`6,047,212
`[11] Patent Number:
`
`Gliner et al.
`[45] Date of Patent:
`*Apr. 4, 2000
`
`[19]
`
`USOO6047212A
`
`[54] EXTERNAL DEFIBRILLATOR CAPABLE OF
`DELIVERING PATIENT IMPEDANCE
`COMPENSATED BIPHASIC VVAVEFORMS
`
`[75]
`
`Inventors: Bradford E. Gliner, Bellevue; Thomas
`D. Lyster, Bothwell; Clinton S. Cole,
`Kirkland; Daniel J. Powers; Carlton B.
`Morgan, both of Bainbridge Island, all
`of Wash.
`
`[73] Assignee: Heartstream, Inc., Seattle, Wash.
`
`[*] Notice:
`
`This patent issued on a continued pros—
`ecution application filed under 37 CFR
`1.53(d), and is subject to the twenty year
`patent
`term provisions of 35 U.S.C.
`154(a)(2).
`
`
`
`21] Appl. No.: 08/946,843
`
`22]
`
`Filed:
`
`Oct. 8, 1997
`
`Related US. Application Data
`
`63] Continuation of application No. 08/803,094, Feb. 20, 1997,
`Pat. No. 5,735,879, which is a continuation of application
`No. 08/103,837, Aug. 6, 1993, abandoned.
`
`
`
`............................................. A61N 1/39
`.......................... 607/7; 607/5; 607/74,
`607/8
`58] Field of Search ........................................ 607/4—8, 74
`
`Int. Cl.7
`51]
`52] US. Cl.
`
`56]
`
`References Cited
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`
`.
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`.
`.
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`OTI IER PUBLICATIONS
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`(List continued on next page.)
`Primary Examiner%ennedy J. Schaetzle
`
`[57]
`
`ABSTRACT
`
`le;.5this invention provides an external defibrillator and
`defibrillation method that automatically compensates for
`patient-to-paticnt impedance differences in the delivery of
`electrotherapeutic pulses for defibrillation and cardiover—
`sion. In a preferred embodiment, the defibrillator has an
`energy source that may be discharged through electrodes on
`the patient to provide a biphasic voltage or current pulse. In
`one aspect of the invention,
`the first and second phase
`duration and initial first phase amplitude are predetermined
`values. In a second aspect of the invention, the duration of
`the first phase of the pulse may be extended if the amplitude
`of the first phase of the pulse fails to fall to a threshold value
`by the end of the predetermined first phase duration, as
`might occur with a high impedance patient. In a third aspect
`of the invention, the first phase ends when the first phase
`amplitude drops below a threshold value or when the first
`phase duration reaches a threshold time value, whichever
`comes first, as might occur with a low to average impedance
`patient. This method and apparatus of altering the delivered
`biphasic pulse thereby compensates for patient impedance
`
`
`differences by changing the nature of the delivered electro—
`
`
`therapeutic pulse, resulting in a smaller, more e icient and
`less expensive defibrillator.
`
`(List continued on next page.)
`
`12 Claims, 7 Drawing Sheets
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`6,047,212
`
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`OTHER PUBLICATIONS
`
`3,782,389
`3,860,009
`3,862,636
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`4,399,818
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`4,473,078
`4,494,552
`4,504,773
`4,574,810
`4,595,009
`4,610,254
`4,619,265
`4,637,397
`4,745,923
`4,800,883
`4,821,723
`4,840,177
`4,848,345
`4,850,357
`4,953,551
`4,998,531
`5,078,134
`5,083,562
`5,097,833
`5,107,834
`5,111,813
`5,111,816
`5,207,219
`5,215,081
`5,222,480
`5,222,492
`5,230,336
`5,237,989
`5,249,573
`5,275,157
`5,306,291
`5,334,219
`5,334,430
`5,352,239
`5,370,664
`5,372,606
`5,385,575
`5,41 1,525
`5,411,526
`5,413,591
`5,431,686
`5,441,518
`5,489,293
`5,507,781
`5,634,938
`
`1/1974
`1/1975
`1/1975
`6/1975
`5/1977
`5/1982
`/1983
`12/1983
`9/1984
`1/1985
`3/1985
`3/1986
`6/1986
`9/1986
`10/1986
`1/1987
`5/1988
`1/1989
`4/1989
`6/1989
`/1989
`7/1989
`9/1990
`3/1991
`1/1992
`1/1992
`3/1992
`7’1992
`5/1992
`5/1992
`5/1993
`6/1993
`6/1993
`6/1993
`7/1993
`7’1993
`10/1993
`1/1994
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`.
