WO2003039648A2 - Defibrillation pacing circuitry - Google Patents

Defibrillation pacing circuitry Download PDF

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Publication number
WO2003039648A2
WO2003039648A2 PCT/IB2002/004476 IB0204476W WO03039648A2 WO 2003039648 A2 WO2003039648 A2 WO 2003039648A2 IB 0204476 W IB0204476 W IB 0204476W WO 03039648 A2 WO03039648 A2 WO 03039648A2
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WO
WIPO (PCT)
Prior art keywords
circuit
resistance
constant voltage
voltage source
patient
Prior art date
Application number
PCT/IB2002/004476
Other languages
French (fr)
Other versions
WO2003039648A3 (en
Inventor
Alan H. Ostroff
Original Assignee
Cameron Health, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Cameron Health, Inc. filed Critical Cameron Health, Inc.
Priority to AU2002363390A priority Critical patent/AU2002363390A1/en
Publication of WO2003039648A2 publication Critical patent/WO2003039648A2/en
Publication of WO2003039648A3 publication Critical patent/WO2003039648A3/en

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Classifications

    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N1/00—Electrotherapy; Circuits therefor
    • A61N1/18—Applying electric currents by contact electrodes
    • A61N1/32—Applying electric currents by contact electrodes alternating or intermittent currents
    • A61N1/38—Applying electric currents by contact electrodes alternating or intermittent currents for producing shock effects
    • A61N1/39—Heart defibrillators
    • A61N1/3906—Heart defibrillators characterised by the form of the shockwave
    • A61N1/3912—Output circuitry therefor, e.g. switches
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N1/00—Electrotherapy; Circuits therefor
    • A61N1/18—Applying electric currents by contact electrodes
    • A61N1/32—Applying electric currents by contact electrodes alternating or intermittent currents
    • A61N1/36—Applying electric currents by contact electrodes alternating or intermittent currents for stimulation
    • A61N1/362—Heart stimulators
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N1/00—Electrotherapy; Circuits therefor
    • A61N1/18—Applying electric currents by contact electrodes
    • A61N1/32—Applying electric currents by contact electrodes alternating or intermittent currents
    • A61N1/38—Applying electric currents by contact electrodes alternating or intermittent currents for producing shock effects
    • A61N1/39—Heart defibrillators
    • A61N1/3956—Implantable devices for applying electric shocks to the heart, e.g. for cardioversion
    • A61N1/3962—Implantable devices for applying electric shocks to the heart, e.g. for cardioversion in combination with another heart therapy
    • A61N1/39622—Pacing therapy

