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`U5005468254A
`
`[19]
`United States Patent
`[11] Patent Number:
`5,468,254
`Nov. 21, 1995
`[45] Date of Patent:
`Hahnet a1.
`
`[54] METHOD AND APPARATUS FOR
`DEFIBRILLATION USING A MULTIPHASIC
`TRUNCATED EXPONENTIAL WAVEFORM
`
`[75]
`
`Inventors: Stephen J. Hahn, Roseville, Minn;
`David K. Swanson, Mountain View,
`Calif.
`
`[73] Assigncc: Cardiac Pacemakers, Inc., St. Paul,
`Minn.
`
`[21] Appl. No.: 97,463
`
`[22]
`
`Filed:
`
`Jul. 26, 1993
`
`
`A61N 1/39
`Int. Cl.6
`[51}
`[52] US. Cl. ............... 607/5
`
`[58] Field of Search ..
`607/5—8
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,821,723
`4,850,357
`4,953,551
`4,998,531
`5,083,562
`5,275,157
`5,352,239
`
`4/1989 Baker, Jr. et a1.
`....... 123/419 D
`7/1989 Bach,.1r.
`..........
`123/419 D
`
`9/1990
`128/419 D
`
`.. 128/419 D
`3/1991
`
`1/1992
`128/419 D
`
`1/1994
`607/6
`....... 607/5
`10/1994
`
`
`OTHER PUBLICATIONS
`
`Dixon et a1., Improved Defibrillation Thresholds With Large
`Contoured Epicardial Electrodes and Biphasic Wave Farms,
`Circulation 76, N0. 5, 1987, pp. 1176—1184.
`Chapman et 31., Comparative Efiicacy of Monophasic and
`Biphasic Truncated Exponential Shocks for Nonthorac-
`otomy Internal Defibrillation in Dogs, Journal of the Ameri-
`can College of Cardiology, v01. 12, No. 3, 1988, pp.
`739—745.
`Jones, et al., Decreased Defibrillatorelnduced Dysfunction
`With Biphasic Rectangular Waveforms, Am. J. Physiol, 247,
`1984, pp. H792~H796.
`Schuder, et 21., Optimal Biphasic Waveform Morphologyfor
`Canine Defibrillation With a Transvenous Catheter and
`Subcutaneous Patch System, Abstracts of the 61st Scientific
`Session, 1988, p. 11—219.
`Primary Examiner—William E. Kamm
`Assistant Examiner—Marianne Parker
`Attorney, Agent, or Finn—Peter Forrest
`[57]
`ABSTRACT
`
`A method and apparatus for converting an arrhythmia of a
`heart using a biphasic truncated exponential waveform
`wherein the first phase is of shorter duration than the second
`phase.
`
`16 Claims, 3 Drawing Sheets
`
`
`
`L|FECOR212-1005
`
`1
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`LIFECOR212-1005
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`

`

`US. Patent
`
`Nov. 21, 1995
`
`Sheet 1 of 3
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`5,468,254
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`VP—
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`0
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`Vp‘
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`O
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`VP—
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`FIG.|
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`~VT
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`”T FIG.2 '
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`0
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`H—o,
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`FIG. 3 PRIOR ART
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`2
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`US. Patent
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`Nov. 21, 1995
`
`Sheet 2 of 3
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`5,468,254
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`Electronic
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`SW-”
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`Circuit
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`Drive
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`Control
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`3
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`

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`US. Patent
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`Nov. 21, 1995
`
`Sheet 3 of 3
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`5,468,254
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`L6
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`4
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`.74
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`I
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`8
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`IO
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`I2
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`FIG. 7
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`FIRST PHASE DURATION
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`60/40
`40/60
`20/80
`FIRST/SECOND PHASE DURATION RATIO
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`
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`FIRST/SECOND PHASE DURATION RATIO
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`5
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`20
`I5
`IO
`SHOCK DURATION MILLISECONDS
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`
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`70
`60
`50
`4O
`3O
`SWITCHING POINT AS‘ °/o OF PEAK VALUE
`75/25
`57/43
`43/57
`32/68
`22/78
`FIRST/SECOND PHASE DURATION RATIO
`
`4
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`5,468,254
`
`1
`METHOD AND APPARATUS FOR
`DEFIBRILLATION USING A MULTIPHASIC
`TRUNCATED EXPONENTIAL WAVEFORM
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention relates, generally, to the field of
`defibrillators. More particularly, it relates to an improvement
`in the efficacy of cardioverter—defibrillators (1CD). Auto—
`matic implantable cardioverter—defibrillators (AICDs) or
`ICDs customarily include a sensor and sensing circuit to
`determine when a therapeutic shock is needed, a control
`circuit
`to determine what
`type of therapeutic shock is
`appropriate, a long-term energy source, such as a battery, a
`short-term energy storage means such as lrigh—voltage
`capacitor, and a circuit for transferring electrical energy first
`from the battery to the capacitor and then from the capacitor
`to the heart by discharging the capacitor in waveforms
`having particular shapes, durations, and sequences, to elec—
`trodes which deliver the energy as a shock to a heart which
`is to be converted.
