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`DEPT OF COMM./ PAT &TM-PTO-436L
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`PHS OF APPLICATION
`1)EDSEPARATELY
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`NOTICE OF ALLOWANCE MAILED,
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`The information disclosed herein may be restricted.
`by the United States Code Title 35, Sections 12(
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`---
`
`4
`
`

`
`United States Patent [19]
`Gliner et al.
`
`[54] ELECTROTHERAPY METHOD UTILIZING
`PATIENT DEPENDENT ELECTRICAL
`PARAMETERS
`
`[75]
`
`Inventors: Bradford E. Giner, Bellevue; Thomas
`D. Lyster, Bothell; Clinton S. Cole,
`Kirkland; Daniel J. Powers; Canlton B.
`Morgan, both of Bainbridge Island, all
`of Wash.
`
`[731 Assignee: Heartstream, Inc., Seattle, Wash.
`
`[]Notice:
`
`The term of this patent shall not extend
`beyond the expiration date of Pat. No.
`5,601,612.
`
`Appl. No.: 691,755
`Aug. 2, 1996
`
`Filed:
`
`Related U.S. Application Data
`
`IContinuation of Ser. NQ. 103,837, Aug. 6,1993, abandoned,
`. . . . . . . . . . . . . . . . . . . . . A61N 1139
`C1.6 ..................
`I It.
`1 U.S. Cl.....................................
`607/1; 6074
`607/4, 5, 6, 7,
`I Field of Search.......................
`607/8, 2, 39, 40, 42.43-46, 48, 50, 58,
`62. 74
`
`References Cited
`U.S. PATENT DOCUMENT'S
`3,211,154 10/1965 Becker et al..
`3/1966 Caywood et al.
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`
`(List continued on next page.)
`
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`0491649 A
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`11111
`11111
`
`11111 11111
`11111
`ill 11111li11111
`11111111
`11111
`
`US005749905A
`[ill Patent Number:
`[45] Date of Patent:
`
`5,749,905
`*May 12, 1998
`
`2083363
`93/16759
`94/21327
`94/22530
`WO 95/05215
`WO 95/09673
`WO 95132020
`
`3/ 1982
`9/1993
`9/1994
`10/1994
`211995
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`United Kingdom
`WIPO.
`WI PO
`WIPO.
`WIPO.
`WIPO.
`WIPO.
`
`OTHER PUBLICATIONS
`Winide et al., "Improved low energy defibrillation efficacy
`in man using a biphasic truncated exponential waveform"'
`JACC9(2): 142A (1987).
`(List continued on next page.)
`
`Primary Examiner-William E. Kamm
`Assistant Examiner-Kennedy J. Schaetzle
`Attorney; Agent, or Firmn-James R. Shay; Cecily Anne
`Snyder
`
`ABSTRACT
`[57]
`This invention provides an external defibrillator and defibril-
`lation method that automatically compensates for patient-
`to-patient impedance differences in the delivery of electro-
`therapeutic pulses for defibrillation and cardioversion. 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 differ-
`ences by changing the nature of the delivered electrothera-
`peutic pulse, resulting in a smaller, more efficient and less
`expensive defibrillator.
`
`11 Claims, 7 Drawing Sheets
`
`V NATE 'TH-155
`
`LiFHPOLPI
`
`RAC F
`
`5
`
`

