Abstract:
The disclosure provides methods and apparatus for simultaneously providing protection to an implantable medical device, such as an extra-cardiac implantable defibrillator (EID), while allowing efficacious therapy delivery via an external defibrillator (e.g., an automated external defibrillator, or AED). Due to the orientation of the electrodes upon application of therapy via, for example, via an AED the structure of the EID essentially blocks therapy delivery. In addition, but for the teaching of this disclosure sensitive circuitry of an EID can be damaged during application of external high voltage therapy thus rendering the EID inoperable. EIDs are disclosed that are entirely implantable subcutaneously with minimal surgical intrusion into the body of the patient and provide distributed cardioversion-defibrillation sense and stimulation electrodes for delivery of cardioversion-defibrillation shock and pacing therapies across the heart when necessary. Configurations include one hermetically sealed housing with one or, optionally, two subcutaneous sensing and cardioversion-defibrillation therapy delivery leads or alternatively, two hermetically sealed housings interconnected by a power/signal cable. The housings are generally dynamically configurable to adjust to varying rib structure and associated articulation of the thoracic cavity and muscles. Further the housings may optionally be flexibly adjusted for ease of implant and patient comfort. One aspect includes partially insulating a surface of an EID that faces away from a heart while maintaining a major conductive surface facing the heart.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of chronically implantable medical devices; in particular, the invention relates to methods and apparatus to selectively shunt externally-delivered defibrillation energy delivered to a subject who has an extra-cardiac implantable defibrillator (EID) to preserve the EID and to allow the externally-delivered defibrillation waveform and its accompanying therapeutic energy to reach the myocardium. 
       BACKGROUND OF THE INVENTION 
       [0002]    Both automated external defibrillators (AEDs) and implantable cardioverter-defibrillators (ICDs) are becoming increasing available and it is estimated that as a result many thousands of individuals have received life-saving defibrillation therapy. 
         [0003]    More recently non-transvenous, extra-cardiac ICDs—herein EIDs (whether or not such devices include cardioversion capability)—have begun to be developed and might become as widespread as ICDs are today. As a result, the possibility exists that an individual having an EID might receive high energy defibrillation therapy from an AED. 
         [0004]    The inventors suggest that for a number of reasons such therapy could cause more harm than good unless preventative measures are incorporated into the EID. 
         [0005]    Prior U.S. Pat. No. 5,999,398 to Makl et al. issued 7 Dec. 1999 (the &#39;398 patent) entitled, “Feed-through Assembly having Varistor and Capacitor Structure,” is hereby incorporated herein by reference. In the &#39;398 patent a filter structure is proposed that includes both varistor and capacitive characteristic thereby providing purportedly effective transient suppression and interference filtering with a single package. Although not central to the present invention, prior U.S. Pat. No. 6,253,105 to Leyde entitled, “Method for Delivering Defibrillation Energy,” is also incorporated herein by reference in its entirety. 
       SUMMARY 
       [0006]    The present invention provides methods and apparatus for simultaneously providing protection to an implantable medical device, such as an extra-cardiac implantable defibrillator (EID), and allowing efficacious therapy delivery via an external defibrillator (e.g., a manual or an automated external defibrillator, or AED). Due to the orientation of the electrodes upon application of therapy via, for example, an AED the structure of the EID can essentially block therapy delivery. In addition, sensitive circuitry of an EID can be damaged thus rendering the EID inoperable. 
         [0007]    In one form of the invention, an EID includes a pair of high voltage-capacity defibrillation electrodes defining at least one defibrillation vector through a volume of myocardial tissue and at least one voltage shunting device (e.g., a varistor such as a metal oxide varistor, or MOV). As is known in the electronic arts a varistor is a voltage dependent, nonlinear device that has electrical characteristics similar to a pair of Zener diodes mounted back-to-back. Basically, a varistor shunts transient electrical currents away from circuitry by presenting a low resistance path in the presence of overvoltage situations. They are the most broadly applied technology, protecting vulnerable circuit components in applications whether low or high energy and current ratings are required. Commercially available varistors are available with operating voltages from 2.5V to 2800VDC and 3.5-3500VDC from companies such as Littelfuse, Inc. of Des Plaines, Ill. The Littelfuse company sells MOVs composed mainly of zinc oxide with small amounts of bismuth, manganese, cobalt, and other metal oxides that work by absorbing voltage surges and dissipating the energy as heat. These MOVs are available with peak current ratings ranging from 40 A to 70,000 A and peak energy ratings ranging from 0.1 J to 10,000 J. Certain Littelfuse MOVs are designed to suppress transient voltages such as lightning and other high level transients found in industrial and AC line applications. For the purposes of shunting energy for an AED applied to a subject implanted with an EID the peak energies vary but range from about 100 J to about 200 J. 