Definitions

  • the present invention relates to apparatus and methods useful in connection with performing electrical cardioversion/defib ⁇ llation and optional pacing of the heart
  • Defib ⁇ llation/cardioversion is a technique employed to counter arrhythmic heart conditions including some tachycardias in the atria and/or ventricles.
  • electrodes are employed to stimulate the heart with electrical impulses or shocks, of a magnitude substantially greater than pulses used in cardiac pacing
  • Defibnllation/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/defib ⁇ llators (ICDs).
  • ICDs implantable cardioverter/defib ⁇ llators
  • 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 supe ⁇ or vena cava and, into one or more endocardial areas of the heart
  • Such electrode systems are called mtravascular 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 mtravascular or transvenous electrodes, employed either alone, in combination with other mtravascular or transvenous electrodes, or in combination with an epicardial patch or subcutaneous electrodes.
  • Compliant epicardial defib ⁇ llator 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.
  • V-Fib p ⁇ ma ⁇ ly vent ⁇ cular fibrillation
  • transvenous ICD systems also increase cost and require specialized lnterventional 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.
  • the automatic external defib ⁇ llator (AED) AEDs employ the use of cutaneous patch electrodes, rather than implantable lead systems, to effect defib ⁇ llation 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 defib ⁇ llation AEDs can be nearly as effective as an ICD for defib ⁇ llation if applied to the victim of vent ⁇ cular fib ⁇ llation promptly, l e , within 2 to 3 minutes of the onset of the vent ⁇ cular fib ⁇ llation AED therapy has great appeal as a tool for diminishing the ⁇ sk of death in public venues such as in air flight
  • an AED must be used by another individual, not the person suffe ⁇ ng from the potential fatal rhythm It is more of a public health tool than a patient-specific tool like an ICD Because >75% of cardiac arrests occur
  • the invention relates in various aspects to methods and apparatus for selectively converting a defib ⁇ llator circuit or circuit for delive ⁇ ng a defib ⁇ llating pulse to a patient into circuitry suitable for providing a constant current, useful, e g , in pacing applications.
  • Fig 1 is a schematic view of a conventional defib ⁇ llator circuit
  • Fig 2 is a circuit schematic of an illustrative embodiment of the present invention
  • Fig 3 is a circuit schematic of an alternate embodiment
  • Fig 4 is a circuit schematic of a second alternate embodiment
  • Fig 5 is a circuit schematic illustrating one approach to controlling switching in the embodiment of Fig 4 Detailed Description of Illustrative Embodiments
  • Fig 1 illustrates a conventional "H-b ⁇ dge" defib ⁇ llator circuit 11
  • the circuit 11 includes a capacitor Ci which is charged to a high voltage and four switches H 1; H 2 , L,, L 2 .
  • the capacitor CI and switches H,, H 2 , L L 2 are used to create either a monophasic voltage pulse or a biphasic voltage pulse (Fig 2) across a patient represented by resistance R PA ⁇
  • the switches H b H 2 , L b L 2 may be MOSFETs, IGBTs, or SCRs (silicon controlled rectifiers)
  • a first pair of switches, e g a first pair of switches, e g .
  • Hi and L 2 may be closed to create a first positive pulse 13 Then all of the switches, Hi, H 2 , L,, L 2 , are turned off du ⁇ ng a "center pulse" delay pe ⁇ od d, At the end of the delay pe ⁇ od di, the switches H 2 and Li are both closed, thereby reversing the current through the patient R PA ⁇ to produce the negative voltage pulse 17
  • digital logic is employed to control the sequencing of the switches H H 2 , L,, L 2
  • the order of the pulses can be inverted, l e
  • the negative going pulse 17 can be produced before the positive going pulse 13
  • the direction of the pulses 13, 17 is, e g , 1 to 20 millisecond and the inter-pulse delay dj is, e g , one millisecond
  • Fig 3 illustrates circuitry which may operate as a defib ⁇ llator circuit du ⁇ ng a first selected interval and as a constant current source du ⁇ ng a second selected interval
  • the constant current may be useful, for example, in providing a "pacing" current to a patient R PAT
  • the high side switches Hi , H 2 employed in Fig 3 may be IGBTs, MOSFETs,
  • Such high side switches Hi, H 2 may be controlled in any suitable manner such as, for example, with pulse transformers, opto-couplers, photo-voltaic generators, or in accordance with the teachings of the application filed on even date herewith on behalf of the same inventor and entitled Simplified Defib ⁇ llator Output Circuit, herein incorporated by reference
  • Digital logic suitable for controlling such circuitry to achieve switching may comprise a programmed microprocessor, microcontroller, discrete logic, or other forms of digital logic control
