Patent Publication Number: US-2016228715-A9

Title: Implantable medical device fixation

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/096,881, filed Apr. 28, 2011 entitled “IMPLANTABLE MEDICAL DEVICE FIXATION”, herein incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to fixation techniques for implantable medical devices. 
     BACKGROUND 
     Medical devices such as electrical stimulators, leads, and electrodes are implanted to deliver therapy to one or more target sites within the body of a patient. To ensure reliable electrical contact between the electrodes and the target site, fixation of the device, lead, or electrodes is desirable. 
     A variety of medical devices for delivering a therapy and/or monitoring a physiological condition have been used clinically or proposed for clinical use in patients. Examples include medical devices that deliver therapy to and/or monitor conditions associated with the heart, muscle, nerve, brain, stomach or other organs or tissue. Some therapies include the delivery of electrical signals, e.g., stimulation, to such organs or tissues. Some medical devices may employ one or more elongated electrical leads carrying electrodes for the delivery of therapeutic electrical signals to such organs or tissues, electrodes for sensing intrinsic electrical signals within the patient, which may be generated by such organs or tissue, and/or other sensors for sensing physiological parameters of a patient. 
     Medical leads may be configured to allow electrodes or other sensors to be positioned at desired locations for delivery of therapeutic electrical signals or sensing. For example, electrodes or sensors may be carried at a distal portion of a lead. A proximal portion of the lead may be coupled to a medical device housing, which may contain circuitry such as signal generation and/or sensing circuitry. In some cases, the medical leads and the medical device housing are implantable within the patient. Medical devices with a housing configured for implantation within the patient may be referred to as implantable medical devices (IMDs). 
     Implantable cardiac pacemakers or cardioverter-defibrillators, for example, provide therapeutic electrical signals to the heart, e.g., via electrodes carried by one or more implantable medical leads. The therapeutic electrical signals may include pulses for pacing, or shocks for cardioversion or defibrillation. In some cases, a medical device may sense intrinsic depolarizations of the heart, and control delivery of therapeutic signals to the heart based on the sensed depolarizations. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate therapeutic electrical signal or signals may be delivered to restore or maintain a more normal rhythm. For example, in some cases, an IMD may deliver pacing stimulation to the heart of the patient upon detecting tachycardia or bradycardia, and deliver cardioversion or defibrillation shocks to the heart upon detecting fibrillation. 
     Leadless IMDs may also be used to deliver therapy to a patient, and/or sense physiological parameters of a patient. In some examples, a leadless IMD may include one or more electrodes on its outer housing to deliver therapeutic electrical signals to patient, and/or sense intrinsic electrical signals of patient. For example, leadless cardiac devices, such as leadless pacemakers, may also be used to sense intrinsic depolarizations and/or other physiological parameters of the heart and/or deliver therapeutic electrical signals to the heart. A leadless cardiac device may include one or more electrodes on its outer housing to deliver therapeutic electrical signals and/or sense intrinsic depolarizations of the heart. Leadless cardiac devices may be positioned within or outside of the heart and, in some examples, may be anchored to a wall of the heart via a fixation mechanism. 
     SUMMARY 
     In general, this disclosure describes remotely-deployable active fixation tines for fixating IMDs or their components, such as leads, to patient tissues. As referred to herein an “IMD component,” may be an entire IMD or an individual component thereof. Examples of IMDs that may be fixated to patient tissues with remotely-deployable active fixation tines according to this disclosure include leadless pacemakers and leadless sensing devices. 
     Active fixation tines disclosed herein may be deployed from the distal end of a catheter located at a desired implantation location for the IMD or its component. As further disclosed herein, active fixation tines provide a deployment energy sufficient to permeate a desired patient tissue and secure an IMD or its component to the patient tissue without tearing the patient tissue. This disclosure includes active fixation tines that allow for removal from a patient tissue followed by redeployment, e.g., to adjust the position of the IMD relative to the patient tissue. As different patient tissues have different physical and mechanical characteristics, the design of active fixation tines may be coordinated with patient tissue located at a selected fixation site within a patient. Multiple designs may be used to optimize fixation for a variety of patient tissues. 
     In one example, the disclosure is directed to an assembly comprising: an implantable medical device; and a set of active fixation tines attached to the implantable medical device. 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 implantable medical device to a hooked position in which the active fixation tines bend back towards the implantable medical device. The active fixation tines are configured to secure the implantable medical device to a patient tissue when deployed while the distal ends of the active fixation tines are positioned adjacent to the patient tissue. 
     In another example, the disclosure is directed to a kit for implanting an implantable medical device within a patient, the kit comprising: the implantable medical device; a set of active fixation tines attached to the implantable medical device. 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 implantable medical device to a hooked position in which the active fixation tines bend back towards the implantable medical device. The active fixation tines are configured to secure the implantable medical device to a patient tissue when deployed while the distal ends of the active fixation tines are positioned adjacent to the patient tissue. The kit further comprises a catheter forming a lumen sized to receive the implantable medical device and hold the active fixation tines in the spring-loaded position, wherein the lumen includes an aperture that is adjacent to the distal end of the catheter; and a deployment element configured to initiate deployment of the active fixation tines while the implantable medical device is positioned within the lumen of the catheter. Deployment of the active fixation tines while the implantable medical device is positioned within the lumen of the catheter causes the active fixation tines to pull the implantable medical device out of the lumen via the aperture that is adjacent to the distal end of the catheter. 
     In another example, the disclosure is directed to a method comprising: obtaining an assembly comprising an implantable medical device and a set of active fixation tines attached to the implantable medical device; positioning the distal ends of the active fixation tines adjacent to a patient tissue; and deploying the active fixation tines from a spring-loaded position in which distal ends of the active fixation tines point away from the implantable medical device to a hooked position in which the active fixation tines bend back towards the implantable medical device to secure the implantable medical device to the patient tissue. 
     The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  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. 2  is a conceptual diagram illustrating another example therapy system comprising an IMD coupled to a plurality of leads that may be used to monitor one or more physiological parameters of a patient and/or provide therapy to the heart of a patient. 
         FIGS. 3A-3B  illustrate the leadless IMD of  FIG. 1  in further detail. 
         FIGS. 4A-4B  illustrate an assembly including the leadless IMD of  FIG. 1  and a catheter configured to deploy the leadless IMD of  FIG. 1   
         FIGS. 5A-5H  illustrate techniques for securing the leadless IMD of  FIG. 1  to a patient tissue using the catheter of  FIG. 4A-4B . 