`
`Development of implantable cardioverter—defibrillators
`since their introduction in the mid 1980s, has not only been
`directed toward improving their reliability in terms of deliv-
`ering defibrillation pulses when fibrillation of the heart is
`detected, but also toward increasing their efficacy. That is, to
`apply to a heart the minimum amount of energy necessary to
`ensure conversion. By decreasing the amount of energy
`required for conversion or defibrillation, the physical size of
`the implanted automatic defibrillator can be decreased by
`reducing the physical size of the battery, the capacitor, and
`other components. A decrease in energy requirements also
`means that even if the defibrillator is with some degree of
`frequency called upon to defibrillate a heart, the battery will
`have a longer life. Thus, extending the period of time before
`which the defibrillator must be replaced.
`Advances in reducing the energy required for defibrilla-
`tion have been made in the past in various ways. The
`electrodes delivering the defibrillation shocks to the heart
`have been improved. It has also been found that shocks of
`particular shapes, durations, and polarities are more effective
`in defibrillating the heart. This invention relates to further
`improvements in the shape of the shocks.
`2. Description of Related Art Including Information Dis-
`closed Under Secs. 1.97499
`
`In a paper entitled: DECREASED DEFIBRILIATOR-
`INDUCED DYSFUNCTION WITH BIPHASIC RECTAN-
`GULAR WAVEFORMS; by Janice L. Jones and Ronald E.
`Jones, AM. J. Physiol. 247 (Heart Circ. Physiol. 16);
`H792—H796, 1984, a study is reported on the characteristics
`of the negative second portion of a biphasic waveform
`which best ameliorates postshock dysfunction. The study
`was based on the use of chick embryo cultured myocardial
`cells. The article concluded that “the negative tail can only
`partially reverse the deleterious efiects of the leading portion
`of the waveform and that this effect can be produced either
`by a low amplitude undershoot that lasts for a long time or
`by a higher amplitude undershoot that lasts for a shorter
`time.” While this work did find that some waveshapes with
`the second phase longer than the first reduced dysfunction,
`dysfunction has never been shown to have a impact on
`defibrillation efficacy.
`The paper Improved Defibrillation Thresholds with Large
`Contoured Epicardial Electrodes and Biphasic Waveforms,-
`by Ellen G. Dixon, circulation 76, No 5 1176—1184, 1987, is
`
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`primarily concerned with the testing of large contoured
`patch electrodes on dogs. The electrodes were tested with
`monophasic, biphasic, and triphasic waveforms. Further, the
`biphasic waveforms were tested with the first phase being
`both longer than and shorter than the second phase. It is
`reported that biphasic waveforms, with the durations of the
`first and second phases equal, have a significantly lower
`threshold voltage than a monophasic waveform.