`
`.
`.
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`.
`
`.
`
`.
`
`.
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`.
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`.
`
`.
`
`.
`
`.
`
`.
`
`.
`
`..
`
`
`FOREIGN PATENT DOCUMENTS
`0437104 A1
`0457604
`0 491 649 A2
`0507504 A1
`2070435
`2083363
`W0 93/16759
`WO 94/21327
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`
`.
`.
`.
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`
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`
`
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`al. “Ventricular defibrillation with square
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`transthoracic cardioversion”
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`(1965).
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`and cardioversion: clinical studies using an automated
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`lar
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`Report (Jill. 22, 1991).
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`for First Medical Semi—Automatic
`Defibrillator (1994), Spacelabs.
`
`2
`
`
`
`6,047,212
`Page 3
`
`Product Brochure for the Shock Advisory System (1987),
`Physio—Control, 11811 Willow Road Northeast, PO. Box
`97006, Redmond WA 98073.9706.
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`(Jun—Jul. 1984).
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`current pathways for transvenous cardioversion in rapid
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`1987).
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`1987).
`Schuder “The role of an engineering oriented medical
`research group in developing improved methods and devices
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`Amer Soc. Artif. Int. Organs XVIII:514—519 (1972).
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`switched, one—cycle quasisinusoidal waveform with criti-
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`Cir.
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`
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`kg calves” Devices & lee/1. Meeting NIH (1981).
`7
`Schuder et al. “Waveform dependency in defibrillation’
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`)
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`(1992).
`
`3
`
`
`
`US. Patent
`
`Apr. 4,2000
`
`Sheet 1 0f7
`
`6,047,212
`
`VOLTAGE
`
`
`
`VOLTAGE
`
`
`
`
`
`VOLTAG E
`
`VTHRESH tTHRESH
`
`fA
`
`H30
`
`4
`
`
`
`US. Patent
`
`Apr. 4,2000
`
`Sheet 2 0f7
`
`6,047,212
`
`10
`
`INITIATE DISCHARGE
`
`
`IN FIRST POLARITY
`
`
`12
`
`
`
`NO
`
`VOLTAGE < VTHRESH
`?
`
`YES
`
`STOP DISCHARGE
`IN FIRST PHASE
`
`WAIT FOR
`
`INTERIM TIME G
`
`CHANGE
`POLARITY
`
`16
`
`20
`
`22
`
`
`
`
`18
`
`
`
`RESUME DISCHARGE
`FOR SECOND PHASE
`DURATION F
`
`24
`
`STOP DISCHARGE
`
`FIG. 3
`
`5
`
`
`
`US. Patent
`
`Apr. 4,2000
`
`Sheet 3 0f7
`
`6,047,212
`
`VOLTAGE
`
`A
`
`VTHRESH‘
`
`————————————— I+G+i
`
`l
`
`tTHRESH
`
`VOLTAGE
`
`fA
`
`VTHRESHI
`
`tTHRESH
`
`VOLTAG E
`
`A
`
`VTHRESHi
`
`6
`
`
`
`US. Patent
`
`Apr. 4,2000
`
`Sheet 4 0f7
`
`6,047,212
`
` 50
`INITIATE DISCHARGE
`IN FIRST POLARITY
`
`
`
`YES
`
`STOP DISCHARGE
`IN FIRST PHASE
`
`VOLTAGE < V'THHESH
`
`58
`
`60
`
`62
`
`64
`
`WAIT FOR
`INTERIM TIME G
`
`CHANGE
`POLARITY
`
`
`
`
`
`
`
`
`
`
`RESUME DISCHARGE
`FOR SECOND PHASE
`
`DURATION F
`
`
`
`STOP DISCHARGE
`
`FIG. 6
`
`7
`
`
`
`US. Patent
`
`Apr. 4,2000
`
`Sheet 5 0f7
`
`6,047,212
`
`
`
`INITIATE DISCHARGE
`IN FIRST POLARITY
`
`90
`
`
`
`
`VOLTAGE < VTHRESH
`VOLTAGE < VTHRESH
`
`
`
`
`
`
`CHANGE
`POLARITY
`
`STOP DISCHARGE
`OF FIRST PHASE
`
`
`WAIT FOR
`
`
`INTERIM TIME G
`
`FIG. 9
`
`95
`
`96
`
`RESUME DISCHARGE
`FOR SECOND PHASE
`DURATION F
`
`
`
`
`
`STOP DISCHARGE
`
`97
`
`98
`
`8
`
`
`
`US. Patent
`
`Apr. 4, 2000
`
`Sheet 6 0f 7
`
`6,047,212
`
`
`
`ELECTRODE
`36
`
`E
`
`LECTRODE
`36
`
`CONNECTOR
`34
`
`ENERGY
`SOURCE
`32
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`6,047,212
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`1
`EXTERNAL DEFIBRILLATOR CAPABLE OF
`DELIVERING PATIENT IMPEDANCE
`COMPENSATED BIPHASIC WAVEFORMS
`
`This application is a continuation of application Ser. No.