`
`5,749,905
`Page 2
`
`U.S. PATENT DOCUMENTS
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`3,782,389
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`4,473,078
`4,494,552
`4,504,773
`4,574,810
`4,595,009
`4,610,254
`4,619265
`4,637,397
`4,745,923
`4,800,883
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`4,850,357
`4,953,551
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`5,230,336
`5,237,989
`5,249,573
`5,275,157
`5,306,291
`5,.334,219
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`5,352,239
`5,370,664
`5,372,606
`5,385,575
`5,411,525
`5,411,526
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`5,489,293
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`3/1986
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`5/1 988
`1/1989
`4/1989
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`7/1989
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`3/1991
`1/1992
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`Miani et al.
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`Jones et at.
`Winstrom .
`Winstrom,.
`Baker, Jr. et al.
`Charbonnmer et al.
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`Bach, Jr.
`Mehra et at.
`B6cchi et at.
`Heilman et at.
`de Coriolis et at.
`Campos.
`Ideker et al.
`Charbonnier et al.
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`Adams et al.
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`Fain et al. .
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`OTHER PUBLICATIONS
`
`Anderson et al. "The efficacy of trapezoidal wave forms for
`ventricular defibrillation" Chest70(2):298-300 (19,76).
`Jones et al. "Decreased defibrillator-induced dysfunction
`with biphasic rectangular waveforms" Am. J. Physiol.
`247:H792-796 (1984).
`Jones et al. "Improved defibrillator waveform. safety factor
`with biphasic waveforms" Am. J. Physiol. 245 :H60-65
`(1983).
`Schuder et al. "Defibrillation of 100 kg calves with asym-
`metrical, bi-directional, rectangular pulses" Can. Res.
`18:419-426 (1984).
`Schuder &t al' 'Transthoracic ventricular defibrillation in the
`100 kg calf wil~ symmetrical one-cycle bidirectional rect-
`angular wave stimuli" IEEE Trans. BME 30(7):4 15422
`(1983).
`
`Schuder et al."Ultrahigh--energy hydrogen thyratron/SCR
`bidirectional waveform defibrillator" Med. & Bio. Eng. &
`Comput. 20:419-424 (1982).
`Schuder et al. 'Transthoracic ventricular defibrillation with
`Square-wave
`cycle" Cir
`stimuli;
`one-hallf
`Res.
`XV:258-264 (1964).
`Schuder et al. "Waveform dependency in defibrillating 100
`kg calves" Devices & Tech. Meeting NIH (1981).
`Tang et al. "Ventricular defibrillation using biphasic: wave-
`forms of different phasic duration" PACE 10:Abstract No. 47
`(1987).
`Tang et al. "Strength duration curve for ventricular defibril-
`lation using biphasic waveforms" PACE, 10: Abstract No. 49
`(1987).
`Wetherbee et al. "Subcutaneous patch electrode-A means
`to obviate thoracotomy for implantation of the automatic
`cardioverter defibrillation
`system?" Circ. 72:111-384,
`Abstract No. 1536 (1985).
`Kerber et at. "Energy, current, and sucess in defibrillation
`and cardioversion: clinical studies using an automated
`impedance-based method of energy adjustment." Cire.
`77(5):1038-1046 (1988).
`Lerman et al. "Current-based versus energy-based ventricu-
`lar
`A
`defibrillation:
`prospective
`JA CC
`study"
`12(5):1259-1264 (1988).
`Alferness, et al
`'The influence of shock waveforms on
`defibrillation efficacy" IEEE Engineering in Medicine and
`Biology pp. 25-27 (Jun. 1990).
`Bilie et al. "Predicting and validating cardiothoracic current
`flow using finite element modeling" PACE 15:563, Abstract
`219 (Apt. 1992).
`Chapman et al. "Non-thoracotomy internal defibrillation:
`Improved efficacy with biphasic
`shocks" Circulation
`76:3 12, Abstract No. 1239 (1987),
`Cooper et al. 'Temporal separation of the two pulses of
`single capacitor biphasic and dual monophasic waveforms"
`Circulation 84(4):612: Abstract No. 2433 (1991).
`Cooper et al. 'The effect of phase separation of biphasic
`waveform. defibrillation" PACE 16:47 1482 (Mar. 1993).
`Cooper et al. 'The effect of temporal separation of phases on
`biphasic waveform defibrillation efficacy" The Annual inter-
`national Conference of the IEEE Engineering in Medicine
`and Biology 13(2):0766-0767 (1991).
`Crampton et al. "Low energy ventricular defibrillation and
`miniature defibrillators" JAMA 235(21):2284 (1976).
`Dahlback et al. "Ventricular defibrillation with square
`waves" The Lancet (Jul. 2, 1966).
`Echt et al. "Biphasic waveform is more efficacious than
`monophasic waveform. for
`transthoracic cardioversion"
`PACE 16:914, Abstract No. 256 (Apr. 1993).
`Feeser et al. "Strength-duration and probability of success
`curves for defibrillation with biphasic waveforms" Circula-
`tion 82(6):2128-2141 (1990).
`Guse et al. "Defibrillation with low voltage using a left
`ventricular catheter and four cutaneous patch electrodes in
`dogs" PACE 14:443-451 (Mar. 1991).
`Jones et al. "Defibrillator waveshape optimization" Devices
`and Tech Meeting NIH (1982).
`,
`Jones et al. "Reduced excitation threshold in potassium
`depolarized myocardial cells with symmetrical biphasic
`waveforms" 1I Mol. Cell. Cardiol, 17(39):XXVII, Abstract
`No. 39 (1985).
`Jude et al. "Fundamentals of cardiopulmonary resuscitation"
`F.A. Davis & Company. Philadelphia, PA, pp. 98-104
`(1965).
`
`6
`
`

