer sheath  520 . Specifically, a clinician, from the proximal end of inner sheath  540 , may remotely push tether  550  distally relative to the distal end of outer sheath  520  to push IMD  380  out distal opening  522  of outer sheath  520 , e.g., by holding tether  550  in place and retracting outer sheath  520  ( FIG. 15C ). As shown in  FIG. 15C , a portion of expandable fixation element  381  of IMD  380  is expanded from a collapsed position to an expanded position as IMD  380  passes out of distal opening  522  of the distal end of outer sheath  520 . In the expanded position, expandable fixation element  381  will secure IMD  380  within the patient, e.g., as described with respect to IMD  17  ( FIG. 4 ) or IMD  15  ( FIG. 5 ). 
     Then the clinician may, again from the proximal end of inner sheath  540 , move enlarged distal portion  532  proximally towards distal opening  522  of outer sheath  520 , past IMD  380  and helical element  552  while helical element  552  remains engaged to the looped fixation element of IMD  380  ( FIG. 15D ). In this manner, IMD  380  is not fully deployed when enlarged distal portion  532  is retracted past IMD  380 . Once the clinician retracts enlarged distal portion  532  proximally past IMD  380  and helical element  552 , the clinician may, again from the proximal end of inner sheath  540 , remotely rotate tether  550  such that helical element  552  releases a looped element of IMD  380  to deploy IMD  380  ( FIG. 15E ). Specifically, in the example illustrated in  FIG. 15E , expandable fixation element  381  is the looped element of IMD  380 , and rotating helical element  552  releases expandable fixation element  381  from inner sheath  540 . 
     At this point, IMD  380  is fully deployed proximate to the target site, e.g., within a vasculature of the patient. However, the clinician may optionally recapture IMD  380  by first grabbing a looped element of IMD  380 , e.g., expandable fixation element  381 , with helical element  552 , and then using tether  550  to pull IMD  380  into the distal end of outer sheath  520 . In this manner, tether  550  may be used to adjust the position of IMD  380  after full deployment, or to remove IMD  380  from the patient after full deployment. As one example, the clinician may decide to remove IMD  380  from the patient after full deployment if electronic testing of IMD  380  produces unsatisfactory results. As another example, the clinician may decide to remove IMD  380  from the patient after full deployment if the clinician determines that expandable fixation element  381  is improperly sized to locate IMD  380  at the target site. In such an example, IMD  380  may be replaced with an IMD including an expandable fixation element with a proper size. As another example, a clinician may use delivery catheter  500  to remove IMD  380  during a subsequent surgical procedure, e.g., once IMD  380  has met or exceeded its projected lifespan. During such a subsequent surgical procedure, IMD  380  could be replaced with a new IMD using the same outer sheath used during the removal of IMD  380 . 
     After IMD  380  is fully deployed proximate to the target site, the clinician retracts tether  550  and enlarged distal portion  532  into inner lumen  524  of outer sheath  520  and withdraws delivery catheter  500  ( FIG. 15F ). 
       FIGS. 16A-16B  illustrate example techniques for intravascular delivery of IMD  380  using delivery catheter  560 . Delivery catheter  560  includes elongated outer sheath  520  and elongated inner sheath  570 . Delivery catheter  560  and outer sheath  520  are sized to traverse a vasculature of the patient, and delivery catheter  560  is configured to carry IMD  380  within a distal portion of inner lumen  524  of outer sheath  520  while traversing the vasculature of the patient. Inner sheath  570  is slidable within outer sheath  520  and includes enlarged distal portion  562  and tether  552 . 
     Delivery catheter  560  is substantially similar to delivery catheter  500 , except that enlarged distal portion  562  includes an inflatable member. In some examples, inner sheath  570  and enlarged distal portion  562  may include a lumen (not shown) configured to receive a guidewire and/or deliver contrast injections during an implantation procedure. For example, the inflatable member of enlarged distal portion  562  may be functionally similar to inflatable member  432  ( FIGS. 13A-13B ) and may optionally include a tapered flexible tip, such as tapered flexible tip  480  ( FIG. 14 ). The other components and features of delivery catheter  560  are substantially similar to those of delivery catheter  500 . For brevity, these components and features are discussed in limited detail with respect to delivery catheter  560 . 
     During an implantation procedure, a clinician first positions delivery catheter  560  such that the distal end of outer sheath  520  is proximate to a target site within the patient via a vasculature accessed during a surgical procedure, as represented by  FIG. 16A . For example, delivery catheter  560  may be advanced into an entry vessel, such as the femoral artery, and then manipulated and navigated through the patient&#39;s vasculature until the distal end of outer sheath  520  proximate to a target site within the patient. In different examples, delivery catheter  560  may be steerable or be configured to traverse a guidewire to be directed to the target site from the access point of the vasculature. The clinician may use imaging techniques, such as fluoroscopy, to monitor the position of outer sheath  520 , inner sheath  570 , and IMD  380  throughout the implantation procedure. 