`08/803,094 filed Feb. 20, 1997, now U.S. Pat. No. 5,735,
`879, which is a continuation of application Ser. No. 08/103,
`837 filed Aug. 6, 1993, now abandoned.
`BACKGROUND OF THE INVENTION
`
`This invention relates generally to an electrotherapy
`method and apparatus for delivering a shock to a patient’s
`heart. In particular, this invention relates to a method and
`apparatus for using an external defibrillator to deliver a
`biphasic defibrillation shock to a patient’s heart through
`electrodes attached to the patient.
`Defibrillators apply pulses of electricity to a patient’s
`heart to convert ventricular arrhythmias, such as ventricular
`fibrillation and ventricular tachycardia,
`to normal heart
`rhythms through the processes of defibrillation and
`cardioversion, respectively. There are two main classifica-
`tions of defibrillators: external and implanted. Implantable
`defibrillators are surgically implanted in patients who have
`a high likelihood of needing electrotherapy in the future.
`Implanted defibrillators typically monitor the patient’s heart
`activity and automatically supply electrotherapeutic pulses
`directly to the patient’s heart when indicated. Thus,
`implanted defibrillators permit the patient to function in a
`somewhat normal fashion away from the watchful eye of
`medical personnel.
`External defibrillators send electrical pulses to the
`fiatient’s heart through electrodes applied to the patient’s
`orso. External defibrillators are useful in the emergency
`room, the operating room, emergency medical vehicles or
`other situations where there may be an unanticipated need to
`rovide electrotherapy to a patient on short notice. The
`advantage of external defibrillators is that they may be used
`on a patient as needed, then subsequently moved to be used
`with another patient. However, because external defibrilla—
`ors deliver their electrotherapeutic pulses to the patient’s
`leart indirectly (i.e., from the surface of the patient’s skin
`rather than directly to the heart), they must operate at higher
`energies, voltages and/or currents than implanted defibril-
`ators. The high energy, voltage and current requirements
`lave made current external defibrillators large, heavy and
`expensive, particularly due to the large size of the capacitors
`or other energy storage media required by these prior art
`devices.
`
`
`
`The time plot of the current or voltage pulse delivered by
`a defibrillator shows the defibrillator’s characteristic wave-
`form. Waveforms are characterized according to the shape,
`polarity, duration and number of pulse phases. Most current
`external defibrillators deliver monophasic current or voltage
`electrotherapeutic pulses, although some deliver biphasic
`sinusoidal pulses. Some prior art implantable defibrillators,
`on the other hand, use truncated exponential, biphasic wave-
`forms. Examples of biphasic implantable defibrillators may
`be found in US. Pat. No. 4,821,723 to Baker, Jr., et al.; U.S.
`Pat. No. 5,083,562 to de Coriolis et al.; U.S. Pat. No.
`4,800,883 to Winstrom; U.S. Pat. No. 4,850,357 to Bach, Jr.;
`and U.S. Pat. No. 4,953,551 to Mehra et al.
`Because each implanted defibrillator is dedicated to a
`single patient, its operating parameters, such as electrical
`pulse amplitudes and total energy delivered, may be effec-
`tively titrated to the physiology of the patient to optimize the
`defibrillator’s effectiveness. Thus, for example, the initial
`
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`voltage, first phase duration and total pulse duration may be
`set when the device is implanted to deliver the desired
`amount of energy or to achieve that desired start and end
`voltage differential (i.e, a constant tilt).