         [0008]    For example, given a nominal 1500V defibrillation energy delivered via an AED a varistor such as an MOV coupled to a conductive feedthrough pin that passes through the housing or shield of an EID will allow up to 1500V to defibrillate the heart and any energy over 1500V will be partially shunted. Thus, the electrical voltage appearing across the terminals of an EID will be limited to less than about 1600V thereby protecting the EID circuitry while allowing external defibrillation therapy to proceed essentially unimpeded. 
         [0009]    The present invention generally relates to implantable medical devices, particularly implantable (cardioverter) defibrillators that are entirely implanted subcutaneously and, more particularly, have no leads or electrodes contacting the heart or extending into the thoracic cavity. 
         [0010]    Apparatuses and methods are disclosed relating to various types of EID&#39;s with geometries, shapes and sizes adapted for subcutaneous or submuscularimplant. In a prophylactic application, for example, some embodiments form EID systems that can be placed completely in the subcutaneous or submuscular position without the need to place leads or electrodes in the vasculature of the patient. One set of embodiments of the invention provides a variety of configurations for delivering cardioversion/defibrillation therapy with a vector of energy controlled by operative circuitry of a non-active-can type EID. In one form of the invention, the EID housing can be conveniently implanted in a surgically-created subcutaneous or submuscular pocket formed over or near a portion of the cardiac notch, or sternum of a patient and adjacent a portion of pectoralis major. 
         [0011]    In yet another embodiment, the EID may be implanted in a pocket formed adjacent a portion of the external abdominal oblique. In another embodiment, the EID housing may be implanted in a pocket formed adjacent a portion of the serratus anterior. 
         [0012]    In one embodiment, the EID electrically couples to one or more elongated, coil-type high voltage electrodes with the electrodes disposed in a location providing defibrillation vectors covering adequate mass of myocardial tissue to achieve defibrillation and deliver pacing therapy. Specifically, leads may be substantially implanted adjacent a portion of the external abdominal oblique; adjacent the cardiac notch; adjacent a portion of the serratus anterior; and adjacent a portion of the latissimus dorsi. 
         [0013]    In one embodiment, more than one high voltage electrodes are implemented with the EID connected to all electrodes. The one or more high voltage electrodes may include a set of coil electrodes disposed in an orientation relative to a patient&#39;s heart that provides several different therapy delivery vectors therebetween. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    These and other features of the present invention will be appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiment of the invention when considered in connection with the accompanying drawings, in which like numbered reference numbers designate like parts throughout the figures thereof. 
           [0015]      FIG. 1A  depicts a multi-planar view of an EID of a first embodiment of the present invention. 
           [0016]      FIG. 1B  illustrates an EID of the first embodiment implanted in a patient. 
           [0017]      FIG. 2A  illustrates a multi-planar view of an EID of a second embodiment of the present invention. 
           [0018]      FIG. 2B  illustrates an EID of the second embodiment implanted in a patient. 
           [0019]      FIG. 3A  illustrates a multi-planar view of a third embodiment of an EID in accordance with the present invention. 
           [0020]      FIG. 3B  illustrates the EID of the third embodiment implanted in a patient. 
           [0021]      FIG. 4A  illustrates the EID of the fourth embodiment in accordance with the present invention. 
           [0022]      FIG. 4B  illustrates the EID of the fourth embodiment implanted in a patient. 
           [0023]      FIG. 4C  illustrates a cross-sectional view of a cable connecting the two parts of an EID of the fourth embodiment in accordance with the present invention. 
           [0024]      FIGS. 4D ,  4 E and  4 F illustrate a cross-sectional view of a patient taken through the thoracic cavity and center of the heart showing the deployment and arrangement of the fourth embodiment EID in accordance with the present invention. 
           [0025]      FIG. 5A  illustrates a multi-planar view of an EID in accordance with a fifth embodiment of the present invention. 
           [0026]      FIGS. 5B and 5C  illustrate a cross-sectional view of a patient taken through the thoracic cavity and center of the heart with the deployment and arrangement of the EID, and the EID of the fifth embodiment implanted in a patient, respectively. 
           [0027]      FIG. 6A  illustrates a multi-planar view of another EID embodiment. 
           [0028]      FIGS. 6B and 6C  illustrate perspective views of an EID showing major internal piece parts of a generic embodiment. 
           [0029]      FIG. 7  illustrates a block diagram of the circuitry of an exemplary EID. 