`
`5,749,905
`Page 3
`
`Knickerbocker et al. "A portable defibrillator" IEEE Trans
`on Power and Apparatus Systems 69:1089-1093 (1963).
`the defibrillator"
`Kuowenhoven "The development of
`Annals of Internal Medicine 71(3):449-458 (1969).
`Langer et al. "Considerations in the development of the
`automatic implantable defibrillator" Medical Instrumenta-
`tion 10(3):163-167 (1976).
`Lindsay et al. "Prospective evaluation of a sequential pacing
`and high energy bi-directional shock algorithm for trans-
`venous cardioversion in patients with ventricular tachycar-
`dia" Circulation 76(3):601-609 (1987).
`Mirowski et al. "Clinical treatment of life threatening yen-
`tricular tachyarrhythmias with the automatic inplantable
`defibrillator" American Heart J. 102(2):265-270 (198 1).
`'Termaination of malignant ventricular
`Mirowski et al.
`arrhythmias with an implanted automatic defibrillator in
`human beings" New Engl J. Med. 303(6):322-324 (1980).
`Podolsky "Keeping the beat alive" U.S. News & World
`Report (Jul. 22, 1991).
`Product Brochure for the Shock Advisory System (1987).
`Physio-Control, 11811 Willow Road Northeast, P.O. Box
`97006. Redmond WA 98073.9706.
`Redd (editor), "Defibrillation with biphasic waveform may
`increase safety, improve survival" Medlines pp. 1-2 (Jun.-
`Jul. 1984).
`Saksena et al. "A prospective evaluation of single and dual
`current pathways for transvenous cardioversion in rapid
`ventricular tachycardia" PACE 10:1130-1141 (Sep.-Oct.
`1987).
`Saksena et al. "Development for future implantable cardio-
`verters and defibrillators" PACE 10:1342-1358 (Nov.-Dec.
`1987).
`Schuder 'The role of an engineering oriented medical
`research group in developing improved methods and devices
`for achieving ventricular defibrillator: The University of
`Missouri experience" PACE 16:95-124 (Jan. 1993).
`
`Schuder et al. "A multielectrode-time sequential laboratory
`defibrillator for the study of implanted electrode systems"
`Amer. Soc, Artif. Int. Organs XVf11:514-519 (1972).
`implanted
`Schuder et al. "Development of automatic
`defibrillator" Devices & Tech Meeting NIf (198 1).
`Stanton et al. "Relationship between defibrillation threshold
`and upper limit of vulnerability in humans" PACE 15:563,
`Abstract 221 (Apr. 1992).
`Tang at al. "Ventricular defibrillation using biphasic wave-
`of phasic duration"
`importance
`forms: The
`.IACC
`13(1):207-214 (1989).
`Walcott et al. "Comparison of monophasic, biphasic, and the
`edmark waveform. for external defibrillation" PACE 15:563,
`Abstract 218 (Apr. 1992).
`Walthen et al. "Improved defibrillation efficacy using four
`nonthoracotomy leads for sequential pulse defibrillation"
`PACE 15:563, Abstract 220 (Apr. 1992).
`Winkle et al. 'The implantable defibrillator in the ventricular
`arrhythmias" Hospital Practice, pp. 149-165 (Mar. 1983).
`Zipes "Sudden cardiac death" Circulation 85(1):160-166
`(1992).
`Schuder et al. "Waveform dependency
`Devices & Tech Meeting NIH (1981).
`Product Brochure for First Medical Semi-Automatic
`Defibrillator (1994), Spacelabs.
`Schuder et al. "One-cycle bidirectional rectangular wave
`shocks for open chest defibrillation in the calf' Abs. Am. Soc.
`Art if. Intern. Organs 9:16 (1981).
`Schuder et al. "Comparison of effectiveness of relay-
`switched, one-cycle quasisinusoiqal waveformn with criti-
`cally damped sinusoid waveform in transthoracic defibril-
`lation of 100-kilogram calves" Medical Instrumentation
`22(6):281-285 (1988).
`
`in defibrillation"
`
`7
`
`

`
`U.S. Patent
`
`May 12, 1998
`
`Sheqt 1 of 7
`
`5,749,905
`
`VOLTAGE$
`
`VOLTAGE
`
`2
`
`VOLTAGE
`
`VTHR ESH}
`
`A.
`
`.t
`
`E-
`
`tO-
`
`FIG. 1
`
`TIME
`
`FIG. 2
`
`TIME
`
`FIG. 4
`
`~~- F
`
`tTHTIME
`
`TIME
`
`8
`
`

