Patent Publication Number: US-2018042553-A1

Title: Implantable Device with a Tail Extension Including Embedded Sensor and Antenna

Description:
BACKGROUND OF THE INVENTION 
     Embodiments herein generally relate to implantable cardiac monitoring devices. 
     An implantable cardiac monitoring (ICM) device is a medical device that is implanted in a patient to, among other things, monitor electrical activity of a heart. An ICM device may record cardiac activity of a patient over time and report such cardiac activity to an external device. The ICM device may optionally perform various levels of sophisticated analysis of the cardiac activity and based thereon perform additional recording operations. The ICM device may also be configured to deliver appropriate electrical and/or drug therapy, and as such is also referred to as an implantable medical device (IMD). Examples of IMDs include pacemakers, cardioverters, cardiac rhythm management devices, defibrillators, and the like. The electrical therapy produced by an IMD may include, for example, pacing pulses, cardioverting pulses, and/or defibrillator pulses. The device is used to both provide treatment for the patient and to inform the patient and medical personnel of the physiologic condition of the patient and the status of the treatment. 
     In general, an ICM includes a battery, memory and electronic circuitry that are hermetically sealed within a metal housing (generally referred to as the “can”). The metal housing typically is formed of titanium and includes a shell with an interconnect cavity, in which the memory, pulse generator and/or processor module reside. The device housing is configured to receive a header assembly. The header assembly comprises a mechanical structure which houses an antenna and a sensing electrode. A feed-through assembly is located at the header receptacle area and is sealed to the device housing to form an interface for conductors to enter/exit the interconnect cavity. 
     However, ICM devices have experienced some limitations. Certain types of ICM devices include one or more sensing electrodes and an antenna that are located within the ICM device. For example, the sensing electrode/electrodes and antenna may be located in the header of the ICM device. Heretofore, the header assembly structure joined to the device limits the length of the antenna and limits the placement of the sensing electrode/electrodes relative to the region of interest. 
     A need remains for improved ICM devices and methods of manufacture thereof. 
     SUMMARY 
     In accordance with embodiments herein, a device is provided for an implantable cardiac monitor device comprising a device housing having a sensing circuit and a radio frequency (RF) communications circuit housed within the housing. The device further comprising a tail extension having a proximal end, a distal end, and an extension body extended there between wherein the proximal end is coupled to the housing. The extension body is formed of a flexible material and includes at least one conductor that includes a proximal end conductively coupled to the sensing and RF communications circuit. At least a portion of the conductor of the tail extension forms an antenna to be utilized by the RF communications circuit to communicate with an external device. Further, an electrode is provided on the tail extension and is conductively coupled to the conductor and the sensing circuit. 
     Optionally, the portion of the conductor that forms the antenna is electrically coupled to the electrode. In at least one embodiment, the conductor consists of a single conductor that both forms the antenna and carries cardiac sensed signals from the electrode. In at least one other embodiment, the conductor may include first and second conductors, wherein the first conductor is coupled to the electrode and is configured to carry cardiac sensed signals, and the second conductor forms the antenna and is configured to carry communications data to and/or from the RF communications circuit. 
     Optionally, the proximal end of the tail extension is joined directly at a non-header interface on a surface of the device housing. The device further comprises a feed-through assembly joined to the device housing. The feed-through assembly including a single conductor extending there through. The single conductor has a proximal end connected to the sensing and RF communications circuits and a distal end projecting from the feed-through assembly. 
     Optionally, the device housing further comprises a filter circuit conductively coupled between the at least one conductor and the sensing and RF communications circuits. The filter circuit is configured to block RF transmissions from reaching the sensing circuit. The filter circuit includes an inductive element and a capacitive element connected in series with one another at an intermediate node. The sensing circuit joins to the intermediate node. The capacitor element is configured to form an open circuit when experiencing cardiac sensing signals and to form a closed circuit when experiencing RF transmissions. The filter further comprises a low pass filter branch configured to pass cardiac sensed signals and a band pass filter branch configured to pass RF communications in a frequency range that includes approximately 2.4 GHz. 
     Optionally, the tail extension of the device comprises a predetermined length between the distal end and the proximal end. The predetermined length is tuned based on the center of frequency of the RF communications bandwidth. 
     In accordance with embodiments herein, a method is provided for implanting a cardiac monitor device comprising positioning a device subcutaneously, the device comprising a device housing having a sensing circuit and a radio frequency (RF) communications circuit housed within the housing. The device further comprises a tail extension having an extension body with a proximal end joined to the device housing. The extension body is formed of a flexible material and includes at least one conductor. The tail extension is positioned in a subcutaneous area with an electrode provided on the tail extension located proximate to a region of interest (ROI) in a heart. The electrode collects cardiac signals from the ROI provided on the tail extension and conveys the RF communications data to an external device using an antenna that is formed from at least a portion of the conductor. 
     Optionally, the method comprises a positioning operation of locating a distal end of the tail extension remote from the device housing and an electrode provided at the distal end of the tail extension located proximate to a region of interest in the heart. The conductor includes a first and second conductor, wherein the first conductor is coupled to the electrode and configured to carry the cardiac signals and the second conductor forms the antenna and is configured to carry the RF communications data to and/or from the RF communications circuit. 
     Optionally, the method comprises filtering the cardiac signals to isolate the RF communications circuit from the cardiac signals sensed over the conductor, and filtering the RF communications to isolate the sensing circuit from the RF communications data carried by the conductor. The collecting and conveying operations utilize a common conductor in the tail extension. The filtering operation comprises low pass filtering to pass the cardiac signals along a sensing branch and band pass filtering to pass RF communications data that is in a frequency range that includes approximately 2.4 GHs, along a communications branch. 
     Optionally, the method comprises inserting the tail extension into a lumen in a tail implant tool and inserting the tail implant tool subcutaneously to a location proximate to the ROI. The tail implant tool is removed while leaving the tail extension implanted. Optionally, the ROI is located proximate to an atrium of the heart such that the cardiac signals include sensed P-waves. 