  • a resistor Ri is inserted in se ⁇ es with the emitter or the source leg of a first low side transistor Q 2 , which is preferably an IGBT or MOSFET Similarly, a resistor
  • R 2 is inserted in series with the emitter or source leg of the second low side transistor Qi A constant voltage is applied across the resistor Ri via a voltage source Si, which applies a voltage
  • the transistors Qi, Q 2 serve the purposes of low side switches, e g , L b L 2 of Fig 1, and the resistors R b R 2 are switched out of the circuit by suitable means, e g , switches
  • the low side transistors Qi, Q 2 may be high voltage IGBT or MOSFETs, ranging from 500 volts to 3,000 volts capacity or greater
  • the threshold voltage V ⁇ of the transistor Q may vary from device to device. Hence, it is typically necessary to calibrate the circuit in production. In calibrating a circuit like that of Fig 2, a known voltage is applied and the current through Ri is measured, typically resulting in a large offset, which is compensated for by the system software
  • the voltage source Si may be constructed using a feedback circuit employing an operational amplifier A, as shown in Fig 4.
  • the op-amp Ai is connected to directly drive the low side transistor Q 2 , which may comp ⁇ se, e g., a MOSFET or IGBT.
  • Use of the operational amplifier A] removes the uncertainty of the threshold voltage V ⁇ so that the current that passes through the resistor Ri is equal to simply V DR] E divided by R .
  • the capacitor voltage V c needs to be appropriately selected according to a number of considerations.
  • the cu ⁇ ent that is programmed to go through the patient will generate a voltage V PAT across the patient.
  • the voltage compliance V C O MP of the current source must be appropriately set
  • the voltage compliance V CO MP IS the voltage V Rt across the resistor Rj plus the minimum operating voltage V ⁇ of the low side transistor Q2. Accordingly, the minimum voltage V HV across the capacitor to current source is defined by the relation-
  • V HV (mm ) V PAT + V CO MP (2)
  • V HV voltage
  • V DRIV E voltage
  • resistors e g , m series with or for Ri
  • Suitable logic circuitry may be provided to select the value of V DRIVE
  • a DAC digital to analog converter
  • a DAC is a circuit that generates different voltages in response to corresponding digital codes provided to it. Such a DAC could be used to d ⁇ ve either an input of the op-amp A, (as illustrated in Fig. 4) or the input (gate) of the transistor Qi As noted above, an advantage of the op-amp Ai is that it removes the V ⁇ term from the VH V equation. Particular parameter ranges for circuitry as configured in Figs. 2 and 3 include 1 to 50 ohms for the resistance R] and 1 to 20 volts for a V DR I V E resulting in a current ranging from 0 to 500 mil amps.
  • FIG. 5 Another illustrative circuit for implementing a cu ⁇ ent source is illustrated in Fig 5.
  • This circuit employs a resistor R 3 connected between the high voltage capacitor Ci and the high side switches Hi, H 3 The resistor R 3 is switched out of the circuit by a switch SW 3 for defib ⁇ llator operation and into the circuit for pacing.
  • the circuit of Fig. 5 is somewhat more energy wasteful but will work with the use of a high voltage switch for SW 3
  • the switches Hi, H 2 ; Li , L 2 are manipulated so as to place the resistor R 3 in se ⁇ es with the output
  • the amount of current may then be selected by the voltage to which the capacitor Q is charged.
  • the patient resistance R PAT vanes from 30-150 ohms
  • selecting a resistor R 3 of anywhere from 500-5000 ohms, l e., a resistance that is much larger than that of the patient results in an approximation of an ideal cu ⁇ ent source
  • a high voltage switch SW 3 is connected across the se ⁇ es resistor R 3 to switch R 3 out of the circuit in order to enter the defib ⁇ llation mode Since the high voltage switch SW 3 is a floating switch, a high side driver 19 is also needed. These considerations render the circuit of Fig. 6 more difficult to implement m an implantable device.
  • the circuits of Figs. 3 and 4 require a switch, e.g , SW, to switch to the defib ⁇ llation mode, but the switch SW, does not have to be a high voltage switch Instead, the switch SW1 need only be a smaller, low voltage device having the capacity to pass the defib ⁇ llation current. In an illustrative circuit, there may be on the order of only 10 volts across SWi, which is advantageous
  • the voltage V DRIV E IS preferably implemented by a DAC, either connected to directly d ⁇ ve the resistor Ri (Fig. 3) or to dnve the resistor R 3 through an op-amp A, (Fig. 4).
  • Provision of a constant cu ⁇ ent has the advantage of maintaining a constant current density across the heart, l ⁇ espective of the electrode interface impedance.