           [0030]      FIG. 8  illustrates a schematic indicating the relative electrical connections of a EID according to the invention as well as the representative couplings of a pair of surface-paddle electrodes of an external defibrillator. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]      FIG. 1A  depicts a multi-planar view of a first embodiment of the present invention. EID  12  is an ovoid, substantially, kidney-shaped housing with connector  14  for attaching a subcutaneous sensing and cardioversion/defibrillation therapy delivery lead  16 . EID  12  may be constructed of stainless steel, titanium or ceramic as described in U.S. Pat. No. 4,180,078 “Lead Connector for a Body Implantable Stimulator” to Anderson and U.S. Pat. No. 5,470,345 “Implantable Medical Device with Multi-layered Ceramic Enclosure” to Hassler, et al. The electronics circuitry of EID  10  (described herein pertaining to  FIG. 21 ) may be incorporated on a polyamide flex circuit, printed circuit board (PCB) or ceramic substrate with integrated circuits packaged in leadless chip carriers and/or chip scale packaging (CSP). In one of the views, the concave construction of EID  12  is illustrated. The minor concavity of the housing of EID  12  follows the natural curve of the patient&#39;s median ribcage at about the cardiac notch. The central curved depression shown in frontal elevation view  10  is ergonomically aligned to minimize patient discomfort when seated, bending over and/or during normal torso movement. 
         [0032]    EID  12  is shown coupled to subcutaneous lead  16 . At connector block  14 , the crescent-shaped connector block  14  enables a reliable curvilinear connection between lead  16  and the curved edge of EID  12 . Lead  16 , like the other leads discussed below, includes an elongated lead body carrying conventional, mutually insulated conductors, each coupled to an electrode. 
         [0033]      FIG. 1B  illustrates EID  12  implanted in patient  20 . Specifically, lead  16  is advanced adjacent the cardiac notch and tunneled subcutaneously from the median implant pocket of EID  12  laterally and posterially to the patient&#39;s back to a location opposite the heart such that the heart  18  is disposed between the EID  12  and the distal end of subcutaneous lead  16 . The implant location of EID  12  and lead  16  is typically subcutaneously above the external abdominal oblique. The distal end of lead  16  is tunneled above the external oblique muscle extending over to a portion of the latissimus dorsi. 
         [0034]      FIG. 2A  is a multi-planar view of EID  30 , a second embodiment of the present invention. EID  30  is a convex, flexible ovaloid-shaped housing with connectors  14  (two shown) for attaching a pair of subcutaneous sensing and cardioversion/defibrillation therapy delivery leads  16 A and  16   b . According to the invention one or more components capable of limiting electrical voltage, such as a metal oxide varistor  15  couples to the leads  16   a  and  16   b  and via a conductor  17  to a source of common electrical reference voltage (e.g., the housing of EID  30 ). 
         [0035]    EID  30  may be constructed of stainless steel, titanium or ceramic. View  10 A is a side view of EID  30  showing the tapered housing  30 , a mid-line flexible joint  32 , connector  14 , lead  16  and active can electrode  38 . The active can electrode  38  allows sensing and cardioversion, defibrillation and/or pacing therapy delivery between the EID  30  and one or both leads  16 A or  16   b . The jointed housing  30  allows physician flexibility in selecting implant locations and accommodates variances in size and weight of patients for implant. Additionally, the flexible housing provides less patient discomfort in sitting, bending over and/or during normal torso movement because the configurations allows dynamic adjustment to the patient&#39;s dynamic and muscular movements. View  10   b  is a top cut-away view of the EID  30  showing the convex construction that promotes ease of subcutaneous implant by following the natural curve of the patient&#39;s lateral ribcage. View  10   c  is a vertical cross section of EID  30  showing internal components that will be described in more detail hereinbelow. Shown in this view are battery  36 , electronics module  37 , high voltage capacitors  34 , flex circuit  35  and flexible housing joint  32 . 
         [0036]      FIG. 2B  illustrates implant position of EID  30  and leads  16 A and  16 B according to a second embodiment of the invention. EID  30  is implanted subcutaneously over a portion of the external oblique muscle laterally outside the 20 th  ribcage of patient  20 . Lead  16 A is tunneled subcutaneously from the lateral implant pocket of EID  30  anterially and medially to the cardiac notch. Further, lead  16   b  is tunneled posterially adjacent the latissimus dorsi, to the patient&#39;s back to a location opposite the heart such that the heart  18  is disposed between the distal end of subcutaneous lead  16 A and the distal end of subcutaneous lead  16   b.    