     In accordance with embodiments herein, a method is provided for providing an implantable cardiac monitor device comprising providing a device comprising a device housing have a sensing circuit and a radio frequency (RF) communications circuit housed within the housing. The method further comprising joining a tail extension to the device housing, the tail extension having an extension body with a proximal end joined to the device housing. The extension body formed of a flexible material and including at least one conductor. An electrode may be positioned on the tail extension to sense cardiac signals when the tail extension is located proximate to a region of interest (ROI) in the heart. A portion of the conductor forms an antenna and tuning the antenna and RF communications circuit to convey RF communications data to an external device utilizing a predetermined communications frequency. 
     Optionally, the tuning operation includes tuning the antenna to utilize the predetermined communications frequency centered at one of 2.4 GHz and 400 MHz. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an implantable cardiac monitoring (ICM) device intended for subcutaneous implantation at a site near the heart in accordance with embodiments herein. 
         FIG. 2  illustrates a side perspective view of the ICM device in accordance with embodiments herein. 
         FIG. 3  illustrates a side sectional view of a tail extension joined to the ICM device in accordance with embodiments herein. 
         FIG. 4  illustrates a side sectional view of a device housing in accordance with embodiments herein. 
         FIG. 5A  illustrates a side sectional view of the tail extension joined to the device housing in accordance with embodiments herein. 
         FIG. 5B . illustrates a side sectional view of the tail extension joined to the device housing in accordance with embodiments herein. 
         FIG. 6A  illustrates a side sectional view of a tail extension separated from a device housing in accordance with embodiments herein. 
         FIG. 6B  illustrates a side sectional view of a tail extension joined to the device housing in accordance with embodiments herein. 
         FIG. 6C  illustrates a flowchart method for implanting and connecting a tail extension and a device housing in accordance with embodiments herein. 
         FIG. 7  illustrates a block diagram of an exemplary ICM device that is configured to be implanted into the patient in accordance with embodiments herein. 
         FIG. 8  illustrates a circuit diagram of the exemplary ICM device of  FIG. 7  in accordance with embodiments herein. 
         FIG. 9  illustrates a block diagram of an alternative exemplary ICM device that is configured to be implanted into the patient in accordance with embodiments herein. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the embodiments herein, devices and methods are described that afford improved placement of a sensing electrode and antenna performance in implantable cardiac monitoring (ICM) devices (including implantable medical devices configured to deliver therapies). In accordance with at least some embodiments, the devices and methods described herein provide improved sensing of cardiac signals as compared to conventional implantable cardiac monitoring devices and eliminate conventional structural limitations. In accordance with at least some embodiments, the devices and methods described herein provide improved wireless communications performance in a compact footprint. 
       FIG. 1  illustrates an implantable cardiac monitoring (ICM) device  100  intended for subcutaneous implantation in a patient at a site near a heart  106 . The ICM device  100  includes a device housing  102  that is joined to a proximal end of a tail extension  104 . The device housing  102  is a solid component that communicates with an external device  108 . The tail extension  104  is a flexible component that receives cardiac signals from the heart  106 . The ICM device  100  is placed in a subcutaneous area of the patient with a distal end of the tail extension  104  located proximate to a region of interest (ROI) in the heart. For example, the ROI is located proximate to an atrium of the heart such that cardiac signals that are collected by the ICM device  100  include P-waves. Optionally, the ROI could be a number of anatomical aspects of a patient, such as the heart wall, the right ventricle, left ventricle, etc. The ICM device  100  may be positioned in various orientations relative to a patient anatomy, such as relative to various aspects of the heart, parallel to the sternum, non-parallel to the sternum, and the like. 
     The device housing  102  includes various other components such as one or more sense circuits for receiving and collecting cardiac signals from one or more combinations of electrodes, a microprocessor for processing the cardiac signals in accordance with various algorithms (e.g., an AF detection algorithm), a memory for temporary storage of electrogram data, a device memory for long-term storage of electrogram data (e.g., based on certain triggering events, such as AF detection), sensors for detecting patient activity and a battery for powering circuits and other components. 
     The ICM device  100  senses near field and/or far field cardiac signals and stores the cardiac signals as electrogram data. The ICM device  100  processes the electrogram (EGM) data in various manners such as to detect physiologic characteristics of interest (e.g., arrhythmias). The ICM device  100  automatically records segments of the electrogram data in temporary or long-term memory based on identification of the characteristics of interest. By way of example, the EGM data may be stored in memory of the ICM device  100  until subsequent transmission to the external device  108 . Electrogram processing and arrhythmia detection is provided for, at least in part, by algorithms embodied in program instructions that are executed by one or more microprocessors. In one configuration, the monitoring device is operative to detect atrial fibrillation. 
       FIG. 2  illustrates a side perspective view of the ICM device  100  in accordance with an embodiment herein.  FIG. 2  illustrates the tail extension  104  and device housing  102  of the ICM device  100  in more detail. 
     The tail extension  104  is elongated to extend along a longitudinal axis (designated by arrow X T ). The tail extension  104  is formed in a tubular shape about the longitudinal axis (designated by the arrow X T ) and is generally circular in shape about a cross sectional axis (designated by arrow Y T ). For example, the tail extension  104  is shaped similarly to a hollow wire. Optionally, the tail extension could be formed with a number of shapes. Additionally, the tail extension  104  is a flexible structure made of a flexible, insulating, biocompatible material (e.g., silicone rubber, polyurethane, etc.). 
     The tail extension  104  comprises a distal end  206  and a proximal end  208 , with an extension body  204  spanning between the distal end  206  and the proximal end  208 . The distal end  206  of the tail extension  104  is located remote from the device housing  102 . The proximal end  208  of the tail extension  104  is positioned near, and coupled to, the device housing  102 . 
