Abstract:
In a cardioverter/defibrillator system, an electrical circuit includes an energy storage device, an output circuit for controlling delivery of pulse therapy from the energy storage device to a patient, and a sensing circuit coupled across the patient to sense the patient&#39;s heart signal. The output circuit may be in the form of an H-bridge switching circuit wherein a pair of switches of the output circuit are simultaneously turned on to discharge residual voltage across the patient that remains after delivery of pulse therapy. Thus, interference with sensing of the patient&#39;s heart signal is avoided.

Description:
RELATED APPLICATIONS  
       [0001]    The present application may find use in systems such as are disclosed in the U.S. patent application entitled “SUBCUTANEOUS ONLY IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER,” having Ser. No. 09/663,607, filed Sep. 18, 2000, pending, and U.S. patent application entitled “UNITARY SUBCUTANEOUS ONLY IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER,” having Ser. No. 09/663,606, filed Sep. 18, 2000, pending, of which both applications are assigned to the assignee of the present application, and the disclosures of both applications are hereby incorporated by reference.  
         [0002]    Applications related to the foregoing applications include a U.S. patent application entitled “DUCKBILL-SHAPED IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR AND METHOD OF USE,” U.S. patent application entitled “CERAMICS AND/OR OTHER MATERIAL INSULATED SHELL FOR ACTIVE AND NON-ACTIVE S-ICD CAN,” U.S. patent application entitled “SUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTION WITH IMPROVED INSTALLATION CHARACTERISTICS,” U.S. patent application entitled “SUBCUTANEOUS ELECTRODE WITH IMPROVED CONTACT SHAPE FOR TRANSTHORACIC CONDUCTION,” U.S. patent application entitled “SUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTION WITH HIGHLY MANEUVERABLE INSERTION TOOL,” U.S. patent application entitled “SUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTION WITH LOW-PROFILE INSTALLATION APPENDAGE AND METHOD OF DOING SAME,” U.S. patent application entitled “SUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTION WITH INSERTION TOOL,” U.S. patent application entitled “METHOD OF INSERTION AND IMPLANTATION FOR IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR CANISTERS,” U.S. patent application entitled “CANISTER DESIGNS FOR IMPLANTABLE CARDIOVERTER-DEFIBRILLATORS,” U.S. patent application entitled “RADIAN CURVED IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR CANISTER,” U.S. patent application entitled “CARDIOVERTER-DEFIBRILLATOR HAVING A FOCUSED SHOCKING AREA AND ORIENTATION THEREOF,” U.S. patent application entitled “BIPHASIC WAVEFORM FOR ANTI-BRADYCARDIA PACING FOR A SUBCUTANEOUS IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR,” and U.S. patent application entitled “BIPHASIC WAVEFORM FOR ANTI-TACHYCARDIA PACING FOR A SUBCUTANEOUS IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR,” the disclosures of which applications are hereby incorporated by reference.  
     
    
     
       FIELD OF THE INVENTION  
         [0003]    The present invention relates generally to defibrillation/cardioversion systems, and more particularly, to a defibrillation/cardioversion system having an H-bridge with a sensing circuit used in pacing and shocking the heart.  
         BACKGROUND OF THE INVENTION  
         [0004]    Defibrillation/cardioversion is a technique employed to counter arrhythmic heart conditions including some tachycardias in the atria and/or ventricles. Typically, electrodes are employed to stimulate the heart with electrical impulses or shocks, of a magnitude substantially greater than pulses used in cardiac pacing.  
           [0005]    Defibrillation/cardioversion systems include body implantable electrodes that are connected to a hermetically sealed container housing the electronics, battery supply and capacitors. The entire system is referred to as implantable cardioverter/defibrillators (ICDs). The electrodes used in ICDs can be in the form of patches applied directly to epicardial tissue, or, more commonly, are on the distal regions of small cylindrical insulated catheters that typically enter the subclavian venous system, pass through the superior vena cava and, into one or more endocardial areas of the heart. Such electrode systems are called intravascular or transvenous electrodes. U.S. Pat. Nos. 4,603,705, 4,693,253, 4,944,300, 5,105,810, the disclosures of which are all incorporated herein by reference, disclose intravascular or transvenous electrodes, employed either alone, in combination with other intravascular or transvenous electrodes, or in combination with an epicardial patch or subcutaneous electrodes. Compliant epicardial defibrillator electrodes are disclosed in U.S. Pat. Nos. 4,567,900 and 5,618,287, the disclosures of which are incorporated herein by reference. A sensing epicardial electrode configuration is disclosed in U.S. Pat. No. 5,476,503, the disclosure of which is incorporated herein by reference.  
