United States Patent I [191
`de Coriolis et al.
`
`[54] METHOD AND APPARATUS FOR
`APPLYING ASYMMETRIC BIPHASIC
`TRUNCATED EXPONENTIAL
`COUNTERSHOCKS
`
`Inventors: Paul E. de Coriolis; John R. Batty,
`Jr., both of Miami, Fla.; Bruce J.
`Shook, North Andover, Mass.
`
`Assignee: Telectronics Pacing Systems, Inc.,
`Englewood, Colo.
`
`Appl. No.: 574,510
`
`Filed:
`
`Aug. 28, 1990
`
`Related US. Application Data
`Continuation of Ser. No. 397.637, Aug. 23, 1989. aban-
`doned, which is a continuation of Ser. No. 145,515,
`Jan. 19, 1988, abandoned.
`
`Int. Cl.5 ............................................... A61N 1/36
`U.S. Cl. .........
`128/419 D
`Field of Search .................................. .. 128/419 D
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`128/421
`3,224,447 12/1965 Becker et a1.
`128/419 D
`3,614,955 10/1971 Mirowski et al.
`...... .. 128/4-05
`4,120,305 10/1978 Rhoads et al.
`..
`. 128/419 PG
`4,498,478
`2/1985 Bourgeois
`128/419 D
`4,576,170
`3/1986 Bradley el al.
`128/419 D
`4,637,397
`1/1987 Jones et al.
`..
`128/419 D
`4,803,883
`1/1989 Winstrom
`128/419 D
`4,821,723 4/1989 Baker, Jr. et al.
`. 128/419 PG
`4,830,006
`5/1989 I-Ialuska et al
`128/419 D
`4,850,357 7/1989
`... .... 128/419 D
`....
`4,953,551
`9/1990 Mehra et al.
`FOREIGN PATENT DOCUMENTS
`
`.
`0281219 9/1988 European Pat. Off.
`OTHER PUBLICATIONS
`
`W. B. Kouwenhoven, Annals of Internal Medicine, vol.
`71, No. 3, 1317- 449-458 (Sep., 1969).
`Langer et al., “Considerations in the Development of
`the Automatic Implantable Defibrillator,” Medical In-
`
`W US00508352 I
`[11] Patent Number:
`5,083,562
`
`[45] Date of Patent:
`
`Jan. 28, 1992
`
`strumentatian: vol. 10, No. 3; May—.lun. 1976; pp.
`163-167.
`Mirowski et a1., “Termination of Malignant Ventricular
`Arrhythmias with an Implanted Automatic Defibrilla-
`tor in Human Beings," New Eng. Journal of Med.:
`8/7/80; pp. 322-324.
`Mirowski et al., “Clinical Treatment of Life—Threaten-
`ing Ventricular Tachyarrhythmias with the Automatic
`Implantable Defibrillator,” Amer. Heart Journal; 8/81,
`pp. 265-270.
`Schuder et al., "Waveform Dependency in Defibrillat-
`ing 100 kg Calves,” Devices & Tech. Meeting, NIH, 1982;
`p. 174.
`J. L. Jones & R. E. Jones, “Defibrillator Waveshape
`Optimization,” Devices & Tech. Meeting, NIH, 1982; p.
`175.
`~
`J. L. Jones & R. E. Jones, “Improved Defibrillator
`Waveform Safety Factor with Biphasic Waveforms,"
`The American Journal of Physiology 7/83; 245:1; pp.
`H60-H65.
`Schuder et al., “Ultrahigh-Energy Hydrogen Thyra-
`tron/SCR Bidirectional Waveform Defibrillator,” Med-
`ical & Biological Engineering & Computing, Jul. 1982, pp.
`419-424.
`R. A. Winkle, “The Implantable Defibrillator in Ven-
`tricular Arrhythmias,” Hospital Practice, Mar. 1983, pp.
`149-165.
`
`(List continued on next page.)
`
`Primary Examiner—Francis Jaworski
`Attorney, Agent, or Firm—Gottlieb, Rackman &
`Reisman
`
`ABSTRACI‘
`[57] .
`A method and apparatus for applying an asymmetric
`biphasic exponential waveform countershock to the
`heart useful in an implantable cardioverter or defibrilla-
`tor, wherein the second phase has a start amplitude of
`substantially one half that of the first phase, and wherein
`the polarity of a capacitor discharging through 21 cur-
`rent path including the heart is reversed. A voltage
`reversing circuit may include a voltage shifter which
`shifts voltage associated with switching elements in the
`circuit to reduce voltage stresses in the switching ele-
`ments.
`
`18 Claims, 4 Drawing Sheets
`
`1
`
`LIFECOR212-1004
`
`

`
`Page 2
`
`OTHER PUBLICATIONS
`
`Schuder et al., “Transthoracic Ventricular Defibrilla-
`tion in the 100 kg Calf with Symmetrical One—Cycle
`Bidirectional Rectangular Wave Stimuli,” IEEE Trans-
`actions on Biomedical Engineering, vol. BME—30, No. 7;
`Jul. 1983; pp. 415-422.
`Schuder et al., “Det'ibrillation of 100 kg Calves with -
`Asymmetrical, Bidirectional, Rectangular Pulses,” Car-
`diovascular Research; l984:l8; pp. 419-426.
`Schuder et al., “One—Cycle Bidirectional Rectangular
`Wave Shocks for Open Chest Defibrillation in‘ the
`Calf,” Abs. Amer. Soc. Artificial Internal Organs; 9:16.
`Schuder et al., “Development of Automatic Implanted
`Defibrillator," Devices & Tech. Meeting, NIH; p. 206
`(1931).
`.
`Tang et al., “Ventricular Defibrillation Using Biphasic
`Waveforms of Different Phasic Duration,” PACE: vol.
`10, Mar.-Apr. 1987.
`Tang et al., “Strength Duration Curve for Ventricular
`
`Defibrillation Using Biphasic Waveforms," The North
`Amer. Society of Pacing and Electrophysiology; May 2.
`1987; p. 49.
`Winkle et al., “Improved Low Energy Defibrillation
`Efficacy in Man Using a Biphasic Truncated Exponen-
`tial Waveform," JACC: vol. 9, No. 2; Feb. 1987; p.
`142A.
`B. D. Lindsay et al., “Prospective Evaluation of a Se-
`quential Pacing and High-Energy Bidirectional Shock
`Algorithm for Transvenous Cardioversion in Patients
`with Ventricular Tachycardia," Therapy and Prevention,
`vol. 76, No. 3 (Sep., 1987).
`S. Saksena et al., “A Prospective Evaluation of Single‘
`and Dual Current Pathways for Transvenous Cardio-
`version in Rapid Ventricular Tachycardia,” PACE,
`vol. 10, pp. 1130-1141 (Sep.—Oct., 1987).
`S. Saksena et al., “Developments for Future Implant-
`able Cardioverters and Defibrillators,” PACE, vol. l0,
`pp. 1342-1358 (Nov.—Dec., 1987).
`
`2
`
`

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`U.S. Patent
`
`Jan. 28, 1992
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`Sheet 1 of 4
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`5,083,562
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`U.S. Patent
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`Jan. 28, 1992
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`Sheet 2 of 4
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`5,083,562
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`U.S. Patent
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`2991
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`Sheet 3 of 4
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`5
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`U.S. Patent
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`Jan. 28, 1992
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`1
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`5,083,562
`
`METHOD AND APPARATUS FOR APPLYING
`ASYMMETRIC BIPHASIC TRUNCATED
`EXPONENTIAI. COUNTERSHOCKS
`
`This application is a continuation of Ser. No. 397,637
`filed Aug. 23, 1989 and now abandoned, which is a
`continuation of Ser. No. 145,515 filed Jan. 19, 1988 and
`now abandoned.
`
`TECHNICAL FIELD
`
`This invention relates to apparatus for applying coun-
`tershocks to the heart. More particularly it relates to
`cardioverters and defibrillators which supply truncated
`exponential pulses, and to implantable devices of this
`type.
`
`BACKGROUND ART
`
`Ventricular arrhythmias are potentially lethal. In the
`instance of chaotic, non-coordinated muscle contrac-
`tion, known as fibrillation, death can ensue within min-
`utes after onset. To convert the fibrillation to an orga-
`nized, slower cardiac rate, an electrical countershock is
`given. A high energy pulse of 400 joules or less is ap-
`plied across the chest wall using an external defibrilla-
`tor. However, such external defibrillators are located in
`hospitals and in rescue vehicles. Because death can
`ensue within ten minutes, medical assistance may arrive
`too late to resuscitate the patient.
`For patients who have survived an episode of ven-
`tricular fibrillation, there is a high probability of reoc-
`currence. In addition, patients who have experienced
`sustained symptomatic ventricular tachycardia are at
`risk in that such arrhythmias may convert to fibrillation.
`It is these patients who benefit from an implantable
`cardioverter or defibrillator.
`An implantable cardioverter or defibrillator must be
`capable of sensing ventricular cardiac electrical activ-
`ity, of determining if the sensed electrical activity is
`ventricular tachycardia or fibrillation and of enabling a
`circuit which then delivers a high energy pulse to elec-
`trodes associated with the heart to perform cardiover-
`sion (a shock in synchronization with the cardiac cycle)
`or defibrillation.
`In prior implantable defibrillators, a truncated expo-
`nential pulse (trapezoidal pulse) of 25 joules or more has
`been utilized. Such a trapezoidal pulse is produced by a
`few external defibrillators but, more commonly, a
`damped sine wave is used. External defibrillators re-
`quire a higher energy source, of up to 400 joules, be-
`cause of energy dissipation through the chest wall.
`Schuder et
`al.
`in “Ultrahigh-Energy Hydrogen
`Thyratron/SCR Bidirectional Waveform Defibrilla-
`tor,” Med & Biol. Eng. & Comput. Vol. 20, pp. 419-424
`(July, 1982), show a symmetrical bi-directional trun-
`cated exponential waveform having less than 10%
`droop, so as to approximate a rectangular waveform,
`and a schematic diagram of an apparatus for generating
`such a waveform. The bi-directional pulse as described
`by Schuder et al.
`is incorporated into a rather large
`external defibrillator. Such bi-directional pulses can
`defibrillate and do not appear to influence adversely the
`outcome of subsequent attempts to defibrillate. How-
`ever, much of the energy stored in the capacitor is lost,
`because the charge on the capacitor at the end of each
`phase must be dumped when the voltage is still a large
`fraction of the initial charge voltage.
`
`2
`In an implantable defibrillator, it is necessary to con-
`serve space so that the implantable unit is not large.
`Further, because battery energy is finite in an implant-
`able unit, any increase in efficiency that can be attained
`by fuller utilization of the stored energy (as measured in
`joules) or reduction in defibrillation threshold is of criti-
`cal importance. Such increase in efficiency extends the
`useful life of the implant, thus reducing the frequency of
`implant replacement and the cost associated with an
`inefficient device due to the cost of the device itself, and
`surgical and hospital expenses incurred as a result of
`replacement. While replacement is a rather simple pro-
`cedure, the patient is nevertheless exposed to the risks
`inherent in any form of surgery. In addition, reduction
`in defibrillation threshold, while saving energy, also has
`the beneficial effect of reducing the discomfort experi-
`enced by the patient when the defibrillation shock is
`applied.
`DISCLOSURE OF THE INVENTION
`
`It is an object of the invention to provide a method
`and apparatus for applying electrotherapy to the heart
`which is energy efficient.
`It is another object of the invention to provide a
`method and apparatus for defibrillating the heart at a
`relatively low defibrillation threshold.
`It is an additional object of the invention to provide
`an apparatus for applying electrotherapy to the heart
`which may be implanted within the body.
`It is a further object of the invention to provide a
`method and apparatus for applying biphasic counter-
`shocks to the heart by using a single capacitor or a
`single capacitor bank for energy storage.
`It is still another object of the invention to provide a
`switching circuit which permits the use of a single ca-
`pacitor bank in an apparatus and method for applying
`electrotherapy to the heart, which circuit minimizes
`voltage stresses on the components therein.
`In accordance with the method of the invention,
`electrotherapy is applied to the heart by way of conduc-
`tive leads electrically connected to electrodes associ-
`ated with the heart. The leads and electrodes conduct
`pulses of electric current to the heart. These pulses
`include a first truncated exponential waveform of a first
`polarity having a first start amplitude and a first end
`amplitude; and a second truncated exponential wave-
`form of a second polarity opposite that of said first
`polarity. The second truncated exponential waveform
`has a second start amplitude and a second end ampli-
`tude. The second start amplitude is lower than the first
`start amplitude. The second start amplitude may be
`substantially equal to the first end amplitude. Preferably
`the second start amplitude is equal to substantially one-
`half of the first start amplitude or is equal to between
`forty and sixty percent of the first start amplitude. The
`first truncated exponential waveform and the second
`truncated exponential waveform are applied by charg-
`ing a capacitor to a voltage corresponding to the first
`start amplitude, discharging the capacitor through the
`leads to a voltage corresponding to the first end ampli-
`tude, reversing polarity of connection to the leads and
`discharging the capacitor to a voltage corresponding to
`the second end amplitude. The capacitor may be dis-
`connected from the leads for a predetermined period of
`time before reversing the polarity of connection of the
`capacitor to the leads.
`Also in accordance with the invention, an apparatus
`for administering electrotherapy to the heart comprises
`
`7
`
`

`
`5,083,562
`
`3
`a recognition means responsive to an electrical signal
`from the heart for determining when the heart is in need
`of electrotherapy, a capacitive energy storage means for
`storing electrical energy to be applied to the heart, an
`energy source means for providing electrical energy to
`the energy storage means, a conductor means for con-
`ducting the stored electrical energy from the energy
`storage means to the heart, and a connection means for
`connecting the conductor means with a first polarity to
`administer a first shock, and with a second polarity
`opposite said first polarity to administer a second shock
`after administering of the first shock.
`The connection means comprises a first switch means
`for connecting a first terminal of the energy storage
`means to a first lead to the heart, a second switch means
`for connecting a second terminal of the energy storage
`means to a second lead to the heart, a third switch
`means for connecting the first terminal of the energy
`storage means to the second lead to the heart, a fourth
`switch means for connecting the second terminal of the
`energy storage means to the first lead to the heart, and
`timing means for closing the first switch means and the
`second switch means at a first time, for opening the first
`switch means and the second switch means at a second
`time after the first time, for closing the third switch
`means and the fourth switch means at a third time after
`the second time, and for opening the third switch means
`and the fourth switch means at a fourth time after the
`third time.
`switch means includes two
`Preferably the first
`switches connected in series. Further,
`the apparatus
`preferably includes a voltage reducing means for reduc-
`ing the voltage across the fourth switch means at a fifth
`time, the fifth time being after the second time and
`before the third time. The voltage reducing means shifts
`the voltage from across the fourth switch means so that
`it is across the first switch means. When the first switch
`means includes two switches connected in series, the
`apparatus may further comprise a first resistor con-
`nected across a first of the two switches, a fifth switch
`means having a first terminal connected to the second
`terminal of the energy storage means and a second resis-
`tor connecting a second terminal of the fifth switch
`means and a connection point between the first of the
`two switches and a second of the two switches. The
`apparatus further comprises a capacitor connected be-
`tween the connection point and the second terminal of
`the energy storage means.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Further objects, features and advantages of our in-
`vention will become apparent upon consideration of the
`following detailed description in connection with the
`drawings, in which:
`_
`FIG. 1 is a simplified, schematic view along a section
`through a thorax showing an implantable defibrillator in
`accordance with invention and its connection to leads
`associated with the heart;
`FIG. 2 is a conceptual block diagram of the implant-
`able defibrillator of FIG. 1;
`FIG. 3 is a conceptual schematic diagram of a circuit
`for use in the apparatus according to the invention;
`FIG. 4 illustrates the waveform of the defibrillatory
`pulse of an implantable defibrillator in accordance with
`the invention;
`FIG. 5 is a detailed schematic diagram of a circuit for
`use in the preferred embodiment of the apparatus ac-
`cording to the invention; and
`
`4
`FIG. 6A to FIG. 61 represent waveforms associated
`with the circuit of FIG. 5.
`
`BEST MODE FOR CARRYING OUT THE
`INVENTION
`
`The invention is described herein with respect to an
`implantable defibrillator which is believed to be its
`primary, most important and most urgent area of appli-
`cation. However, it will be recognized in the art that the
`invention may be applied to external defibrillators and
`to cardioverters as well.
`Referring to FIG. 1, an implantable defibrillator 10 is
`implanted subcutaneously in a patient, generally in an
`abdominal muscle. An epicardial sensing lead 12 includ-
`ing an insulated conductor wire is in electrical continu-
`ity with a ventricular sensing electrode 14 used for
`sensing ventricular electrical activity. Alternatively a
`pervenous sensing lead may be threaded through an
`appropriate vein and positioned in the right ventricle 16
`of the heart 18. A terminal assembly (not shown) at the
`proximal end 20 of sensing lead 12 is inserted into a first
`receptacle (not shown) within the neck 22 of implant-
`able defibrillator 10. The tenninal assembly of sensing
`lead 12, when inserted in its respective receptacle,
`is
`electrically connected to the insulated conductor and to
`the circuitry of the defibrillator 10. This capsulated in
`case 24 is a low impedance battery, using lithium vana-
`dium pentoxide or lithium silver vanadium pentoxide
`chemistry.
`'
`A terminal assembly (not shown) of a second lead 26
`is connected to implantable defibrillator 10 via a second
`receptacle in neck 22 of defibrillator 10. The insulated
`conductor wire of lead 26 is tunneled subcutaneously.
`An electrode pad 28 at the distal end of lead 26 is su-
`tured onto the epicardial surface of the left ventricle 30
`of the heart 18 during a thoracotomy or median sternot-
`omy A subxiphoid approach may also be used. A defi-
`brillatory electrode 32, which acts as a cathode for the
`delivery of the first phase of biphasic defibrillation
`shocks, protrudes slightly_ from the cardiac surface of
`electrode pad 28 of lead 26.
`A base patch 27 having an electrode 29 which is
`preferably epicardial, but as noted below may be subcu-
`taneous, located at the distal end of a third lead 33, has
`a proximal terminal received in a third receptacle (not
`shown) in neck 22 of defibrillator 10. Electrode 29 acts
`as an anode for the delivery of the first phase of biphasic
`defibrillation shocks.
`While lead 26 with its electrode 32 and lead 33 with
`its electrode 29 are epicardial, either lead may be epicar-
`dial or pervenous. Of importance, it is also contem-
`plated that for defibrillation one lead and electrode may
`be pervenous, while the other may be a subcutaneous
`patch. The increased efficiency of defibrillation of an
`asymmetric biphasic truncated exponential pulse in
`accordance with the invention would thus pennit the
`installation of an implantable defibrillator without ne-
`cessitating opening the chest cavity. This greatly in-
`creases the desirability of the apparatus according to the
`invention by reducing the risk of patient morbidity or
`mortality as a result of risks associated with a more
`extensive surgical procedure.
`Referring to FIG. 2, the circuitry 34 within the tita-
`nium case 24 is comprised of a fibrillation detector 36
`(or a tachycardia detector for purposes of cardiover-
`sion) which may include any of several well-known
`arrhythmia detectors and a countershock or defibrilla-
`tion circuit 38. Electrical activity generated by the ven-
`
`45
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`SO
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`

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`5,083,562
`
`5
`tricles 16,30 of the heart 18 is transmitted by the ventric-
`ular sensing electrode I4 to the fibrillation detector 36.
`If the detected ventricular activity is classified as fibril-
`lation, as indicated by a chaotic, non-rhythmic electrical
`activity, defibrillator circuit 38 is enabled.
`Referring to FIG. 3, the pulse forming or defibrillator
`circuit 38 is, in, simplified or conceptual form, com-
`prised of a capacitor 39, a series of switches 40, 42, 44,
`and 46, which appropriately open and close in response
`to signals from a timer circuit 49 .(as described below,
`also with reference to FIG. 4), to deliver defibrillation
`shocks to the heart 18, and optionally, a switch 48 and
`a resistive load 50.
`'
`Referring to FIG. 4, an asymmetric biphasic expo-
`nential waveform 52, as recorded between defibrilla-
`tory electrodes 29 and 32 is measured from a baseline
`54. Waveform 52 has a leading edge 56 of amplitude V1.
`The leading edge is followed by an exponential decay
`58 of 2 to 8 milliseconds (for a time which may prefera-
`bly be equal to approximately 0.7 times the time con- 20
`stant of capacitor 39 and the defibrillation resistance of
`the heart) to a second voltage level V2, at trailing edge
`60, which waveform 52 returns to baseline 54. A zero
`voltage period 62 of 0.1 to 5.0 milliseconds is followed
`by a polarity reversal. The voltage V2 is then applied to
`the heart at leading edge 63 and a second exponential
`decay 64 occurs, which is preferably of the same dura-
`tion, as the first decay 58. The voltage falls to a third
`voltage level V3 at trailing edge 66, from which it re-
`turns to baseline 54.
`Asymmetric biphasic exponential waveform 52 is
`generated by utilizing circuit 38 in the following man-
`ner: capacitor 39 is charged by an energy source (not
`shown in FIG. 3) to a voltage V] of the polarity shown.
`Switches 40 and 42 are closed by timer circuit 49 plac-
`ing capacitor 39 in series with the heart 18. Current
`flows through the heart 18 in the direction of the arrow
`68 discharging the capacitor to voltage V2. Switches
`44, 46 and 48 are open during this time. Then, capacitor
`39 is disconnected from the heart 18 and the voltage
`level of capacitor 39 is held at V2 when timer circuit 49
`causes switches 40 and 42 to open. After a short no
`current interval, current is then caused to flow through
`the heart 18 in the direction as shown by the arrow 70
`when timer circuit 49 causes switch 44 and switch 46 to
`close, thereby discharging capacitor 39 to voltage V3.
`The second phase is truncated when timer circuit 49
`causes switch 44 and switch 46 to open (all others hav-
`ing remained open). Optional switch 48 may then be
`closed causing capacitor 39 to discharge into resistive 50
`load 50.
`,
`Thus, the defibrillatory pulse is actually made up of
`two pulses of current which travel through the heart
`first in one direction and then in the opposite direction
`increasing the probability of depolarizing and then ren-
`dering refractory a larger volume of the cardiac muscle
`fiber. The refractoriness makes it unlikely for fibrilla-
`tion to continue. However, if fibrillation does continue,
`the capacitor is recharged and an additional pulse or
`pulses having the waveform illustrated in FIG. 4 may 60
`be administered.
`Referring to FIG. 5, the conceptual circuit of FIG. 3
`is implemented. FIG. 5 includes a charging circuit 72 of
`conventional type for charging a single capacitor bank
`74, a comparator circuit 77 for providing electrical 65
`signals indicative of the voltage to which capacitor
`bank 74 has been charged and a switching circuit shown
`generally at 78, which is a practical implementation of
`
`6
`the concept of FIG. 3. While timer circuit 49 is not
`described, the necessary waveforms and their relation-
`ships in time are described below with respect to FIG.
`6B to FIG. 6G.
`When fibrillation is detected by fibrillation detector
`36 (FIG. 2) capacitor bank 74 is charged by capacitor
`charging circuit 72. Charging circuit 72 may be any of
`several well-known circuits which have provisions for a
`“soft start" so that the large current demands placed on
`the battery at the beginning of the charge cycle are
`somewhat reduced in magnitude and provisions for low
`battery voltage detection means for temporarily halting
`the charge cycle to permit battery recovery should the
`battery voltage drop to a predetermined low level.
`While capacitor bank 74 would preferably include
`only a single capacitor, the limitations in capacitor tech-
`nology, and in particular the limitations of aluminum
`electrolytic capacitors are such that
`the maximum
`working voltage is not sufficient to withstand the maxi-
`mum voltage to which the capacitor must be charged.
`Thus, capacitor bank 74 may be made up of two capaci-
`tors 74A and 74B which are connected in series and
`charged by charging circuit 72. Specifically, capacitor
`74A is charged by a secondary winding 75A of a step up
`transformer 75 through a diode 76A. In a similar man-
`ner, capacitor 74B is charged by current from second-
`ary winding 75B of transformer 75 through diode 76B.
`A voltage divider including resistors 80 and 82 and a
`smoothing capacitor 84 provide an output voltage to
`comparator circuit 77 which is proportional to, but a
`small fraction of, the voltage across capacitor bank 74.
`Comparator circuit 77 includes an integrated circuit
`comparator 90. The positive supply voltage from the
`battery, appropriately increased in level by, for exam-
`ple, voltage doubling is always supplied to circuit 90.
`However, the negative supply voltage to comparator 90
`is connected through a MOSFET 92 which becomes
`conductive only upon application of an analog power
`enable signal to the gate thereof. Comparator 90 is thus
`turned off most of the time for purposes for saving
`power and turned on only during time periods when a
`voltage “measurement” must be made.
`A sample-and-hold circuit including a MOSFET 94
`and a capacitor 96 provides a reference voltage to one
`input of comparator 90. The other input of comparator
`90 is connected to the junction between resistors 80 and
`82. When the charging cycle is initiated, power is ap-
`plied to comparator 90 through MOSFET 92 and a
`voltage level applied to the drain of MOSFET 94 is
`stored on capacitor 96 by supplying a sample pulse to
`the gate of MOSFET 94. It is desirable to use a sample-
`and-hold circuit, with the storage capacitor thereof
`located in close physical proximity to the comparator
`chip, and when in the hold mode, electrically isolated
`from the remainder of the circuitry of defibrillator 10,
`so that the large currents in the circuitry do not induce
`spurious voltages on the capacitor and detract from the
`accuracy of comparator circuit 77.
`When capacitor bank 74 has charged to the appropri-
`ate voltage level, and the output of comparator 90
`switches states, charging is complete. Charging circuit
`72 is then turned off. A voltage representative of V2 is
`then loaded on to capacitor 96 of comparator circuit 77.
`A shock is initiated by operation of time circuit 49 im-
`mediately for defibrillation or in synchronism with de-
`tected heart activity if cardioversion is being per-
`formed. However, it is desirable to reconfirm the pres-
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`7
`ence of a tachyarrhythmia before timer circuit 49 oper-
`ates to supply a shock.
`During sensing and pacing, defibrillation lead 26 acts
`as a return for electrical conduction to the heart. The
`pacing and/or sensing signals from this lead are con-
`ducted through MOSFET 100 for this purpose. Thus,
`MOSFET 100 must be at least partially switched on at
`all times except duringthe charging of capacitor bank
`74 at which time MOSFET 100 is switched off to pro-
`vide protection against accidental discharge. MOSFET
`100 is turned on by a switching circuit including MOS-
`FETS 102 and 104. When the signal SZN (FIG. 6B) is at
`ground potential MOSFET 104 is off and MOSFET
`102 is turned on. Diode 106 becomes conductive thus
`raising the potential of the gate of MOSFET 100 by
`conducting current through resistor 108. Capacitor 110
`smoothes the voltage at the gate of MOSFET 100 and
`prevents transient impulses from turning on MOSFET
`I00. Capacitor 112 protects MOSFET 100 against cur-
`rents due to electrosurgery by providing a bypass be-
`tween the source and drain thereof.
`While MOSFET 100 is held on so that its internal
`resistance is in the order of 20 to 30 ohms for sensing
`and pacing (when SZN is at logic zero),
`in order to
`conduct a defibrillation pulse MOSFET 100 must be
`put into a very low resistance condition or “turned on
`hard” so that its internal impedance is of a mere fraction
`of an ohm. This is accomplished through the action of
`MOSFETS 114 and 116 as well as capacitor 118. When
`SZBOOSTN from timer circuit 49 is at a logic high
`voltage level, MOSFET 114 is turned off and MOS-
`FET 116 is turned on. Capacitor 118 is charged through
`diode 106 so that the side facing diode 106 is at a voltage
`close to that of the positive supply voltage (the doubled
`battery voltage). As shown in FIG. 6C, for the applica-
`tion of a defibrillation pulse, SZBOOSTN is dropped to
`ground potential by timer circuit 49. This turns off
`MOSFET 116 and turns on MOSFET 114. The result is
`to place capacitor 118 in series with the positive supply
`voltage so that approximately twice that voltage is
`applied across the source and drain of MOSFET 104
`(off at that time). This results in MOSFET 100 being
`turned on hard. Zener diode 120 serves to limit the
`voltage applied between the source and gate of MOS-
`FET 100 so that it is not damaged. FIG. 6A illustrates
`that after capacitor bank 74 has been charged to voltage
`V1, S2N goes low and then S2BOOSTN goes low
`shortly thereafter (FIG. 6B and FIG. 6C). MOSFETS
`102, 104, 114 and 118 may all be on a single type
`CD4007 chip.
`In order to provide a defibrillation shock to the heart,
`it also necessary to turn on SCR 122 and SCR 124. This
`is accomplished when timer circuit 49 provides a pulse
`S1 (not shown in FIG. 6A to FIG. 6H) to the gate of
`MOSFET 126 completing a circuit through the primary
`winding 130A of transformer 130, current limiting resis-
`tor 132 and MOSFET 126. The surge of current
`through primary winding 130A induces a voltage in a
`first secondary winding 13GB and a second secondary
`winding 130C of transformer 130. If the polarities of
`connection of the windings of transformer 130 are as
`indicated in FIG. 5, SCR 122 and SCR 124 are trig-
`gered. False triggering is prevented by capacitor 134
`across secondary 130B and capacitor 136 across second-
`ary 130C.
`When MOSFET 100 and SCRS 122 and 124 have
`been turned on (the state of the latter two devices being
`represented by FIG. 6D) current is conducted through
`
`30
`
`35
`
`55
`
`60
`
`5,083,562
`
`8
`SCR 122, SCR 124, to a lead terminal in neck 22 of
`defibrillator 10, along defibrillation lead 33, through the
`heart 18, along defibrillation lead 26, to another lead
`terminal
`in neck 22 of defibrillator 10, and finally
`through MOSFET 100, all connected in series across
`capacitor bank 74.
`After the output of comparator 90 has changed states
`due to the voltage across capacitor bank 74 being re-
`duced to voltage V2, which may be some fixed fraction
`of the voltage V1,
`timer circuit 49 causes SZN and
`SZBOOSTN to return to a logic high state thus turning
`off MOSFET 100, commutating SCR 122 and SCR 124
`(FIG. 6D) and terminating the initial phase of discharge
`of capacitor bank 74. At this point capacitor bank 74 is
`electrically disconnected from the heart.
`For a period of time following the first phase, timer
`circuit 49 does not interact with the circuit of FIG. 5. In
`other words, there is a delay of 0.1 millisecond to 5
`milliseconds during which no current is delivered to the
`heart 18. After this delay, capacitor bank 74 is recon-
`nected to the heart with its polarity reversed. First,
`timer circuit 49 causes the voltage SSN (FIG. 6E) ap-
`plied to resistor 140 to go from a positive logic level to
`zero volts (ground). Since the emitter of transistor 142 is
`connected to the positive supply voltage,
`this causes
`transistor 142 to turn on which in turn causes triggering
`of SCR 144 by way of current conducted through resis-
`tor 146. Before this occurs, the full voltage of capacitor
`bank 74 appears across SCR 148. However, when SCR
`144 is turned on much of this voltage is slowly shifted to
`appear across SCR 122 due to. the voltage division ac-
`tion of resistor 150 and resistor 152. Typically, resistor
`150 will have a resistance value at least 10 times as great
`as that of resistor 152. The voltage is gradually shifted
`due to the discharge time of a capacitor 154 through the
`series combination of resistor 152 and SCR 144. A prac-
`tical time constant may be on the order of 10 to 50
`microseconds. This shift in voltage is represented by the
`change in the voltage at point A (the connection point
`of SCR 122 and SCR 124) as shown in FIG. 6H. The
`slight jog in voltage at point X of FIG. 6H after the
`shift, occurs when SCR 148 is turned on as described
`below.
`Timer circuit 49 causes the heart 18 to be connected
`to capacitor bank 74 by turning on SCR 148 and SCR
`156. SCR 148 is triggered by changing the voltage S4N
`(FIG. 6F) applied to resistor 158 from a logic high state
`to a logic zero state (ground). thus turning on transistor
`160 (the emitter of which is connected to the positive
`supply voltage) and triggering SCR 148 due to current
`conducted through resistor 162.
`To turn on SCR 156 the voltage S3 at the gate at
`MOSFET 164 is changed from ground to a positive
`logic voltage (FIG. 6G). Current begins to flow
`through the series circuit including the primary wind-
`ing 168A of a transformer 168, current limiting resistor
`166 and MOS