     One or more primary sensing electrodes  210  are positioned at the distal end  206  of the tail extension  104 . One or more secondary sensing electrodes  212 ,  213 ,  215  may be provided on the device housing  102  and/or on the tail extension  104  at an intermediate point(s) along the length of the extension body  204 . The sensing electrodes  210 ,  212 ,  213 ,  215  are configured to receive cardiac signals from the heart  106 . The sensing electrodes  210 ,  212 ,  213 ,  215  may vary in shape and size. 
     The device housing  102  extends along a longitudinal axis  218 . In the embodiment of  FIG. 2 , the device housing  102  is formed in a rectangular shape about the longitudinal axis  218  and in a rectangular shape about a cross sectional axis (designated by the arrow Y D ). However, the device housing  102  may be constructed with alternative shapes (e.g., a tubular shape, a circular shape, a shape similar to pacemakers, ICDs, CRM devices and other implantable devices). The device housing  102  comprises opposed elongated sides  220 ,  222 , a top edge  228 , a bottom edge  230 , a header end  226  and a base end  235 . The base end  235  includes the secondary electrode  212  provided thereon. The opposed elongated sides  220 ,  222  are generally parallel, but may be constructed with alternative contours. The opposed sides  220 ,  222  merge with the top and bottom edges  228  and  230  along smooth beveled regions about the cross sectional axis (designated by the arrow Y D ). 
     In  FIG. 2 , the device housing  102  is constructed in two pieces, namely with a battery portion  223  and an electronics shell portion  225 . The battery portion  223  is formed as a self-contained, hermetically sealed battery. The electronics shell portion  225  encloses the various electronic components and circuits as discussed herein. 
       FIG. 3  illustrates a detailed cross-sectional view of the tail extension  104  joined to the device housing  102  in accordance with an embodiment. The tail extension  104  and the device housing  102  are oriented to extend along the longitudinal axis  218 .  FIG. 3  illustrates the tail extension  104  and header end  226  of the device housing  102  in more detail. 
     The tail extension  104  comprises at least one conductor  302 . The conductor  302  is positioned generally centered within the tail extension  104  and extends longitudinally the length of the extension body  204 . The conductor  302  may be a single strand wire, a multi-strand wire, a filar, or the like. Optionally, the conductor  302  may be formed as a tubular mesh. The tubular mesh may be constructed to include an interior lumen that is configured to receive an insertion tool (e.g., a guide wire, stylet, etc.). 
     The conductor  302  includes a distal end  306  and a proximal end  308 . The distal end  306  of the conductor is conductively joined to the sensing electrode  210 . The proximal end  308  of the conductor is positioned proximate to the device housing  102 . 
     In accordance with embodiments herein, all or a portion of the conductor  302  within the tail extension  104  is also utilized as an antenna  304 . The antenna  304 , for example, may be utilized to support communications in accordance with various wireless protocols such as, but not limited to, a Medical Implant Communications Service (MICS) protocol, a Bluetooth protocol, a Bluetooth Low Energy protocol, a Wi-Fi protocol, and the like. 
     The tail extension  104  (and antenna  304 ) may be constructed with various lengths based upon select criteria. For example, the length of the tail extension  104  may be varied based upon the application for which the ICM device  100  is constructed. For example, different length tail extensions may be used when monitoring P-waves, or monitoring R-waves. Additionally or alternatively, one or more length(s) of tail extensions may be useful when sensing electrical activity of the heart as R-waves or P-waves, whereas one or more other length(s) of tail extensions may be useful when monitoring impedance, such as across various blood pools in the heart and/or in vessels proximate to the heart (e.g., the five great vessels of the heart). In addition, the length of the tail extension may be varied based on a desired combination of anatomical locations to be monitored by the ICM device  100 . For example, it may be desirable to locate the device housing  102  on one side of the heart (e.g., proximate the apex of the heart), while positioning the distal end of the tail extension  104  proximate to a different side/portion of the heart (e.g., proximate to the base of the heart). The length of the tail extension may be varied for other reasons as well. 
     In addition, the length of the tail extension may also be based on characteristics of the antenna  304 . In particular, the length of the antenna is set to a predetermined or select length based on various criteria. For example, the length of the conductor, that defines the antenna  304 , may be set to a predetermined or select length in order to electrically tune the antenna  304  and the RF communications circuit to a predetermined frequency. For example, the length of the antenna may be set to a predetermined length in order to provide a desired antenna performance when communicating RF communications data centered at a select center frequency (e.g., 2.4 GHz, 400 MHz, etc.). Optionally, the length of the antenna may be sized to a different length to communicate over a different frequency. 
     Optionally, more than one conductor may be provided within the tail extension with the conductors conductively isolated from one another (e.g., individually insulated). When more than one conductively insulated conductor is provided within the tail extension, a first conductor (sensor conductor) may be dedicated to carrying sensed signals from the sensing electrode  210  on the distal end, while a second conductor (antenna conductor) may be dedicated to transmitting and receiving communications data. When separate sensor and antenna conductors are provided in the tail extension, the sensor and antenna conductors may be shaped and formed to provide a desired level of performance relative to the corresponding usage. For example, a sensor conductor may be sized differently from an antenna conductor. Additionally or alternatively, the sensor conductor may be formed from a different material than the antenna conductor. Additionally or alternatively, the sensor conductor may utilize a multi-strand filar with a select first diameter, while the antenna conductor is a single strand with a different (e.g., thicker) diameter. 