           [0006]    In addition to epicardial and transvenous electrodes, subcutaneous electrode systems have also been developed. For example, U.S. Pat. Nos. 5,342,407 and 5,603,732, the disclosures of which are incorporated herein by reference, teach the use of a pulse monitor/generator surgically implanted into the abdomen and subcutaneous electrodes implanted in the thorax. This system is far more complicated to use than current ICD systems using transvenous lead systems together with an active can electrode and therefore it has no practical use. It has in fact never been used because of the surgical difficulty of applying such a device (3 incisions), the impractical abdominal location of the generator and the electrically poor sensing and defibrillation aspects of such a system.  
           [0007]    Recent efforts to improve the efficiency of ICDs have led manufacturers to produce ICDs which are small enough to be implanted in the pectoral region. In addition, advances in circuit design have enabled the housing of the ICD to form a subcutaneous electrode. Some examples of ICDs in which the housing of the ICD serves as an optional additional electrode are described in U.S. Pat. Nos. 5,133,353, 5,261,400, 5,620,477, and 5,658,321 the disclosures of which are incorporated herein by reference.  
           [0008]    ICDs are now an established therapy for the management of life threatening cardiac rhythm disorders, primarily ventricular fibrillation (V-Fib). ICDs are very effective at treating V-Fib, but are therapies that still require significant surgery.  
           [0009]    As ICD therapy becomes more prophylactic in nature and used in progressively less ill individuals, especially children at risk of cardiac arrest, the requirement of ICD therapy to use intravenous catheters and transvenous leads is an impediment to very long term management as most individuals will begin to develop complications related to lead system malfunction sometime in the 5-10 year time frame, often earlier. In addition, chronic transvenous lead systems, their reimplantation and removals, can damage major cardiovascular venous systems and the tricuspid valve, as well as result in life threatening perforations of the great vessels and heart. Consequently, use of transvenous lead systems, despite their many advantages, are not without their chronic patient management limitations in those with life expectancies of &gt;5 years. The problem of lead complications is even greater in children where body growth can substantially alter transvenous lead function and lead to additional cardiovascular problems and revisions. Moreover, transvenous ICD systems also increase cost and require specialized interventional rooms and equipment as well as special skill for insertion. These systems are typically implanted by cardiac electrophysiologists who have had a great deal of extra training.  
           [0010]    In addition to the background related to ICD therapy, the present invention requires a brief understanding of a related therapy, the automatic external defibrillator (AED). AEDs employ the use of cutaneous patch electrodes, rather than implantable lead systems, to effect defibrillation under the direction of a bystander user who treats the patient suffering from V-Fib with a portable device containing the necessary electronics and power supply that allows defibrillation. AEDs can be nearly as effective as an ICD for defibrillation if applied to the victim of ventricular fibrillation promptly, i.e., within 2 to 3 minutes of the onset of the ventricular fibrillation.  
           [0011]    AED therapy has great appeal as a tool for diminishing the risk of death in public venues such as in air flight. However, an AED must be used by another individual, not the person suffering from the potential fatal rhythm. It is more of a public health tool than a patient-specific tool like an ICD. Because &gt;75% of cardiac arrests occur in the home, and over half occur in the bedroom, patients at risk of cardiac arrest are often alone or asleep and can not be helped in time with an AED. Moreover, its success depends to a reasonable degree on an acceptable level of skill and calm by the bystander user.  