     Enlarged distal portion  562  is configured to substantially fill inner lumen  524  of outer sheath  520  and close-off distal opening  522  of outer sheath  520  while the inflatable member of enlarged distal portion  562  is inflated. The inflatable member of enlarged distal portion  562  is generally inflated while delivery catheter  560  is advanced to a location proximate a target site within a patient. After positioning the distal end of outer sheath  520  is proximate to a target site within the patient, the clinician deflates the inflatable member of enlarged distal portion  562  and retracts enlarged distal portion  562  proximally into inner lumen  524  of outer sheath  520  to a position that is proximal to IMD  380  within inner lumen  524  of outer sheath  520 . 
     Inner sheath  570  facilitates deployment of IMD  380  out of distal opening  522  of outer sheath  520 . In particular, inner sheath  570  includes tether  550 , which has helical element  552  on its distal end. Tether  550  is remotely controllable from a proximal end of inner sheath  570  to release IMD  380  from the distal end of outer sheath  520 . Specifically, a clinician, from the proximal end of inner sheath  570 , may remotely push tether  550  distally relative to the distal end of outer sheath  520  to push IMD  380  out distal opening  522  of outer sheath  520  ( FIG. 16B ). As shown in  FIG. 16B , expandable fixation element  381  of IMD  380  is expanded from a collapsed position to an expanded position as IMD  380  passes out of distal opening  522  of the distal end of outer sheath  520 . In the expanded position, expandable fixation element  381  will secure IMD  380  within the patient, e.g., as described with respect to IMD  17  ( FIG. 4 ) or IMD  15  ( FIG. 5 ). 
     Once the clinician retracts enlarged distal portion  562  proximally past IMD  380  and helical element  552 , the clinician may, again from the proximal end of inner sheath  570 , remotely rotate tether  550  such that helical element  552  releases a looped element of IMD  380  to deploy IMD  380 . 
     At this point, IMD  380  is fully deployed proximate to the target site, e.g., within a vasculature of the patient. However, the clinician may optionally recapture IMD  380  by first grabbing a looped element of IMD  380 , e.g., expandable fixation element  381 , with helical element  552 , and then using tether  550  to pull IMD  380  into the distal end of outer sheath  520 . In this manner, tether  550  may be used to adjust the position of IMD  380  after full deployment, or to remove IMD  380  from the patient after full deployment. 
       FIGS. 17A-17E  illustrate exemplary techniques for intravascular delivery of IMD  380  with a kit including outer sheath  620  and inner sheath  640 . Inner sheath  640  is configured to carry IMD  380  at its distal end. Inner sheath  640  forms a slit at its distal end to facilitate deployment of IMD  380 . Specifically,  FIGS. 17A-17E  illustrate distal portions of outer sheath  620  and elongated inner sheath  640 . In some examples, outer sheath  620  may be considered to be substantially similar to outer sheath  234 . For example, outer sheath  620  may be included in an assembly with coupling module  206 . In such an example, inner sheath  640  may be slidably coupled to coupling module  226  in a mating assembly, and the distal end of inner sheath  640  may be positioned proximate a target site within a patient in a similar manner that inner sheath  202  is positioned proximate a target site within a patient as described with respect to  FIGS. 8A-8E . 
     While  FIGS. 17A-17E  illustrate implantation techniques using IMD  380 , in different examples, IMD  380  may be substantially similar to IMD  17  ( FIG. 4 ) or IMD  15  ( FIG. 5 ). As one example, the techniques of  FIGS. 17A-17E  may be used to deliver IMD  17  to a position within a heart of a patient, such as a position proximate to an inner wall of the right ventricle, within the right atrium, the left atrium, and/or left ventricle. As another example, the techniques of  FIGS. 17A-17E  may be used to deliver IMD  15  to an intravascular position such as a pulmonary artery or other vasculature of the patient. 
     The distal end of inner sheath  640  is configured to carry IMD  380  through inner lumen  624  of outer sheath  620 , and inner sheath  640  is slidable within inner lumen  624  of outer sheath  620 . Inner sheath  640  facilitates deployment of IMD  380  out of distal opening  622  of outer sheath  620 . In particular, inner sheath  640  forms slit  641 , which allows inner sheath  640  to uncurl to expose the IMD  380  when the distal end of inner sheath  640  passes out of distal opening  622  of outer sheath  620 . The distal end of inner sheath  640  is elastically deformed within inner lumen  624  such that the distal end of inner sheath  640  is biased to uncurl and expose IMD  380  when the distal end of inner sheath  640  passes out of distal opening  622  of outer sheath  620 . 