Abstract

Electrical circuit componentry is switchable into a defibrillator circuit to deliver a constant pacing current to a patient. The circuitry may include a constant current source inserted in a leg of the defibrillator circuit or a resistor of selected value inserted between in high voltage source and the high side of a defibrillator circuit.

Description

Defibπllation Pacing Circuitry
Cross-Reference To Related Applications
The invention of the present application may find application m systems such as are disclosed in U.S. patent application entitled "SUBCUTANEOUS ONLY IMPLANTABLE
CARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER," having Seπal No. 09/663,607, filed September 18, 2000, pending, and U S patent application entitled "UNITARY SUBCUTANEOUS ONLY IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER," having Serial No. 09/663,606, filed September 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.
In addition, the foregoing applications are related to the 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 "BLPHASIC 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
The present invention relates to apparatus and methods useful in connection with performing electrical cardioversion/defibπllation and optional pacing of the heart
Background Of The Invention
Defibπllation/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
Defibnllation/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/defibπllators (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 supeπor vena cava and, into one or more endocardial areas of the heart Such electrode systems are called mtravascular 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 mtravascular or transvenous electrodes, employed either alone, in combination with other mtravascular or transvenous electrodes, or in combination with an epicardial patch or subcutaneous electrodes. Compliant epicardial defibπllator 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.
In addition to epicardial and transvenous electrodes, subcutaneous electrode systems have also been developed For example, U.S Patent 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 defibπllation aspects of such a system
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 m 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
ICDs are now an established therapy for the management of life threatening cardiac rhythm disorders, pπmaπly ventπcular fibrillation (V-Fib) ICDs are very effective at treating V-
Fib, but are therapies that still require significant surgery
As ICD therapy becomes more prophylactic in nature and used in progressively less ill individuals, especially children at πsk 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 tπcuspid 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 >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 lnterventional 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.
In addition to the background related to ICD therapy, the present invention requires a brief understanding of a related therapy, the automatic external defibπllator (AED) AEDs employ the use of cutaneous patch electrodes, rather than implantable lead systems, to effect defibπllation 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 defibπllation AEDs can be nearly as effective as an ICD for defibπllation if applied to the victim of ventπcular fibπllation promptly, l e , within 2 to 3 minutes of the onset of the ventπcular fibπllation AED therapy has great appeal as a tool for diminishing the πsk of death in public venues such as in air flight However, an AED must be used by another individual, not the person suffeπng from the potential fatal rhythm It is more of a public health tool than a patient-specific tool like an ICD Because >75% of cardiac arrests occur in the home, and over half occur m 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
What is needed therefore, especially for children and for prophylactic long term use for those at πsk of cardiac arrest, is a combination of the two forms of therapy which would provide prompt and near-certain defibπllation, 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/defibπllator that is of simple design and can be comfortably implanted in a patient for many years
Moreover, it has appeared advantageous to the inventor to provide the capability in such improved circuitry to provide a signal suitable for pacing when the circuitry is not operating in a defibπllation mode.
Summary Of The Invention
Accordingly, the invention relates in various aspects to methods and apparatus for selectively converting a defibπllator circuit or circuit for deliveπng a defibπllating pulse to a patient into circuitry suitable for providing a constant current, useful, e g , in pacing applications.
Brief Description Of The Drawings