`Furthermore,
`the defibrillation threshold voltage and
`energy were reported to be significantly higher for biphasic
`waveforms in which the relationship of the duration of the
`first to the second phase were 25/75 and 35/65, compared to
`50/50, even though the initial voltage of the first phase was
`of a greater magnitude than the second, with the trailing
`voltage of the first phase being equal
`to the beginning
`voltage of the second phase.
`Thus, while this paper was primarily directed to research
`with respect to electrodes, it does present data indicating that
`the duration of the first phase of a biphasic waveform should
`be equal to or longer than that of the second phase. This
`conclusion was based upon an earlier postulation by Jones et
`
`
`a. that the first phase conditions the heart cells to allow more
`
`
`e ‘ective defibrillation by the second phase.
`The paper, Comparative Efi‘icacy of Monophasic and
`Biphasic Truncated Exponential Shocks for Nomhorac-
`otomy Internal Defibrillation in Dogs; by Peter D. Chap-
`man, et al, Journal of the American College of Cardiology,
`
`Vol. 12, No. 3, September 1988 pages 739—745, reports the
`
`
`encacies of monophasic and biphasic truncated exponential
`shocks in dogs. The monophasic shocks were compared with
`biphasic shocks having relative P1 (first phase) versus PZ
`(second phase) durations of (50 and 50%, 75 and 25%, 90
`and 10%, 25 and 75%, 10 and 90%) It was concluded that
`biphasic shocks with P1 (initial positive phase) longer than
`P2 (terminal negative phase) markedly reduced energy
`requirements for nonthoracotomy canine defibrillation and
`may, therefore, facilitate development of nonthoracotomy
`devices for clinical applications. The paper further reports
`that biphasic pulses with the second phase longer than the
`first phase (25 and 75% and 10 and 90% configurations)
`resulted in energy thresholds that were significantly higher
`than even those for monophasic shocks.
`The paper: Optimal Biphasic Waveform Morphology for
`Canine Cardiac Defibrillation with a Transvenous Catheter
`and Subcutaneous Patch System, by John C. Schuder, et al,
`Circulation, vol 78, 114219, 1988 set forth that previous
`studies have shown that biphasic waveforms are generally
`superior to monophasic waveforms for achieving canine
`ventricular defibrillation. It further reports on additional
`tests directed at determining the significance of the duration
`of the initial phase. All of the tests for this study were
`conducted with a 10 millisecond truncated exponential
`waveform shock, and with the final current equal to 25% of
`the initial current. The timing of the polarity reversal was
`changed such that initial pulse durations of l, 3, 5, 7 and 9
`milliseconds were tested. The study concluded that
`ten
`millisecond biphasic truncated exponential waveforms are
`more effective with an initial pulse duration of 5 to 7
`milliseconds, i.e., equal to or greater than the duration of the
`second phase.
`US. Pat. No. 4,850,357—issued Jul. 25, 1989, and
`entitled: BIPHASIC PULSE GENERATOR FOR AN
`IMPLANTABLE DEFIBRILLATOR;
`is directed toward a
`circuit for delivering biphasic pulses without the need to
`short circuit the high voltage capacitor, which stores the
`energy for the pulses, at the end of a pulse. While not
`
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`5,468,254
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`3
`elaborating on the relative durations of the first and second
`phases of the pulse, the phases are shown to be equal, with
`the initial voltage of the second phase being equal to the
`terminal voltage of the first phase.
`US. Pat. No. 4,821,723—issued Apr. 18, 1989, and
`entitled: BIPHASIC WAVEFORMS FOR DEFIBRILLA-
`TION is directed toward a method and apparatus for defibril-
`lating a heart with a biphasic shock having an initial phase,
`the duration of which is at least slightly greater than the
`duration of the second phase. Further, the first phase of the
`biphasic waveform commences with a voltage magnitude
`equal to or greater than the initial voltage level of the second
`phase.
`the paper: Transthoracic Ventricular
`Referring to
`Defibrillation in the 100 Kg calf with Symmetrical One-
`Cycle Bidirectional Rectangular Wave Stimuli; IEEE Trans
`Biomed. Eng. 30: 415, 1983, and to the paper: Defibrillator
`of 100 Kg Calves with Asymmetrical Bidirectional Rectan—
`gular Pulses; Cardiovasa Res. 419, 1984, it is stated in U.S.