`
`U.S. Patent
`U.S. Patent
`
`May 12, 1998
`May 12, 1998
`
`Sheef 2 of 7
`Sheet 2 of 7
`
`iL
`
`5,749,905
`I 5,749,905
`
`I
`
`INITIATE DISCHARGE
`IN FIRST POLARITY
`
`”N
`
`O
`
`I
`
`7
`
`YES
`
`STOP DISCHARGE
`IN FIRST PHASE
`
`WAIT FOR
`INTERIM TIME G
`
`CHANGE
`POLARITY
`
`RESUME DISCHARGE
`FOR SECOND PHASE
`DURATION F
`
`STOP DISCHARGE
`
`FIG. 3
`
`9
`
`

`
`U.S. Patent
`
`May 12, 1998
`
`Sheet) of 7
`
`5,749,905
`
`VOLTAGE
`
`VTHRESHI']
`
`VTHRfESH-
`
`VOLTAGE
`
`A
`VTHRESHF-
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`
`- -
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`FIG. 7
`
`TIME
`
`TIME
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`TIME
`
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`
`

`
`U.S. Patent
`
`U.S PaentMay
`12, 1998
`
`Sheet 4 of 7
`
`5,749,905
`
`FIG. 6
`
`11
`
`

`
`U.S. Patent
`
`May 12, 1998
`
`Sheet 5 of 7
`
`5,749,905
`
`INITIATE DISCHARGE
`IN FIRST POLARITY
`
`STOP DISCHARGE
`OF FIRST PHASE
`
`FIG. 9
`
`RESUME DISCHARGE
`FOR SECOND PHASE
`DURATION F
`
`12
`
`

`
`U.S. Patent
`U.S. Patent
`
`2,1m.M
`899
`May 12, 1998
`1I
`
`Sheet 6 of 7
`Shee\t 6 of 7
`
`5,749,905
`
`I 30
`
`FIG. 10
`
`13
`
`