         FIGS. 6A-6B  illustrate an active fixation tine showing measurements used to calculate performance characteristics of the active fixation tine. 
         FIGS. 7A-7D  illustrate exemplary tine profiles. 
         FIG. 8  is a functional block diagram illustrating an example configuration of an IMD. 
         FIG. 9  is a block diagram of an example external programmer that facilitates user communication with an IMD. 
         FIG. 10  is a flowchart illustrating techniques for implanting an implantable medical device within a patient. 
     
    
    
     DETAILED DESCRIPTION 
     Active fixation tines disclosed herein may be useful to secure an implantable medical device (IMD) including any components thereof, such as a medical lead, to a patient tissue during minimally invasive surgery. Minimally invasive surgery, such as percutaneous surgery, permits IMD implantation with less pain and recovery time than open surgery. However, minimally invasive surgery tends to be more complicated than open surgery. For example, forming device fixation requires a surgeon to manipulate instruments remotely, e.g., within the confines of an intravascular catheter. With techniques for remote deployment and fixation of IMDs it can be difficult to ensure adequate fixation while minimizing tissue damage. The active fixation tines disclosed are suitable for securing an IMD to a patient tissue. In addition, active fixation tines disclosed herein also allow for simple removal from a patient tissue without tearing the patient tissue followed by redeployment, e.g., to adjust the position of the IMD after first securing the IMD to the patient tissue. 
     In one example, active fixation tines disclosed herein may be deployed from the distal end of a catheter positioned by a clinician at a desired implantation location for the IMD. As further disclosed herein, active fixation tines provide a deployment energy sufficient to permeate a desired patient tissue and secure an IMD to the patient tissue without tearing the patient tissue. As different patient tissues have different physical and mechanical characteristics, the design of active fixation tines may be configured according to the properties of the patient tissue located at a selected fixation site within a patient. Multiple designs may be made for a variety of patient tissues, and available for selection based on the patient tissue at the fixation site. 
     Although various examples are described with respect to cardiac leads and leadless IMD, the disclosed active fixation tines may be useful for fixation of a variety of implantable medical devices in a variety of anatomical locations, and fixation of cardiac leads and leadless IMD is described for purposes of illustration. The described techniques can be readily applied securing catheters and other medical leads, e.g., for neurostimulation. As examples, medical leads with active fixation tines may be used for cardiac stimulation, gastric stimulation, functional electrical stimulation, peripheral nerve stimulation, spinal cord stimulation, pelvic nerve stimulation, deep brain stimulation, or subcutaneous neurological stimulation as well as other forms of stimulation. In addition, described techniques can be readily applied to IMDs including sensors, including leadless IMDs and IMDs with medical leads. As examples, IMDs including sensors and active fixation tines may include one or more of the following sensors: a pressure sensor, an electrocardiogram sensor, an oxygen sensor (for tissue oxygen or blood oxygen sensing), an accelerometer, a glucose sensor, a potassium sensor, a thermometer and/or other sensors. 
       FIG. 1  is a conceptual diagram illustrating an example therapy system  10 A 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 . Therapy system  10 A includes IMD  16 A, which is coupled to programmer  24 . IMD  16 A may be an implantable leadless pacemaker that provides electrical signals to heart  12  via one or more electrodes (not shown in  FIG. 1 ) on its outer housing. Additionally or alternatively, IMD  16 A may sense electrical signals attendant to the depolarization and repolarization of heart  12  via electrodes on its outer housing. In some examples, IMD  16 A provides pacing pulses to heart  12  based on the electrical signals sensed within heart  12 . 
     IMD  16 A includes a set of active fixation tines to secure IMD  16 A to a patient tissue. In the example of  FIG. 1 , IMD  16 A is positioned wholly within heart  12  proximate to an inner wall of right ventricle  28  to provide right ventricular (RV) pacing. Although IMD  16 A is shown within heart  12  and proximate to an inner wall of right ventricle  28  in the example of  FIG. 1 , IMD  16 A may be positioned at any other location outside or within heart  12 . For example, IMD  16 A 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  16 A may include other stimulation functionalities. For example, IMD  16 A may provide atrioventricular nodal stimulation, fat pad stimulation, vagal stimulation, or other types of neurostimulation. In other examples, IMD  16 A may be a monitor that senses one or more parameters of heart  12  and may not provide any stimulation functionality. In some examples, system  10 A may include a plurality of leadless IMDs  16 A, e.g., to provide stimulation and/or sensing at a variety of locations. 
     As discussed in greater detail with respect to  FIGS. 3A-5H , IMD  16 A 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  16 A to be removed from a patient tissue followed by redeployment, e.g., to adjust the position of IMD  16 A relative to the patient tissue. For example, a clinician implanting IMD  16 A may reposition IMD  16 A during an implantation procedure if testing of IMD  16 A indicates a poor electrode-tissue connection. 
       FIG. 1  further depicts programmer  24  in wireless communication with IMD  16 A. In some examples, programmer  24  comprises a handheld computing device, computer workstation, or networked computing device. Programmer  24 , shown and described in more detail below with respect to  FIG. 9 , includes a user interface that presents information to and receives input from a user. It should be noted that 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  16 A. For example, the user may interact with programmer  24  to retrieve physiological or diagnostic information from IMD  16 A. A user may also interact with programmer  24  to program IMD  16 A, e.g., select values for operational parameters of the IMD  16 A. For example, the user may use programmer  24  to retrieve information from IMD  16 A 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  16 A regarding other sensed physiological parameters of patient  14  or information derived from sensed physiological parameters, such as intracardiac or intravascular pressure, activity, posture, tissue oxygen levels, blood 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  16 A regarding the performance or integrity of IMD  16 A or other components of system  10 A, or a power source of IMD  16 A. As another example, the user may interact with programmer  24  to program, e.g., select parameters for, therapies provided by IMD  16 A, such as pacing and, optionally, neurostimulation. 
     IMD  16 A 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  16 A implant site in order to improve the quality or security of communication between IMD  16 A and programmer  24 . 
       FIG. 2  is a conceptual diagram illustrating another example therapy system  10 B 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 . Therapy system  10 B includes IMD  16 B, which is coupled to medical leads  18 ,  20 , and  22 , and programmer  24 . As referred to herein, each of IMD  16 B and medical leads  18 ,  20  and  22  may be referred to generally as an IMD. In one example, IMD  16 B may be an implantable pacemaker that provides electrical signals to heart  12  via electrodes coupled to one or more of leads  18 ,  20 , and  22 . IMD  16 B is one example of an electrical stimulation generator, and is configured attached to the proximal end of medical leads  18 ,  20 , and  22 . In other examples, in addition to or alternatively to pacing therapy, IMD  16 B may deliver neurostimulation signals. In some examples, IMD  16 B may also include cardioversion and/or defibrillation functionalities. In other examples, IMD  16 B may not provide any stimulation functionalities and, instead, may be a dedicated monitoring device. Patient  14  is ordinarily, but not necessarily, a human patient. 