`In contrast, because external defibrillator electrodes are
`not in direct contact with the patient’s heart, and because
`external defibrillators must be able to be used on a variety of
`patients having a variety of physiological differences, exter-
`nal defibrillators must operate according to pulse amplitude
`and duration parameters that will be effective in most
`patients, no matter what
`the patient’s physiology. For
`example,
`the impedance presented by the tissue between
`external defibrillator electrodes and the patient’s heart varies
`from patient to patient, thereby varying the intensity and
`waveform shape of the shock actually delivered to the
`patient’s heart for a given initial pulse amplitude and dura—
`tion. Pulse amplitudes and durations effective to treat low
`impedance patients do not necessarily deliver elTective and
`energy efficient treatments to high impedance patients.
`Prior art external defibrillators have not fully addressed
`the patient variability problem. One prior art approach to this
`problem was to provide the external defibrillator with mul-
`tiple energy settings that could be selected by the user, A
`common protocol
`for using such a defibrillator was to
`attempt defibrillation at an initial energy setting suitable for
`defibrillating a patient of average impedance, then raise the
`energy setting for subsequent defibrillation attempts in the
`event that the initial setting failed. The repeated defibrilla-
`tion attempts require additional energy and add to patient
`risk. What is needed, therefore, is an external defibrillation
`method and apparatus that maximizes energy efficiency (to
`minimize the size of the required energy storage medium)
`and maximizes therapeutic efficacy across an entire popu-
`lation of patients.
`SUMMARY OF THE INVENTION
`
`This invention provides an external defibrillator and
`defibrillation method that automatically compensates for
`patient—to—patient impedance differences in the delivery of
`electrotherapeutic pulses for defibrillation and cardiover-
`sion. In a preferred embodiment, the defibrillator has an
`energy source that may be discharged through electrodes on
`the patient to provide a biphasic voltage or current pulse. In
`one aspect of the invention,
`the first and second phase
`duration and initial first phase amplitude are predetermined
`values. In a second aspect of the invention, the duration of
`the first phase of the pulse may be extended if the amplitude
`of the first phase of the pulse fails to fall to a threshold value
`by the end of the predetermined first phase duration, as
`might occur with a high impedance patient. In a third aspect
`of the invention, the first phase ends when the first phase
`amplitude drops below a threshold value or when the first
`phase duration reaches a threshold time value, whichever
`comes first, as might occur with a low to average impedance
`patient. This method and apparatus of altering the delivered
`biphasic pulse thereby compensates for patient impedance
`
`dilIerences by changing the nature of the delivered electro-
`
`
`therapeutic pulse, resulting in a smaller, more e icient and
`less expensive defibrillator.
`BRIEF DESCRIPTION OF THE DRAWIVGS
`
`
`
`FIG. 1 is a schematic representation of a low—tilt biphasic
`electrotherapeutic waveform according to a first aspect of
`this invention.
`
`FIG. 2 is a schematic representation of a high-tilt biphasic
`electrotherapeutic waveform according to the first aspect of
`this invention.
`
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`FIG. 3 is a flow chart demonstrating aart of an electro-
`herapy method according to a second aspect of this inven—
`ion.
`
`6,047,212
`
` Jart of an electro-
`
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`4
`energy delivery requirements affect defibrillator battery and
`capacitor choices.
`We have determined that, for a given patient, externally-
`applied truncated exponential biphasic waveforms defibril-
`late at lower voltages and at lower total delivered energies
`than externally-applied monophasic waveforms. In addition,
`we have determined that there is a complex relationship
`between total pulse duration, first to second phase duration
`ratio, initial voltage, total energy and total tilt.
`Up to a point, the more energy delivered to a patient in an
`electrotherapeutic pulse, the more likely the defibrillation
`attempt will succeed. Low-tilt biphasic waveforms achieve
`effective defibrillation rates with less delivered energy than
`high-tilt waveforms. However,
`low-tilt waveforms are
`energy inefficient, since much of the stored energy is not
`delivered to the patient. On the other hand, defibrillators
`delivering high-tilt biphasic waveforms deliver more of the
`stored energy to the patient than defibrillators delivering
`low—tilt waveforms while maintaining high efficacy up to a
`certain critical tilt value. Thus, for a given capaci or, a given
`initial voltage and fixed phase durations, high impedance
`patients receive a waveform with less total energy and lower
`peak currents but better conversion properties 3er unit of
`energy delivered, and low impedance patients receive a
`waveform with more delivered energy and higher peak
`
`currents. There appears to be an optimum tilt range in which
`high and low impedance patients will receive e “ective and
`efficient therapy. An optimum capacitor charged to a prede-
`termined voltage can be chosen to deliver an e ective and
`efficient waveform across a population of patien s having a
`variety of physiological differences.