           [0012]    What is needed therefore, especially for children and for prophylactic long term use for those at risk of cardiac arrest, is a combination of the two forms of therapy which would provide prompt and near-certain defibrillation, like an ICD, but without the long-term adverse sequelae of a transvenous lead system while simultaneously using most of the simpler and lower cost technology of an AED. What is also needed is a cardioverter/defibrillator that is of simple design and can be comfortably implanted in a patient for many years.  
           [0013]    Typically, ICDs generate an electrical shock by charging a capacitance system to a high voltage from a low voltage power source and oscillator circuit. Then, the power source is switched out of the circuit and the electrical charge stored in the capacitance system is discharged through electrodes implanted in a patient.  
           [0014]    Typical discharge waveforms used with ICDs include monophasic, biphasic or multiphasic waveforms delivered as capacitance discharges. A monophasic waveform is comprised of a single monotonically decaying electrical pulse typically truncated before complete discharging of the capacitance system.  
           [0015]    Biphasic waveforms are comprised of a decaying electrical pulse having a pair of decaying electrical phases of opposite polarity. To generate a biphasic pulse, an H-bridge switch circuit is used, which is connected to the implanted electrodes. The H-bridge switches the polarity of the two phases. In generating the biphasic pulse, a first phase is discharged from the capacitance system, similar to a monophasic pulse. When the first pulse is truncated, the H-bridge switch circuit immediately reverses the discharge polarity of the capacitance system as seen by the electrodes to generate the second phase of the biphasic waveform being of opposite polarity.  
           [0016]    An H-bridge may be used in defibrillators that deliver high voltage electrical pulses, or shock, and also lower energy pacing pulses to a patient. After a shock or pacing energy is delivered to a patient, normally there is residual voltage on implanted electrodes on the patient such that the sensing ability of those electrodes is reduced, thus hindering the observation of a heart signal through an electrocardiogram.  
           [0017]    What is needed, therefore, is a defibrillator with an H-bridge switch circuit such that residual voltage is dissipated from electrodes after a shock or pacing energy is delivered to a patient so that sensing activity is not affected.  
         SUMMARY OF THE INVENTION  
         [0018]    An electrical circuit for a cardioverter-defibrillation system includes an energy storage device such as a capacitor, an output circuit for controlling delivery of defibrillation pulses from the energy storage device to a patient, and a sensing circuit coupled across the patient to sense the patient&#39;s heart signal. The output circuit may be in the form of an H-bridge switching circuit wherein a pair of switches of the output circuit are simultaneously turned on to discharge residual voltage across the patient that remains after delivery of defibrillation pulses. Thus, interference with sensing of the patient&#39;s heart signal is avoided. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    For a better understanding of the invention, reference is now made to the drawings where like numerals represent similar objects throughout the figures where:  
         [0020]    [0020]FIG. 1 is a schematic diagram of a typical ICD circuit including an H-bridge output circuit; and  
         [0021]    [0021]FIG. 2 is a schematic diagram of an H-bridge with sensing circuitry according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    Referring first to FIG. 1, a schematic diagram of a typical ICD circuit including an H-bridge output circuit is illustrated. Circuit  10  includes a battery power source  12 ; a double secondary fly back transformer  15 ; a transistor switch  14 ; rectifying diodes  16 ,  18 ; high voltage storage capacitors  20 ,  22 ; circuit control  50 ; an output circuit  30  having four legs arranged in the form of an “H” (an “H-bridge  30 ”), each leg of the H-bridge  30  having switches  32 ,  34 ,  36 , and  38 , respectively; and cardiac electrodes  40 ,  42 .  
         [0023]    The H-bridge  30  is connected to cardiac electrodes  40 ,  42 , and is used to generate a biphasic pulse. The H-bridge  30  switches the polarity of the two phases. A first phase is discharged from the high voltage storage capacitors  20 ,  22  by activating switches  32  and  38 . Then the first phase is truncated, and the H-bridge  30  activates switches  36  and  34 , and reverses the discharge polarity of the high voltage storage capacitors  20 ,  22  from the point of view of the cardiac electrodes  40 ,  42 , to generate the second phase of the waveform with opposite polarity.  
         [0024]    Referring now to FIG. 2, a schematic diagram of an H-bridge with sensing circuitry according to an embodiment of the present invention is illustrated. An energy storage capacitor  62  is connected to an H-bridge  60 . A sensing circuit  80  is connected across a patient at nodes  78  and  79  of the H-bridge  60 .  
         [0025]    It should be appreciated that a variety of H-bridge output circuits such as the one described with respect to FIG. 1 may be used within the scope of the present invention. Furthermore, it should be noted that additional semiconductor switches may be incorporated in each leg of the H-bridge to reduce the voltage that must be switched by each switch.  
         [0026]    Although FIG. 2 shows a single energy storage capacitor  62 , it is well-understood in the art that a bank of capacitors may be used, or any other energy storage device. The energy storage capacitor  62  can be charged to a range of voltage levels, with the selected level depending on the patient and other parameters. The typical maximum voltage necessary for ICDs using most biphasic waveforms is approximately 750 Volts with an associated maximum energy of approximately 41 Joules. For subcutaneous ICDs, the maximum voltages used may be in the range of about 50 to about 3150 Volts and are associated with energies of about 0.5 to about 350 Joules. The energy storage capacitor  62  may be controlled to deliver either defibrillation or pacing energy, and could range from about 25 to about 200 micro farads for a subcutaneous ICD.  
         [0027]    After charging to a desired level, the energy stored in capacitor  62  may be delivered to the patient in the form of a defibrillation pulse or pacing energy. H-bridge  60  is provided as an output circuit to allow the controlled transfer of energy from the energy storage capacitor  62  to the patient.  
         [0028]    Each leg of the H-bridge  60  contains a solid-state switch  64 ,  66 ,  68 , and  70 . Switches  64 ,  66 ,  68 , and  70  may be silicon controlled rectifiers (SCRs), insulated gate bipolar transistors (IGBTs), or MOSFETs. H-bridge  60  further includes electrodes  74  and  76  coupled to a patient.  
         [0029]    Switches  64  and  68  are coupled to the positive lead of the energy storage capacitor  62  via bridge line  65 . It should be noted that a protective circuit (not shown) with inductive and resistive properties may be added, for example, at bridge line  65  between the positive lead of the capacitor  62  and the switch  64  to limit current and voltage changes from the storage capacitor  62  during a defibrillation pulse. Switches  66  and  70  are coupled to the negative lead of the energy storage capacitor  62  via a bridge line  67 . The patient is connected to the left side of the H-bridge by a line  63  and to the right side of the H-bridge by a line  69 . Line  63  is connected to electrode  76  and line  69  is connected to electrode  74 .  
         [0030]    By selectively switching on pairs of switches in the H-bridge, a biphasic defibrillation pulse may be applied to the patient. Embodiments of the present invention may also use monophasic or multiphasic defibrillation pulses. The switches in the H-bridge are biased with a voltage that allows them to remain turned-on even when conducting low current.  
         [0031]    When the energy storage capacitor  62  is charged to a selected energy level, the switches  64  and  70  may be turned on to connect the energy storage capacitor  62  with lines  63  and  69  for the application of a first phase of a defibrillation pulse to the patient. The stored energy travels from the positive terminal of the energy storage capacitor  62  on line  65 , through switch  64  and line  63 , across the patient, and back through line  69  and switch  70  to the negative terminal of the capacitor. The first phase of the biphasic pulse is therefore a positive pulse. Before the energy storage capacitor  62  is completely discharged, the switch  70  is biased off to prepare for the application of the second phase of the biphasic pulse. Once the switch  70  is biased off, switch  64  will also become non-conductive because the voltage falls to zero.  