`Pat. No. 4,821,723 that: “Schuder and his associates were
`able to defibrillate 100 Kg calves using symmetrical bipha-
`sic rcetangular waveforms at a lower range of energy and
`current, and to achieve a higher percentage of successful first
`shock defibrillations than with monophasic waveforms.
`Those same investigators obtained good results with asym—
`metrical biphasic waveforms in which the amplitude of the
`second phase of the shock was smaller than that of the first
`phase, and the two phases were of equal duration.” This
`patent also sets forth the theory that the duration of the first
`phase of a biphasic waveform may have a significant effect
`on the extent of conditioning. It is further stated: “It appears
`that a short first phase, relative to the second phase, may be
`of insuflicient duration to allow a conditioning process to be
`completed.” As was previously set forth with respect to the
`Dixon paper, this study is based on the earlier postulation by
`Jones et al that the first phase conditions the heart cells to
`allow more efiective defibrillation by the second phase.
`Other efforts to reduce the size of an implantable defibril-
`lator have been directed toward improvement of the elec—
`trodes through which pulses are applied to the heart for
`defibrillation purposes. US Pat. No. 4,953,551—issued on
`Sep. 4, 1990 and entitled: METHOD OF DEFIBRILLATING
`A HEART: is primarily directed toward an improvement in
`the electrodes. However, the patent also advocates the use of
`an asymmetrical biphasic waveform. The asymmetrical
`waveform set forth is one in which the first and second
`phase, are of equal duration, but in which the initial voltage
`of the second phase is equal to the final voltage of the first
`phase, (voltage decays during the pulses on an exponential
`basis).
`US. Pat. No. 4,998,53l—issued on Mar. 12, 1991, and
`entitled:
`IMPLANTABLE N-PHASIC DEFIBRILLATOR
`OUTPUT BRIDGE CIRCUIT discloses a means for gener-
`ating not only biphasic, but also monophasic, multi-phase or
`sequential defibrillation pulses. The patent is not particularly
`concerned with, nor does it discuss, the eflicacy of biphasic
`pulses nor is it concerned with the relative durations of the
`first and second pulses.
`US. Pat. No. 5,083,562 issued on Jan. 28, 1992, and
`entitled: METHOD AND APPARATUS FOR APPLYING
`ASYMMETRIC BIPHASIC TRUNCATED EXPONENTIAL
`COUNTERSHOCKS, sets forth a defibrillation therapy in
`which a first
`truncated exponential waveform of a first
`polarity has a first phase start amplitude and a first phase end
`amplitude and, a second truncated exponential waveform of
`second polarity opposite that of the first polarity, has a
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`second phase start amplitude and a second phase end ampli-
`tude. The second phase start amplitude being lower than the
`first phase start amplitude and in a disclosed embodiment
`being substantially equal to the first end amplitude. Further,
`the second phase start amplitude is equal to substantially one
`half of the first phase start amplitude. This patent further
`teaches that the first and second phases are preferably of
`equal duration.
`While there are many patents and papers in addition to
`those set forth above which relate to biphasic waveforms,
`the inventor is unaware of any which teach the advantage of
`the first phase being shorter than the second.
`
`SUMMARY OF THE INVENTION
`
`It is an object of this invention to provide a method and
`apparatus for improving the efficacy of defibrillators which
`provide biphasic truncated exponential waveforms. It is a
`further object of the method of this invention to provide
`biphasic defibrillation pulses to a heart wherein the relative
`durations of the phases have a predetermined relationship to
`each other, such that
`the energy, voltage, and current
`required for defibrillation is reduced from that required prior
`to the applicant’s invention. It is a further object of this
`invention provide a defibrillator which generates biphasic
`defibrillation pulses in accordance with the applicant’s
`invention wherein the energy, voltage, and current required
`for defibrillation is reduced from that which has previously
`been considered necessary.
`In accordance with this invention, a method and apparatus
`of defibrillation is provided in which multiphasic pulses are
`applied to a heart with the first pulse being of a shorter
`duration than the second pulse. More particularly, in accor—
`dance with this invention, a defibrillator is provided wherein
`a biphasic,
`truncated exponential waveform is generated
`from a single capacitor discharge in which the first pulse is
`shorter in duration than the subsequent phase or phases.
`Further, a method of defibrillation is provided wherein a
`defibrillator provides a biphasic pulse, the duration ratio of
`the first pulse to the subsequent pulse in a preferred embodi-
`ment being approximately 40 to 60.
`In its broader aspects; the method and apparatus of this
`invention is generally applicable to converting arrhythmias
`of the heart, particularly both atrial and ventricular tachy-
`cardia arrhythmias.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 shows the exponentially decaying waveform of a
`capacitor discharge into a resistive load.
`FIG. 2 shows a similar exponentially decaying waveform
`of the discharge of a capacitor into a resistive load, but
`which has been truncated.
`
`FIG. 3 shows the typical biphasic waveform of the output
`of a defibrillator in accordance with the teachings of the
`prior art as set forth above.
`FIG. 4 shows the biphasic waveform generated by a
`defibrillator in accordance with this invention.
`
`FIG. 5 is a block diagram of a pulse generator for an
`implantable defibrillator which is capable of delivering
`biphasic truncated exponential waveform in accordance
`with this invention.
`
`FIG. 6 is a biphasic defibrillator waveform in accordance
`with this invention such as could be delivered by the
`generator of FIG. 5.
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`5,468,254
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`5
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`FIG. 7 is a chart setting forth the probability of a suc—
`cessful defibrillation while varying the relative durations of
`first and second phases of an 80% tilt biphasic waveform
`with the overall waveform length being held at a constant 20
`milliseconds.
`
`FIG. 8 is a chart comparing the energy required for
`successful dcfibrillation for 80% tilt waveforms of four
`different fixed durations with the duration of the first phase
`with respect to the second phase being 40/60% in accor-
`dance with this invention, and 60/40% a now generally
`accepted duration relationship.
`FIG. 9 is a chart comparing energy requirements with
`respect to the switching point from first to second phase as
`a percentage of peak voltage for a biphasic waveform.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`Generally, all current implantable defibrillation pulse gen-
`erators and even some external generators use a capacitor
`discharge to deliver pulse energy to a heart to be defibril—
`lated. The discharge of a charged capacitor into a resistive
`load, which is representative of a heart and attached defibril-
`lation electrodes, results in a waveform with an exponential
`decay from a peak voltage VP as shown in FIG. 1. When a
`capacitor discharge is used for defibrillation, prior experi—
`mentation has established that stopping or truncating the
`discharge at a voltage V7, before it reaches too low a level,
`as shown in FIG. 2, results in more efficacious defibrillation.
`Further, as set forth in the description of related art set forth
`above, it has been shown that causing a reversal of polarity
`during the discharge to produce a so-ealled biphasic wave-
`form, as shown in FIG. 3, results in improved defibrillation
`efficacy as compared to a “monophasic” waveform such as
`shown in FIG. 2. Referring to FIG. 3, during the first phase,
`which has a duration D1, the discharge voltage decays from
`VP to V5. The polarity of the pulse applied to the heart is
`then reversed, with the voltage decaying during the second
`phase of a duration D2 from —V5 to —VT. In accordance with
`the prior art, duration D1 is typically equal to duration D2,
`or longer, such as the 50/50% relationship shown in FIG. 3.
`A biphasic defibrillation waveform such as shown in FIG. 3
`may be generated by any number of circuits, one of which
`is set forth in U.S. Pat. No. 4,850,357 issued Jul. 25, 1989
`and assigned to the assignee of this application. The teach-
`ings of this patent while briefly set forth hereinafter, are
`incorporated herein by reference as an example of the type
`of circuit which might be used in practicing the teachings of
`this invention.
`
`As set forth above in the description of related art, it has
`been the consensus of those skilled in the art that making the
`first phase of a biphasic waveform at least equal
`to or
`preferably longer in duration than the second phase provides
`the most effective defibrillation, all other parameters being
`the same. Further, biphasic waveforms have most frequently
`been used with the initial voltage of the second phase equal
`to the terminal voltage of the first phase, such that consid-
`erably less energy is conveyed through the electrodes to the
`heart by the second phase as compared to the first phase.
`This type of biphasic waveform facilitates the use of a single
`energy storage capacitor. However, if two capacitors are
`utilized, the initial amplitudes of each phase could be of
`equal magnitude or in any desired ratio of magnitudes.
`The applicant has now determined, contrary to the teach-
`ings of the prior art as set forth above, that the defibrillation
`efiicacy of biphasic waveform pulses are improved by
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`making the first phase shorter than the second. That is it has
`been determined, for instance, that the waveform shown in
`FIG. 4, wherein D1 and D2 are 40% and 60% respectively of
`the total pulse duration, has lower defibrillation strength
`requirements than the previously used biphasic waveform
`having equal first and second phases as is shown in FIG. 3,
`or longer first phases.
`Having determined that the defibrillation strength require-
`ments are lower with a shorter first phase, it is now possible:
`to reduce the size of the pulse generator, to increase the
`defibrillator safety margin, to provide a therapy which is
`more easily tolerated by the patient and to provide a higher
`implant success rate due to the more efficacious therapy,
`particularly when endocardial leads are used.
`A biphasic pulse generator of the type shown in the
`above~mentioned U.S. Pat. No. 4,850,357 may,
`through
`circuit component adjustments, deliver pulses in accordance
`with the applicant’s invention. However, the same has; not
`been previously done, wherein to do so was contrary to that
`which has been consistently taught in the prior art.
`Referring to FIG. 5, a block diagram of a biphasic pulse
`generator of the type set forth in the just mentioned patent
`is shown. FIG. 5 corresponds in general to FIG. 2 of the just
`mentioned patent. Control circuit 10 is
`a Four State
`Sequencer specifically designed to provide a first wait
`duration DWI, a first pulse duration D1, a second wait
`duration DW2, and a second pulse duration D2 by monitoring
`the voltage on capacitor 12 and providing timed signals to
`electronic switches 14 and 16 via drive circuits 18 and 20
`respectively. An output waveform from the circuit of FIG. 5
`in accordance with this invention is shown in FIG. 6. When
`drive circuit 18 is set high,
`the electronic switch 14 is
`allowed to conduct and a thyristor 22 is turned “on”, such
`that the charge stored on capacitor 12 is delivered to the
`heart across electrodes 24 and 26 in a first polarity. After a
`first phase duration D1 as determined by output sense circuit
`28, drive circuit 18 is forced low turning off electronic
`switch 14 and thyristor 22. After a short delay Dm, pref.
`erably less than 500 milliseconds, drive circuit 20 is set high
`turning on electronic switch 16 and a thyristor 30 thus,
`providing current of the opposite polarity to the heart. After
`second phase duration D2 determined by output sense circuit
`28, the electronic switch 16 is turned off which turns off
`thyristor 30. While the circuit of FIG. 5 is shown to provide
`wait durations DWI and DW in FIG. 6, the duration DW2
`being significantly less than D1 or D2, it is not shown in the
`waveform of FIGS. 1—4.
`
`In summary, the electronic switch 14 conducts to steer the
`low voltage side of the main storage capacitor 12 to elec—
`trode 26 while the high side is connected to electrode 24.
`Electronic switch 16 conducts to steer the low voltage side
`of the main storage capacitor 12 to electrode 24 while the
`high side is connected to electrode 26 The thyristor 22, when
`switch “on”, provides the high voltage to electrode 2’1. The
`thyristor 30, when switched “on”, provides the high voltage
`to electrode 26.
`
`Output sense circuit 28 monitors the output across elec-
`trodes 24 and 26. When the output voltage across electrodes
`24 and 26 falls to a predetermined level, the output sense
`circuit 28 will signal the control circuit 10, which then forces
`drive circuit 18 low. This shuts otf electronic switch 14 and,
`therefore,
`thyristor 22. When the voltage applied across
`electrodes 24 and 26 falls to a still lower predetermined
`value, the output sense circuit 28 again signals the control
`circuit 10. This forces drive circuit 20 to be switched to a
`low, which shuts off electronic switch 16 and, therefore, the
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`5,468,254
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`7
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`thyristor 30.
`The high voltage isolation transformers 32 and 34 are
`used to isolate the thyristor drive circuits and prevent the
`transmission of undesired currents to them. Also, the high
`voltage isolation transformers are used to separate one
`section of the system from undesired influences of the other
`section.
`
`The applicant has conducted studies with swine which
`establish the efficacy of a biphasic pulse having a shorter
`first phase in accordance with his invention. As hereinafter
`set forth, specific groups of different biphasic defibrillation
`waveforms were applied to each swine, with the data used
`to fit defibrillation probability of success curves for each
`waveform in each swine. The fifty percent probability of
`success was obtained for each waveform from the curves.
`Using appropriate statistical tests, the fifty percent probabil-
`ity of success levels were compared for significant differ-
`ences. Six swine were tested in each study.
`
`STUDY 1
`
`FIG. 7 is a bar chart showing the results of a study
`comparing the energy levels required for 50% probability of
`successful defibrillation for 20 millisecond, 80% tilt bipha-
`sic waveforms, wherein the duration of the first phase of the
`waveform was 4, 8 or 12 milliseconds. The length of each
`bar and the values above each bar represent a normalized
`value of the energy required for the 50% probability of
`successful defibrillation under otherwise like conditions.
`The 80% tilt referred to in FIG. 7 is determined by dividing
`the difierencc between the initial voltage and the final
`voltage by the initial voltage and expressing that value as a
`percentage. That is, if the initial voltage is 100 and the final
`voltage is 20, the tilt is 80%. Referring to FIGS. 1—4, and 6
`showing defibrillator waveforms, VP is the initial (or peak)
`voltage and VT is the final (or trailing) voltage of the
`waveforms.
`
`Prior to the applicant’s invention, for a waveform with a
`total duration of 20 milliseconds, a minimum of 10 milli-
`seconds duration for the first phase has been commonly
`used. This was based upon the teachings of the prior art that
`the first phase should be equal to or longer than the second.
`As set forth in FIG. 7, for a very short first phase duration,
`that is 4 milliseconds, the energy levels required for suc-
`cessful dcfibrillation were increased dramatically over that
`for a 12 millisecond first phase duration as used in the past.
`However, a significantly lower energy level resulted when
`comparing the previously used 12 millisecond first phase to
`an 8 millisecond first phase in accordance with this inven-
`tion. This difference was statistically significant at
`the
`p§0.05 level.
`
`STUDY 2
`
`FIG. 8 is a bar chart showing the results of a study
`conducted with 6 swine which considered biphasic wave-
`forms of 5, 10, 15 and 20 milliseconds total durations and
`80% tilt. The energy requirements for the 40—60% first phase
`to second phase durations of this invention were compared
`to those for 60—40% durations used in the past at each of the
`four total durations. The length of each of the bars and the
`values above each bar in FIG. 8 represents a per unit value
`of the energy required for fifty percent probability of suc-
`cessful defibrillation under otherwise like conditions. The
`pulse durations shown in milliseconds are the total durations
`for the biphasic waveforms. It should be noted that the
`energy requirements at all four total durations are lower for
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`8
`the 40—60 ratio in accordance with this invention compared
`to those for the 60—40% waveforms previously used. Energy
`requirements are reduced an average of 30 percent. Statis-
`tically significant differences were found at the 10, 15 and 20
`millisecond total durations pulses.