     During an implantation procedure, a clinician may first position outer sheath  620  such that distal opening  622  of outer sheath  620  is proximate to a target site within the patient via a vasculature accessed during a surgical procedure, as represented by  FIG. 17A . For example, outer sheath  620  may be advanced into an entry vessel, such as the femoral vein and then manipulated and navigated through the patient&#39;s vasculature distal opening  622  of outer sheath  620  is proximate to a target site within the patient. In different examples, outer sheath  620  may be steerable or be configured to traverse a guidewire to be directed to the target site from the access point of the vasculature. The clinician may use imaging techniques, such as fluoroscopy, to monitor the position of outer sheath  620 , inner sheath  640 , and IMD  380  throughout the implantation procedure. 
     After locating distal opening  622  of outer sheath  620  proximate to a target site within the patient, the clinician may remotely push inner sheath  640  distally relative to outer sheath  620  to expose the distal end of inner sheath  640  and IMD  380 , e.g., by holding inner sheath  640  in place and retracting outer sheath  620 . IMD  380  includes expandable fixation element  381 , which is deployable from a collapsed position to an expanded position secure the IMD  380  proximate a target site within a patient. When exposed, a portion of expandable fixation element  381  may assume the expanded position, as shown in  FIG. 17B . 
     As shown in  FIG. 17B , IMD  380  is partially deployed from inner sheath  640 . Only a portion of expandable fixation element  381  has assumed the expanded position. At this point, the clinician may retract the distal end of inner sheath  640  into inner lumen  624  of outer sheath  620  to return the IMD  380  to inner lumen  624  of outer sheath  620 . When the distal end of inner sheath  640  and IMD  380  are returned to inner lumen  624  of outer sheath  620 , the distal end of inner sheath  640  curls and the expanded portion of expandable fixation element  381  resumes collapsed position to fit within inner lumen  624  of outer sheath  620 . 
     As one example, the clinician may partially deploy IMD  380  and perform electronic testing of IMD  380 , as sensing elements of IMD  380 , such as a pressure sensor, may be exposed when IMD  380  is partially deployed. The clinician may decide to remove IMD  380  from the patient after partial deployment if testing results are unsatisfactory or if the clinician determines that expandable fixation element  381  is improperly sized to locate IMD  380  at the target site. In such an example, IMD  380  may be replaced with an IMD including an expandable fixation element with a proper size. 
     As represented by  FIG. 17C , once IMD  380  is fully exposed, at least a portion of expandable fixation element  381  of IMD  380  has assumed the expanded position. In order to fully deploy IMD  380  from inner sheath  640 , the clinician then retracts inner sheath  640  into inner lumen  624  of outer sheath  620  after the portion of expandable fixation element  381  assumes the expanded position ( FIG. 17D ). This causes the distal end of outer sheath  620  to interact with expandable fixation element  381  to slide IMD  380  out of inner lumen  624  of outer sheath  620  ( FIG. 17E ). In an example, the clinician may hold inner sheath  640  in place while advancing outer sheath  620  to cause the distal end of outer sheath  620  to interact with expandable fixation element  381  to slide IMD  380  out of inner lumen  624  of outer sheath  620 . 
       FIGS. 18A-18C  illustrate techniques for measuring the size of vasculature  700  using deployment receptacle  340  of  FIGS. 10A-10D  with IMD  380  partially deployed from deployment receptacle  340 . As shown in  FIG. 18A , the distal end of deployment receptacle  340 , which includes deployment bay  342 , is delivered adjacent a target site within vasculature  700  through outer sheath  320 . As one example, vasculature  700  may be a pulmonary artery or other vasculature of the patient. 
     Tether  350  with helical element  352  is then used to partially deploy IMD  380  from distal opening  322  of outer sheath  320 . As shown in  FIG. 18B , a portion of expandable fixation element  381  assumes the expanded position when IMD  380  is partially deployed from the distal opening. 
     Outer sheath  320 , deployment receptacle  340  and the partially deployed IMD  380  is then advanced within the vasculature ( FIG. 18C ). A clinician monitors the process of outer sheath  320 , deployment receptacle  340  and the partially deployed IMD  380 . Specifically, the clinician monitors vasculature  700  and/or the expanded portion of expandable fixation element  381  for deflection to determine when the size of the expanded portion of expandable fixation element  381  corresponds to the size of vasculature  700 . For example, vasculature  700  may be tapered, and IMD  380  may be configured to best fit when the size of the expanded portion of expandable fixation element  381  corresponds to the size of vasculature  700 . In this manner, the expanded portion of expandable fixation element  381  may be used to measure the size of vasculature  700  to determine a target side for deployment of IMD  380  within vasculature  700 . Deflection by either vasculature  700  or the expanded portion of expandable fixation element  381  may indicate the size of the expanded portion of expandable fixation element  381  corresponds to the size of vasculature  700 . 
