Abstract:
Apparatus and methods for generating an induction waveform for performing threshold testing in an implantable medical device are disclosed. Such tests may be performed during the implant procedure, or during a device checkup procedure, or routinely during the lifetime of the device. The threshold test may include induction of an arrhythmia (such as ventricular fibrillation) followed by delivery of therapy at various progressively-increasing stimulation parameters to terminate the arrhythmia. As such, the capability to induce fibrillation within the device is desired. Induction of the arrhythmias may be accomplished via delivery of a relatively low energy shock or through delivery of an induction stimulation pulse to the cardiac tissue timed concurrently with the vulnerable period of the cardiac cycle.

Description:
FIELD 
       [0001]    The present invention relates generally to implantable medical devices. In particular, the disclosure relates to methods, components, and circuits for therapy delivery by the implantable medical devices. 
       BACKGROUND 
       [0002]    The human anatomy includes many types of tissues that can either voluntarily or involuntarily, perform certain functions. After disease, injury, or natural defects, certain tissues may no longer operate within general anatomical norms. For example, organs such as the heart may begin to experience certain failures or deficiencies. Some of these failures or deficiencies can be diagnosed, corrected or treated with an implantable medical device (IMD). For example, an implanted IMD may detect an arrhythmia, such as ventricular fibrillation, and deliver one or more electrical pulses to stop the arrhythmia and allow the heart to reestablish a normal sinus rhythm. 
         [0003]    Examples of such IMDs include subcutaneous implantable cardioverter/defibrillator (SICD) systems that provide synchronous cardioversion shocks and/or asynchronous defibrillation shocks and subcutaneous pacemaker/cardioverter/defibrillator (SPCD) systems that provide additional staged therapies of anti-tachyarrhythmia pacing, synchronous cardioversion shocks and asynchronous defibrillation shocks. In general, the IMDs deliver a first pulse at a first energy level upon detecting an arrhythmia and, if the arrhythmia is not stopped, deliver additional pulses at increasing energy levels until the arrhythmia is stopped or the programmed progression of pulses has been exhausted. 
         [0004]    Typically, threshold testing is performed to evaluate the effectiveness of an IMD in ending episodes of arrhythmia. For example, the energy levels or waveforms of pulses delivered by the IMD, the sensitivity of the IMD to detect ventricular fibrillation, or the position of the electrodes used to deliver the pulses, can be configured as necessary to assure the effectiveness of the IMD. The threshold testing may be performed during the implantation process, during subsequent follow-up sessions and/or during the automatic configuration sessions initiated by the IMD. One method of testing an IMD&#39;s capability to reliably defibrillate the heart involves induction of an episode of an arrhythmia in the patient&#39;s heart, and then allowing the IMD to detect and terminate the induced arrhythmia. The IMD itself has the capability of inducing arrhythmia during the threshold testing procedures. 
         [0005]    The IMD induces an arrhythmia by delivering a pulse during the period of vulnerability within a cardiac cycle, e.g., during or near the T-wave, delivering a high frequency pulse train, delivering direct current, or other known methods for inducing the fibrillation. The clinician may program the stimulation parameters for the induction attempt, such as the timing, amplitude, or other characteristics of a T-wave shock. If the induction attempt fails, the new stimulation parameters are used for another induction attempt. 
         [0006]    When an induction attempt succeeds, the IMD can fail to detect the arrhythmia, or fail to stop the arrhythmia. In such cases, the detection algorithm or the pulse progression must be modified. The process repeats until successful arrhythmia induction, detection, and defibrillation occur such that the effectiveness of the IMD is confirmed. 
         [0007]    The process of confirming the effectiveness of an IMD can be time and resource consuming both for the clinician and for the IMD. For example, a clinician programs the IMD to execute an initial arrhythmia detection algorithm, and programs an initial progression of pulses to be delivered in response to a detected arrhythmia. The clinician then programs the IMD to induce the heart to fibrillate, so that the programmed detection algorithm and pulse progression can be tested. During automatic configuration sessions, the device may also consume a large amount of the internally stored energy to perform those functions. Accordingly, there remains a need for improved circuits and methods for therapy delivery. 
       SUMMARY 
       [0008]    It is desired to perform threshold testing to evaluate the efficacy of the therapy delivered by a subcutaneous or substernal implantable medical device (SIMD). The threshold testing may be performed during the implant procedure, or during an IMD checkup procedure, or during automatic configurations of the IMD over the lifetime of the device. 
         [0009]    In accordance with embodiments of the present disclosure, the SIMD includes circuits for generating and delivering therapy and arrhythmia induction stimulation pulses. The circuits include an output circuit having a plurality of switches that are arrayed in the shape of a “H” to form a H-bridge circuit. In some embodiments, the output circuit further includes at least one opt-in circuit that is coupled in parallel to one of the switches. The output circuit is configured such that the opt-in circuit is utilized to bypass the switch element of the H-bridge during delivery of arrhythmia induction stimulation pulses. 