         [0037]      FIG. 3A  is a multi-planar view of a third embodiment of EID  40 . EID  40  is an elongated slender ellipsoid with sections of partially articulating dynamic segments having surface mounted subcutaneous sensing and cardioversion/defibrillation therapy delivery electrodes  44  and  46 . EID  40  may be constructed of stainless steel, titanium or ceramic or equivalent. View  10 A is a top view of EID  40  showing the segmented construction (at  42 ). One or more of the segmented portions  45  can be adapted to house, for instance, high voltage defibrillation circuitry  47 . According to the invention, an energy limiting component (or components)  43  can couple to an electrical reference (e.g., a ground or common electrical potential for the device, such as a portion of the metallic housing) and to a conductor  49  that couples to a high voltage electrode  44  (at  44 ′). Electrodes  44  and  46  located at opposite ends of EID  40  are typically 100 mm 2  to 1000 mm 2 . View  10   b  is a further top view showing the dynamic flexibility of EID  40  in which it assumes a dynamically adjustable, compressive and tensile opposing surfaces when implanted outside the thoracic cavity over the ribs. Specifically, in its normal position, EID  40  is substantially flat both at the top and bottom surfaces. However, when implanted, EID  40  dynamically forms a concave and convex surface at the flat top and segmented bottom surfaces when tunneled into the subcutaneous regions of the thoracic cavity above the ribs or the intercostals region therebetween. As illustrated in  FIG. 3B , EID  40  dynamically adjusts to wrap around the ribcage with electrode  44  anterior to the cardiac notch and the EID  40  is positioned such that electrode  46  is laterally located in opposition to electrode  46  thereby positioning heart  18  between the electrodes. The dynamic configurability of EID  40  creates an external surface that is convex and slightly bent in two directions and at the same time twisted on its long axis to closely fit over the ribs. 
         [0038]      FIG. 4A  is an illustration of the fourth embodiment of the present invention. EID  50  housings are connected by an interconnecting lead  52  containing power, control, sensing and therapy delivery conductors. The EID  50  contains integrated subcutaneous sensing and cardioversion/defibrillation therapy delivery electrodes. EID  50  may be constructed of stainless steel, titanium or ceramic. View  10 A is a cross sectional view through one of the EID  50  housings showing the concave inner surface to enable un-obtrusive subcutaneous implant because the oval profile of EID  50  is designed to follow the natural curve of the patient&#39;s median cardiac notch and posterior ribcage. Integrated electrodes (not shown) located on the inner surfaces of EID  50  are typically 100 mm 2  to 1000 mm 2  in active area. View  10   b  is a top view of EID  50  showing a convex domed top and a substantially flat bottom. According to the invention one or more components capable of limiting electrical current, such as a metal oxide varistor  15  couples to the leads  16   a  and  16   b  and via a conductor  17  to a source of common electrical reference voltage (e.g., the housing of EID  30 ). 
         [0039]      FIG. 4B  illustrates EID  50  implanted in patient  20 . Specifically, EID  50  is implanted outside the ribcage with a first EID  50  housing anterior to the cardiac notch and the other EID  50  housing tunneled and positioned posterially in relation to heart  18 . 
         [0040]      FIG. 4C  illustrates a cross-sectional view of the interconnecting cable  52 . The outer sheath of cable  52  consists of a urethane or equivalent sheath  232  with an inner insulation  236  of HP Silicone. The power, control and sensing conductors  230  are wrapped with ETFE while the defibrillation conductors  234  are constructed of Ag/MP35N and wrapped with ETFE and reinforced with tensile material. 
         [0041]      FIGS. 4D ,  4 E and  4 F illustrate cross-sectional views taken through the thoracic cavity and center of the heart showing the deployment and implant of EID  50 .  FIG. 4D  shows a tunneling tool  56  entering the patient&#39;s body  20  at a first incision anterior to the cardiac notch, tunneled laterally and posterially to exit at a second incision in the patient&#39;s back adjacent a portion of the latissimus dorsi. The EID  50  and interconnecting cable  52  are attached to the tunneling tool  56 , which is retracted, and the EID  50  and cable  52  are pulled into a posterior implant location as shown in  FIG. 4E . The second housing of EID  50  is attached to the interconnecting cable  52  and placed into an implant pocket anterior to the cardiac notch as shown in  FIG. 4F . 
         [0042]      FIG. 5A  illustrates a fifth embodiment of the present invention. EID  50  consists of two rounded beetle-shaped housings connected by an interconnecting lead  62 . The EID  60  may be constructed of stainless steel, titanium or ceramic. Excess length and a strain relief loop are provided in cable  62 . The EID  60  contains integrated subcutaneous sensing and cardioversion/defibrillation therapy delivery electrodes  66 . Suture loops  64  are provided on each housing to enable the fixation of each housing in a predetermined location for proper stimulation and to prevent device migration. As is shown in the top view, EID  60  housing includes a concave inner surface to enable a compliant subcutaneous movement by the canisters following the natural curve of the patient&#39;s median cardiac notch and posterior ribcage. Integrated electrodes  66  located on the inner surfaces of canisters  60  are typically 100 mm 2  to 1000 mm 2  in active area. According to the invention one or more components capable of limiting electrical current, such as a metal oxide varistor  15  couples to the leads  16   a  and  16   b  and via a conductor  17  to a source of common electrical reference voltage (e.g., the housing of EID  30 ). The component  15  can be disposed upon a portion of a hybrid circuit board (not shown) and in order to increase temperature dissipation, a layer or block of a material capable of functioning as a heat sink can be applied under the component  15 . In one embodiment, a layer of copper is disposed under the component  15  and electrically isolated from the other circuitry and active components of the EID  30 . In addition, known types of capacitive filtering components can be used in addition to the component  15 . In one form of the invention, discoidal capacitors are integrated into a feedthrough assembly to reduce or eliminate electronic interference from entering the housing  60 . 