     In an example, the clinician may use fluoroscopy to view vasculature  700  and/or the expanded portion of expandable fixation element  381  while advancing outer sheath  320 , deployment receptacle  340  and the partially deployed IMD  380  within vasculature  700 . The clinician may also inject a contrast dye within vasculature  700  to aid in the monitoring of the expanded portion of expandable fixation element  381  and vasculature  700 . 
     Once the clinician determines when the size of the expanded portion of expandable fixation element  381  corresponds to the size of vasculature  700 , the clinician may deploy IMD  380  within vasculature  700  in accordance with the techniques described with respect to  FIGS. 10A-10D . 
       FIG. 19  illustrates techniques for measuring the size of vasculature  700  using outer sheath  620  and inner sheath  640  of  FIGS. 17A-17E  with IMD  380  partially deployed from inner sheath  640 . The distal end of inner sheath  640  is delivered adjacent a target site within vasculature  700  through outer sheath  320 . As one example, vasculature  700  may be a pulmonary artery or other vasculature of the patient. 
     Inner sheath  640  is then used to partially deploy IMD  380  from distal opening  622  of outer sheath  620 . As shown in  FIG. 19 , a portion of expandable fixation element  381  assumes the expanded position when IMD  380  is partially deployed from distal opening  622  of outer sheath  620 . 
     Outer sheath  620 , inner sheath  640  and the partially deployed IMD  380  is then advanced within the vasculature. A clinician monitors the process of outer sheath  620 , inner sheath  640  and the partially deployed IMD  380 . Specifically, the clinician monitors vasculature  700  and/or the expanded portion of expandable fixation element  381  for deflection to determine when the size of the expanded portion of expandable fixation element  381  corresponds to the size of vasculature  700 . For example, vasculature  700  may be tapered, and IMD  380  may be configured to best fit when the size of the expanded portion of expandable fixation element  381  corresponds to the size of vasculature  700 . In this manner, the expanded portion of expandable fixation element  381  may be used to measure the size of vasculature  700  to determine a target side for deployment of IMD  380  within vasculature  700 . Deflection by either vasculature  700  or the expanded portion of expandable fixation element  381  may indicate the size of the expanded portion of expandable fixation element  381  corresponds to the size of vasculature  700 . 
     In an example, the clinician may use fluoroscopy to view vasculature  700  and/or the expanded portion of expandable fixation element  381  while advancing outer sheath  620 , inner sheath  640  and the partially deployed IMD  380  within vasculature  700 . The clinician may also inject a contrast dye within vasculature  700  to aid in the monitoring of the expanded portion of expandable fixation element  381  and vasculature  700 . 
     Once the clinician determine when the size of the expanded portion of expandable fixation element  381  corresponds to the size of vasculature  700 , the clinician may deploy IMD  380  within vasculature  700  in accordance with the techniques described with respect to  FIGS. 17A-17E . 
       FIG. 20  illustrates techniques for measuring the size of vasculature  700  using delivery catheter  500  of  FIGS. 16A-16B . As described previously, delivery catheter  500  includes inner sheath  540  with inflatable distal portion  562 . As shown in  FIG. 20 , the distal end of outer sheath  520  is delivered adjacent a target site within vasculature  700 . As one example, vasculature  700  may be a pulmonary artery or other vasculature of the patient. 
     Inflatable distal portion  562  may be inflated during the insertion of delivery catheter  500  as described with respect to  FIGS. 16A-16B . Inflatable distal portion  562  may be used to measure the size of vasculature  700 . For example, a clinician may monitor vasculature  700  during the insertion of delivery catheter  500 ; once vasculature  700  is occluded by inflatable distal portion  562 , the clinician will know that the size of vasculature  700  at that location corresponds to the inflated size of inflatable distal portion  562 . In some examples, the clinician may adjust the inflated size of inflatable distal portion  562  to measure the size of vasculature  700 . In other examples, the clinician may maintain the inflated size of inflatable distal portion  562  to find the location within vasculature  700  that corresponds to the inflated size of inflatable distal portion  562 . In any event, once the clinician has found a suitable target site within vasculature  700 , the clinician may deploy IMD  380  from delivery catheter  500  in accordance with the techniques described with respect to  FIGS. 16A-16B . 
       FIG. 21  is a flowchart illustrating techniques for measuring the size of a vasculature using a partially deployed IMD. For example, the techniques of  FIG. 21  may be performed using deployment receptacle  340  of  FIGS. 10A-10D  or using outer sheath  620  and inner sheath  640  of  FIGS. 17A-17E . In a further example, the techniques of  FIG. 21  may be performed using the system for intravascular delivery of an IMD described with respect to  FIGS. 6-8E . 