         [0010]    Exemplary methods for the threshold testing may include induction of an arrhythmia (such as ventricular fibrillation) followed by delivery of therapy at various progressively increasing stimulation parameters to terminate the arrhythmia. The induction of the arrhythmias may be accomplished via delivery of a relatively low energy induction stimulation pulse or through delivery of two or more induction stimulation pulses to the cardiac tissue timed concurrently with the vulnerable period of the cardiac cycle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The following drawings are illustrative of particular embodiments of the invention and therefore do not limit the scope of the invention, but are presented to assist in providing a proper understanding. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description. The present invention will hereinafter be described in conjunction with the appended drawings, wherein like numerals denote like elements, and: 
           [0012]      FIG. 1  is a front view of a patient implanted with an implantable cardiac system; 
           [0013]      FIG. 2  is a side view of the patient implanted with the implantable cardiac system; 
           [0014]      FIG. 3  is a block diagram of a subcutaneous/substernal SIMD of the implantable cardiac system; 
           [0015]      FIG. 4  illustrates an exemplary schematic showing a portion of the operational circuitry of the SIMD in accordance with an embodiment of the disclosure; 
           [0016]      FIG. 5  shows an exemplary schematic diagram of the output circuit, such as functionally shown in  FIG. 4 ; and 
           [0017]      FIG. 6  is a diagram illustrating arbitrary input signals and resulting delivered waveforms using, for example, the output circuit that is schematically shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Currently, implantable medical devices (IMD), such as the implantable cardioverter/defibrillator (ICD), use endocardial or epicardial leads which extend from the ICD housing through the venous system to the heart. Electrodes positioned by the leads in or adjacent to the heart are used for therapy delivery and sensing functions. Cardioversion and defibrillation shocks are generally applied between a coil electrode carried by one of the leads and the ICD housing, which acts as an active can electrode. A subcutaneous or substernal implantable medical device (SIMD) differs from the more commonly used IMD in that the housing and leads are typically implanted subcutaneously such that the sensing and therapy are accomplished subcutaneously. The SIMD does not require leads to be placed in or on the cardiac tissue. Instead, the SIMD makes use of one or more electrodes on the housing, together with one or more leads that carry one or more electrodes for therapy delivery and/or sensing, which are implanted in the subcutaneous or substernal space. 
         [0019]    The disclosure describes techniques, components, devices, and methods for performing threshold testing of an SIMD. In this disclosure, the stimulation pulses that are delivered for the threshold testing will be referred to as arrhythmia induction stimulation pulses (or simply as induction stimulation pulses). 
         [0020]    In this disclosure, “substernal space” refers to the region defined by the undersurface between the sternum and the body cavity but not including the pericardium. In other words, the region is dorsal to the sternum and ventral to the ascending aorta. The substernal space may alternatively be referred to by the terms “retrosternal space” or “mediastinum” or “infrasternal” as is known to those skilled in the art and includes the region referred to as the anterior mediastinum. For ease of description, the term substernal space will be used in this disclosure, it being understood that the term is interchangeable with any of the other aforementioned terms. 
         [0021]    In this disclosure, the term “extra-pericardial” space refers to the region around, but not in contact with, the outer heart surface. The region defined as the extra-pericardial space includes the gap, tissue, bone, or other anatomical features around the perimeter of, and adjacent to, but not in contact with the pericardium. 
         [0022]    In this disclosure therapy stimulus pulse information such as stimulus amplitude, duration, rate, and/or waveform type (e.g., mono-phasic, bi-phasic, tri-phasic, or multi-phasic, etc.) and the like are included under the rubric of “stimulation parameters.” 
         [0023]      FIGS. 1-2  are conceptual diagrams of a patient  12  implanted with an exemplary implantable cardiac system  10 .  FIG. 1  is a front view of patient  12  implanted with implantable cardiac system  10 .  FIG. 2  is a side view patient  12  with implantable cardiac system  10 . 
         [0024]    Implantable cardiac system  10  includes a subcutaneous/substernal implantable medical device (SIMD)  14  connected to a lead  18 . The lead  18  may be utilized for sensing and/or to provide an electrical stimulation pulse such as pacing or defibrillation. Lead  18  includes electrodes  32  and  34  that may be configured for delivery of the stimulation pulse. In addition, or alternatively, the electrodes  32 ,  34  may be configured for sensing. 
         [0025]    SIMD  14  may provide stimulation pulse therapy and/or sense electrical activity of heart  26  via a combination of delivery/sensing vectors that include combinations of electrodes  32  and  34  and the housing or can electrode of SIMD  14 . For example, SIMD  14  may deliver therapy or obtain electrical signals sensed using a delivery/sensing vector between electrodes  32  and  34 , or using a delivery/sensing vector between electrode  32  and the conductive housing or can electrode of SIMD  14 , or using a delivery/sensing vector between electrode  34  and the conductive housing or can electrode of SIMD  14 , or a combination thereof. In this manner, sensing and stimulation pulses including defibrillation therapy, ATP therapy or post shock pacing (or other pacing therapy) may be provided in an ICD system without entering the vasculature or the pericardial space, nor making intimate contact with the heart. 
         [0026]    The electrodes  32  and  34  may be located near a distal end of lead  18 . Electrodes  32  and  34  may comprise ring electrodes, hemispherical electrodes, coil electrodes, helix electrodes, or other types of electrodes, or combination thereof. Electrodes  32  and  34  may be the same type of electrodes or different types of electrodes. 
         [0027]    The lead body of lead  18  also includes one or more elongated electrical conductors (not illustrated) that extend through the lead body from the connector assembly of SIMD  14  provided at a proximal lead end to electrodes  32 ,  34 . The lead body of lead  18  may be formed from a non-conductive material, including silicone, polyurethane, fluoropolymers, mixtures thereof, and other appropriate materials, and shaped to form one or more lumens within which the one or more conductors extend. However, the techniques are not limited to such constructions. 