         [0043]      FIG. 5B  illustrates a cross-sectional view through the thoracic cavity and the center of the heart  18  showing the implant location for EID  60 . Specifically, a first housing of EID  60  is implanted anterior to the cardiac notch and a second housing of EID  60  located posterially. Interconnecting cable  62  containing power, control, sensing and therapy delivery conductors is located between the EID  60  housing as shown. 
         [0044]      FIG. 5C  illustrates EID  60  implanted in patient  20 . As discussed hereinabove, EID  60  is subcutaneously implanted with the two housings carrying exposed large surface electrodes. The positioning is such that a major potion of the myocardium of heart  18  is located between the two electrodes  66  on each housing of EID  60 . 
         [0045]      FIG. 6A  is a plan side view of a subcutaneous cardioverter-defibrillator  10  of a ninth embodiment of the present invention. Canister  100  is an ovaloid-shaped housing with a connector  14  for attaching  1  or  2  subcutaneous sensing and cardioversion/defibrillation therapy delivery leads. This design allows great flexibility in device placement and location. Canister  100  may be constructed of stainless steel, titanium or ceramic. The electronics circuitry of subcutaneous cardioverter-defibrillator  10  (described later in relation to  FIG. 21 ) may be incorporated on a polyamide flex circuit, printed circuit board (PCB) or ceramic substrate with integrated circuits packaged in leadless chip carriers and/or chip scale packaging (CSP). View  10 A is an end view of subcutaneous cardioverter-defibrillator  100  showing the connector  14 , suture loops  102  (2 shown) and antenna  106 . Suture loops  102  are provided on housing  100  to allow the fixation of housing in a fixed pocket location for proper stimulation and to prevent device migration. 
         [0046]      FIG. 6B  is a plan view showing the component parts/elements of the EID  100  of  FIG. 6A . Components shown include, battery  77 , electronics module  76 , tantalum capacitors  76  (3 shown), transformer  75 , antenna  106  and connector  14 . 
         [0047]      FIG. 6C  is a perspective view showing an alternative embodiment of the major piece parts/elements of the EID  100  of  FIG. 6A . Components shown include, battery  77 , electronics module  78 , aluminum capacitors  76  (4 shown), transformer  75 , antenna  106  and connector  14 . 
         [0048]    The electronic circuitry employed in the EID (as described above in relation to the various embodiments shown in  FIG. 1-15 ) can take any of the known forms that detect a tachyarrhythmia from the sensed EGM and provide cardioversion/defibrillation shocks as well as post-shock pacing as needed. A simplified block diagram of such circuitry adapted to function employing the first and second and, optionally, the third cardioversion-defibrillation electrodes as well as the EGM sensing and pacing electrodes described above is set forth in  FIG. 7 . It will be understood that the simplified block diagram does not show all of the conventional components and circuitry of such ICDs including digital clocks and clock lines, low voltage power supply and supply lines for powering the circuits and providing pacing pulses or telemetry circuits for telemetry transmissions between the ICD and an external programmer or monitor. 
         [0049]      FIG. 7  illustrates the electronic circuitry, low voltage and high voltage batteries within the hermetically sealed housings. The low voltage battery  353  is coupled to a power supply (not shown) that supplies power to the ICD circuitry and the pacing output capacitors to supply pacing energy in a manner well known in the art. The low voltage battery can comprise one or two conventional LiCF x , LiMnO 2  or LiI 2  cells. The high voltage battery  312  can comprise one or two conventional LiSVO or LiMnO 2  cell. 
         [0050]    In  FIG. 7 , EID functions are controlled by means of stored software, firmware and hardware that cooperatively monitor the EGM, determine when a cardioversion-defibrillation shock or pacing is necessary, and deliver prescribed cardioversion-defibrillation and pacing therapies. The block diagram of  FIG. 7  incorporates circuitry set forth in commonly assigned U.S. Pat. No. 5,163,427 “Apparatus for Delivering Single and Multiple Cardioversion and Defibrillation Pulses” to Keimel; U.S. Pat. No. 5,188,105 “Apparatus and Method for Treating a Tachyarrhythmia” to Keimel and U.S. Pat. No. 5,314,451 “Replaceable Battery for Implantable Medical Device” to Mulier for selectively delivering single phase, simultaneous biphasic and sequential biphasic cardioversion-defibrillation shocks typically employing an ICD IPG housing electrode coupled to the COMMON output  332  of high voltage output circuit  340  and one or two cardioversion-defibrillation electrodes disposed in a heart chamber or cardiac vessel coupled to the HVI and HV-2 outputs ( 313  and  323 , respectively) of the high voltage output circuit  340 . The circuitry of the subcutaneous EID of the present invention can be made simpler by adoption of one such cardioversion-defibrillation shock waveform for delivery simply between the first and second cardioversion-defibrillation electrodes  313  and  323  coupled to the HV-I and HV-2 outputs respectively. Or, the third cardioversion-defibrillation electrode  332  can be coupled to the COMMON output as depicted in  FIG. 7  and the first and second cardioversion-defibrillation electrodes  313  and  323  can be electrically connected in to the HV-1 and the HV-2 outputs, respectively, as depicted in  FIG. 7 . 