     First, a distal end of an elongated outer sheath forming an inner lumen with a distal opening is positioned adjacent a target site within a vasculature of a patient ( 702 ). Then an IMD is partially deployed from the distal opening of the outer sheath ( 704 ). The IMD includes an expandable fixation element expandable from a collapsed position to an expanded position, and at least a portion of the expandable fixation element assumes the expanded position when the implantable medical device is partially deployed from the distal opening. 
     After the IMD is partially deployed from the distal opening of the outer sheath, the distal end of the outer sheath with the implantable medical device partially deployed from the distal opening is advanced within the vasculature ( 706 ). While advancing the distal end of the outer sheath with the implantable medical device partially deployed from the distal opening, at least one of the vasculature and the portion of the expandable fixation element is monitored for deflection to determine when the size of the portion of the expandable fixation element corresponds to the size of the vasculature ( 708 ). Monitoring at least one of the vasculature and the portion of the expandable fixation element for deflection may include using fluoroscopy to view at least one of the vasculature and the portion of the expandable fixation element while advancing the distal end of the outer sheath within the vasculature. In addition, monitoring at least one of the vasculature and the portion of the expandable fixation element for deflection may also include injecting a contrast dye within the vasculature. 
     After determining the size of the portion of the expandable fixation element corresponds to the size of the vasculature, the techniques may include fully releasing the implantable medical device to deploy the implantable medical device within the vasculature ( 710 ). 
     In examples in which the IMD includes a pressure sensor, the techniques may further include monitoring pressure within the vasculature with the pressure sensor with the implantable medical device partially deployed from the distal opening to test the functionality of IMD at that location, and after verifying the functionality of implantable medical device at that location, fully releasing the implantable medical device to deploy the implantable medical device within the vasculature. For example, such a testing may include receiving an indication of the monitored pressure from the IMD with an external programmer, such as programmer  24  ( FIG. 1 ). 
       FIGS. 22-24C  illustrate example techniques for intravascular delivery of IMD  380  using delivery catheter  800 . Delivery catheter  800  includes tether  850 , which forms loop  851  to engage a looped element of the IMD  380 , such as an expandable fixation element of IMD  380 .  FIG. 22  illustrates a portion of delivery catheter  800  near the distal end of delivery catheter  800  whereas  FIGS. 23A-23D  illustrate deployment of IMD  380  from the distal end of delivery catheter  800 .  FIGS. 24A-24C  illustrate deployment handle  860  at the proximal end of delivery catheter  800 . Deployment handle  860  may be operated by a clinician during an implantation procedure to remotely deploy IMD  380  from the distal end of delivery catheter  800  as shown in  FIGS. 23A-23D . In particular, deployment handle  860  may be used to retract outer sheath  820  to expose IMD  380  while inner sheath  830  holds IMD  380  at a target location within the patient. 
     As shown in  FIG. 22 , delivery catheter  800  includes elongated outer sheath  820  forming inner lumen  821  with distal opening  822 . Outer sheath  820  is sized to traverse a vasculature of the patient. Delivery catheter  800  further includes elongated inner sheath  830 . Elongated inner sheath  830  includes stopper  840 , which is configured to engage a proximal side of IMD  380  to preclude IMD  380  from being located at a more proximal position than stopper  840  within inner lumen  821  of outer sheath  820 . In one example, stopper  840  may substantially fill inner lumen  821  of outer sheath  820 . In different examples, distal side  842  of stopper  840  may have a concave shape or be substantially flat. 
     Inner sheath  830  further includes tether  850 , which is configured to form loop  851  on distal side  842  of stopper  840 . Loop  851  is configured to engage a looped element of IMD  380  (not shown in  FIG. 22 ) to couple IMD  380  to inner sheath  830 . In different examples, tether  850  may be formed from a suture-like thread material or from a shape memory alloy such as Nitinol. 
     As described in further detail with respect to  FIGS. 23A-24C , inner sheath  830  and stopper  840  are slidable relative to outer sheath  820 . In particular, stopper  840  is slidable between a position that is proximally located relative to distal opening  822  of outer sheath  820  and a position in which at least a portion of stopper  840  is distally located relative to distal opening  822  of outer sheath  820 . In one example, while positioning the distal end of catheter  800  proximate a target site within a vasculature of a patient during an implantation procedure, stopper  840  may be located entirely within inner lumen  821  of outer sheath  820  such that IMD  380  fits within inner lumen  821  of outer sheath  820  at a position distal to the position of stopper  840  within inner lumen  821  of outer sheath  820 . Further, tether  850  forms loop  851  on distal side  842  of stopper  840 . Loop  851  engages a looped element of IMD  380  (not shown in  FIG. 22 ) to couple IMD  380  to inner sheath  830  during the positioning of the distal end of catheter  800  proximate the target site within the vasculature of the patient during the implantation procedure. 