     Optionally, when separate sensor and antenna conductors are utilized, the antenna conductor may be formed with a different length than the sensor conductor. For example, in some applications, it may be desirable for the antenna conductor to have a length that is a fraction of a wavelength of the center communications frequency (e.g., one quarter wavelength in length). In addition, it may be desirable for the overall length of the tail extension to be longer that the predetermined length for the antenna conductor. In the foregoing example, the sensor conductor may extend the full length of the tail extension X (corresponding to a sensor spacing or sensor placement parameter), while the antenna conductor may extend a shorter length Y (corresponding to a timing related parameter). Utilizing separate conductors for sensing and for the antenna provides more flexibility to tailor the design parameters of the sensing characteristics and the antenna characteristics. 
     In  FIG. 3 , the header end  226  of the device housing  102  comprises a housing mounting surface  216 . The housing mounting surface  216  is a non-header interface that comprises a recessed chamber  232 . The recessed chamber  232  is positioned to open on to the housing mounting surface  216 . The recessed chamber  232  is shaped and dimensioned to receive a feed-through assembly  214  on the device housing  102 . In the present example, the recessed chamber  232  is rectangular in shape, although alternative shapes may be used based on the shape of the feed-through assembly  214 . A passage  234  extends from the recessed chamber  232  through the feed-through assembly  214  along the longitudinal axis  218 . 
     The size and/or shape of the proximal end  208  of the tail extension  104  corresponds to the size and/or shape of the header end  226  of the device housing  102 . For example, the proximal end  208  of the tail extension  104  may be formed in a hollow-rectangular shape to form a cap over the device housing  102  at the header end  226  when joined. Optionally, alternative shapes may be used based on the shape and/or size of the housing mounting surface  216  of the device housing  102 . The hollow-rectangular shape of the proximal end  208  of the tail extension may be joined to the device housing by a press fit onto the device housing  102 . Additionally or alternatively, the proximal end  208  of the tail extension may be joined to the device housing by alternative methods. 
     In  FIG. 3 , a collar  310  is joined to the feed-through assembly  214  of the device housing  102  along the longitudinal axis  218 . The collar  310  comprises a distal end  320  and a proximal end  322 . The distal end  320  is positioned facing the sensing electrode  210  of the tail extension  104 . The proximal end  322  of the collar joins to the feed-through assembly  214 . The collar  310  may be joined to the feed-through assembly by various methods. For example, the collar  310  may be joined to the feed-through assembly by welding, over-molding, mechanical fasteners, or the like. For example, the collar  310  may be welded to the feed-through assembly  214  at a weld joint  316 . 
     Furthermore, the collar  310  comprises one or more openings. The distal end  320  of the collar  310  comprises a distal-opening  312 . The distal-opening  312  is sized and shaped to receive the proximal end  308  of the conductor  302  of the tail extension  104 . The proximal end  322  of the collar  310  comprises a proximal-opening  314 . The proximal-opening  314  is sized and shaped to receive the passage  234  of the feed-through assembly  214 . 
     According to an embodiment, the collar  310  is configured with a third opening  318 . The third opening  318  may be positioned perpendicular to the longitudinal axis  218  and may be constructed to intersect with the distal-opening  312 . The third opening  318  comprises a set-screw  319 . The set-screw  319  of the third opening  318  joins the proximal end  308  of the conductor  302  to the collar  310  to electrically couple the conductor  302  with the collar  310 . 
       FIG. 4  illustrates a detailed cross-sectional view of the device housing  102  formed in accordance with an embodiment. The device housing  102  is oriented to extend along the longitudinal axis  218 .  FIG. 4  illustrates the battery portion  223  and electronics shell portion  225  in more detail. The distal end of the battery portion  223  includes the secondary electrode  212 , while a proximal end  227  includes a shell reception opening sized and shaped to receive an end  229  of the electronics shell portion  225 . A battery feed-through  231  is provided at the interface between the electronics shell portion  225  and battery portion  223 . Conductors  233  extend through the battery feed-through  231  to provide electrical connections to a battery  406 . A weld joint  404  hermetically seals the electronics shell portion  225  to the battery portion  223 . 
       FIG. 4  further illustrates the interface between the feed-through assembly  214  and the electronics shell portion  225  at the housing mounting surface  216 . A weld joint  402  hermetically seals the feed-through assembly  214  to the proximal end of the electronics shell portion  225 . In the foregoing example, welds are provided at component interfaces, however it is understood that alternative attachment techniques may be utilized. 
     The electronics shell portion  225  includes various circuits, components and memory based on the individual operations and application for which the ICM device  100  is configured. In  FIG. 4 , an example electronics board  408  includes one or more processors  409 , memory  411 , sensing circuit  413 , and RF communications circuit  415 , as well as other electrical components. Examples of the various electronic components may include sensing circuitry to sense cardiac signals of interest, one or more processors to perform monitoring operations, transceiver circuitry to communicate with external devices and other components as described herein. 
     The sensing circuit  413  and RF communications circuit  415  are electrically coupled with a single conductor  410  at a proximal end  414 . A distal end  412  of the single conductor  410  extends from the electronics board  408  through the passage  234  of the feed-through assembly  214 . For example, in the present example, the single conductor  410  is a single-strand wire that is electrically coupled to the electronic components on the electronics board  408  (e.g. the sensing circuit  413  and RF communications circuit  415 ). The single conductor  410  projects outward from the feed-through assembly  214  to provide a connection point to be conductively joined to the proximal end of one or more conductors within the tail extension  104 . 
     Returning to  FIG. 3 , the single conductor  410 , extending from the electronics shell portion  225  of the device housing  102 , may be received into the proximal-opening  314  of the collar  310 . The single conductor  410  may be conductively coupled to the collar  310 . For example, the single conductor  410  may be conductively coupled to the collar  310  by a weld joint  315 . Additionally or alternatively, the single conductor may be joined to the collar using alternative attachment techniques. The collar  310  interconnects the proximal end  308  of the conductor  302  of the tail extension  104  with the single conductor  410  extending from the device housing  102 . For example, the collar  310  conductively couples the proximal end  308  of the conductor  302  with the sensing circuit  413  and RF communications circuit  415 . The collar  310  enables electrical communication between the sensing electrode  210  of the tail extension  104  and the electronic components of the device housing  102 . 