         [0051]    The cardioversion-defibrillation shock energy and capacitor charge voltages can be intermediate to those supplied by ICDs having at least one cardioversion-defibrillation electrode in contact with the heart and most AEDs having cardioversion-defibrillation electrodes in contact with the skin. The typical maximum voltage necessary for ICDs using most biphasic waveforms is approximately 750 Volts with an associated maximum energy of approximately 40 Joules. The typical maximum voltage necessary for AEDs is approximately 2000-5000 Volts with an associated maximum energy of approximately 200-360 Joules depending upon the model and waveform used. The ICD of the present invention uses maximum voltages in the range of about 700 to about 3150 Volts and is associated with energies of about 25 Joules to about 210 Joules. The total high voltage capacitance could range from about 50 to about 300 microfarads. 
         [0052]    Such cardioversion-defibrillation shocks are only delivered when a malignant tachyarrhythmia, e.g., ventricular fibrillation is detected through processing of the far field cardiac EGM employing one of the available detection algorithms known in the ICD art. 
         [0053]    In  FIG. 7 , pacer timing/sense amplifier circuit  378  processes the far field EGM SENSE signal that is developed across a particular EGM sense vector defined by a selected pair of the electrodes  332 ,  313  and, optionally, electrode  323  if present as noted above. The selection of the sensing electrode pair is made through the switch matrix/MUX  390  in a manner disclosed in the commonly assigned U.S. Pat. No. 5,331,966 “Subcutaneous Multi-Electrode Sensing System, Method and Pacer” to Bennett, et al patent to provide the most reliable sensing of the EGM signal of interest, which would be the R wave for patients who are believed to be at risk of ventricular fibrillation leading to sudden death. The far field EGM signals are passed through the switch matrix/MUX  390  to the input of a sense amplifier in the pacer timing/sense amplifier circuit  378 . Bradycardia is typically determined by an escape interval timer within the pacer timing circuit  378  or the timing and control circuit  344 , and pacing pulses that develop a PACE TRIGGER signal applied to the pacing pulse generator  392  when the interval between successive R-waves exceeds the escape interval. Bradycardia pacing is often temporarily provided to maintain cardiac output after delivery of a cardioversion-defibrillation shock that may cause the heart to slowly beat as it recovers function. 
         [0054]    Detection of a malignant tachyarrhythmia is determined in the timing and control circuit  344  as a function of the intervals between R-wave sense event signals that are output from the pacer timing/sense amplifier circuit  378  to the timing and control circuit  344 . 
         [0055]    Certain steps in the performance of the detection algorithm criteria are cooperatively performed in a microcomputer  342 , including microprocessor, RAM and ROM, associated circuitry, and stored detection criteria that may be programmed into RAM via a telemetry interface (not shown) conventional in the art. Data and commands are exchanged between microcomputer  342  and timing and control circuit  344 , pacer timing/amplifier circuit  378 , and high voltage output circuit  340  via a bi-directional data/control bus  346 . The pacer timing/amplifier circuit  378  and the timing and control circuit  344  are clocked at a slow clock rate. The microcomputer  342  is normally asleep, but is awakened and operated by a fast clock by interrupts developed by each it-wave sense event or on receipt of a downlink telemetry programming instruction or upon delivery of cardiac pacing pulses to perform any necessary mathematical calculations, to perform tachycardia and fibrillation detection procedures, and to update the time intervals monitored and controlled by the timers in pace/sense circuitry  378 . The algorithms and functions of the microcomputer  342  and timer and control circuit  344  employed and performed in detection of tachyarrhythmias are set forth, for example, in commonly assigned U.S. Pat. No. 5,991,656 “Prioritized Rule Based Apparatus for Diagnosis and Treatment of Arrhythmias” to Olson, et al and U.S. Pat. No. 5,193,535 “Method and Apparatus for Discrimination of Ventricular Tachycardia from Ventricular Fibrillation and for Treatment Thereof” to Bardy, et al, for example. Particular algorithms for detection of ventricular fibrillation and malignant ventricular tachycardias can be selected from among the comprehensive algorithms for distinguishing atrial and ventricular tachyarrhythmias from one another and from high rate sinus rhythms that are set forth in the &#39;656 and &#39;535 patents. 