     Medical leads  18 ,  20 ,  22  extend into the heart  12  of patient  14  to sense electrical activity of heart  12  and/or deliver electrical stimulation to heart  12 . In the example shown in  FIG. 2 , right ventricular (RV) lead  18  extends through one or more veins (not shown), the superior vena cava (not shown), right atrium  26 , and into right ventricle  28 . RV lead  18  may be used to deliver RV pacing to heart  12 . Left ventricular (LV) 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 . LV lead  20  may be used to deliver LV pacing to 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 . RA lead  22  may be used to deliver RA pacing to heart  12 . 
     In some examples, system  10 B may additionally or alternatively include one or more leads or lead segments (not shown in  FIG. 2 ) that deploy one or more electrodes within the vena cava or other vein, or within or near the aorta. Furthermore, in another example, system  10 B may additionally or alternatively include one or more additional intravenous or extravascular leads or lead segments that deploy one or more electrodes epicardially, e.g., near an epicardial fat pad, or proximate to the vagus nerve. In other examples, system  10 B need not include one of ventricular leads  18  and  20 . 
     One or more of medical leads  18 ,  20 ,  22  may include a set of active fixation tines to secure a distal end of the medical lead to a patient tissue. The inclusion of active fixation tines for each medical leads  18 ,  20 ,  22  is merely exemplary. One or more of medical leads  18 ,  20 ,  22  could be secured by alternative techniques. For example, even though each of medical leads  18 ,  20  and  22  is shown with a set of active fixation tines to secure a distal end of the medical lead, LV lead  20 , which extends through one or more veins and the vena cava and into the right atrium  26  of heart  12 , may instead be fixed using passive fixation. 
     The active fixation tines in set active fixation tines attached to a medical lead 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 the distal end of the medical lead be removed from a patient tissue followed by redeployment, e.g., to adjust the position of the distal end of the medical lead relative to the patient tissue. For example, a clinician implanting IMD  16 B may reposition the distal end of a medical lead during an implantation procedure if testing of IMD  16 B indicates a poor electrode-tissue connection. 
     IMD  16 B may sense electrical signals attendant to the depolarization and repolarization of heart  12  via electrodes (described in further detail with respect to  FIG. 4 ) coupled to at least one of the leads  18 ,  20 ,  22 . In some examples, IMD  16 B provides pacing pulses to heart  12  based on the electrical signals sensed within heart  12 . The configurations of electrodes used by IMD  16 B for sensing and pacing may be unipolar or bipolar. 
     IMD  16 B may also provide neurostimulation therapy, defibrillation therapy and/or cardioversion therapy via electrodes located on at least one of the leads  18 ,  20 ,  22 . For example, IMD  16 B may deliver defibrillation therapy to heart  12  in the form of electrical pulses upon detecting ventricular fibrillation of ventricles  28  and  32 . In some examples, IMD  16 B may be programmed to deliver a progression of therapies, e.g., pulses with increasing energy levels, until a fibrillation of heart  12  is stopped. As another example, IMD  16 B may deliver cardioversion or anti-tachycardia pacing (ATP) in response to detecting ventricular tachycardia, such as tachycardia of ventricles  28  and  32 . 
     As described above with respect to IMD  16 A of  FIG. 1 , programmer  24  may also be used to communicate with IMD  16 B. In addition to the functions described with respect to IMD  16 A of  FIG. 1 , a user may use programmer  24  to retrieve information from IMD  16 B regarding the performance or integrity of leads  18 ,  20  and  22  and may interact with programmer  24  to program, e.g., select parameters for, any additional therapies provided by IMD  16 B, such as cardioversion and/or defibrillation. 
     Leads  18 ,  20 ,  22  may be electrically coupled to a signal generator and a sensing module of IMD  16 B 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 B. In some examples, a single connector, e.g., an IS-4 or DF-4 connector, may connect multiple electrical contacts to connector block  34 . In addition, in some examples, leads  18 ,  20 ,  22  may be mechanically coupled to connector block  34  with the aid of set screws, connection pins, snap connectors, or another suitable mechanical coupling mechanism. 
     The configuration of system  10 B illustrated in  FIG. 2  is merely one example. In other examples, a system may include epicardial leads and/or patch electrodes instead of or in addition to the transvenous leads  18 ,  20 ,  22  illustrated in  FIG. 2 . Further, IMD  16 B need not be implanted within patient  14 . In examples in which IMD  16 B is not implanted in patient  14 , IMD  16 B 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 . For each of these examples, any number of the medical leads may include a set of active fixation tines on a distal end of the medical lead in accordance with the techniques described herein. 
     In addition, in other examples, a system may include any suitable number of leads coupled to IMD  16 B, and each of the leads may extend to any location within or proximate to heart  12 . For example, other examples of systems may include three transvenous leads located as illustrated in  FIG. 2 , and an additional lead located within or proximate to left atrium  36 . Other examples of systems may include a single lead that extends from IMD  16 B into right atrium  26  or right ventricle  28 , or two leads that extend into a respective one of the right ventricle  28  and right atrium  26 . Any electrodes located on these additional leads may be used in sensing and/or stimulation configurations. In each of these examples, any number of the medical leads may include a set of active fixation tines on a distal end of the medical lead in accordance with the techniques described herein. 
       FIGS. 3A-3B  illustrate leadless IMD  16 A of  FIG. 1  in further detail. In the example of  FIGS. 3A and 3B , leadless IMD  16 A includes tine fixation subassembly  100  and electronic subassembly  150 . Tine fixation subassembly  100  is configured to anchor leadless IMD  16 A 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  16 A, 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 an insulating coating. Examples of insulating coatings include parylene, urethane, PEEK, or polyimide among others. Electronic subassembly  150  further includes anode  162 , which may include a low polarizing coating, such as titanium nitride, iridium oxide, ruthenium oxide among others. The entirety of the housings of control electronics  152  and battery  160  are electrically connected to one another, but only anode  162  is uninsulated. 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 . Alternatively, anode  162  may be electrically isolated from the other portions of the housings of control electronics  152  and battery  160 . 
     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 catheter  200  ( FIG. 5A ) used to position IMD  16 A during an implantation procedure. Tine fixation subassembly interface  153  and feedthrough pin  154  are located at the distal end of electronic subassembly  150 . Tine fixation subassembly interface  153  includes three tabs that interlock with tine fixation subassembly  100 . 
     As best illustrated in  FIG. 3B , tine fixation subassembly  100  includes fixation element  102 , header body  112 , header cap  114 , locking tab  120 , electrode  122 , monolithic controlled release device (MCRD)  124  and filler cap  126 . Fixation element  102  includes a set of four active fixation tines  103  that are deployable from a spring-loaded position in which distal ends 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 the hooked position in  FIG. 3A . As discussed in further detail with respect to  FIGS. 4A-5H , active fixation tines  103  are configured to secure IMD  16 A to a patient tissue, e.g., a tissue inside the heart or outside the heart, when deployed while the distal ends of active fixation tines  103  are positioned adjacent to the patient tissue. In different examples, active fixation tines  103  may be positioned adjacent to patient tissue such that distal ends  109  penetrate the patient tissue prior to deployment, positioned adjacent to patient tissue such that distal ends  109  contact but do not penetrate the patient tissue prior to deployment or positioned adjacent to patient tissue such that distal ends  109  are near to but do not contact or penetrate the patient tissue prior to deployment. 
     Fixation element  102  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 one example, fixation element  102  including active fixation tines  103  and base  111 , may be manufactured by cutting fixation element  102  as a unitary component from a hollow tube of Nitinol, bending the cut tube to form the hooked position shape of active fixation tines  103  and heat-treating fixation element  102  while holding active fixation tines  103  in the hooked position. Sharp edges of fixation element  102  may be rounded off to improve fatigue loading and reduce tearing of patient tissue during deployment and retraction of active fixation tines  103 . 
     In some examples, all or a portion of fixation element  102 , such as active fixation tines  103 , may include one or more coatings. For example, fixation element  102  may include a radiopaque coating to provide visibility during fluoroscopy. In one such example, fixation element  102  may include one or more radiopaque markers. As another example, fixation element  102  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  16 A during an explantation procedure, which may occur many years after the implantation of IMD  16 A. 
     During assembly of IMD  16 A, prior to being mounted to electronic subassembly  150 , fixation element  102  may be mounted in a header including header body  112  and header cap  114 . For example, fixation element  102  may be mounted such that one tine extends though each of holes  113  in header body  112 . Then header cap  114  is positioned over base  111  of fixation element  102  and secured to header body  112 . As an example, header body  112  and header cap  114  may be fabricated of a biocompatible polymer such as polyether ether ketone (PEEK). Header body  112  and header cap  114  may function to electrically isolate fixation element  102  from electronic subassembly  150  and feedthrough pin  154 . In other examples, fixation element  102  itself may be used as an electrode for stimulation and/or sensing a physiological condition of a patient and may electrically connect to control electronics  152 . 
     During assembly of IMD  16 A, once fixation element  102  is assembled with header body  112  and header cap  114 , fixation element  102 , header body  112  and header cap  114  are mounted to the tabs of tine fixation subassembly interface  153  on electronic subassembly  150  by positioning header body  112  over the tabs of tine fixation subassembly interface  153  and rotating header body  112  to interlock header body  112  with the tabs of tine fixation subassembly interface  153 . Feedthrough pin  154  extends through the center of header body  112  once header body  112  is secured to tine fixation subassembly interface  153 . 
     During assembly of IMD  16 A, after header body  112  is secured to tine fixation subassembly interface  153 , locking tab  120  is positioned over feedthrough pin  154 . As an example, locking tab  120  may be fabricated of a silicone material. Next, electrode  122  is positioned over locking tab  120  and feedthrough pin  154 , and then mechanically and electrically connected to feedthrough pin  154 , e.g., using a laser weld. As an example, electrode  122  may comprise a biocompatible metal, such as an iridium alloy or a platinum alloy. 
     MCRD  124  is located within recess  123  of electrode  122 . In the illustrated example, MCRD  124  takes the form of a cylindrical plug. In other examples, an MCRD band may positioned around the outside of the electrode rather than configured as a cylindrical plug. MCRD  124  may be fabricated of a silicone based polymer, or other polymers. MCRD  124  may incorporate an anti-inflammatory drug, which may be, for example, the sodium salt of dexamethasone phosphate. Because MCRD  124  is retained within recess  123  of electrode  122 , migration of the drug contained in MCRD  124  is limited to the tissue in contact with the distal end of electrode  122 . Filler cap  126  is positioned over electrode  122 . As an example, filler cap  126  may be fabricated of a silicone material and positioned over both electrode  122  and locking tab  120  during assembly of IMD  16 A. 
     As different patient tissues have different physical and mechanical characteristics, active fixation tines  103  may be specifically designed to perform with patient tissues having specific characteristics. For example, active fixation tines  103  may be designed to provide a selected fixation force, designed to penetrate to a particular depth of a patient tissue, designed to penetrate to a particular layer of patient tissue (as different tissue layers may have different mechanical properties) and/or designed to facilitate removal and redeployment from the patient tissue without tearing the patient tissue, either on deployment or removal. Multiple designs of active fixation tine  103  may be used to optimize fixation for a variety of patient tissues. The design of active fixation tine  103  is discussed in further detail with respect to  FIGS. 6A-6B . In addition, the specific design of tine fixation subassembly  100  is not germane to the operation of active fixation tines  103 , and a variety of techniques may be used to attach a set of active fixation tines to an IMD. 
       FIG. 4A  illustrates assembly  180 , which includes leadless IMD  16 A and catheter  200 , which is configured to remotely deploy IMD  16 A. Catheter  200  may be a steerable catheter or be configured to traverse a guidewire. In any case, catheter  200  may be directed within a body lumen, such as a vascular structure to a target site in order to facilitate remote positioning and deployment of IMD  16 A. In particular, catheter  200  forms lumen  201 , which is sized to receive IMD  16 A at the distal end of catheter  200 . For example, the inner diameter of lumen  201  at the distal end of catheter  200  may be about the same size as the outer diameter of IMD  16 A. When IMD  16 A is positioned within lumen  201  at the distal end of catheter  200 , lumen  201  holds active fixation tines  103  in the spring-loaded position shown in  FIG. 4A . In the spring-loaded position, active fixation tines  103  store enough potential energy to secure IMD  16 A to a patient tissue upon deployment. 
     Lumen  201  includes aperture  221 , which is positioned at the distal end of catheter  200 . Aperture  221  facilitates deployment of IMD  16 A. Deployment element  210  is positioned proximate to IMD  16 A in lumen  201 . Deployment element  210  configured to initiate deployment of active fixation tines  103 . More particularly, a clinician may remotely deploy IMD  16 A by pressing plunger  212 , which is located at the proximal end of catheter  200 . Plunger  212  connects directly to deployment element  210 , e.g., with a wire or other stiff element running through catheter  200 , such that pressing on plunger  212  moves deployment element  210  distally within lumen  201 . As deployment element  210  moves distally within lumen  201 , deployment element  210  pushes IMD  16 A distally within lumen  201  and towards aperture  221 . Once the distal ends  109  of active fixation tines  103  reach aperture  221 , active fixation tines  103  pull IMD  16 A out of lumen  201  via aperture  221  as active fixation tines  103  move from a spring-loaded position to a hooked position to deploy IMD  16 A. The potential energy released by active fixation tines  103  is sufficient to penetrate a patient tissue and secure IMD  16 A to the patient tissue. 
     Tether  220  is attached to delivery tool interface  158  (not shown in  FIG. 4A ) of IMD  16 A and extends through catheter  200 . Following deployment of IMD  16 A, a clinician may remotely pull IMD  16 A back into lumen  201  by pulling on tether  220  at the proximal end of catheter  200 . Pulling IMD  16 A back into lumen  201  returns active fixation tines  103  to the spring-loaded position from the hooked position. The proximal ends of active fixation tines  103  remain fixed to the housing of IMD  16 A as active fixation tines  103  move from the spring-loaded position to the hooked position and vice-versa. Active fixation tines  103  are configured to facilitate releasing IMD  16 A from patient tissue without tearing the tissue when IMD  16 A is pulled back into lumen  201  by tether  220 . A clinician may redeploy IMD  16 A with deployment element  210  by operating plunger  212 . 
       FIG. 4B  is a sectional view of the distal end of assembly  180  in which IMD  16 A is positioned within lumen  201 . Lumen  201  holds active fixation tines  103  in a spring-loaded position. Distal ends  109  of active fixation tines  103  are indicated in  FIG. 4B . As shown in  FIG. 4B , the four active fixation tines  103  are positioned substantially equidistant from each other in a circular arrangement. As best seen in  FIG. 3A , active fixation tines  103  are oriented outwardly relative to the circular arrangement. 
     Positioning active fixation tines  103  substantially equidistant from each other in a circular arrangement creates opposing radial forces  222  when active fixation tines  103  are deployed in unison. This allows the combined forces of active fixation tines  103  acting on the distal end of catheter  200  to pull IMD  16 A about perpendicularly out of aperture  221 . When the active fixation tines are deployed while aperture  221  and distal ends  109  of active fixation tines  103  are positioned adjacent to a patient tissue, the forces of active fixation tines  103  acting on the distal end of catheter  200  combine to pull IMD  16 A straight out from aperture  221  and directly towards the patient tissue. While IMD  16 A includes a set of four active fixation tines, a set of more or less than four active fixation tines may be used. For example, as few as two active fixation tines may provide opposing radial forces  222 ; however, a set of at least three active fixation tines may provide better directional consistency in the deployment of an IMD such as IMD  16 A. 
     Distal ends  109  of active fixation tines  103  include substantially flat outer surfaces to register active fixation tines  103  on the inner surface of lumen  201 . The flat outer surfaces of active fixation tines  103  help ensure that the interaction between active fixation tines  103  and the inner surface of lumen  201  during deployment of IMD  16 A provides opposing radial forces  222 . 
       FIGS. 5A-5H  illustrate example techniques for securing IMD  16 A to patient tissue  300  using catheter  200 . As an example, patient tissue  300  may be a heart tissue, such as the inner wall of the right ventricle. For simplicity, a set of only two active fixation tines  103  are shown in each of  FIGS. 5A-5H ; however, the described techniques for securing IMD  16 A to patient tissue  300  are equally applicable to IMDs including a set of more than two active fixation tines  103 . 
       FIG. 5A  illustrates IMD  16 A within lumen  201  of catheter  200 . Lumen  201  holds active fixation tines  103  in a spring-loaded position in which distal ends  109  of active fixation tines  103  point away from IMD  16 A. Aperture  221  is positioned adjacent patient tissue  300 . The distal end  202  of catheter  200  may not pressed forcefully into patient tissue  300 , as pressing patient tissue  300  would alter the mechanical characteristics of patient tissue  300 . As active fixation tines  103  may be designed accordingly to the mechanical characteristics of patient tissue  300 , altering the mechanical characteristics of patient tissue  300  may undesirably alter the interaction of active fixation tines  103  and patient tissue  300  during deployment of active fixation tines  103 . In other examples, it may be desirable to alter the mechanical characteristics of patient tissue  300  for deployment, by significantly pressing on patient tissue  300  during deployment or by otherwise altering the mechanical characteristics of patient tissue  300 , to achieve a desired interaction (e.g., tissue permeation, fixation depth, etc.) between patient tissue  300  and active fixation tines  103  during deployment of active fixation tines  103 . 
       FIG. 5B  illustrates IMD  16 A shortly after a clinician remotely activated active fixation tines  103  using deployment element  210  by pressing on plunger  212  ( FIG. 4A ). As the clinician pressed plunger  212 , deployment element  210  pushed IMD  16 A distally within lumen  201 . Once the distal ends  109  of active fixation tines  103  reached aperture  221 , active fixation tines  103  began to pull IMD  16 A out of lumen  201  via aperture  221 . Distal ends  109  of active fixation tines  103  then penetrated patient tissue  300 .  FIG. 5B  illustrates active fixation tines  103  in a position after distal ends  109  of active fixation tines  103  penetrated patient tissue  300  and shortly after beginning the transition from a spring-loaded position to a hooked position. 
       FIGS. 5B-5F  illustrates active fixation tines  103  as they move from a spring-loaded position in which distal ends  109  of active fixation tines  103  point away from IMD  16 A to a hooked position in which distal ends  109  of active fixation tines  103  bend back towards IMD  16 A.  FIGS. 5D-5F  illustrate active fixation tines  103  in hooked positions. In  FIG. 5D , distal ends  109  of active fixation tines  103  remain embedded in patient tissue  300 , whereas  FIGS. 5E-5F  illustrate distal ends  109  of active fixation tines  103  penetrating out of patient tissue  300 . 
     As active fixation tines  103  move from a spring-loaded position to a hooked position, potential energy stored in active fixation tines  103  is released as IMD  16 A is pulled from lumen  201  via aperture  221 . In addition, active fixation tines  103  penetrate patient tissue  300  to secure IMD  16 A to patient tissue  300  such that electrode  123  ( FIG. 5E ) contacts patient tissue  300  within the center of the circular arrangement of active fixation tines  103 . Active fixation tines  103  provide a forward pressure of electrode  123  onto tissue  300  to assure good electrode-tissue contact. 
     As active fixation tines  103  pull IMD  16 A from lumen  201 , tether  220 , which is attached to delivery tool interface  158  of IMD  16 A is exposed, e.g., as shown in  FIG. 5E . Following deployment of IMD  16 A, a clinician may remotely pull IMD  16 A back into lumen  201  by pulling on tether  220  at the proximal end of catheter  200 . For example, the clinician may perform a test of IMD  16 A to evaluate a performance characteristic of electrode  123  while the IMD  16 A is secured to patient tissue  300  as shown in  FIG. 5E . If the test of IMD  16 A indicates inadequate performance, the clinician may decide to redeploy IMD  16 A. Pulling IMD  16 A back into lumen  201  releases IMD  16 A from patient tissue  300  and returns IMD  16 A to the position shown in  FIG. 5A . From this position a clinician may reposition IMD  16 A as desired and redeploy IMD  16 A. 
     As shown in  FIG. 5F , once IMD  16 A is secured to patient tissue  300  in the desired position, the clinician may release IMD  16 A from tether  220 . For example, the clinician may sever tether  220  at the proximal end of catheter  200  and remove tether  220  from delivery tool interface  158  by pulling on one of the severed ends of tether  220 . As shown in  FIG. 5G , once IMD  16 A is released from tether  220 , the clinician may remove catheter  200 , leaving IMD  16 A secured to patient tissue  300 . As shown in  FIG. 5H , active fixation tines  103  may continue to migrate to a lower-potential energy hooked position over time. However, any of the hooked positions of active fixation tines  103  as shown in  FIGS. 5D-5G  may be sufficient to adequately secure IMD  16 A to patient tissue  300 . 
     While the techniques of  FIGS. 5A-5H  are illustrated with respect to IMD  16 A, the techniques may also be applied to a different IMD, such as a medical lead including a set of active fixation tines like medical leads  18 ,  20 ,  22  of IMD  16 B ( FIG. 2 ). For example, such a medical lead may extend through a catheter during an implantation procedure. As such, deploying a medical lead may not require a separate deployment element within the catheter. Instead, simply pushing on the medical lead at the proximal end of the catheter may initiate deployment of a set of active fixation tines at the distal end of the medical lead by pushing the active fixation tines attached to the distal end of the medical lead out of the distal end of the catheter. Similarly retracting a medical lead for redeployment may not require a tether, but may instead simply involve pulling on the medical lead at the proximal end of the catheter. 
       FIGS. 6A-6B  illustrate one active fixation tine  103  and further illustrate measurements used to calculate performance characteristics of active fixation tine  103 . In particular,  FIG. 6A  illustrates a cross-section of active fixation tine  103  with width  104  and thickness (T)  105 .  FIG. 6B  illustrates a side-view of active fixation tine  103  with tine length (L)  106 , tine radius (r)  107  and tine angle  108 . 
     The design of active fixation tine  103  is based on many criteria. As one example, an active fixation tine must penetrate a patient tissue when extended in the spring-loaded position. To meet this criteria, length  106  must be large enough to overcome the elasticity of the patient tissue such that distal end  109  of active fixation tine  103  permeates the patient tissue before active fixation tine  103  starts to bend significantly when deployed. For example, active fixation tine  103  will start to bend significantly when deployed once the curved portion of active fixation tine  103  reaches aperture  221  in distal end  202  of catheter  200  ( FIG. 4A ). 
     If distal end  109  of active fixation tine  103  were pointed, this would reduce the insertion force; however, adding a sharp point to active fixation tine  103  may cause tearing of patient tissue during deployment and removal of active fixation tine  103 . For this reason, distal end  109  of active fixation tine  103  may be rounded. As one example, tine thickness  105  may be between about 0.005 inches and about 0.010 inches. In a further example, tine thickness  105  may be between about 0.006 inches and about 0.009 inches. In some examples, a tine may include a ball on its distal end to further resist tearing of patient tissue. One such example is shown in  FIG. 7C . 
     As another example, the straight section providing length  106  of active fixation tine  103  must provide a column strength great enough to resist buckling from the force of the patient tissue before distal end  109  of active fixation tine  103  permeates the patient tissue. Column strength is dependent on length  106 , width  104  and thickness  105 , whereas the force required to permeate a patient tissue is dependent on mechanical properties of the tissue and the cross-sectional area of distal end  109  of active fixation tine  103 . In addition, active fixation tine  103  may be designed to buckle before penetrating a particular tissue layer deeper than a targeted tissue layer. For example, when attaching to endocardial tissue, a tine may be designed to buckle before penetrating an epicardial layer of heart tissue to prevent penetrating an epicardial layer of heart tissue during deployment. 
     As another example, a set of active fixation tines may be designed to provide a selected holding force, which may also be referred to as the pull force required to remove a deployed set of active fixation tines from patient tissue (or other material). As one example, a holding force of between 1 and 5 newtons (N) or between 2 and 3 N may be suitable for securing IMD  16 A within heart  12  ( FIG. 1 ), while facilitating removal of the set of active fixation tines without tearing patient tissue. 
     Releasing an IMD from the tissue without tearing the tissue by pulling the implantable medical device away from the tissue includes, pulling on the implantable medical device to stretch the tissue until the tissue stiffness matches the tine straightening force, further pulling on the implantable medical device until the tines straighten without tearing the tissue, and continued pulling on the implantable medical device once the tines have straightened sufficiently to remove the tines from the patient tissue. The pulling distance required to release the tines from the tissue is longer than the length og the tines because of the elasticity of the tissue. For an example, in an example wherein the tines 7 mm long, removing the tines from the tissue may require pulling the IMD 12-20 mm away from the tissue. 