`This invention is a defibrillator and defibrillation method
`that takes advantage of this relationship between waveform
`tilt and total energy delivered in high and low impedance
`patients. In one aspect of the invention,
`the defibrillator
`operates in an open loop, i.e., without any feedback regard—
`ing patient
`impedance parameters and with preset pulse
`phase durations. The preset parameters of the waveforms
`shown in FIGS. 1 and 2 are therefore the initial voltage Aof
`the first phase of the pulse, the duration E of the first phase,
`the interphase duration G, and the duration F of the second
`phase. The terminal voltage B of the first phase, the initial
`voltage C of the second phase, and the terminal voltage D of
`the second phase are dependent upon the physiological
`parameters of the patient and the physical connection
`between the electrodes and the patient.
`the total
`For example,
`if the patient
`impedance (i.e.,
`impedance between the two electrodes) is high, the amount
`of voltage drop (exponential decay) from the initial voltage
`Ato the terminal voltage B during time E will be lower (FIG.
`1) than if the patient impedance is low (FIG. 2). The same
`is true for the initial and terminal voltages of the second
`phase during time F. The values of A, E, G and F are set to
`optimize defibrillation and/or cardioversion efficacy across a
`population of patients. Thus, high impedance patients
`receive a low-tilt waveform that is more effective per unit of
`delivered energy, and low impedance patients receive a
`high—tilt waveform that delivers more of the stored energy
`and is therefore more energy efficient.
`Another feature of biphasic waveforms is that waveforms
`with relatively longer first phases have better conversion
`properties than waveforms with equal or shorter first phases,
`provided the total duration exceeds a critical minimum.
`Therefore, in the case of high impedance patients, it may be
`desirable to extend the first phase of the biphasic waveform
`(while the second phase duration is kept constant)
`to
`increase the overall efficacy of the electrotherapy by deliv-
`
`
`
`FIG. 4 is a schematic representation 0 a biphasic wave-
`orm delivered according to the second aspect of this inven—
`ion.
`
`FIG. 5 is a schematic representation 0 a biphasic wave-
`orm delivered according to the second aspect of this inven—
`ion.
`
`FIG. 6 is a flow chart demonstrating dart of an electro-
`herapy method according to a third aspect of this invention.
`FIG. 7 is a schematic representation 0 ‘ a biphasic wave-
`orm delivered according to the third aspect of this inven-
`ion.
`
`FIG. 8 is a schematic representation 0 l a biphasic wave-
`orm delivered according to the third aspect of this inven-
`ion.
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`FIG. 9 is a flow chart demonstrating
`herapy method according to a combination of the second .
`and third aspects of this invention.
`FIG. 10 is a block diagram of a defibrillator system
`according to a preferred embodiment of this invention.
`FIG. 11 is a schematic circuit diagram of a defibrillator
`system according to a preferred embodiment of this inven-
`tion.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`FIGS. 1 and 2 illustrate the patient—to—patient differences
`that an external defibrillator design must take into account.
`These figures are schematic representations of truncated
`exponential biphasic waveforms delivered to two different
`patients from an external defibrillator according to the
`electrotherapy method of this invention for defibrillation or
`cardioversion. In these drawings, the vertical axis is voltage,
`and the horiaontal axis is time. The principles discussed here
`are applicable to waveforms described in terms of current
`versus time as well, however.
`The waveform shown in FIG. 1 is called a low-tilt
`waveform, and the waveform shown in FIG. 2 is called a
`high-tilt waveform, where tilt H is defined as a percent as
`follows:
`
`
`MIelDI
`
`=
`
`,
`x100
`
`As shown in FIGS. 1 and 2, Ais the initial first phase voltage
`and D is the second phase terminal voltage. The first phase
`erminal voltage B results from the exponential decay over
`ime of the initial voltage A through the patient, and the
`second phase terminal voltage D results from the exponen—
`ial decay of the second phase initial voltage C in the same
`manner. The starting voltages and first and second phase
`
`durations of the FIG. 1 and FIG. 2 waveforms are the same,
`
`
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`he differences in end voltages B and D reflect di erenoes in
`datient impedance.