`
`STUDY 3
`
`FIG. 9 is a bar chart showing the results of a study
`conducted with 6 swine which considered biphasic wave-
`forms having a fixed 10 millisecond total duration and an
`80% tilt. The switch point between the first and second phase
`was varied and is expressed in the bar chart of FIG. 9 as
`occurring at a certain percentage of the initial voltage. The
`length of each bar and the value above each bar in FIG. 9
`represents the value of the energy required for the same
`probability of successful defibrillation under otherwise like
`conditions normalized to the energy value at the 40% of
`peak voltage switching point. The switching point at 40%
`results in the previously used 60/40% biphasic waveform.
`Switching points greater than 40% decrease the relative
`duration of the first phase and increase the relative duration
`of the second phase. As shown in FIG. 9, increasing the
`switching point to 50% results in a 26% lower energy
`requirement than the previously used 40% switching point.
`The 50% of peak voltage switching point yields the pre-
`ferred 40/60% biphasic waveform ratio in accordance with
`this invention.
`
`In accordance with the teachings of the circuit shown in
`FIG. 5, the switching point in accordance with the previ-
`ously used 60/40% ratio is at 40% of the initial voltage. In
`accordance with this invention, a preferred value of the
`switching point is at 50% of the initial voltage. Similarly, the
`termination voltage is determined by output sense circuit 28
`as a percentage of the initial voltage which would remain
`unchanged at 20% of the initial value of V.
`Biphasic truncated exponential pulses in accordance with
`this invention may also be provided by pulse generators
`wherein the relative durations of the first and second pulses
`are determined by a ratio-metric control circuit. That is, the
`first phase is still terminated at a voltage determined as a
`percentage of the initial voltage, but the duration of the first
`phase is measured and the second phase is terminated when
`its duration reaches a certain percentage of the duration of
`the first phase. Such a pulse generator is set forth in
`copending application Ser. No. 07/951,232, filed Sep. 25,
`1992, entitled METHOD AND APPARATUS FOR GENER-
`ATING MULTIPHASIC DEFIBRILLATION WAVEFORMS
`BASED ON PULSE WIDTH RATIOS, which is assigned to
`the assignee of this application, and the teachings of which
`application are incorporated herein by reference.
`Thus,
`in accordance with this invention, an improved
`method and apparatus for defibrillation of a heart is pro-
`vided. The defibrillation method of this invention has
`improved performance characteristics, such that a defibril—
`lator operated in accordance with this invention may be
`made smaller, compared to prior art defibrillations,
`the
`
`defibrillator safety margin may be increased and a higher
`
`
`
`implant success rate realized due to the more e‘icacious
`therapy provided by the invention.
`It should be apparent to those skilled in the art that what
`has been described is considered at present to be the pre-
`ferred embodiment of the defibrillation method and appa-
`ratus of this invention. In accordance with the Patent Stat-
`utes, changes may be made in the defibrillation method and
`apparatus without actually departing from the true spirit and
`
`8
`
`

`

`5,468,254
`
`9
`this invention is
`scope of this invention. For instance,
`applicable to any biphasic or multiphasic waveform, with
`different shapes, i.e., square, ramp, triangle, sinusoidal, etc.,
`of various tilts, and without regard to the particular circuit
`which develops the waveform In the case of multiphasic
`waveforms, the first phase would be shorter than either the
`second phase or a combination of ensuing phases including
`the second. Further, this invention is not limited to defibril-
`lation applications, but is generally applicable to converting
`arrhythrnias of the heart, particularly both atrial and ven-
`tricular
`tachycardia arrhythmias. Finally, cardioverting
`shocks in accordance with this invention may be applied to
`the heart through either internal or external electrodes.
`The appended claims are intended to cover all such
`changes and modifications which fall in the true spirit and
`scope of this invention.
`We claim:
`1. A method for defibrillating a heart by applying elec—
`trical energy shocks to the heart through electrodes, the
`method comprising:
`A. applying a first shock having a first duration and a first
`polarity to the heart through the electrodes,
`B. applying at least a second shock, follwing said first
`shock, to the heart through the electrodes, said second
`shock having a second polarity opposite to said first
`polarity, and having a second duration wh