         [0056]    The detection algorithms are highly sensitive and specific for the presence or absence of life threatening ventricular arrhythmias, e.g., ventricular tachycardia (V-TACH) and ventricular fibrillation (V-FIB). Another optional aspect of the present invention is that the operational circuitry can detect the presence of atrial fibrillation (A FIB) as described in Olson, W. et al. “Onset And Stability For Ventricular Tachyarrhythmia Detection in an Implantable Cardioverter and Defibrillator,” Computers in Cardiology (1986) pp. 167-170. Detection can be provided via R-R Cycle length instability detection algorithms. Once A-FIB has been detected, the operational circuitry will then provide QRS synchronized atrial cardioversion/defibrillation using the same shock energy and wave shapes used for ventricular cardioversion/defibrillation. 
         [0057]    Operating modes and parameters of the detection algorithm are programmable and the algorithm is focused on the detection of V-FIB and high rate V-TACH (&gt;240 bpm). 
         [0058]    Although the EID of the present invention may rarely be used for an actual sudden death event, the simplicity of design and implementation allows it to be employed in large populations of patients at modest risk with modest cost by medical personnel other than electrophysiologists. Consequently, the EID of the present invention includes the automatic detection and therapy of the most malignant rhythm disorders. As part of the detection algorithm&#39;s applicability to children, the upper rate range is programmable upward for use in children, known to have rapid supraventricular tachycardias and more rapid V-FIB. 
         [0059]    When a malignant tachycardia is detected, high voltage capacitors  356 ,  358 ,  360 , and  362  are charged to a pre-programmed voltage level by a high-voltage charging circuit  364 . It is generally considered inefficient to maintain a constant charge on the high voltage output capacitors  356 ,  358 ,  360 ,  362 . Instead, charging is initiated when control circuit  344  issues a high voltage charge command HVCHG delivered on line  345  to high voltage charge circuit  364  and charging is controlled by means of bi-directional control/data bus  366  and a feedback signal VCAP from the HV output circuit  340 . High voltage output capacitors  356 ,  358 ,  360  and  362  may be of film, aluminum electrolytic or wet tantalum construction. 
         [0060]    The negative terminal of high voltage battery  312  is directly coupled to system ground. Switch circuit  314  is normally open so that the positive terminal of high voltage battery  312  is disconnected from the positive power input of the high voltage charge circuit  364 . The high voltage charge command HVCHG is also conducted via conductor  349  to the control input of switch circuit  314 , and switch circuit  314  closes in response to connect positive high voltage battery voltage EXT B+ to the positive power input of high voltage charge circuit  364 . Switch circuit  314  may be, for example, a field effect transistor (FET) with its source-to-drain path interrupting the EXT B+ conductor  318  and its gate receiving the HVCHG signal on conductor  345 . High voltage charge circuit  364  is thereby rendered ready to begin charging the high voltage output capacitors  356 ,  358 ,  360 , and  362  with charging current from high voltage battery  312 . 
         [0061]    High voltage output capacitors  356 ,  358 ,  360 , and  362  may be charged to very high voltages, e.g., 700-3150V, to be discharged through the body and heart between the selected electrode pairs among first, second, and, optionally, third subcutaneous cardioversion-defibrillation electrodes  313 ,  332 , and  323 . In accordance with certain aspects of the present invention a metal oxide varistor  400 , 402  electrically couples intermediate a high voltage electrode (e.g.,  313 , 323 ) and a source of reference voltage (e.g., internal circuitry of the EID). Thus, in the event that external defibrillation therapy is delivered to a patient having an EID, the defibrillation energy passes to the myocardium and does not shunt to the EID thereby possibly damaging the EID and/or limiting the defibrillation energy delivered to the patient. Another voltage-limiting component  404  can be placed across the pacing sensing amplifier(s)  378  used to sense far field cardiac wavefronts. This component  404  thus protects the amplifiers  378  from damage during application of external defibrillation therapy. 
         [0062]    The details of the voltage charging circuitry are also not deemed to be critical with regard to practicing the present invention; one high voltage charging circuit believed to be suitable for the purposes of the present invention is disclosed. High voltage capacitors  356 ,  358 ,  360 , and  362  are charged by high voltage charge circuit  364  and a high frequency, high-voltage transformer  368  as described in detail in commonly assigned U.S. Pat. No. 4,548,209 “Energy Converter for Implantable Cardioverter” to Wielders, et al. Proper charging polarities are maintained by diodes  370 ,  372 ,  374  and  376  interconnecting the output windings of high-voltage transformer  368  and the capacitors  356 ,  358 ,  360 , and  362 . As noted above, the state of capacitor charge is monitored by circuitry within the high voltage output circuit  340  that provides a VCAP, feedback signal indicative of the voltage to the timing and control circuit  344 . Timing and control circuit  344  terminates the high voltage charge command HVCHG when the VCAP signal matches the programmed capacitor output voltage, i.e., the cardioversion-defibrillation peak shock voltage. 
         [0063]    Timing and control circuit  344  then develops first and second control signals NPULSE  1  and NPULSE  2 , respectively, that are applied to the high voltage output circuit  340  for triggering the delivery of cardioverting or defibrillating shocks. In particular, the NPULSE  1  signal triggers discharge of the first capacitor bank, comprising capacitors  356  and  358 . The NPULSE  2  signal triggers discharge of the first capacitor bank and a second capacitor bank, comprising capacitors  360  and  362 . It is possible to select between a plurality of output pulse regimes simply by modifying the number and time order of assertion of the NPULSE  1  and NPULSE  2  signals. The NPULSE  1  signals and NPULSE  2  signals may be provided sequentially, simultaneously or individually. In this way, control circuitry  344  serves to control operation of the high voltage output stage  340 , which delivers high energy cardioversion-defibrillation shocks between a selected pair or pairs of the first, second, and, optionally, the third cardioversion-defibrillation electrodes  313 ,  323 , and  332  coupled to the HV-1, HV-2 and optionally to the COMMON output as shown in  FIG. 7 . 
         [0064]    Thus, EID  10  monitors the patient&#39;s cardiac status and initiates the delivery of a cardioversion-defibrillation shock through a selected pair or pairs of the first, second and third cardioversion-defibrillation electrodes  313 ,  323  and  332  in response to detection of a tachyarrhythmia requiring cardioversion-defibrillation. The high HVCHG signal causes the high voltage battery  312  to be connected through the switch circuit  314  with the high voltage charge circuit  364  and the charging of output capacitors  356 ,  358 ,  360 , and  362  to commence. Charging continues until the programmed charge voltage is reflected by the VCAP signal, at which point control and timing circuit  344  sets the HVCHG signal low terminating charging and opening switch circuit  314 . Typically, the charging cycle takes only fifteen to twenty seconds, and occurs very infrequently. The EID  10  can be programmed to attempt to deliver cardioversion shocks to; the heart in the manners described above in timed synchrony with a detected R-wave or can be programmed or fabricated to deliver defibrillation shocks to the heart in the manners described above without attempting to synchronize the delivery to a detected R-wave. Episode data related to the detection of the tachyarrhythmia and delivery of the cardioversion-defibrillation shock can be stored in RAM for uplink telemetry transmission to an external programmer as is well known in the art to facilitate in diagnosis of the patient&#39;s cardiac state. A patient receiving the EID  10  on a prophylactic basis would be instructed to report each such episode to the attending physician for further evaluation of the patient&#39;s condition and assessment for the need for implantation of a more sophisticated and long-lived EID. 
         [0065]      FIG. 8  illustrates a schematic indicating the relative electrical connections of an EID  800  according to the invention as well as the representative couplings of a pair of surface-paddle electrodes of an external defibrillator  802  (e.g., an automated external defibrillator or a manually operated emergency technician-operated defibrillator) and the paddle electrodes  804 , 806  coupled thereto. In  FIG. 8  various inherent sources of electrical impedances are represented schematically (e.g., interface between a patient&#39;s skin and the paddle electrodes  804 , 806  as well as the inter-electrode impedances). In the embodiment depicted in  FIG. 8 , a single high-voltage coil electrode  808  couples via elongated conductor  809  to high voltage defibrillation circuitry  340  disposed within the EID  800 . The conductor  809  enters the shield or “can”  811  of the EID via a hermetically sealed conductive feedthrough  810 . Also coupled to this interconnected circuit is a voltage-limiting component  400  (e.g., a metal oxide varistor) which in turn couples to a source of electrical reference such as the metallic shield or “can”  811 . 
         [0066]    An electrode used to sense far field cardiac activity, such as the coil electrode  808  and/or an additional electrode  812  couples via conductor  814  to amplifier circuitry  378  and conductor  814  also couples to a voltage-limiting component  404  (e.g., a metal oxide varistor) according to the invention. As is known in the art the feedthrough(s)  810  typically provide electrical insulation from the traditionally conductor can  811  and are oftentimes disposed intermediate the can and a connector component that provides a reliable means of coupling the conductors  809 , 814  to said feethroughs  810 . 
         [0067]    Also, in one embodiment a surface portion of a metallic housing that faces away from a heart for an EID according to the invention can be coated with dielectric material (or otherwise insulated) and a portion facing the heart can be actively electrified. Thus, the non-conductive surface acts to reduce the energy dissipated by the MOV which allows a relatively smaller MOV to be used. 
         [0068]    It will be apparent from the foregoing that while particular embodiments of the invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.