`Prior art disclosures of the use of truncated exponential
`biphasic waveforms in implantable defibrillators have pro—
`vided little guidance for the design of an external defibril-
`ator that will achieve acceptable defibrillation or cardiover-
`sion rates across a wide population of patients. The
`defibrillator operating voltages and energy delivery require—
`ments affect the size, cost, weight and availability of com—
`:Ionents. In particular, operating voltage requirements affect
`he choice of switch and capacitor technologies. Total
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`ering a more efficacious waveform and to increase the total
`amount of energy delivered. FIGS. 3—5 demonstrate a
`defibrillation method according to this second aspect of the
`invention in which information related to patient impedance
`is fed back to the defibrillator to change the parameters of
`the delivered electrotherapeutic pulse.
`FIG. 3 is a flow chart showing the method steps following
`the decision (by an operator or by the defibrillator itself) to
`apply an electrotherapeutic shock to the patient through
`electrodes attached to the patient and charging of the energy
`source, e. g., the defibrillator’s capacitor or capacitor bank, to
`the initial first phase voltage A. Block 10 represents initia—
`tion of the first phase of the pulse in a first polarity.
`Discharge may be initiated manually by the user or auto-
`matically in response to patient heart activity measurements
`(_e.g., ECG signals) received by the defibrillator through the
`electrodes and analyzed by the defibrillator controller in a
`manner known in the art.
`least a
`Discharge of the first phase continues for at
`threshold time tTmSH, as shown by block 12 of FIG. 3. If,
`at the end of time IIHRLW, the voltage measured across the '
`energy source has not dropped below the minimum first
`phase terminal voltage threshold VTHRESH, first phase dis-
`charge continues, as shown in block 14 of FIG. 3. For high
`impedance patients, this situation results in an extension of
`the first phase duration beyond tTHRESH, as shown in FIG. 4,
`until
`the measured voltage drops below the threshold
`VTHRESH. Discharge then ends to complete the first phase, as
`represented by block 16 of FIG. 3. If, on the other hand, the
`patient has low impedance, the voltage will have dropped
`below VTHRESH when the time threshold is reached, result—
`ing in a waveform like the one shown in FIG. 5.
`At the end of the first phase, and after a predetermined
`interim period G, the polarity of the energy source connec-
`tion to the electrodes is switched, as represented by blocks
`18 and 20 of FIG. 3. Discharge of the second phase of the
`biphasic pulse then commences and continues for a prede—
`termined second phase duration F, as represented by block
`26 of FIG. 3, then ceases. This compensating electrotherapy
`method ensures that the energy is delivered by the defibril-
`lator in the most efficacious manner by providing for a
`minimum waveform tilt and by extending the first phase
`duration to meet the requirements of a particular patient.
`Because this method increases the waveform tilt for high
`impedance patients and delivers more of the energy from the
`energy source than a method without compensation,
`the
`defibrillator’s energy source can be smaller than in prior art
`external defibrillators, thereby minimizing defibrillator size,
`weight and expense. It should be noted that the waveforms
`shown in FIGS. 4 and 5 could be expressed in terms of
`current versus time using a predetermined current threshold
`value without departing from the scope of the invention.
`FIGS. 678 illustrate a third aspect of this invention that
`prevents the delivered waveform from exceeding a maxi—
`mum tilt (i.e., maximum delivered energy) in low impedance
`patients. As shown by blocks 52 and 54 in FIG. 6, the first
`phase discharge stops either at the end of a predetermined
`time tTHRESH or when the first phase voltage drops below
`V'THRESH. The second phase begins after an interim period
`G and continues for a preset period F as in the second aspect
`of the invention. Thus, in high impedance patients, the first
`phase ends at time tTHRESH, even if the voltage has not yet
`fallen below V‘THRESH, as shown in FIG. 7. In low imped-
`ance patients, on the other hand,
`the first phase of the
`delivered waveform could be shorter in duration than the
`time tlmxfl, as shown in FIG. 8.
`Once again, the waveforms shown in FIGS. 7 and 8 could
`be expressed in terms of current versus time using a prede-
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