     During the implantation procedure, one the distal end of catheter  800  is positioned proximate the target site within the vasculature of the patient, the clinician may partially retract outer sheath  820  to expose IMD  380 , but tether remains engaged with the looped element of IMD  380 . If the clinician is satisfied with the position of IMD  380 , the clinician may then further retract outer sheath  820  such that at least a portion of stopper  840  is distally located relative to distal opening  822  of outer sheath  820 . When stopper is located in this position, tether  850  releases the looped element of IMD  380 . 
     Specifically, one end of tether  850  is fixed to stopper  840  and the second end of tether  850  includes bead  852 . When tether  850  forms the loop, bead  852  is located within inner lumen  821  of outer sheath  820  proximal to stopper  840 . In this position, bead  852  is pinched between an inner surface of outer sheath  820  and a proximal side of stopper  840 . Retracting outer sheath  820  such that at least a portion of stopper  840  is distally located relative to distal opening  822  of outer sheath  820  serves to free bead  852  from between an inner surface of outer sheath  820  and the proximal tapered surface of stopper  840  to open lopped  851 . 
     In the example shown in  FIG. 22 , the proximal side of stopper  840  is tapered from an inner diameter to an outer diameter of stopper  840 . Bead  852  provides a tapered profile configured to register with the taper of the proximal side of stopper  840  and the inner surface of the outer sheath  820 . The tapered profile of bead  852  may mitigate binding between outer sheath  820 , bead  852  and stopper  840  as compared to a bead having a different shape, such as a sphere shape. As also shown in the example shown in  FIG. 22 , stopper  840  includes groove  849  adjacent to outer sheath  820 . Groove  849  is configured to receive tether  850  when tether  850  forms loop  851 . This may further mitigate binding, i.e., binding between outer sheath  820 , tether  850  and stopper  840 . 
     As shown in  FIG. 22 , inner sheath  830  includes inner shaft  831 , which extends from a proximal end of inner sheath  830  to a distal end of inner sheath  830  including through stopper  840 . Inner sheath  830  further includes outer shaft  832  that extends from the proximal end of inner sheath  830  to a position within stopper  840  such that a distal end of outer shaft  832  is within stopper  840 . In an example, outer shaft  832  is shrink tubing formed over inner sheath  830 . In one example, stopper  840  is an overmold that encapsulates the distal end of outer shaft  832  and a portion of inner shaft  831  to fix the position of outer shaft  832  relative to inner shaft  831 . 
     Outer shaft  832  provides stiffness to inner sheath  830  between the proximal end of inner sheath  830  and stopper  840 . The stiffness provided by shaft  832  may mitigate buckling of inner sheath  830  during an implantation procedure. Meanwhile the configuration of inner sheath  830  provides a smaller diameter distal to stopper  840 , which increases the space available for IMD  380  within inner lumen  821  of outer sheath  820 , and reduces the outer diameter of outer sheath  820  needed for outer sheath  820  to contain both inner sheath  830  and IMD  380  within the distal portion of inner lumen  821 . 
     As one example, the inner diameter of outer sheath  820  may be 13 French (0.13 inches) or less and the outer diameter of outer sheath  820  may be about 3 French (0.03 inches) greater than the inner diameter of outer sheath  820 , i.e., 16 French or less. In some examples, the body portion of IMD  380  including a sensor may have a cross-sectional thickness of about 10 French (0.10 inches) and the entirety of IMD  380  including fixation element  381  may provide a cross-sectional profile thickness of about 12 French (0.12 inches) when fixation element  381  is in a fully-collapsed position. In an alternative configuration, a delivery catheter similar to delivery catheter  800  may be modified to be tipless, i.e., without enlarged distal portion  835 . In such a configuration, inner sheath  830  would terminate at stopper  840 , and the diameter of outer sheath  820  could be further reduced as the distal portion of inner lumen  821  would only have to be large enough to contain IMD  380  and not also contain inner shaft  831 . The dimensions provided herein are merely examples, and the particular sizes of the components discussed herein may be modified to account for different size IMDs and/or different target sites and access routes within a patient. 
       FIGS. 23A-24C  illustrate exemplary techniques for intravascular delivery of IMD  380  using delivery catheter  800 .  FIGS. 23A-23D  illustrate deployment of IMD  380  from the distal end of delivery catheter  800 , whereas  FIGS. 24A-24C  illustrate deployment handle  860  at the proximal end of delivery catheter  800 . Delivery catheter  800  includes elongated outer sheath  820  and elongated inner sheath  830  in a substantially coaxial arrangement. Elongated outer sheath  820  and elongated inner sheath  830  each extend from deployment handle  860  to the distal end of delivery catheter  800 . Delivery catheter  800  and outer sheath  820  are sized to traverse a vasculature of the patient, and delivery catheter  800  is configured to carry IMD  380  within a distal portion of inner lumen  821  of outer sheath  820  while traversing the vasculature of the patient. Inner sheath  830  is slidable within outer sheath  820  and includes enlarged distal portion  835  and tether  850 . Enlarged distal portion  835  provides an inflatable member and flexible tapered tip  837 , e.g., as described with respect to  FIG. 14 . In other examples, a tapered distal end, e.g., as described with respect to  FIGS. 15A-15F  may be used in place of the inflatable member. In some examples, inner sheath  830  and enlarged distal portion  835  may include a lumen (not shown) configured to receive a guidewire and/or deliver contrast injections during an implantation procedure. 
     IMD  380  includes expandable fixation element  381 , which is deployable from a collapsed position to an expanded position secure the IMD  380  proximate a target site within a patient. While  FIGS. 23A-23D  illustrate implantation techniques using IMD  380 , in different examples, IMD  380  may be substantially similar to IMD  17  ( FIG. 8 ) or IMD  15  ( FIG. 8 ). As one example, the techniques of  FIGS. 23A-24C  may be used to deliver IMD  17  to a position within a heart of a patient, such as a position proximate to an inner wall of the right ventricle, within the right atrium, the left atrium, and/or left ventricle. As another example, the techniques of  FIGS. 23A-24C  may be used to deliver IMD  15  to an intravascular position such as a pulmonary artery or other vasculature of the patient. 
     During an implantation procedure, a clinician first positions delivery catheter  800  such that the distal end of outer sheath  820  is proximate to a target site within the patient via a vasculature accessed during a surgical procedure, as represented by  FIG. 23A . For example, delivery catheter  800  may be advanced into an entry vessel, such as the femoral artery, and then manipulated and navigated through the patient&#39;s vasculature until the distal end of outer sheath  820  proximate to a target site within the patient. In different examples, delivery catheter  800  may be steerable or be configured to traverse a guidewire to be directed to the target site from the access point of the vasculature. The clinician may use imaging techniques, such as fluoroscopy, to monitor the position of outer sheath  820 , inner sheath  830 , and IMD  380  throughout the implantation procedure. As shown in  FIG. 22 , outer sheath  820  includes marker band  824 , which is visible to the clinician during imaging. As one example, marker band  824  may be a gold band. Bead  852  and/or stopper  840  may also include radiopaque materials to aid in visibility of catheter  800  during an implantation procedure. In some examples, bead  852  and stopper  840  may have different radiopaque materials such that bead  852  and stopper  840  such that a clinician can clearly distinguish bead  852  and stopper  840  from one another during imaging. In further examples, delivery catheter  800  may have an internal lumen for contrast injections. As one example, the internal lumen for contrast injections may be a guidewire lumen. 
     Enlarged distal portion  835  is configured to substantially fill inner lumen  821  of outer sheath  820  and close-off distal opening  822  of outer sheath  820  when inflated, for example, while delivery catheter  800  is advanced to a location proximate a target site within a patient. 
     After positioning the distal end of outer sheath  820  is proximate to the target site within the patient, the clinician operates deployment handle  860  ( FIGS. 24A-24C ) to deploy IMD  380 . As shown in  FIG. 24A , deployment handle is located at the proximal end of outer sheath  820  and inner sheath  830  (not indicated in  FIG. 24A ). Deployment handle  860  includes sheath retraction mechanism  870 , which facilitates selectively retracting outer sheath  820  relative to inner sheath  830  to facilitate remote deployment of IMD  380  out of the distal opening of the inner lumen of the outer sheath. 
     Deployment handle  860  includes body  862 , which forms grip surfaces  864  to improve the controllability of deployment handle  860  by a clinician. Sheath retraction mechanism  870  includes slidable deployment button  872 , which is configured to selectively retract outer sheath  820  relative to inner sheath  830  when moved from a distal position on body  862  (as shown in  FIG. 24A ) to a more proximal position on the body (as shown in  FIGS. 24B and 24C ). Deployment button  872  may be directly coupled to outer sheath  820 , whereas body  862  may be directly coupled to inner sheath  830  via guidewire port  886 . Body  862  includes slot  873 , which allows slidable deployment button  872  to be outside body  862  while being connected to outer sheath  820 . Sheath retraction mechanism  870  further includes positive stops  875 , which individually register with deployment button  872  such that a clinician can incrementally retract outer sheath  820  if desired. 
     Deployment handle  860  further includes partial deployment lock button  874 . Partial deployment lock button  874  is configured to selectively prevent deployment button  872  from being moved to a position configured to fully release the IMD  380  from the inner sheath, i.e., a position in which bead  852  is released to open loop  851 . 
     As mentioned above, deployment handle  860  includes guidewire port  886 . Inner sheath  830  includes a guidewire lumen (not shown) extending throughout the length of inner sheath  830 , the guidewire lumen being configured to slidably receive a guidewire. Guidewire port  886  is in substantial alignment with the guidewire lumen of inner sheath  830 . Guidewire port  886  facilitates removal of a guidewire from within the guidewire lumen by pulling the guidewire proximally out of guidewire port  886  and the guidewire lumen of inner sheath  830 . In some examples, guidewire port  886  may include a one-way valve to prevent patient fluids from flowing through guidewire lumen of inner sheath  830  and out of guidewire port  886  once the distal end of catheter  800  is inserted within a patient. 
     Deployment handle  860  further includes flushing check valve  882 . Flushing check valve  882  is a one-way valve that facilitates flushing outer sheath  820  to remove air from within outer sheath  820  prior to inserting the distal end of catheter  800  within a patient to mitigate a risk of emboli within the patient. The one-way configuration of check valve  882  also serves to prevent patient fluids from flowing through inner lumen  821  of outer sheath  820  and out of check valve  882  once the distal end of catheter  800  is inserted within a patient. 
     Deployment handle  860  further includes inflation port  884 , which is configured to exchange an inflation media via inflation media tube  885 . The inflation port is used to selectively inflate inflatable member  835  on the distal end of inner sheath  830 . 
     As mentioned previously, after positioning the distal end of outer sheath  820  is proximate to the target site within the patient, as represented by  FIG. 23A , the clinician evaluates inflation media from inflatable member  835  via inflation port  884  deployment handle  860 , as represented by  FIG. 23B . 
     Then the clinician operates deployment handle  860  to deploy IMD  380 . As shown in  FIG. 24A , slidable deployment button  872  is in its most distal position, which coincides with the distal end of outer sheath  820  being in its most distal position as shown in  FIGS. 23A and 23B . From this position, the clinician moves slidable deployment button  872  in a proximal direction relative to body  862  of deployment handle  860  as represented by  FIG. 24B . This retracts outer sheath  820  relative to inner sheath  830  such that stopper  840  of pushes IMD  380  out of distal opening  822  of outer sheath  820  as represented by  FIG. 23C . However, because the position of inner sheath  830  has been maintained within the patient while outer sheath  820  is retracted, the position of IMD  380  is also maintained within the patient while outer sheath  820  is retracted. In this manner, retracting outer sheath  820  rather than extending inner sheath  830  to push IMD  380  out of distal opening  822  of outer sheath  820  allows a clinician to locate IMD  380  at a target deployment site before IMD  380  is actually deployed. 
     As shown in  FIG. 23C , all or a portion of expandable fixation element  381  of IMD  380  is expanded from a collapsed position to an expanded position as IMD  380  passes out of distal opening  822  of the distal end of outer sheath  820 . In the expanded position, expandable fixation element  381  will secure IMD  380  within the patient, e.g., as described with respect to IMD  17  ( FIG. 4 ) or IMD  15  ( FIG. 8 ). 
     Partial deployment lock button  874  of deployment handle  860  selectively prevents deployment button  872  from being moved to a position configured to fully-release IMD  380  from inner sheath  830 , i.e., a position in which bead  852  is released to open loop  851 . As shown in  FIG. 24B , partial deployment lock button  874  is engaged prevents slidable deployment button  872  from moving further in a proximal direction. 
     If a clinician is not satisfied with the position of IMD  380  after partial deployment, as represented by  FIG. 23C  and  FIG. 24B , the clinician may advance outer sheath  820  relative to inner sheath  830  by moving deployment button  872  to its distal position as shown in  FIG. 24A . This relocates IMD  380  within inner lumen  821  of outer sheath  820  via distal opening  822  of outer sheath  820 . The clinician may then optionally redeploy IMD  380 , e.g., at a different position within the patient. 
     Once a clinician is satisfied with the position of IMD  380  after partial deployment, as represented by  FIG. 23C  and  FIG. 24B , the clinician may open partial deployment lock button  874  and move deployment button  872  to a more proximal position as represented by  FIG. 24C . This further retracts outer sheath  820  in a more proximal direction relative to inner sheath  830  such that stopper  840  is positioned distally relative to distal opening  822  of outer sheath  820  to open tether loop  851  by releasing bead  852  and release expandable fixation element  381 , a looped element of IMD  380 , from inner sheath  830  as represented by  FIG. 23D  to fully deploy IMD  380 . Once IMD  380  is fully deployed, a clinician may withdraw catheter  800  from the patient. 
     While deployment handle  860  has been described specifically with respect to catheter  800 , the techniques disclosed with respect to deployment handle  860  may also be used with a variety of alternate catheter designs, including those disclosed herein. 
     Various examples of the disclosure have been described. These and other examples are within the scope of the following claims.