         [0028]    The one or more elongated electrical conductors contained within the lead bodies of leads  16  and  18  may engage with respective ones of electrodes  32 ,  34 . The respective conductors may electrically couple to circuitry, such as a therapy module or a sensing module, of SIMD  14  via connections in connector assembly, including associated feedthroughs. The electrical conductors transmit therapy from a therapy module within SIMD  14  to one or more of electrodes  32 ,  34  and transmit sensed electrical signals from one or more of electrodes  32 ,  34  to the sensing module within SIMD  14 . 
         [0029]    In the example illustrated in  FIGS. 1-2 , SIMD  14  is implanted subcutaneously on the left midaxillary of patient  12 . SIMD  14  may, however, be implanted at other subcutaneous locations on patient  12 . The lead  18  may be inserted through an incision  2  or  4  on the patient&#39;s body for subcutaneous and/or extrapericardial implantation as will be described in more detail below. 
         [0030]    Lead  18  includes a proximal end that is connected to SIMD  14  and a distal end that includes one or more electrodes. Lead  18  may be implanted within the mediastinum such that one or more electrodes  32  and  34  are located over a cardiac silhouette of the ventricle as observed via fluoroscopy. In the example illustrated in  FIGS. 1-2 , lead  18  is located substantially centered under sternum  22 . Lead  18  extends subcutaneously from SIMD  14  toward xiphoid process  20 . At a location near xiphoid process  20  lead  18  bends or turns and extends superior upward in the substernal space. In one example, lead  18  may be placed in the mediastinum  36  and, more particularly, in the anterior mediastinum. The anterior mediastinum is bounded laterally by pleurae, posteriorly by pericardium, and anteriorly by sternum  22 . In other instances, however, lead  18  may be implanted such that it is offset laterally from the center of sternum  22 . Alternatively, lead  18  may be placed such that a therapy vector between one of electrodes  32 , 34  and a housing or can electrode of SIMD  14  is substantially across the ventricle of heart  26 . Although described herein as being implanted in the substernal space, the mediastinum, or the anterior mediastinum, lead  18  may be implanted in other extra-pericardial locations. 
         [0031]    The configuration described above in  FIGS. 1-2  is directed to providing ventricular pacing via lead  18 . In situations in which atrial pacing is desired in addition to or instead of ventricular pacing, lead  18  may be positioned further superior. A pacing lead configured to deliver pacing pulses to both the atrium and ventricle may have more electrodes. For example, the pacing lead may have one or more electrodes located over a cardiac silhouette of the atrium as observed via fluoroscopy and one or more electrodes located over a cardiac silhouette of the ventricle as observed via fluoroscopy. In some instances, two substernal pacing leads may be utilized with one being an atrial pacing lead implanted such that the electrodes are located over a cardiac silhouette of the atrium as observed via fluoroscopy and the other being a ventricle pacing lead being implanted such that the electrodes are located over a cardiac silhouette of the ventricle as observed via fluoroscopy. 
         [0032]    SIMD  14  may include a housing that forms a hermetic seal that protects components of SIMD  14 . The housing of SIMD  14  may be formed of a conductive material, such as titanium. SIMD  14  may also include a connector assembly (also referred to as a connector block or header) that includes electrical feedthroughs through which electrical connections are made between conductors within lead  18  and electronic components included within the housing. Housing may enclose one or more processors, memories, transmitters, receivers, sensors, sensing circuitry, therapy circuitry and other appropriate components as is known in the art. SIMD  14  is configured to be implanted in a patient, such as patient  12 . 
         [0033]    As shown in  FIG. 1 , an anchoring mechanism  40  may be provided along the lead body to couple the lead  18  at an access point  4  through which the distal end of the lead  18  is inserted into the substernal space. The access point  4  is any location that provides access into the substernal space. In one exemplary embodiment, the access point  4  is adjacent to or below the xiphoid process (also referred to as “subxiphoid”). The access point may also be at the notch (not shown) that connects the xiphoid process to the sternum. In other embodiments, the substernal space may also be accessed through the manubrium. The anchoring mechanism  40  is fixedly-coupled to cartilage, musculature, tissue or bone at the entry point into the substernal space at or near the access point at which site the body of the lead  18  transitions from the subcutaneous tissue into the substernal space of patient  12 . 
         [0034]    The examples illustrated in  FIGS. 1-2  are exemplary in nature and should not be considered limiting of the techniques described in this disclosure. In other examples, SIMD  14  and lead  18  may be implanted at other locations. For example, SIMD  14  may be implanted in a subcutaneous pocket in the right chest. In this example, lead  18  may be extend subcutaneously from the device toward the manubrium of the sternum and bend or turn and extend subcutaneously inferiorily from the manubrium of the sternum, substantially parallel with the sternum. 
         [0035]    In addition, it should be noted that system  10  may not be limited to treatment of a human patient. In alternative examples, system  10  may be implemented in non-human patients, e.g., primates, canines, equines, pigs, and felines. These other animals may undergo clinical or research therapies that may benefit from the subject matter of this disclosure. 
         [0036]      FIG. 3  is a block diagram of SIMD  14  that is connected to a patient  12 . The SIMD  14  includes a control circuit  60  that is connected to at least two energy storage capacitors  62   a , and  62   b  (collectively “ 62 ”) via a charging circuit  64 . During the operation of the SIMD  14 , the control circuit  60  controls various functions of the SIMD  14  such as stimulation pulse delivery or sensing. For example, the control circuit  60  controls the delivery of stimulation pulses for threshold testing to maximize the efficiency of the therapy delivered based on a selected one or more therapy programs, which may be stored in memory. Among other things, the control circuit  60  issues signals to regulate the charging circuit  64  to charge the energy storage capacitors  62  to a desired voltage level. Feedback on the voltage level of the energy storage capacitors  62  is provided to the control circuit  60 . A power source  58  is provided in SIMD  14  and may be coupled to charging circuit  64  to provide the energy that is utilized to generate the stimulation pulses. 