`
`U.S. Patent
`U.S. Patent
`
`May 12, 1993
`May 12, 1998
`
`ehS
`7fD7m.
`Sheet 7 of 7
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`5,749,905
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`
`1
`ELECTROTHERAPY METHOD UTILIZING
`PATIENT DEPENDENT ELECTRICAL
`PARAMETERS
`
`This application is a CONTINUATION of application
`Scr. No. 08/103,837 filed Aug. 6, 1993 now abandoned.
`
`BACKGROUND OF THE iVENTION
`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 arrhythrnias. such as ventricular
`to normal heart
`tachycardia,
`fibrillation and ventricular
`the processes of defibrillation and
`rhythms through
`cardioversion, respectively. There are two main classifica-
`tions of defibrillators: external and implanted. Implantable
`defibrfflators 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 eiectrotherapeutic pulses
`to the patient's heart when indicated. Thus,
`directly
`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
`patient's heart through electrodes applied to the patient's
`torso. 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
`provide electrotherapy to a patient short notice. The advan-
`tage of external defibrillators is that they may be used on a
`patient as needed, then subsequently moved to he used with
`another patient. However, because external defibrillators
`deliver their electrotherapeutic pulses to the patient's heart
`indirectly (i.e., from the surface of the patient's skin rather
`to the heart), they must operate at higher
`than directly
`energies, voltages and/or currents than implanted defibril-
`lators. The high energy, voltage and current requirements
`have 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 imnplantable defibrillators,
`on the other hand, use truncated exponential, biphasic wave-
`forms. Examples of biphasic implantable defibrillators may
`be found in U.S. Pat. No. 4,821.723 to Baker, Jr., et al.; U.S.
`to de Coriolis et al.; U.S. Pat. No.
`Pat. No. 5.083.562
`4.800,883 to Winstrom; U.S. Pat. No. 4,850,357 to Bach, Jr.;
`and U.S. Pat. No. 4,953.55 1 to Mehra et al.
`Because each implanted defibrillator is dedicated to a
`single patient. its operating parameters, such as electrical
`pulse ampfttd&s 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
`voltage, first phase duration and total pulse duration may be
`set when the device is implanted to deliver the desired
`
`20
`
`amount oif-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
`5 external defibrillators must be able to be used on a variety of
`patients having a variety of physiological differences, exter-
`nal defibriflators must operate according to pulse amplitude
`and duration parameters that will be effective in most
`patients, no matter what the patient's physiology. For
`10 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
`15 impedance patients do not necessarily deliver effective 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 muli-
`tiple energy settings that could be selected by the user. A
`cornmon 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
`25 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
`minimnize the size of the required energy storage medium)
`30 and maximizes therapeutic efficacy across an entire popu-
`lation of patients.
`SUMMARY OF THE IVENTION
`This invention provides an external defibrillator and
`35 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
`40 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
`45 of the first phase of the pulse falls 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
`so 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-
`55 therapeutic pulse, resulting in a smaller, more efficient and
`less expensive defibrillator.
`BRIEF DESCRHYFION OF THE DRAWINGS
`FIG. 1 is a schematic representation of a low-tilt biphasic
`60 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.
`FIG. 3 is a flow chart demonstrating part of an electro-
`therapy method according to a second aspect of this inven-
`tion.
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`FIG. 4 is a schematic representation of a biphasic wave-
`form delivered according to the second aspect of this inven-
`tion.
`FIG. 5 is a schematic representation of a biphasic wave-
`form delivered according to the second aspect of this inven-
`tion.
`FIG. 6 is a flow chart demonstrating part of an electro-
`therapy method according to a third aspect of this invention.
`FIG. 7 is a schematic representation of a biphasic wave-
`form delivered according to the third aspect of this inven-
`tion.
`FIG. 8 is a schematic representation of a biphasic wave-
`form delivered according to the third aspect of this inven-
`tion.
`FIG. 9 is a flow chart demonstrating part of an electro-
`therapy 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 preferr ed 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
`to the
`patients from an external defibrillator according
`electrotherapy method of this invention for defibrillation or
`cardioversion. in these drawings, the vertical axis is voltage,
`and the horizontal 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:
`
`H =! W D X10
`PAT tOO
`
`As shown in FIGS. 1 and 2, A is the initial first phase voltage
`and 1) is the second phase terminal voltage. The first phase
`terminal voltage B results from the exponential decay over
`time of the initial voltage A through the patient, and the
`second phase terminal voltage D results from the exponen-
`tial 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;
`the differences in end voltages B and D reflect differences in
`patient impedance.
`Prior art disclosures of the use of truncated exponential
`biphasic waveforms in imnplantable defibrillators have pro-
`vided little guidance for the design of an external defibril-
`lator that will achieve acceptable defibrillation or cardiover-
`sioa rates across a wide population of patients. The
`defibrillator operating voltages and energy delivery require-
`ments afL.ect the size, cost, weight and availability of com-
`ponents. in particular, operating voltage requirements affect
`the choice' oh switch and capacitor technologies. Total
`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 low6r 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
`5 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
`10 high-tilt waveforms. However, 10w-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
`15 certain critical tilt value. Thus, for a given capacitor, 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 per unit of
`energy delivered, and low impedance patients receive a
`20 waveform with more delivered energy and higher peak
`currents. There appears to be an optimum tilt range in which
`high and low impedance patients W~ill receive effective and
`efficient therapy. An optimum capacitor charged to a prede-
`termined voltage can be chosen to deliver an effective and
`25 efficient waveform. across a population of patients having a
`variety of physiological differences.
`IThis 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
`30 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 A of
`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
`40 between the electrodes and the patient.
`For example, if the patient impedance (i.e., the total
`impedance between the two electrodes) is high, the amount
`of voltage drop (exponential decay) from the initial voltage
`A to the terminal voltage B during time E will be lower (HIG.
`45 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 E. 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
`50 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
`55 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
`to
`is kept constant)
`60 (while the second phase duration
`increase the overall efficacy of the -electrotherapy by deliv-
`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
`65 invention in which information related to patient impedance
`is fed back to the defibrillator to change the parameters of
`the delivered electrotherapeutic pulse.
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`HIG. 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 intia-
`dion of the first phase of the pulse in a first polarity.
`Discharge my 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.
`Discharge of the first phase continues for at least a
`threshold time trHRESH1 as shown by block 12 of FIG. 3. If,
`at the end of time t7REH 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 HIG. 3. For high
`impedance patients, this situation results in an extension of
`the first phase duration beyond tT,,,,H, as shown in HIG. 4,
`until the measured voltage drops below the threshold
`VTpxH Discharge then ends to complete the first phase, as
`represented by block 16 of HIG. 3. If, on the other hand, the
`patient has low impedance. the voltage will have dropped
`below VTH ..SFl 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 HIG. 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
`22 of HIG. 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. 6-8 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 dr