       FIG. 5A  illustrates an alternative example of a connection mechanism for interconnecting the conductor  302  of the tail extension  104  to the electronic components of the device housing  102 . A collar  506  is joined to the feed-through assembly  214 . A distal end  512  of the collar  506  is positioned facing away from the feed-through assembly  214 . A proximal end  510  of the collar is positioned facing towards the feed-through assembly  214 . In the embodiment of  FIG. 5A , the collar  506  comprises a collar-passage  508 . The collar-passage  508  is open along the longitudinal axis  218 . The collar-passage  508  is sized and shaped to receive the conductor  302  of the tail extension  104  on the distal end  512  and to receive the passage  234  of the feed-through assembly  214  on the proximal end  510 . 
     The single conductor  410  is electrically coupled to the electronic components of the device housing  102  at the proximal end  414  (not shown). The distal end  412  of the single conductor  410  extends from the device housing through the feed-through assembly  214  along the longitudinal axis  218 . The distal end  412  of the single conductor  410  is conductively joined to a pin  514 . The pin  514  extends from the distal end  412  of the single conductor  410  towards the collar  506  along the longitudinal axis  218 . The conductor  302  of the tail extension  104  may be conductively coupled with the pin  514  at a weld joint  502 . For example, the pin  514  conductively couples the proximal end  308  of the conductor  302  with the sensing circuit  413  and RF communications circuit  415 . 
       FIG. 5B  illustrates an alternative example of a connection mechanism for interconnecting the conductor of the tail extension  104  to the electronic components of the device housing  102 . In accordance with an embodiment in  FIG. 5B , a single conductor  522  (corresponding to the conductor  302  of  FIG. 3 ) extends within the tail extension  104  along the longitudinal axis  218 . All or a portion of the conductor  522  within the tail extension  104  is also utilized as an antenna  524  (corresponding to the antenna  304  of  FIG. 3 ). The single conductor  522  is conductively joined to the sensing electrode  210  on the distal end of the tail extension  104  (not shown). The single conductor  522  extends between the sensing electrode  210  and the device housing  102 . In the present embodiment, the single conductor  522  extends through the passage  234  of the feed-through assembly  214  and into the device housing  102 . The single conductor  522  is electrically coupled to the electronic components of the device housing  102  (not shown). For example, the single conductor  522  is conductively joined to the sensing electrode  210  at a distal end, and conductively joined to the electronic components of the device housing  102  at a proximal end. 
     The previous embodiments illustrate three examples demonstrating how the conductor of the tail extension  104  may be conductively coupled to the electronic components of the device housing  102 . However, it is understood that alternative attachment techniques may be utilized. The conductor of the tail extension  104 , electrically coupled with the electronic components of the device housing  102 , enables electrical communication between the sensing electrode  210  of the tail extension and the electronic components of the device housing  102 . The conductor  302 , electrically coupled to the sensing electrode  210 , performs the operations of conveying collected cardiac sensing signals from the ROI in the heart to the sensing circuitry  413  of the device housing. The portion of the conductor  302  that forms the antenna  304  performs the operations of communicating the RF communications data to and/or from the RF communications circuitry  415  of the device housing  102  and the external device  108 . 
       FIGS. 6A and 6B  illustrate an embodiment of a tail extension  604  and a device housing  602  formed in accordance with an alternative embodiment. The tail extension  604  and device housing  602  are configured to be subcutaneously positioned at a ROI of the heart prior to the tail extension  604  being joined to the device housing  602 .  FIG. 6A  illustrates the tail extension  604  separated from the device housing  602 .  FIG. 6B  illustrates the tail extension  604  joined to the device housing  602 .  FIGS. 6A and 6B  will be discussed in detail together in connection with  FIG. 6C . 
       FIG. 6C  illustrates a flowchart for a process for separately subcutaneously implanting the tail extension  604  and the device housing  602 , and then joining the tail extension  604  to the device housing  602  after implant. 
     The tail extension  604  (corresponding to the tail extension  104  of  FIG. 2 ) comprises a distal end  606  and a proximal end  608 , with an extension body  607  spanning between the distal end  606  and the proximal end  608 . The distal end  606  of the tail extension  604  is located remote from the device housing  602 . A primary sensing electrode  600  (corresponding to the primary sensing electrode  210  of  FIG. 2 ) is positioned at the distal end  606  of the tail extension  604 . The proximal end  608  of the tail extension  604  is positioned near, and configured to be coupled to, the device housing  602 . 
     A single conductor  601  (corresponding to the conductor  302  of  FIG. 3 ) extends within the tail extension  604  along a longitudinal axis  618 . For example, the single conductor  601  extends between the sensing electrode  600  and the proximal end  608  of the tail extension  604 . All or a portion of the conductor  601  within the tail extension  604  is also utilized as an antenna  612  (corresponding to the antenna  304  of  FIG. 3 ). 
     A header gasket  616  forms a cap over the device housing  602  at the header end  626 . The header gasket  616  is a hollow piece that is sized and/or shaped to fit over the device housing  602 . The header gasket  616  is a flexible structure made of a flexible, insulating, biocompatible material (e.g., silicone rubber, polyurethane, etc.). The header gasket  616  comprises a receiving slot  603  that is sized and/or shaped to receive the proximal end  608  of the tail extension  604 . For example, the receiving slot  603  of the header gasket  616  receives the proximal end  608  of the tail extension  604  in order to join the tail extension  604  to the device housing  602 . Optionally, the header gasket  616  may be omitted entirely. 