     Tine holding force may be considered the sum of tine straightening forces (to move the active fixation tines from the hooked position to the spring-loaded position) plus forces between the tine and the patient tissue, including frictional forces and forces that resist straightening of the tine in the patient tissue. Using finite element analysis, validated by actual testing, the following transfer function of the pull force required to remove a set of four active fixation tines deployed in cardiac tissue was determined, wherein C 1 :C 8  each represents a constant greater than zero: 
       Pull Force=− C   1   +C 2* T−C   3   *L+C   4   *r−C   5   *T*L−C   6   *T*r−C   7   *L*r+C   8   *T*L*r    (Equation 1)
 
     A sensitivity analysis using a Pareto Chart of Effects on the importance of the different factors of Equation 1 indicated that pull force is most sensitive to tine thickness (59%), followed by tine radius (38%). Pull force showed the least sensitivity to tine length (3%). In addition, the interaction between thickness and radius was also important, whereas the other interactions were less significant. 
     In some examples, thickness greater than 0.009 inches or less than 0.003 inches may not be able to produce a pull forces suitable for securing IMD  16 A within heart  12  ( FIG. 1 ). Of course, in other examples, e.g., using a different selected holding forces, or assuming different material properties of active fixation tines  103  and/or of patient tissue tine thickness of greater than 0.009 inches or less than 0.003 inches may be suitable. 
     One additional design factor is fatigue loading, e.g., fatigue loading resulting from movement of a patient. For example, active fixation tines  103  may be designed to secure IMD  16 A to patient heart  12  for a period of eighteen or more years. During that time, active fixation tines  103  may experience about 600 million heart beats from heart  12 . In addition, sharp corners are detrimental to withstanding fatigue loading; for this reason, corners of active fixation tines  103  may be rounded, e.g., as best shown in  FIG. 6A . 
       FIGS. 7A-7D  illustrate exemplary profiles of the distal ends of different active fixation tine designs. In particular,  FIG. 7A , illustrates rectangular profile  410  that provides a consistent width through its distal end  412 . A tine providing rectangular profile  410  may also provide a generally consistent thickness. As an example, rectangular profile  410  is consistent with the profile of active fixation tines  103 . 
       FIG. 7B  illustrates profile  420 , which includes an increased width at its distal end  422 . A tine providing profile  420  may also provide a generally consistent thickness. Profile  420  may provide an increased insertion force and reduced column strength relative to tine profile  410 . In addition, a tine providing profile  420  may reduce tearing of patient tissue during insertion and removal relative to a tine providing tine profile  410 . 
       FIG. 7C  illustrates profile  430 , with includes an enlarged distal tip  432 . Enlarged distal tip  432  is wider and thicker than the rest of a tine providing profile  430 . A tine including enlarged distal tip  432  may reduce tearing of patient tissue during insertion and removal relative to a tine providing tine profile  410 . 
       FIG. 7D  illustrates profile  440 , which includes an increased width at its distal end  442 . A tine providing profile  440  may also provide a generally consistent thickness. Profile  440  also includes a series of apertures  444 . After implantation, a tine including apertures  444  may provide a significant increase in holding strength relative to tine providing profile  410  as patient tissue grows around apertures  444 . In addition, tine profile  440  may provide an increased insertion force and reduced column strength relative to tine profile  410 . 
       FIG. 8  is a functional block diagram illustrating one example configuration of IMD  16 A of  FIGS. 1 and 3  or IMD  16 B of  FIG. 2  (referred to generally as IMD  16 ). In the example illustrated by  FIG. 8 , IMD  16  includes a processor  80 , memory  82 , signal generator  84 , electrical sensing module  86 , telemetry module  88 , and power source  89 . Memory  82  may include computer-readable instructions that, when executed by processor  80 , cause IMD  16  and processor  80  to perform various functions attributed to IMD  16  and processor  80  herein. Memory  82  may be a computer-readable storage medium, including any volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other digital or analog media. 
     Processor  80  may include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or integrated logic circuitry. In some examples, processor  80  may include multiple components, such as any combination of one or more microprocessors, one or more controllers, one or more DSPs, one or more ASICs, or one or more FPGAs, as well as other discrete or integrated logic circuitry. The functions attributed to processor  80  in this disclosure may be embodied as software, firmware, hardware or any combination thereof. Processor  80  controls signal generator  84  to deliver stimulation therapy to heart  12  according to operational parameters or programs, which may be stored in memory  82 . For example, processor  80  may control signal generator  84  to deliver electrical pulses with the amplitudes, pulse widths, frequency, or electrode polarities specified by the selected one or more therapy programs. 
     Signal generator  84 , as well as electrical sensing module  86 , is electrically coupled to electrodes of IMD  16  and/or leads coupled to IMD  16 . In the example illustrated in  FIG. 8 , signal generator  84  is configured to generate and deliver electrical stimulation therapy to heart  12 . For example, signal generator  84  may deliver pacing, cardioversion, defibrillation, and/or neurostimulation therapy via at least a subset of the available electrodes. In some examples, signal generator  84  delivers one or more of these types of stimulation in the form of electrical pulses. In other examples, signal generator  84  may deliver one or more of these types of stimulation in the form of other signals, such as sine waves, square waves, or other substantially continuous time signals. 
     Signal generator  84  may include a switch module and processor  80  may use the switch module to select, e.g., via a data/address bus, which of the available electrodes are used to deliver stimulation signals, e.g., pacing, cardioversion, defibrillation, and/or neurostimulation signals. The switch module may include a switch array, switch matrix, multiplexer, or any other type of switching device suitable to selectively couple a signal to selected electrodes. 
     Electrical sensing module  86  monitors signals from at least a subset of the available electrodes, e.g., to monitor electrical activity of heart  12 . Electrical sensing module  86  may also include a switch module to select which of the available electrodes are used to sense the heart activity. In some examples, processor  80  may select the electrodes that function as sense electrodes, i.e., select the sensing configuration, via the switch module within electrical sensing module  86 , e.g., by providing signals via a data/address bus. 
     In some examples, electrical sensing module  86  includes multiple detection channels, each of which may comprise an amplifier. Each sensing channel may detect electrical activity in respective chambers of heart  12 , and may be configured to detect either R-waves or P-waves. In some examples, electrical sensing module  86  or processor  80  may include an analog-to-digital converter for digitizing the signal received from a sensing channel for electrogram (EGM) signal processing by processor  80 . In response to the signals from processor  80 , the switch module within electrical sensing module  86  may couple the outputs from the selected electrodes to one of the detection channels or the analog-to-digital converter. 