         [0037]    The control circuit  60  may include any type of circuitry that can issue control signals for controlling the various functions of SIMD  14 . For example, the control circuit  60  is generally representative of a processor and associated memory. The memory, for example, may include computer readable instructions that, when executed by processor, cause the components of the SIMD  14  to perform various functions attributed to them. For example, the memory may include any non-transitory, computer-readable storage media including any combination of one or more of a volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or other digital media with the sole exception being a transitory propagating signal. 
         [0038]    To facilitate therapy delivery, the SIMD  14  may deliver the therapy in response to sensed physiological conditions. As such, SIMD  14  may include sense circuitry  74  that is coupled to the control circuit  60 . For example, sense circuitry  74  may include one or more sense amplifier circuits receiving cardiac signals to monitor the heart (e.g., sense evoked responses of the heart), such as described, for example, in U.S. Pat. No. 5,861,013, to Peck et al., or, for example, in U.S. Pat. No. 5,117,824, to Keimel et al., entitled Apparatus for Monitoring Electrical Physiologic Signals,” both of which are incorporated herein by reference in their entirety. 
         [0039]    The functions attributed to SIMD  14  herein may be embodied as one or more processors, hardware, firmware, software, or any combination thereof. Depiction of different features as discrete modules or components is intended to highlight different functional aspects and does not necessarily imply that such modules must be realized by separate hardware or software components. Rather, functionality associated with one or more modules may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. For example, sensing circuitry  74  for receiving and converting analog electrical signals received from other SIMD modules or sensors may be implemented in hardware and software included in control circuit  60 . 
         [0040]    After charging to a desired level, the energy stored in the energy storage capacitors  62  may be delivered to the patient  12  in the form of stimulation pulses. Control circuit  60  may be connected to the output circuit  72  to control the delivery of the stimulation pulses. The application of appropriate control signals causes the output circuit  72  to deliver the energy received from the capacitors  62  in the form of stimulation pulses. The energy is delivered to the patient  12  attached to the SIMD  14  over a set of electrodes that may be selected from one or more of the electrodes on the lead(s) (e.g.,  32 ,  34 ), and/or the can/housing electrode. The control circuit  60  may verify the integrity of the output circuit  72  before and during the transfer of the stimulation pulse. In accordance with aspects of the present application, some of the components of SIMD  14  are disclosed in  FIGS. 4-5 . 
         [0041]    To avoid unnecessarily obscuring the inventive aspects of the disclosure, it should be understood that numerous other components of SIMD  14  have not been shown. Examples of such additional components and/or circuit configurations for the operational circuitry employed in SIMD  14  can take any of the known forms that detect a tachyarrhythmia from the sensed ECG and provide cardioversion/defibrillation shocks as well as post-shock pacing as needed while the heart recovers. Such components include circuitry for powering and controlling various sensing and therapy delivery functions. An exemplary simplified block diagram of such circuitry adapted to function employing the sensing and therapy delivery electrodes described herein is set forth in U.S. Pat. No. 7,647,095, “Method and Apparatus for Verifying a Determined Cardiac Event in a Medical Device Based on Detected Variation in Hemodynamic Status” to Bhunia and in U.S. Pat. No. 8,155,740, “Constant Current Pacing Apparatus and Pacing Method” to Wanasek, which are both incorporated herein by reference in their entirety. It will be understood that the simplified block diagram does not show all of the conventional components and circuitry of such devices 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 SIMD  14  and an external programmer. 
         [0042]      FIG. 4  illustrates an exemplary schematic showing a portion of the operational circuitry of SIMD  14  in accordance with an embodiment of the disclosure. The output circuit  72  allows the controlled delivery of energy from the energy storage capacitors  62  to the patient  12 . The energy may be delivered to provide one or more functions of the SIMD  14  such as therapy delivery, or threshold testing for pacing or defibrillation therapies. Collectively, the energy delivered for various therapies (such as pacing, defibrillation) and threshold testing is performed in accordance with a treatment regimen that may be pre-programmed into SIMD  14  (such as the aforementioned processor/memory) and updated during the operational life of the SIMD  14 , as is known in the art. 
         [0043]    The output circuit  72  includes four switches  80 ,  82 ,  84 , and  86  that are interconnected. As is shown in  FIG. 4 , the switches  80 ,  82 ,  84 , and  86  are arrayed to define a configuration that is commonly referred to as a “H-bridge”. In other words, the four interconnected switches are arrayed having switches  80  and  84  that are connected to the high side and switches  82  and  86  that are connected to the low side. 
         [0044]    Output circuit  72  further includes opt-in circuit  76  and opt-in circuit  78  that are electrically coupled to the switches  80 - 86 . In one embodiment, the opt-in circuit  76  is connected in parallel with switch  82  such that a first end is connected at an intersection point of the interconnected terminals for switches  80  and  82  and a second end is connected to the other terminal of switch  82 . The opt-in circuit  78  is connected in parallel with switch  86  such that a first end is connected at an intersection point of the interconnected terminals for switches  84  and  86  and a second end is connected to the other terminal of switch  86 . As will be described in more detail below, the opt-in circuits  76 ,  78  include conducting devices that when configured in accordance with predetermined criteria will cause a stimulation pulse to bypass the switches  82  and  86  during one or more operations of the SIMD  14 . 