     In  FIG. 6C , beginning at  652 , a needle  605  is inserted into a subcutaneous position at a ROI of the heart. The needle  605  is maneuvered until a distal end of the needle  605  is located at a final subcutaneous location of interest (not shown). The final subcutaneous location could be a ROI proximate to the heart. For example, the final location may be positioned on one side of the heart (e.g., proximate the base of the heart). Additionally or alternatively, the final location may be any other ROI that is proximate to the heart. Optionally, the final location may be proximate to another anatomy of interest, such as an ROI in or near the lungs, an organ, a portion of the nervous system, the spine, the brainstem, the brain, etc. 
     At  654 , the distal end  606  of the tail extension  604  is loaded into the needle  605 . For example, the distal end  606  with the sensing electrode  600  is inserted through the needle  605  and discharged from the distal end of the needle in order to position the sensing electrode  600  at the final subcutaneous location. Optionally, the tail extension  604  may be loaded into the needle before or after the needle is inserted to the ROI. At  656 , the needle  605  is removed from the final subcutaneous position leaving the tail extension  604  implanted and the sensing electrode  600  at the final subcutaneous position. 
     At  658 , the device housing  602  is implanted to a device retention subcutaneous position. Various methods and tools may be used to implant the device housing  602 . For example, a sheath, catheter and the like may be introduced through a vein or artery to a device retention position. Optionally, the device housing  602  may be located in a pocket through an open incision (e.g., at a sub-clavicle pocket). For example, it may be desirable to position the device housing  602  on a different side/portion of the heart (e.g., proximate the apex of the heart) from the distal end  606  of the tail extension  604 . At  660 , once the device housing  602  is implanted at the desired location, the implanted tail extension  604  is joined to the implanted device housing  602 . The device housing  602  and the tail extension  604  may be secured to one another in various manners. For example, device housing  602  and the tail extension  604  may be secured to one another in a manner similar to attaching a proximal end of a lead to a pacemaker, cardioverter defibrillator, neuro-stimulation device and the like. 
       FIG. 6B  illustrates the tail extension  604  joined to the header gasket  616 . In the embodiment of  FIG. 6B , a collar  610  (corresponding to the collar  310  of  FIG. 3 ) is joined to a feed-through assembly  614  of the device housing  602  along the longitudinal axis  618 . The collar receives the conductor  601  of the tail extension at a distal end  620 . The collar receives a single conductor  611  (corresponding to the single conductor  410  of  FIG. 3 ) extending from the device housing  602  through a passage  634  of the feed-through assembly  614  of the device housing  602  at a proximal end  622 . 
     The collar  610  is configured with an opening  617  perpendicular to the longitudinal axis  618 . The opening  617  comprises a set-screw  619 . The header gasket  616  flexes to allow access to the collar  610  in order to join the conductor  601  of the tail extension  604  to the collar  610  with the set-screw  619 . The set-screw  619  of the opening  617  joins the conductor  601  to the collar  610  to electrically couple the conductor  601  with the collar  610 . The collar  610  interconnects the conductor  601  of the tail extension  604  with the single conductor  611  extending from the device housing  602 . The collar  610  enables electrical communication between the sensing electrode  600  of the tail extension  604  and the electronic components of the device housing  602 . Once the conductor  601  is joined to the collar  610 , the header gasket  616  rebounds to the original position. In the original position, the proximal end  608  of the tail extension  604  is held to the header gasket  616  by a press fit into the receiving slot  603 . 
     The previous embodiment illustrates one example demonstrating how the conductor of the tail extension  604  may be conductively coupled to the electronic components of the device housing  602  after the tail extension  604  and the device housing  602  have been individually subcutaneously implanted. However, it is understood that alternative attachment techniques may be utilized. 
       FIG. 7  illustrates a block diagram of the ICM device  100  that is configured to be subcutaneously implanted into the patient. Optionally, the ICM device  100  may be provided as an external device that is worn, held or otherwise located proximate to the patient during operation. The ICM device  100  may be implemented to monitor ventricular activity alone, or both ventricular and atrial activity through sensing circuitry. The ICM device  100  has the device housing  102  to hold the electronic/computing components. The device housing  102  (which is often referred to as the “can”, “case”, “encasing”, or “case electrode”) may be programmably selected to act as an electrode for certain sensing modes. 
     In the present embodiment, the sensing electrode  210  positioned on the tail extension  104  is coupled to a terminal  708 . The device housing  102  further includes a connector (not shown) with at least one terminal  710  and preferably a second terminal  712 . The terminals  710 ,  712  may be coupled to additional sensing electrodes on the device housing, on the tail extension, or located otherwise. For example, the terminals  710 ,  712  may be coupled to the sensing electrodes  212 ,  213  and/or  215  of  FIG. 2 . Additionally or alternatively, the terminals  710 ,  712  may be connected to one or more leads having one or more electrodes provided thereon, where the electrodes are located in various locations about the heart. The type and location of each electrode may vary. 
     The ICM device  100  is configured to be placed subcutaneously utilizing a minimally invasive approach. Subcutaneous electrodes are provided on the device housing  102  to simplify the implant procedure and eliminate a need for a transvenous lead system. The sensing electrodes may be located on opposite sides/ends of the device and designed to provide robust episode detection through consistent contact at a sensor-tissue interface. The ICM device  100  may be configured to be activated by the patient or automatically activated, in connection with recording subcutaneous ECG signals. 
     The ICM device  100  includes a programmable microcontroller  714  that controls various operations of the ICM device  100 , including cardiac monitoring. Microcontroller  714  includes a microprocessor (or equivalent control circuitry), RAM and/or ROM memory, logic and timing circuitry, state machine circuitry, and I/O circuitry. The microcontroller  714  also performs the operations in connection with collecting cardiac activity data and analyzing the cardiac activity data to identify episodes of interest. Microcontroller  714  includes an arrhythmia detector  716  that is configured to analyze cardiac activity data to identify potential AF episodes as well as other arrhythmias (e.g. Tachcardias, Bradycardias, Asystole, etc.). 