     During pacing, escape interval counters maintained by processor  80  may be reset upon sensing of R-waves and P-waves with respective detection channels of electrical sensing module  86 . Signal generator  84  may include pacer output circuits that are coupled, e.g., selectively by a switching module, to any combination of the available electrodes appropriate for delivery of a bipolar or unipolar pacing pulse to one or more of the chambers of heart  12 . Processor  80  may control signal generator  84  to deliver a pacing pulse to a chamber upon expiration of an escape interval. Processor  80  may reset the escape interval counters upon the generation of pacing pulses by signal generator  84 , or detection of an intrinsic depolarization in a chamber, and thereby control the basic timing of cardiac pacing functions. The escape interval counters may include P-P, V-V, RV-LV, A-V, A-RV, or A-LV interval counters, as examples. The value of the count present in the escape interval counters when reset by sensed R-waves and P-waves may be used by processor  80  to measure the durations of R-R intervals, P-P intervals, P-R intervals and R-P intervals. Processor  80  may use the count in the interval counters to detect heart rate, such as an atrial rate or ventricular rate. In some examples, a leadless IMD with a set of active fixation tines may include one or more sensors in addition to electrical sensing module  86 . For example, a leadless IMD may include a pressure sensor and/or an oxygen sensor (for tissue oxygen or blood oxygen sensing). 
     Telemetry module  88  includes any suitable hardware, firmware, software or any combination thereof for communicating with another device, such as programmer  24  ( FIGS. 1 and 2 ). Under the control of processor  80 , telemetry module  88  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. Processor  80  may provide the data to be uplinked to programmer  24  and receive downlinked data from programmer  24  via an address/data bus. In some examples, telemetry module  88  may provide received data to processor  80  via a multiplexer. 
     In some examples, processor  80  may transmit an alert that a mechanical sensing channel has been activated to identify cardiac contractions to programmer  24  or another computing device via telemetry module  88  in response to a detected failure of an electrical sensing channel. The alert may include an indication of the type of failure and/or confirmation that the mechanical sensing channel is detecting cardiac contractions. The alert may include a visual indication on a user interface of programmer  24 . Additionally or alternatively, the alert may include vibration and/or audible notification. Processor  80  may also transmit data associated with the detected failure of the electrical sensing channel, e.g., the time that the failure occurred, impedance data, and/or the inappropriate signal indicative of the detected failure. 
       FIG. 9  is a functional block diagram of an example configuration of programmer  24 . As shown in  FIG. 9 , programmer  24  includes processor  90 , memory  92 , user interface  94 , telemetry module  96 , and power source  98 . Programmer  24  may be a dedicated hardware device with dedicated software for programming of IMD  16 . Alternatively, programmer  24  may be an off-the-shelf computing device running an application that enables programmer  24  to program IMD  16 . 
     A user may use programmer  24  to select therapy programs (e.g., sets of stimulation parameters), generate new therapy programs, or modify therapy programs for IMD  16 . The clinician may interact with programmer  24  via user interface  94 , which may include a display to present a graphical user interface to a user, and a keypad or another mechanism for receiving input from a user. 
     Processor  90  can take the form of one or more microprocessors, DSPs, ASICs, FPGAs, programmable logic circuitry, or the like, and the functions attributed to processor  90  in this disclosure may be embodied as hardware, firmware, software or any combination thereof. Memory  92  may store instructions and information that cause processor  90  to provide the functionality ascribed to programmer  24  in this disclosure. Memory  92  may include any fixed or removable magnetic, optical, or electrical media, such as RAM, ROM, CD-ROM, hard or floppy magnetic disks, EEPROM, or the like. Memory  92  may also include a removable memory portion that may be used to provide memory updates or increases in memory capacities. A removable memory may also allow patient data to be easily transferred to another computing device, or to be removed before programmer  24  is used to program therapy for another patient. Memory  92  may also store information that controls therapy delivery by IMD  16 , such as stimulation parameter values. 
     Programmer  24  may communicate wirelessly with IMD  16 , such as using RF communication or proximal inductive interaction. This wireless communication is possible through the use of telemetry module  96 , which may be coupled to an internal antenna or an external antenna. An external antenna that is coupled to programmer  24  may correspond to the programming head that may be placed over heart  12 , as described above with reference to  FIG. 1 . Telemetry module  96  may be similar to telemetry module  88  of IMD  16  ( FIG. 8 ). 
     Telemetry module  96  may also be configured to communicate with another computing device via wireless communication techniques, or direct communication through a wired connection. Examples of local wireless communication techniques that may be employed to facilitate communication between programmer  24  and another computing device include RF communication according to the 802.11 or Bluetooth® specification sets, infrared communication, e.g., according to the IrDA standard, or other standard or proprietary telemetry protocols. In this manner, other external devices may be capable of communicating with programmer  24  without needing to establish a secure wireless connection. An additional computing device in communication with programmer  24  may be a networked device such as a server capable of processing information retrieved from IMD  16 . 
     In some examples, processor  90  of programmer  24  and/or one or more processors of one or more networked computers may perform all or a portion of the techniques described in this disclosure with respect to processor  80  and IMD  16 . For example, processor  90  or another processor may receive one or more signals from electrical sensing module  86 , or information regarding sensed parameters from IMD  16  via telemetry module  96 . In some examples, processor  90  may process or analyze sensed signals, as described in this disclosure with respect to IMD  16  and processor  80 . 
       FIG. 10  is a flowchart illustrating techniques for implanting an implantable medical device within a patient. The techniques of  FIG. 10  are described with respect to IMD  16 A, but are also applicable to other IMDs, such as deployment of leads associated with IMD  16 B. First, assembly  180 , which includes leadless IMD  16 A and catheter  200 , is positioned to a location within the patient, such as right ventricle  28  or a vasculature of the patient ( 502 ). Next, IMD  16 A is deployed from catheter  200  to the location within the patient, such as right ventricle  28  or a vasculature of the patient ( 504 ). For example, the clinician may push on plunger  212  to deploy IMD  16 A. 
     The clinician evaluates whether IMD  16 A is adequately fixated and positioned within the patient ( 506 ). For example, the clinician may use fluoroscopy to evaluate whether IMD  16 A is adequately fixated and positioned within the patient. If the clinician determines IMD  16 A is inadequately positioned within the patient, the clinician operates catheter  200  to recapture IMD  16 A by pulling on tether  220  ( 508 ). Then, the clinician either repositions distal end of catheter  200  or replaces IMD  16 A with another IMD better suited for the implantation location ( 510 ). Then step  502  (see above) is repeated. 
     Once the clinician determines IMD  16 A is adequately fixated within the patient ( 506 ), the clinician operates catheter  200  to fully release IMD  16 A within the patient, e.g., by cutting tether  220  ( 512 ). Then, the clinician withdraws catheter  200 , leaving IMD  16 A secured within the patient ( 514 ). 
     Various examples of the disclosure have been described. These and other examples are within the scope of the following claims.