         [0045]    As shown in  FIG. 4 , the intersection of the switches  80 - 86  includes HV-A and HV-B terminals that couple the output circuit  72  to the therapy delivery and sense electrodes. The SIMD  14  delivers stimulation pulses to patient  12  typically through at least two electrodes (e.g., via one or more of electrodes  32 ,  34  ( FIGS. 1 ,  2 ), and/or the can electrode) that are coupled to terminal HV-A and terminal HV-B according to predetermined therapy or other treatment regimens. 
         [0046]    Switches  80  and  84  are coupled through a discharge switch  88  to a positive terminal of the energy storage capacitors  62 . Discharge switch  88  is controlled by control circuit  60  to be biased in a conducting (closed) state and remain in the conducting state during discharge of the capacitors  62 . Switches  82  and  86  are coupled to a negative terminal of the energy storage capacitors  62 . The selection of one or more of the switches  80 ,  82 ,  84 , and  86  under control of control circuit  60  may be used to provide one or more functions. For example, selection of certain switches in one or more configurations may be used to provide one or more types of stimulation pulses such as pacing, cardioversion, or defibrillation, or may be used for threshold testing, or may be used to remove DC polarization of the tissue-to-electrode interface, etc. 
         [0047]    In accordance with aspects of this disclosure, the switches  80 ,  82 ,  84 ,  86  are biased into one of a conducting or non-conducting state to deliver one or more stimulation pulses to effect various predetermined operations of the SIMD  14 . The switches  80 - 86  may be implemented as power semiconductor devices that may be operated as an electronic switch. Examples of power semiconductor devices include thyristors, silicon controlled rectifiers (SCRs), insulated-gate bipolar transistors (IGBTs), metal oxide semiconductor field effect transistors (MOSFETs such as N-channel or P-Channel MOSFETs, BiMOSFETs), either employed alone or in electrical series with high voltage thyristers or “triacs” or having a current blocking component such as a diode connected across the source and drain, a body diode, etc, associated therewith. The discharge switch  88  may be implemented as a power semiconductor device operated as an electronic switch (including for example an IGBT, or a BiMOSFET, or any of the other devices disclosed herein). As is known in the art, such semiconductor devices may be switched into conduction based on signals that are issued by control circuit  60  alone, or by dedicated drive circuits which respond to low voltage control signals, or a combination of both. 
         [0048]    Control circuit  60  issues control signals that cause the output circuit  72  to be configured in one of a plurality of configurations each of which is selected to provide one or more functions of the SIMD  14 . For example, in accordance with an embodiment, the SIMD  14  provides a biphasic therapy stimulation pulse to the patient in the following manner. With reference to  FIG. 4 , the opt-in circuits  76  and  78  are biased in a non-conducting (open) state during delivery of the therapy stimulation pulse. Once the energy storage capacitors  62  are charged to a selected energy level, the switches  80 ,  86 , and  88  are biased in a conducting state in order to provide a path from the capacitors  62  to electrodes (e.g.,  32 ,  34 ) for the application of a first phase of a therapy stimulation pulse to the patient  12 . The stored energy discharged by capacitors  62  is conducted from the positive terminal of the capacitors, through switch  80 , across the patient  12 , back through switch  86  to the negative terminal of the capacitors  62 . The first phase of the biphasic pulse therefore applies a positive pulse from the electrode  32  to the electrode  34 . 
         [0049]    Before the energy storage capacitors  62  are completely discharged, switches  80  and  86  are biased in a non-conducting state in preparation for application of the second phase of the biphasic pulse. For example, biasing the switches  80 - 86  in a non-conducting state may be achieved by opening switch  88  to shut off the current flow in accordance with one embodiment. In other embodiments, the control circuit  60  may issue signals to control the biasing of each of the switches. 
         [0050]    After the end of the first phase of the biphasic therapy stimulation pulse, the switches  88 ,  84  and  82  are biased in a conducting state to start the second phase of the biphasic pulse. Switches  84  and  82  provide a path to apply a negative therapy stimulation pulse to the patient  12 . With reference to  FIG. 4 , the energy travels from the positive terminal of the capacitors  62 , through switch  84 , across the electrodes  34 ,  32  coupled to the patient  12 , and out through switch  82  to the negative terminal of the capacitors  62 . The polarity of the second phase of the therapy stimulation pulse is therefore opposite in polarity to the first phase of the pulse. 
         [0051]    In another example, the SIMD  14  will perform threshold testing to determine optimum stimulation parameters for therapy delivery. SIMD  14  induces fibrillation of heart  12 , e.g., ventricular fibrillation, to test the effectiveness of the SIMD  14  in detecting and stopping the fibrillation. SIMD  14  induces fibrillation according to a fibrillation induction protocol included in the patient&#39;s treatment regimen, and is capable of employing a plurality of fibrillation protocols to induce fibrillation. Exemplary fibrillation induction protocols include delivery of an electrical pulse to heart  12  during the T-wave of a cardiac cycle, delivery of a high-frequency pulse train, and delivery of direct current. The disclosure is not limited to the exemplary induction protocols, and SIMD  14  can induce fibrillation according to any of a number of fibrillation induction protocols known in the art. SIMD  14  detects fibrillation employing one or more fibrillation detection techniques known in the art. SIMD  14  can be programmed to deliver a progression of therapies, e.g., pulses with increasing energy levels, until the detected fibrillation of heart  12  is stopped. 