For a better understanding of the invention, reference is now made to the drawings where like numerals represent similar objects throughout the figures and wherein Fig 1 is a schematic view of a conventional defibπllator circuit, Fig 2 is a circuit schematic of an illustrative embodiment of the present invention,
Fig 3 is a circuit schematic of an alternate embodiment, Fig 4 is a circuit schematic of a second alternate embodiment,
Fig 5 is a circuit schematic illustrating one approach to controlling switching in the embodiment of Fig 4 Detailed Description of Illustrative Embodiments
Fig 1 illustrates a conventional "H-bπdge" defibπllator circuit 11 The circuit 11 includes a capacitor Ci which is charged to a high voltage and four switches H1; H2, L,, L2. The capacitor CI and switches H,, H2, L L2 are used to create either a monophasic voltage pulse or a biphasic voltage pulse (Fig 2) across a patient represented by resistance RPAτ In vaπous applications, the switches Hb H2, Lb L2, may be MOSFETs, IGBTs, or SCRs (silicon controlled rectifiers) To create a biphasic waveform such as that shown in Fig 2, a first pair of switches, e g ,
Hi and L2, may be closed to create a first positive pulse 13 Then all of the switches, Hi, H2, L,, L2, are turned off duπng a "center pulse" delay peπod d, At the end of the delay peπod di, the switches H2 and Li are both closed, thereby reversing the current through the patient RPAτ to produce the negative voltage pulse 17 Typically, digital logic is employed to control the sequencing of the switches H H2, L,, L2 In such case, the order of the pulses can be inverted, l e , the negative going pulse 17 can be produced before the positive going pulse 13 In illustrative applications, the direction of the pulses 13, 17 is, e g , 1 to 20 millisecond and the inter-pulse delay dj is, e g , one millisecond
Fig 3 illustrates circuitry which may operate as a defibπllator circuit duπng a first selected interval and as a constant current source duπng a second selected interval The constant current may be useful, for example, in providing a "pacing" current to a patient RPAT
As m Fig 1 , the high side switches Hi , H2 employed in Fig 3 may be IGBTs, MOSFETs,
SCRs, or other suitable switches Such high side switches Hi, H2 may be controlled in any suitable manner such as, for example, with pulse transformers, opto-couplers, photo-voltaic generators, or in accordance with the teachings of the application filed on even date herewith on behalf of the same inventor and entitled Simplified Defibπllator Output Circuit, herein incorporated by reference Digital logic suitable for controlling such circuitry to achieve switching may comprise a programmed microprocessor, microcontroller, discrete logic, or other forms of digital logic control In the circuit of Fig 3, a resistor Ri is inserted in seπes with the emitter or the source leg of a first low side transistor Q2, which is preferably an IGBT or MOSFET Similarly, a resistor
R2 is inserted in series with the emitter or source leg of the second low side transistor Qi A constant voltage is applied across the resistor Ri via a voltage source Si, which applies a voltage
VDRIVE to the gate (or base) of the first low side transistor Q2 Duπng operation of the circuit of Fig 2 as a defibπllator, the transistors Qi, Q2 serve the purposes of low side switches, e g , Lb L2 of Fig 1, and the resistors Rb R2 are switched out of the circuit by suitable means, e g , switches
SW, and SW2 Duπng pacing operation of the circuit of Fig 3, a suitable switching signal Si is applied to switch resistor R] into the circuit In an illustrative application of the circuitry of Fig 3, the low side transistors Qi, Q2 may be high voltage IGBT or MOSFETs, ranging from 500 volts to 3,000 volts capacity or greater In the circuit of Fig 2, the voltage across the resistor Ri is defined by the equation: VRI = VQR VE - Vτ (1) where Vτ is the fixed (constant) threshold voltage of the low side transistor Qi Thus, if V R]VE IS
15 volts, and Vτ is in the range of 2-6 volts, VR] is in the range of 13 to 9 volts Accordingly, a constant voltage is applied across the resistor Rb resulting m a constant cuπent lj_ through the resistor Rj, and hence through the patient Rpat
As those skilled in the art may appreciate, the threshold voltage Vτ of the transistor Q, may vary from device to device. Hence, it is typically necessary to calibrate the circuit in production. In calibrating a circuit like that of Fig 2, a known voltage is applied and the current through Ri is measured, typically resulting in a large offset, which is compensated for by the system software
In order to avoid calibration, the voltage source Si may be constructed using a feedback circuit employing an operational amplifier A, as shown in Fig 4. The op-amp Ai is connected to directly drive the low side transistor Q2, which may compπse, e g., a MOSFET or IGBT. Use of the operational amplifier A] removes the uncertainty of the threshold voltage Vτ so that the current that passes through the resistor Ri is equal to simply VDR] E divided by R . Thus, one can either drive the transistor Q2 with a voltage source and calibrate the system for the Vτ of the transistor Q2 or use an op-amp circuit to remove the error created by the threshold voltage Vτ of the transistor Q2