     A switch  718  is optionally provided to allow selection of different electrode configurations connected to the terminals  710 ,  712  under the control of the microcontroller  714 . The switch  718  is controlled by a control signal from the microcontroller  714 . Optionally, the switch  718  may be omitted and the I/O circuits directly connected to the housing electrode and a second electrode. 
     The ICM device  100  is further equipped with telemetry circuitry  704 . The telemetry circuitry  704  uses high frequency modulation, for example uses RF or Bluetooth telemetry protocols. The telemetry circuitry  704  may include one or more transceivers. For example, the telemetry circuitry  704  may be coupled to the antenna  304  in the tail extension  104  that transmits communications signals in a high frequency range that will travel through the body tissue in fluids without stimulating the heart or being felt by the patient. 
     The ICM device  100  includes sensing circuitry  702  selectively coupled to one or more electrodes that perform sensing operations to detect cardiac activity data indicative of cardiac activity. For example, the sensing circuitry  702  may be coupled to the conductor  302  in the tail extension that conveys sensing signals from the ROI in the heart in a low frequency range. The sensing circuitry  702  may include dedicated sense amplifiers, multiplexed amplifiers, or shared amplifiers. It may further employ one or more low power, precision amplifiers with programmable gain and/or automatic gain control, bandpass filtering, and threshold detection circuit to selectively sense the cardiac signal of interest. 
     An isolating filter circuit  706  is electrically coupled between the terminal  708  and the telemetry circuitry  704  and the sensing circuitry  702 . The isolating filter circuit  706  receives low frequency cardiac signal data communicated by the conductor  302  and high frequency RF communications data communicated by the antenna  304  through the terminal  708 . The isolating filter circuit  706  is provided to pass or block the data communicated over the single-strand conductor  302  and antenna  304 . The isolating filter circuit  706  passes the low frequency data to the sensing circuitry  702  and blocks the low frequency data from the telemetry circuitry  704 . For example, the isolating filter receives low frequency sensed cardiac signals from the conductor  302  of the tail extension  104 . The isolating filter circuit  706  passes the sensed cardiac signals to the sensing circuitry  702 . The isolating filter circuit  706  blocks the sensed cardiac signals from the telemetry circuitry  704 . 
     The isolating filter circuit  706  passes high frequency data to the telemetry circuitry  704  and blocks the high frequency data from the sensing circuitry  702 . For example, the isolating filter circuit  706  receives high frequency RF communications data from the antenna  304  of the tail extension  104 . The isolating filter circuit  706  passes the high frequency RF communications data to the telemetry circuit  704 . The isolating filter circuit  706  blocks the RF communications data from the sensing circuitry  702 . 
       FIG. 8  provides a detailed illustration of the isolating filter circuit  706  of  FIG. 7 . The isolating filter circuit  706  includes an inductive element  802  and a capacitive element  806  that are connected in series with one another. An intermediate node  804  is positioned between the inductive element  802  and the capacitive element. The sensing circuitry  702  is conductively joined to the intermediate node. The capacitive element  806  is configured to form an open circuit when experiencing low frequency cardiac sensing signals from the sensing electrode  210  of the tail extension  104 . For example, the open circuit enables the sensing signals to electrically flow towards the sensing circuitry  702 . Alternatively, the capacitor  806  is configured to form a closed circuit when experiencing high frequency RF transmissions to and/or from the antenna. The closed circuit capacitive element  806  passes the high frequency RF transmissions to a ground  812  and blocks the high frequency RF transmissions from the sensing circuitry  702 . Optionally, alternative circuitry may be used to filter the low frequency data from the high frequency data provided on a single-strand conductor and antenna. 
     Returning to  FIG. 7 , by way of example, the external device  108  may represent a bedside monitor installed in a patient&#39;s home and utilized to communicate with the ICM device  100  while the patient is at home, in bed or asleep. The external device  108  may be a programmer used in the clinic to interrogate the device, retrieve data and program detection criteria and other features. The external device  108  may be a device that can be coupled over a network (e.g. the Internet) to a remote monitoring service, medical network and the like. The external device  108  facilitates access by physicians to patient data as well as permitting the physician to review real-time ECG signals while being collected by the ICM device  100 . 
     The microcontroller  714  is coupled to a memory  722  by a suitable data/address bus  726 . The programmable operating parameters used by the microcontroller  714  are stored in memory  722  and used to customize the operation of the ICM device  100  to suit the needs of a particular patient. Such operating parameters define, for example, detection rate thresholds, sensitivity, automatic features, arrhythmia detection criteria, activity sensing or other physiological sensors, and electrode polarity, etc. The operating parameters of the ICM device  100  may be non-invasively programmed into the ICM device  100  through the telemetry circuitry  704 . The telemetry circuitry  704  allows intracardiac electrograms and status information relating to the operation of the ICM device  100  to be sent to the external device  108  through an established communication link  724 . 
     The ICM device  100  may further include magnet detection circuitry (not shown), coupled to the microcontroller  714 , to detect when a magnet is placed over the unit. A magnet may be used by a clinician to perform various test functions of the ICM device  100  and/or to signal the microcontroller  714  that the external device  108  is in place to receive or transmit data to the microcontroller  714  through the telemetry circuitry  704 . 
     The ICM device  100  can further include one or more physiologic sensor  728 . Such sensors are commonly referred to (in the pacemaker arts) as “rate-responsive” or “exercise” sensors. The physiological sensor  728  may further be used to detect changes in the physiological condition of the heart, or diurnal changes in activity (e.g., detecting sleep and wake states). Signals generated by the physiological sensors  728  are passed to the microcontroller  714  for analysis and optional storage in the memory  722  in connection with the cardiac activity data, markers, episode information and the like. While shown as included within the ICM device  100 , the physiologic sensor(s)  728  may be external to the ICM device  100 , yet still be implanted within or carried by the patient. Examples of physiologic sensors might include sensors that, for example, activity, temperature, sense respiration rate, pH of blood, ventricular gradient, activity, position/posture, minute ventilation (MV), and so forth. 