         [0052]    Accordingly, techniques of the present invention utilize the output circuit  72  in a predetermined arrangement to deliver the stimulation pulse(s) for the therapy delivery (such as an induction stimulation pulse) in accordance with the patient&#39;s treatment regimen. In response to charging the energy storage capacitors  62  to a selected energy level, the opt-in circuit  78  and switches  80 ,  88  are biased in a conducting state in order to provide a path from the capacitors  62  to electrodes  32 ,  34  for the application of a first phase of a stimulation pulse to the patient  12 . Biasing of the opt-in circuit  78  will be described in more detail below. The stored energy travels from the positive terminal of the capacitors, through switch  80 , across the patient  12 , back through opt-in circuit  78  to the negative terminal of the capacitors  62 . Switches  82 ,  84 , and  86  are biased in a non-conducting state during the first phase. As such, the lower half of the H-bridge is bypassed in this first phase. 
         [0053]    After the end of the first phase of the stimulation pulse delivery, the opt-in circuit  76  and switches  88 ,  84  are biased in a conducting state for the second phase of the biphasic stimulation pulse. Biasing of the opt-in circuit  76  will be described in more detail below. Switch  84  and the opt-in circuit  76  provide a path to apply a negative stimulation pulse to the patient  12 . With reference to  FIG. 4 , the energy travels from the positive terminal of the capacitors  62 , through switch  84 , across the electrodes  34 ,  32  coupled to the patient  12  and back through opt-in circuit  76  to the negative terminal of the capacitors  62 . The polarity of the second phase of the stimulation pulse is therefore opposite in polarity to the stimulation pulse delivered in the first phase. 
         [0054]    Exemplary embodiments of the output circuit  72  of  FIG. 4  are shown in  FIG. 5 . As described in reference to  FIG. 4 , the four output switches  80 ,  82 ,  84 , and  86  allow the transfer of energy from the energy storage capacitors  62 . Switches  80 - 84  may be implemented as any of the power semiconductor devices mentioned above. In such an embodiment, the semiconductor devices may be controlled by the control circuit  60  from a conducting to a non-conducting state. The four output switches  80 ,  82 ,  84 , and  86  can be switched from a non-conducting to a conducting condition based on control signals provided by the control circuit  60 . 
         [0055]      FIG. 5  shows an exemplary schematic diagram of an output circuit  72 , such as functionally shown in  FIG. 4 . Table 1, below, shows the operation of the output circuit  72  of  FIG. 5  to provide various functions of SIMD  14 , with A and B representing the direction of current flow during a given function. Table 1 shows the biasing of the switches  76 - 88  of the output circuit  72  during delivery of therapy stimulation pulses denoted by the references “A-B Delivery,” “B-A Delivery,” “A-B Post Shock Pace,” “B-A Post Shock Pace” and the induction stimulation pulse denoted by the references “A-B Induction,” and “B-A Induction”. 
         [0000]    
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 A-B 
                 B-A 
                 A-B 
                 B-A 
                 A-B Post 
                 B-A Post 
               
               
                 Switch 
                 Delivery 
                 Delivery 
                 Induction 
                 Induction 
                 Shock Pace 
                 Shock Pace 
               
               
                   
               
             
             
               
                 88 
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                 80 
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                 84 
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                 82 
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                 86 
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                 76 
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                 78 
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         [0056]    Control circuit  60  controls the delivery of stimulation pulses to patient  12  coupled to the output circuit  72  by one or more electrodes  32 ,  34 , or the housing electrode. In the exemplary schematic diagram shown in  FIG. 5 , the output circuit  72  includes H-bridge circuit  104 , terminals HV-A and HV-B, and opt-in circuits  76 ,  78 . The control circuit  60  controls the charging circuit  64  ( FIG. 4 ) to generate the input signal for a stimulation pulse, e.g., pacing, defibrillation, or threshold testing, to patient  12  based on a selected one or more programs, which may be stored in memory. For example, control circuit  60  provides control output signals for control of the switches of the H-bridge  104 . Further, for example, control circuit  60  controls the generation of arbitrary input signals for generation of the stimulation pulses. As such, the control circuit  60 , alone or in conjunction with other circuit components such as the charging circuit  64  and a power source (not shown in  FIG. 5 ), may function as a stimulation pulse generator. Such components may correspond to conventionally known stimulation pulse generators such as that described in U.S. Pat. No. 8,340,762, issued to Vonk et al and incorporated herein by reference in its entirety. 
         [0057]    The H-bridge circuit  104  is connected between a high side  106  and a low side  108 . The H-bridge circuit  104  includes first and second legs  110 ,  112  connected between the high side  106  and low side  108  thereof. The first leg  110  of the H-bridge circuit  104  includes first and second switches  80 ,  82  and the second leg  112  of the H-bridge circuit  104  includes third and fourth switches  84 ,  86 . The first switch element  80  is connected towards the high side  106  and the second switch element  82  is connected towards the low side  108 . Further, the third switch element  84  is connected towards the high side  106  and the fourth switch element  86  is connected towards the low side  108 . 