During constant current source operation of the circuit of Fig. 3, the appropπate high side switch is on to permit current flow In addition, the capacitor voltage Vc needs to be appropriately selected according to a number of considerations. First, the cuπent that is programmed to go through the patient will generate a voltage VPAT across the patient. Then, in order to make the current source work, the voltage compliance VCOMP of the current source must be appropriately set In the case of Fig 3, the voltage compliance VCOMP IS the voltage VRt across the resistor Rj plus the minimum operating voltage Vτ of the low side transistor Q2. Accordingly, the minimum voltage VHV across the capacitor to current source is defined by the relation-
VHV (mm ) = VPAT + VCOMP (2)
The higher VHV IS above VHV (mm ), the closer the current source will approach an ideal current source Another consideration in setting VHV is power consumption The amount of current IR] can be vaπed by varying the voltage VDRIVE or by switching in different resistors, e g , m series with or for Ri From an implementation point of view, it is less attractive to switch in a resistor because such switching requires adding transistors or other switching devices It is more efficient to simply vary the voltage VDR]VE Suitable logic circuitry may be provided to select the value of VDRIVE A DAC (digital to analog converter) is one example of such logic circuitry. As those skilled in the art will appreciate, a DAC is a circuit that generates different voltages in response to corresponding digital codes provided to it. Such a DAC could be used to dπve either an input of the op-amp A, (as illustrated in Fig. 4) or the input (gate) of the transistor Qi As noted above, an advantage of the op-amp Ai is that it removes the Vτ term from the VHV equation. Particular parameter ranges for circuitry as configured in Figs. 2 and 3 include 1 to 50 ohms for the resistance R] and 1 to 20 volts for a VDRIVE resulting in a current ranging from 0 to 500 mil amps.
Another illustrative circuit for implementing a cuπent source is illustrated in Fig 5. This circuit employs a resistor R3 connected between the high voltage capacitor Ci and the high side switches Hi, H3 The resistor R3 is switched out of the circuit by a switch SW3 for defibπllator operation and into the circuit for pacing.
The circuit of Fig. 5 is somewhat more energy wasteful but will work with the use of a high voltage switch for SW3 In the circuit of Fig 5, the switches Hi, H2; Li, L2 are manipulated so as to place the resistor R3 in seπes with the output The amount of current may then be selected by the voltage to which the capacitor Q is charged. As an example, assuming the patient resistance RPAT vanes from 30-150 ohms, selecting a resistor R3 of anywhere from 500-5000 ohms, l e., a resistance that is much larger than that of the patient, results in an approximation of an ideal cuπent source The approximation is- i = VHv_ (3) R + RPAτ
While creation of a current source according to Fig. 5 is relatively easy, switching the circuit to the defibπllation mode is more complex. As shown in Fig 6, a high voltage switch SW3 is connected across the seπes resistor R3 to switch R3 out of the circuit in order to enter the defibπllation mode Since the high voltage switch SW3 is a floating switch, a high side driver 19 is also needed. These considerations render the circuit of Fig. 6 more difficult to implement m an implantable device.
In contrast, the circuits of Figs. 3 and 4 require a switch, e.g , SW, to switch to the defibπllation mode, but the switch SW, does not have to be a high voltage switch Instead, the switch SW1 need only be a smaller, low voltage device having the capacity to pass the defibπllation current. In an illustrative circuit, there may be on the order of only 10 volts across SWi, which is advantageous
Thus, only a low voltage switch need be used in the circuits of Figs. 3 and 4. No low voltage driver is necessary since the switch SWi is referenced to ground and can therefore be driven directly A high side dπver circuit is unnecessary In either of the circuits of Fig. 3 or
Fig 4, the voltage VDRIVE IS preferably implemented by a DAC, either connected to directly dπve the resistor Ri (Fig. 3) or to dnve the resistor R3 through an op-amp A, (Fig. 4).
Provision of a constant cuπent has the advantage of maintaining a constant current density across the heart, lπespective of the electrode interface impedance.
While the present invention has been descπbed above in terms of specific embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the following claims are intended to cover various modifications and equivalent methods and structures included within the spiπt and scope of the invention.

Claims

What is claimed is
1. The apparatus comprising an H-bπdge defibrillator circuit, and means switchable into and out of said circuit for causing a constant cuπent to flow through a patient connected to said circuit.
2. The apparatus of claim 1 wherein said means for causing comprises means for imposing a constant voltage across a resistance located in the low side of said circuit.
3. The apparatus of claim 2 wherein said means for imposing compπses a constant voltage source.
4. The apparatus of claim 2 wherein said voltage source applies a constant voltage to a control terminal of a switching device
5. The apparatus of claim 4 wherein said resistance is located in a leg of said switching device
6. The apparatus of claim 3 wherein said constant voltage source comprises a digital to analog converter (DAC)
7 The apparatus of claim 6 wherein said constant voltage source further comprises a feedback circuit.
8. The apparatus of claim 4 wherein said constant voltage source comprises a digital to analog converter (DAC).
9. The apparatus of claim 8 wherein said constant voltage source further compπses a feedback circuit.
10. The apparatus of claim 1 wherein said means for causing comprises a resistance connected between a high voltage source and a high side of said defibrillator circuit
11. The apparatus of claim 10 wherein the value of said resistance is selected to be much larger than the resistance of said patient
12 The apparatus of claim 10 further including a switch operable to switch said resistance in and out of said circuit
13. The apparatus of claim 12 further including a high side dπver circuit for activating said switch
14. The apparatus comprising circuit means for delivering a defibπllating cuπent pulse to a patient, and means switchable into and out of connection with said circuit means for causing a constant cuπent to flow through said patient during a peπod when a cuπent pulse is not being delivered
15. The apparatus of claim 14 wherein said means for causing compπses means for imposing a constant voltage across a resistance located in a low side of said circuit means
16. The apparatus of claim 15 wherein said means for imposing comprises a constant voltage source
17. The apparatus of claim 16 wherein said voltage source applies a constant voltage to a control terminal of a switching device
18. The apparatus of claim 17 wherein said resistance is located in a leg of said switching device
19. The apparatus of claim 16 wherein said constant voltage source compπses a digital to analog converter (DAC)
20 The apparatus of claim 19 wherein said constant voltage source further compnses a feedback circuit.
21. The apparatus of claim 17 wherein said constant voltage source comprises a digital to analog converter (DAC).
22. The apparatus of claim 21 wherein said constant voltage source further compπses a feedback circuit.
23. The apparatus of claim 14 wherein said means for causing comprises a resistance connected between a high voltage source and a high side of said defibrillator circuit.
24. The apparatus of claim 23 wherein the value of said resistance is selected to be much larger than the resistance of said patient
25. The apparatus of claim 23 further including a switch operable to switch said resistance in and out of said circuit.
26. The apparatus of claim 25 further including a high side driver circuit for activating said switch.
27. A method comprising the steps of employing a defibπllator circuit to deliver a defibπllating energy pulse to a patient; and reconfiguπng said circuit in response to at least one switching signal to deliver a constant cuπent to said patient.
28. The method of claim 27 wherein said step of reconfiguring compπses switching a resistance into a selected portion of said defibπllator circuit
29. The method of claim 28 wherein said resistance is switched into a leg of a low side switching device
30. The method of claim 29 wherein said resistance is switched into seπes between a high voltage source and a high side of said defibrillator circuit.
31. The method of claim 28 further including the step of applying a constant voltage across said resistance
32. The method of claim 31 wherein a digital to analog converter is employed in said step of applying a constant voltage.
33. The method of claim 31 wherein a feedback circuit is employed said step of applying a constant voltage.
34. Electronic circuitry comprising: a switching device connected in a circuit to assist in sending a cuπent pulse through a patient; a resistance connected to be switchable in and out of a leg of said switching device; and a source of constant voltage adapted to be applied across said resistance
35. The circuit of claim 34 wherein said source of constant voltage compπses a digital to analog converter
36. The circuit of claim 34 wherein said source of constant voltage compπses a feedback circuit.
37. The circuit of claim 35 wherein said source of constant voltage further compπses a feedback circuit
38. Electronic circuitry comprising: a high voltage source; a switching device connected to assist in sending a cuπent pulse through a patient; and a resistance switchable into and out of connection between said high voltage source and said switching device and having a value selected to cause a constant cuπent to flow therethrough when said resistance is switched into said connection. 39 The circuitry of claim 38 wherein said high voltage source comprises a capacitor 40. The circuitry of claim 39 further including a high side dπver for controlling the switching of said resistance into and out of said connection
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US9283398B2 (en) 2016-03-15
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US20030088282A1 (en) 2003-05-08
US9522284B2 (en) 2016-12-20
WO2003039648A3 (en) 2004-02-05
US20090210021A1 (en) 2009-08-20
US20050288714A1 (en) 2005-12-29
US20160166841A1 (en) 2016-06-16
US6952608B2 (en) 2005-10-04
US7522957B2 (en) 2009-04-21

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