         [0032]    After the end of the first phase of the biphasic defibrillation pulse, switches  68  and  66  are switched on to start the second phase of the biphasic pulse. Switches  68  and  66  provide a path to apply a negative defibrillation pulse to the patient. The energy travels from the positive terminal of the energy storage capacitor  62  on line  65 , through switch  68  and line  69 , across the patient, and back through line  63  and switch  66  to the negative terminal of the energy storage capacitor. The polarity of the second phase of the defibrillation pulse is therefore opposite in polarity to the first phase of the biphasic pulse. The end of the second phase of the biphasic pulse may be truncated by switching on switch  64  to provide a shorted path for the remainder of the capacitor energy through switches  64  and  66 . Digital logic (not shown) may be used to control the sequencing of the switches  64 ,  66 ,  68 , and  70  such that the polarity can be inverted so that the first phase is negative instead of positive. The digital logic generally controls the timing, the duration of each phase and the inter phase delay.  
         [0033]    Sensing circuit  80  is connected to H-bridge  60  across the patient at nodes  78  and  79 . Sensing circuit  80  includes a sense amplifier  96  that senses differentially and is capacitively coupled across the patient. The sense amplifier  96  has a negative lead connected to node  79  in the H-bridge  60  through a capacitor  82 . A resistor  84  is connected to capacitor  82  between ground and node  81  in a high-pass filter of approximately 0.5-20 Hz. Resistor  84  may range in value between approximately 10 KΩ and 500 KΩ. A resistor  92  is connected between node  81  and node  103 . A capacitor  94  and a resistor  102  are connected in parallel at node  103  as a low pass filter of approximately 30-150 Hz. It should be appreciated that there could be multiple low pass filters as well as multiple high pass filters connected to the negative lead of the sense amplifier  96 .  
         [0034]    The sense amplifier  96  has a positive lead connected to node  78  via a capacitor  86 . A resistor  88  is connected to capacitor  86  between ground and node  87  in a high-pass filter of approximately 0.5-20 Hz. A resistor  91  is connected between node  87  and node  99 . A capacitor  100  and a resistor  98  are connected in parallel at node  99  as a low pass filter of approximately 30-150 Hz. It should be appreciated that there could be multiple low pass filters as well as multiple high pass filters connected to the positive lead of the sense amplifier  96 . Furthermore, an embodiment of the sensing circuit may comprise digital logic for overall control of the sensing circuit.  
         [0035]    The sensing circuit  80  allows constant observation of heart signals as an electrocardiogram. When it is time to deliver therapy, a shock or pacing energy is delivered as required. Switches  64 ,  70 ,  68 , and  66  of the H-bridge  60  are sequenced to deliver monophasic, biphasic, or multiphasic pulses. During shock or even during pacing, as soon as the therapy pulse is completed, there may be a residual voltage that remains on electrodes  74  and  76  as they are not simply resistors. Capacitances may be involved in the patient such that after a pacing pulse or defibrillation shock there are residual voltages. The residual voltages could, when present, limit the time that it takes for the differential sensing amplifier  96  to recover and allow proper continuing observation of the heart signal and determine whether the heart has returned to a normal rhythm or whether there is still an arrhythmia. Thus, the amplifier needs to recover as soon as possible, for example, in much less than a second, and the voltages have to be within the common mode operating range of the amplifier as soon as possible.  
         [0036]    To improve the post-shock or post-pacing recovery time on the amplifiers, switches  66  and  70  of the H-bridge  60  are turned on at the same time to discharge any residual voltage across the patient. By turning on or closing both switches  66  and  70 , the voltage across the electrodes  76  and  74  is effectively shorted out and the residual voltage across the patient is removed. If there are any capacitances involved in series or in parallel with the patient, all that energy is dissipated.  
         [0037]    After a monophasic, biphasic or multiphasic pacing pulse, or a shock is delivered, switches  66  and  70  are closed sometime after the end of the pulse, for example, after approximately 50 microseconds to 10 milliseconds, for a period of approximately 10 microseconds to up to approximately a second. This will dissipate the residual voltage across the patient, and improve the recovery time of the sense amplifier. Embodiments of the present invention allows the sensing to be done from the H-bridge. To dissipate energy, additional external switches may be used, however, using the switches of the H-bridge itself saves the complexity of using external switches.  
         [0038]    Numerous characteristics and advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many aspects, only illustrative. Changes may be made in details, particularly in matters of shape, size and arrangement of parts without exceeding the scope of the invention. The invention&#39;s scope is defined in the language in which the appended claims are expressed.