         [0058]    The opt-in circuit  76  is connected to the first leg  110  while the opt-in circuit  78  is connected to the second leg  112 . Opt-in circuit  76  may include a resistor  120  that is connected in series with a power semiconductor device  122 , such for example, as the transistors discussed above. Opt-in circuit  78  may include a resistor  124  connected in series with a power semiconductor device  126 , such for example, as the transistors discussed above. The opt-in circuit  76  is connected in parallel with the switch element  82  while the opt-in circuit  78  is connected in parallel with the switch element  86 . In the example implementation illustrated in  FIG. 5 , the drain of the power semiconductor device  122  is connected at the node between the switches  80  and  82  while the source is connected to the low side  108  through resistor  120 . Similarly, the drain of the power semiconductor device  126  is connected at the node between the switches  84  and  86  while the source is connected to the low side  108  through resistor  124 . 
         [0059]    Each of the switches  80 - 84  may be implemented having substantially identical switch components. In one or more embodiments, any switch configuration may be used. However, the disclosure herein is not limited by this particular configuration and non-identical switches may be utilized in one or more different embodiments. The commands/operations for issuing control signals to control the opening and closing of the switches  80 - 86  may correspond to any techniques known in the art, such as software/firmware implementations, and will not be discussed in detail for the ease of description. Furthermore, one will recognize that, although FETs are used and described herein, one or more functions provided by such FETs may be implemented using other types of transistor devices (e.g., IGBTs) and that the present description is not limited to only the use of FETs or MOSFETs. 
         [0060]    A patient  12  is connectable to the first leg  110  of the H-bridge circuit  104  at a node located between the first and second switches  80 ,  82  (e.g., first node HV-A) and to the second leg  112  of the H-bridge circuit  104  at a node located between the third and fourth switches  84 ,  86  (e.g., second node HV-B). 
         [0061]    In operation, the output circuit  72  will utilize a plurality of configurations of the output circuit  72  to provide various functions of SIMD  14 . For example, the switches  80 - 86  may be configured in a first configuration selected from a plurality of configurations to deliver a pacing therapy. In a further example, a second configuration selected from a plurality of configurations may be utilized to deliver a defibrillation therapy. In another example, a third configuration selected from a plurality of configurations may be utilized to deliver a stimulation pulse for performing a threshold testing to optimize a given therapy such as a defibrillation therapy. In yet another example, the switches  80 - 86  may be manipulated for removal of the DC polarization. That is, an initial pulse for a delivered therapy may be a negative polarity pulse discharged through first and second selected switches of H-bridge circuit  104 , and thereafter, the DC polarization of the tissue electrode interface is removed by applying a pulse through third and fourth switches of H-bridge circuit  104 . 
         [0062]    Each of the plurality of configurations is implemented based on the biasing of the individual switches  80 - 86 . The biasing of the switches  80 - 86  is performed under control of control circuit  60 , which issues control signals to open and close the switches based on a predetermined program that may be stored in memory of the control circuit  60 . 
         [0063]    Control circuit  60  issues control signals that cause the output circuit  72  to be configured in one of a plurality of configurations, each of which is selected to provide one or more functions of the SIMD  14 . In accordance with one embodiment of this disclosure, one of those functions is performing threshold testing to determine optimum stimulation parameters for delivery of therapy, such as pacing or defibrillation. 
         [0064]    Although threshold testing has been described in conjunction with conventional implantable medical devices, the inventors have discovered that providing these waveforms using conventional techniques in a subcutaneous implantable device, such as SIMD  14 , poses various challenges. Among those conventional techniques are solutions that utilize the full H-bridge for the threshold testing, thereby requiring components that increase the complexity and current consumption of the circuitry. Moreover, utilizing the conventional techniques in the subcutaneous implantable devices requires higher energies and durations for inducing the arrhythmia, relative to the transvenous implantable devices. 
         [0065]    In accordance with one or more embodiments of the present disclosure, the threshold testing includes delivery of stimulation pulses to induce an arrhythmia in patient  12 . The threshold testing may be performed during portions of operation of the SIMD  14  when therapy is not being provided. Accordingly, the present disclosure provides output circuit  72  that includes the H-bridge  104  and opt-in circuits  76  and  78 . The output circuit  72  is selectively configured to provide the various functions of SIMD  14  such as delivery of stimulation pulses and/or sensing. In one or more configurations, the output circuit  72  is operated in a predefined configuration to deliver stimulation pulses for threshold testing to maximize the efficiency of the therapy delivery. 
         [0066]    In operation, the output circuit  72  is configured to utilize one switch element on a first half of the H-bridge circuit  104  to select the appropriate positive electrode for the phase of the delivered waveform and one of the power semiconductor devices of the opt-in circuits  76 ,  78  is used to select the negative electrode thereby bypassing the second half of the H-bridge circuit  104 . In this configuration, the combination of the series connected resistor and power semiconductor device of the selected one of the opt-in circuits  76 ,  78  provides current regulation for an induction waveform that is delivered to induce an arrhythmia in patient  12 . 
         [0067]    In an exemplary embodiment, the threshold testing may be performed by delivering a stimulation pulse in the form of a relatively low energy shock (commonly referred to as a T-shock) or one or more stimulation pulses to the cardiac tissue of patient  12  during the vulnerable period of the cardiac cycle. The one or more stimulation pulses may have a frequency about 10 Hz to about 100 Hz, and preferably with a frequency of about 25 Hz to about 85 Hz, and more preferably with a frequency of about 40 Hz to about 70 Hz, and even more preferably with a frequency of about 50 Hz to about 60 Hz, and most preferably with a frequency of about 50 Hz. The techniques for identifying the vulnerable period of the cardiac cycle are known and include that described in U.S. Pat. No. 8,064,996, issued to Belk et al and incorporated herein by reference in its entirety. The output circuit  72  is configured in one of several configurations to deliver the stimulation pulse(s) for the threshold testing at a period that is synchronized or concurrent or coincident with the identified vulnerable period of the cardiac cycle. The at least one stimulation pulse is generated from the energy stored in the energy storage capacitors  62  that are charged to a selected energy level. 
         [0068]    In a first configuration, the opt-in circuit  78  may be used in conjunction with switch  88  and switch  80  of the H-bridge circuit  104  for the threshold testing. The switches  88  and  80  are biased in a conducting state in order to provide a path from the capacitors  62  to electrodes  32 ,  34  for the application of a first phase of a stimulation pulse to the patient  12 . The stored energy travels from the positive terminal of the capacitors, through switches  88  and  80 , across the patient  12 , and back through opt-in circuit  78  to the negative terminal of the capacitors  62 . 
         [0069]    In this first configuration, the return current path defined through opt-in circuit  78  to the negative terminal on the lead  18  completes the conduction path for the induction stimulation pulse delivered to the patient  12  across the electrodes. The control circuit  60  issues a control signal to cause the power semiconductor device  126  to work in conjunction with the series-connected resistor  124  to provide current regulation for the induction waveform. The control signal issued by control circuit  60  causes the semiconductor device  126  to be operated in the linear region (also known as triode mode/ohmic mode). For example, in the case of a semiconductor device  126  implemented as a MOSFET transistor, the MOSFET will operate like a resistor, due to the control signal being configured to control the gate voltage relative to both the source and drain voltages. Switches  82 ,  84 , and  86  are biased in a non-conducting state during the first phase. As such, the lower half of the H-bridge is bypassed in this first phase. 
         [0070]    In another configuration, the opt-in circuit  76  may be used in conjunction with switch  88  and switch  84  of the H-bridge circuit  104  for the threshold testing. The opt-in circuit  76  and switches  88 ,  84  may be biased in a conducting state for a second phase of the stimulation pulse. As such, the delivery path extends from the positive terminal of the capacitors  62 , through switches  88  and  84 , across the electrodes  34 ,  32  coupled to the patient  12 , with the path being completed at the negative terminal of the capacitors  62  via opt-in circuit  76 . The polarity of the second phase of the stimulation pulse is therefore opposite in polarity to the stimulation pulse delivered in the first phase. 
         [0071]    In this configuration, the control circuit  60  issues signals to control operation of the semiconductor device  122  in the triode mode as described above. As such, the series-coupled semiconductor device  122  and resistor  120  will function to provide current regulation as described above with reference to semiconductor device  126  and resistor  124 . 
         [0072]    Alternative embodiments may provide for delivery of biphasic stimulation pulses. In such embodiments, the output circuit  72  may be configured to deliver a first phase of a stimulation pulse having a first polarity by using one of the opt-in circuits  78 ,  76  (in conjunction with the corresponding switch  80 ,  84  as described above), followed by delivery of a second phase of a stimulation pulse having a second polarity by using the other of the opt-in circuits  76 ,  78  (in conjunction with the corresponding switch  84 ,  80  as described above). 
         [0073]    In accordance with some embodiments of the present disclosure, the H-Bridge circuit  104  is configured such that only two switches of the H-Bridge  104  (e.g.,  80  and  84 ) are utilized in conjunction with the opt-in circuits  76 ,  78  for bi-phasic delivery of a stimulation pulse for threshold testing. In other embodiments, a pair of opt-in circuits (not shown) may be coupled in a similar manner as circuits  76 ,  78  to the switches  82  and  86  for bi-phasic delivery of a stimulation pulse for threshold testing. In embodiments where monophasic delivery is desired, then only one of the switches is utilized. Utilizing one of the opt-in circuits  76 ,  78  in the delivery of the threshold testing stimulation pulse to bypass the switches that is coupled in parallel with the selected opt-in circuit enables current regulation of the stimulation pulse that facilitates a reduction in the duration and/or energy levels required to generate an effective stimulation pulse. Nevertheless, having all four switches  80 - 86  allows a plurality of functions of the SIMD  14  to be performed, such as, therapy (pacing or defibrillation) stimulation pulse delivery and removal of the DC polarization. 
         [0074]      FIG. 6  is a diagram illustrating arbitrary input signals (e.g., selected input waveforms) and the resulting waveforms that are delivered using, for example, output circuit  72 , such as schematically shown in  FIG. 5 . As used herein, the term “arbitrary” input signal refers to the ability to select any shape of input (e.g., voltage waveform, static DC level input, shaped waveform, etc.) for use in generating a resulting current waveform (e.g., that generally follows the same shape (e.g., a ramped voltage input waveform being used to generate a ramped delivered current waveform, a static input used to deliver a proportional current, etc.)). 
         [0075]    A static input signal (e.g., a 1.5 V static input) delivered by capacitors  62  may be utilized to generate a proportional delivered waveform (e.g., measured across a 500 ohm load). For example, when the output circuit  72  is operational and a static input waveform  200  is applied, with the switches  80  and  86  being selected, a stimulus pulse or delivered waveform  202  having a first polarity (e.g., a positive polarity) is applied to the patient  12 . Further, for example, when the output circuit  72  is operational with the static input waveform  200  being applied, and the switches  82  and  84  are selected, a stimulus pulse or delivered waveform  204  having a second polarity (e.g., a negative polarity) is applied to the patient  12 . 
         [0076]    The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment. It should also be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the disclosure as set forth in the appended claims and the legal equivalents thereof.