     A battery  730  provides operating power to all of the components in the ICM device  100 . The battery  730  is capable of operating at low current drains for long periods of time. The battery  730  also desirably has a predictable discharge characteristic so that elective replacement time can be detected. As one example, the ICM device  100  employs lithium/silver vanadium oxide batteries. The battery  730  may afford various periods of longevity (e.g. three years or more of device monitoring). In alternate embodiments, the batter  730  could be rechargeable. See for example, U.S. Pat. No. 7,294,108, Cardiac event microrecorder and method for implanting same, which is hereby incorporated by reference. 
       FIG. 9  illustrates a block-diagram of an alternative embodiment of the circuitry of an ICM device  900 . The ICM device  900  has a device housing  901  to hold the electronic/computing components. The ICM device  900  is equipped with telemetry circuitry  904  and sensing circuitry  902 . The telemetry circuitry  904  may include one or more transceivers. The sensing circuitry  902  is selectively coupled to one or more electrodes that perform sensing operations to detect cardiac activity data indicative of cardiac activity. The sensing circuitry  902  may include dedicated sense amplifiers, multiplexed amplifiers, or shared amplifiers. It may further employ one or more low power, precision amplifiers with programmable gain and/or automatic gain control, bandpass filtering, and threshold detection circuit to selectively sense the cardiac signal of interest. 
     In the present embodiment, the sensing electrode  210  positioned on the tail extension  104  is coupled to a terminal  908 . The device housing  102  further includes a connector (not shown) with at least one terminal  910  and preferably a second terminal  912 . The terminals  910 ,  912  may be coupled to additional sensing electrodes positioned on the device housing, on the tail extension, or located otherwise. For example, the terminals  910 ,  912  may be coupled to the sensing electrodes  212 ,  213 , and/or  215  of  FIG. 2 . Additionally or alternatively, the terminals  910 ,  912  may be connected to one or more leads having one or more electrodes provided thereon, where the electrodes are located in various locations about the heart. The type and location of each electrode may vary. 
     A switch  906  is electrically coupled between the terminals  908 ,  910 ,  912  and the telemetry circuitry  904  and the sensing circuitry  902 . In the present example, the switch  906  is provided to allow selection of different electrode configurations connected to the terminals  908 ,  910 , and  912  under the control of a microcontroller  914 . The switch  906  is controlled by a control signal from the microcontroller  914 . The switch  906  is used to determine the sensing polarity of the cardiac signal by selectively closing the appropriate electrical circuitry paths. For example, the switch  906  comprises a low pass filter branch and a band pass filter branch (not shown). The low pass filter branch is configured to pass the low frequency cardiac sensed signals from the sensing electrode  210  coupled to the conductor  302  of the tail extension  104  to the sensing circuitry  902 . The band pass filter branch is configured to pass the high frequency RF communications, in a frequency range that includes approximately 2.4 GHz, from the antenna  304  of the tail extension  104  to the telemetry circuitry  904 . 
     A physician implants the ICM device  100  into a subcutaneously location in the patient via a tail implant tool. The tail implant tool comprises a lumen with a distal end and a proximal end. The lumen is sized and shaped to receive the tail extension  104  of the ICM device  100  within the lumen. The distal end  206  of tail extension  104  is inserted into the distal end of the lumen of the tail implant tool. The physician inserts the distal end of the lumen into a subcutaneous area of the patient located proximate to the ROI in the heart. For example, the physician may insert the distal end of the lumen of the tail implant tool into the patient in order to position the sensing electrode  210  of the tail extension  104  proximate one side of the heart. The physician may position the device housing  102  proximate an opposite side of the heart. Optionally, the physician may position the tail extension  104  and the device housing  102  on the same side of the heart. The tail implant tool releases the tail extension  104 , and is removed from the patient while leaving the tail extension  104  of the ICM device  100  implanted in the patient. Optionally, alternative tools may be utilized to implant the ICM device  100  into the patient. 
     The various methods as illustrated in the FIGS. and described herein represent exemplary embodiments of methods. The methods may be implemented in software, hardware, or a combination thereof. In various of the methods, the order of the steps may be changed, and various elements may be added, reordered, combined, omitted, modified, etc. Various of the steps may be performed automatically (e.g., without being directly prompted by user input) and/or programmatically (e.g., according to program instructions). 
     Various modifications and changes may be made as would be obvious to a person skilled in the art having the benefit of this disclosure. It is intended to embrace all such modifications and changes and, accordingly, the above description is to be regarded in an illustrative rather than a restrictive sense. 
     The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims. 
     Other variations are within the spirit of the present disclosure. Thus, while the disclosed techniques are susceptible to various modifications and alternative constructions, certain illustrated embodiments thereof are shown in the drawings and have been described above in detail. It should be understood, however, that there is no intention to limit the invention to the specific form or forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions and equivalents falling within the spirit and scope of the invention, as defined in the appended claims. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the disclosed embodiments (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. The term “connected,” when unmodified and referring to physical connections, is to be construed as partly or wholly contained within, attached to or joined together, even if there is something intervening. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein and each separate value is incorporated into the specification as if it were individually recited herein. The use of the term “set” (e.g., “a set of items”) or “subset” unless otherwise noted or contradicted by context, is to be construed as a nonempty collection comprising one or more members. Further, unless otherwise noted or contradicted by context, the term “subset” of a corresponding set does not necessarily denote a proper subset of the corresponding set, but the subset and the corresponding set may be equal. 
     All references, including publications, patent applications and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     It is to be understood that the subject matter described herein is not limited in its application to the details of construction and the arrangement of components set forth in the description herein or illustrated in the drawings hereof. The subject matter described herein is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. While the dimensions, types of materials and coatings described herein are intended to define the parameters of the invention, they are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. §112(f), unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure.