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`[11] Patent Number:
`[19]
`United States Patent
`5,607,454
`Cameron et a1.
`Mar. 4, 1997
`[45] Date of Patent:
`
`lllIlIlllllllllllllllllllllIlllllllllIlllllllllllllillllllllllllllllllllll
`U8005607454A
`
`[54] ELECTROTIIERAPY METHOD AND
`APPARATUS
`
`[75]
`
`Inventors: David Cameron. Beanie; Thomas D.
`Lyster, Bothell; Daniel J. Powers,
`Bainbridge Island; Bradford E. Gliner,
`Bellevue; Clinton S. Cole, Seattle;
`Carlton B. Morgan, Bainbridge Island.
`all of Wash.
`
`[73] Assignee: Heartstream, Inc., Seattle, Wash.
`
`[21] Appl.ND.: 227,553
`[22]
`Filed:
`Apr. 14,1994
`
`Related U.S. Application Data
`
`[63] Continuation-impart of Ser. No. 103,837, Aug. 6, 1993.
`[51]
`
` ...... AGIN 1/39
`[52] U.S. C1.
`........................ 607/5; 607/7; 607/6; 607/74;
`607/62
`
`. 607/2. 4. 5-7.
`607/62, 74
`
`field of Search .
`
`[58]
`
`[56]
`
`3,211,154
`3,241,555
`3,706,313
`3,782,389
`3,860,009
`3,862,636
`3,886,950
`
`References Cited
`U.S. PATENT DOCUMENTS
`10/1965 Becker :1 n1.
`.
`3/1966 Caywood et al. .
`12/1972 Miltmi et a].
`.
`..... 607/8
`
`1/1974 Bell ......
`607/8
`1/1975 Ballet al.
`.
`607/5
`
`1/1975 Bell etal.
`.
`6/1975 Ukkestad et
`
`(List continued on next page.)
`FOREIGN PATENT DOCUMENTS
`0281219
`9/1983 European Pat. 03', .
`0315368
`5/1989 European PM. Ofl’.
`.
`0353341
`2/1990 European P81. 03. .
`0437104
`7/1991
`European Pat. 01f. ,
`0507504 10/1992 European Pat. 01f. .
`2070435
`9/1931 United Kingdom .
`2083363
`3/1932 United Kingdom .
`W093I16759
`9/1993 WIPO .
`
`9/1994 WIPO.
`WON/21327
`WOW/22530 10/1994 WIPO.
`OTHER PUBLICATIONS
`
`, Alfemess at 8.1., “The influence of shock waveforms on
`defibrillation efficacy," IEEE Engineering in Medicine and
`Biology, pp. 25—27 (Jun. 1990).
`Anderson at 211,, “The efiicacy of trapezoidal wave forms for
`ventricular defibrillation," Chest, 70(2):298—300 [1976).
`Blilie et al., “Predicting and validating cardiothoracic cur-
`rent flow using finite element modeling," PACE, 152563,
`abstract 219 (Apr. 1992).
`(List continued on next page.)
`Primary Examiner—Marvin M. Lateef
`Assistant Examiner—Kennedy .l. Schaetzle
`Altamey, Agent. or Firm—Morrison & Foerster
`[57]
`ABSTRACT
`
`An electrotherapy method and apparatus for delivering a
`multiphasic waveform from an energy source to a patient.
`The preferred embodiment of the method comprises the
`steps of charging the energy source to an initial level;
`discharging the energy source across the electrodes to
`deliver electrical energy to the patient in a multiphasic
`waveform; monitoring a patient-depehdent electrical param-
`eter during the discharging step; shaping the waveform of
`the delivered electrical energy based on a value of the
`monitored electrical parameter, wherein the relative duration
`of the phases of the multiphasic waveform is dependent on
`the value of the monitored electrical parameter. The pre-
`ferred apparatus comprises an energy source; two electrodes
`adapted to make electrical contact with a patient; 3 connect-
`ing mechanism forming an electrical circuit with the energy
`source and the electrodes when the electrodes are attached to
`a patient; and a controller operating the connecting mecha-
`nism to deliver electrical energy from the energy source to
`the electrodes in a multiphasic waveform the relative phase
`durations of which are based on an electrical parameter
`monitored during delivery of the electrical energy. The
`preferred defibrillator apparatus weighs less than 4 pounds
`and has a volume less than 150 cubic inches. and most
`preferably, weighs approximately three pounds or less and
`has a volume of approximately 141 cu. in.
`
`59 Claims, 4 Drawing Sheets
`
`
`
`
`
`5
`
`

`

`5,607,454
`Page 2M
`
`US. PATENT DOCUMENTS
`
`.
`
`.......................... 607/5
`5/1977 Pantridge et a1.
`4,023,573
`511982 Charbcnnier et a1.
`4,328,808
`.
`4,419,998 12/1983 Heath .
`4,473,078
`9/1984 Angel .......................................... 607/6
`4,494,552
`1/1985 Heath .
`4,504,773
`3/1985 Suzuki et 81..
`4,574,810
`3/1986 Lemar: .
`4,595,009
`6/1986 Leinders .
`4,610,254
`9/1986 Morgan et al. .
`4,619,265 10/1986 Morgan et a1.
`.
`4,637,397
`1/1987 Jones et a].
`.
`4,745,923
`5/1983 Winstrorn .
`4,800,883
`1/1989 Winstrom .
`4,821,723
`4/1989 Baker, Jr. et al. .
`4,840,177
`6/1989 Charbcnnier et a1.
`4,848,345
`7/1989 Zenkich .
`.
`4,550,357
`7/1989 Bach, Jr.
`4,953,551
`9/1990 Mehra et a1.
`,
`4,998,531
`3/1991 130ch et al.
`.
`5,078,134
`1/1992 Heilman et a1.
`5,083,562
`1/1992 dc Coriolis et al. .
`5,107,834
`4/1992 Ideker etal..
`5,111,813
`5/1992 Charbnnuier et a].
`5,111,816
`5/1992 Pless etal..
`5,207,219
`5/1993 Adams etal”
`5,215,081
`6/1993 Ostmff .
`5,222,480
`6/1993 Couche et a1..
`5,222,492
`6/1993 Morgan et 111,,
`5,230,336
`7/1993 Fain et al. ................................... 607/7
`5,237,989
`8/1993 Morgan et a1.
`.
`5.249.573 10/1993 Fincko et a1.
`............................... 607/6
`5,275,157
`1/1994 Morgan et a1.
`.
`5,305,291
`4/1994 K1011 et a1.
`.
`5,334,219
`8/1994 Kroll .
`607/5
`5,352,239
`10/1994 Flees
`
`5,370,664 12/1994 Morgan e
`..
`5,372,606 12/1994 Lang et a1.
`.................................. 607/8
`OTHER PUBLICATIONS
`
`.
`
`.
`
`Chapman et al., "Non-thoracotomy internal defibrillation:
`Improved eflicacy with biphasic shocks," Circulation,
`762312, 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 311., "the efl’ect of phase. separation on biphasic
`waveform defibrillation,"mCE, 16:471—482 (Mar. 1993).
`Cooper et al., “The effect of temporal separation of phases
`on biphasic waveform defibrillation efficacy,” The Annual
`International Conference ofthe IEEE Engineering in Medi-
`cine and Biology Society, 13(2):0766-0767 (1991).
`Crampton et al,, “Low-energy ventricular defibrillation and
`miniature defibrillators," JAMA, 235(21):2284 (1976).
`Dahlhack
`et
`al.,
`“Ventricular
`defibrillation with
`square—waves," The Lancet (Jul. 2, 1966).
`Eeht et 31., “Biphasic waveform is more efficacious than
`monophasic waveform for transthuracic cardioversion,"
`MOE, 16:914. abstract no. 2.56 (Apr. 1993).
`Feeser et 31.. "Strength—duration and probability of success
`curves for defibrillation with biphasic waveforms," Circu-
`lation, “(822128-2141 (1990).
`Guse et al., “Defibrillation with low voltage using a left
`ventricular catheter and four cutaneous patch electrodes in
`dogs,” PACE, 143143451 (Mar. 1991).
`Jones et al., “Decreased defibrillatorwinduced dysfunction
`with biphasic rectangular waveforms," Am J. Physiol,
`247:H792—796 (1984).
`
`"Defibrillator wavesbape optimization,”
`31.,
`et
`Jones
`Devices and Tech. Meeting, NIH (1982).
`Jones et al., “Improved defibrillator waveform safety factor
`with hiphasic waveforms." Am, J. Phyriol., 245516065
`(1983).
`Jones at 211,, “Reduced excitation threshold in potassium
`depolarized myocardial cells with symmetrical biphasic
`waveforms," J. Mal. Cell. Cardiol, 17(39):XXVII, abstract
`no. 39 (1985).
`Jude et al., “Fundamentals of Cardiopulmonary Resuscita-
`tion," EA. Davis Company. Philadelphia PA, pp. 98-104
`(1965).
`Kerber et a1., “Energy, current, and success in defibrillation
`and cardioversion: Clinical studies using an automated
`impedance—based method of energy adjustment," Circula-
`tion. 77(5):1038—1046 (1988).
`Knickerbocker et al., “A portable defibrillator,”IEEE Trans:
`on Power and Apparatus Systems, 69: 1089—1093 (1963).
`Kouwenhoven, “The development of the defibrillator,"
`Annals af/ntemal Medicine, 71(3):449—458 (1969).
`Langer ct a1., “Considerations in the development of the
`automatic implantable defibrillator," Medical Instrumenta-
`tion, 10(3):163-167 (1976).
`Lennan et a1. “Current—based versus energy—based ventricu-
`lar
`deflbrillation:
`A
`prospective
`study,"
`JA CC,
`12(5):1259—1264 (1988).
`Lindsay et al., “Prospective evaluation of a sequential pac-
`ing and high—energy bi—directional shock algorithm for
`transvenous cardioversion in patients with ventricular tachy»
`cardia," Circulation, 76(3):601—609 (1987).
`Mirowski et al., “Clinical
`treatment of life threatening
`ventricular tachyarrhythmias with the aut0matic implantable
`defibrillator," American Heart Journal, 102(2):265—270
`(1981).
`‘Termination of malignant ventricular
`211,,
`Mirowski ct
`arrhythmias with an implanted automatic defibrillator in
`human beings," The New England Jaumal of Medicine,
`303(6):322—324 (1980).
`Podolsky, “Keeping the beat alive,“ US, News & World
`Report (Jul. 22, 1991).
`Product Brochure for First Medic Semi‘Automatic Defibril—
`lators (1994), Spaoelabs Medical Products, 15220 NE. 40th
`Street, P.O. Box 97013. Redmond, WA 98073-9713.
`Product Brochure for the Shock Advisory System (1987),
`Physio—Control, 11811 Willows Road Northeast, PO. Box
`97006, Redmond, WA 98073-9706.
`Redd (editor), “Defibrillation with biphasic waveform may
`increase safety,
`improve survival,” Met/lines, 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 (Sept—Oct.
`1987).
`Sakstma at £11.. “Developments for future implantable car-
`dioverters
`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 defibrillation: The University of
`Missouri experience," PACE, 16:95—124 (Jan. 1993).
`of
`Schuder
`et
`a1.
`“Comparison
`of
`effectiveness
`relay—switched, one-cycle quasisinusoidal waveform with
`critically damped sinusoid waveform in transthoracic
`dcfibrillation of 100461on calves,” Medical Instrumen—
`tation, 22(6):281—285 (1988).
`
`6
`
`

`

`5,607,454
`Page 3
`
`Schuder et a1.. "A multielectrode—tirne sequential laboratory
`defibrillator for the study of implanted electrode systems,"
`Amer: Soc. Am]? Int. Organs, XVIII:514—519 (1972).
`Schuder et al., “Defibrillation of 100 kg calves with asym-
`metrical. bi—directional, rectangular pulses,“ Card. Res.
`18:419-426 (1984).
`Schuder ct
`511.. "Development of automatic implanted
`defibrillator.” Devices & Then. Meeting NIH (1981).
`Schuder et al., “One-cycle bi—directional rectangular wave
`shocks for open chest defibrillation in the calf," Abs. Am.
`Soc. Amf. Intern. Orgm. 9:16.
`Schuder et a1.. "llnnsthoracic ventricular defibrillatiou in
`the 100 kg calf with symmetrical one-cycle lat—directional
`rectangular
`wave
`stimuli,"
`IEEE
`Trans.
`EME,
`30(7):415—422 (1983).
`Schuder et a1., '"l'ransthoracic ventricular defibrillation with
`square—wave stimuli: Onewhalf cycle, one-cycle, and mul-
`ticycle waveforms," Circ. Rest, XV:258—264 (1964).
`Schuder et al., “Ultrahigh—cnergy hydrogen thyratronlSCR
`Iii—directional waveform defibrillator," Med. 3: Biol. Eng. &
`Compile. 20:419—424 (1982).
`Schuder et a1., “Waveform dependency in defibrillating 100
`kg Calves," Devices & Tech. Meeting NIH (1982).
`Schuder et a1., “Waveform dependency in defibrillation,"
`Devices & Tech. Meeting NIH (1981).
`Stanton ct 21., “Relationship between defibrillation threshold
`and upper limit of vulnerability in humans," PACE, 152563,
`abstract 22] (Apr. 1992).
`Tang et a1.. “Strength duration curve for ventricular defibril-
`lation using biphasic waveforms,“ PACE, 10: abstract no. 49
`(Aug. 1987).
`
`Tang et al., “Venuicular dcfibrillation using biphasic wave-
`forms of different phasic duration." PACE. 10: abstract no.
`47 (Mar—Apr. 1987).
`Tang et at, “Ventricular defibrillation using biphasic wave-
`forms: The importance of phasic duration,"
`JACC,
`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).
`Wathen et 31., “Improved defibrillation eflicacy using four
`nonthoracotomy leads for sequential pulse defibrillation,"
`PACE, 15:563. abstract 220 (Apr. 1992).
`Wetberbee et al., “Subcutaneous patch electrode—A means
`to obviate thoracotomy for implantation of the automatic
`implantable cardioverter defibrillation system?” Ciro,
`72:384. abstract no. 1536 (1985).
`'Winkle “The implantable defibrillator in ventricular arrhythe
`ntias." Hospital Practice. pp. 149—165 (Mar. 1983).
`Winkle et al, “Improved low energy defibrillation efficacy
`in man using a biphasic truncated exponential waveform,”
`JACC, 9(2):l42A (1987).
`Zipes, “Sudden cardiac death," Circulation. 85(1):160—166
`(1992).
`Product information for Model H MSA Portable Defibrilla-
`tor (Bulletin No. 1108—2); 4 pp.
`Product information for MSA Portable Defibrillator (Bulle-
`tin No. 1108—1); 4 pp.
`
`
`
`7
`
`

`

`
`
`U.S. Patent
`
`Mar. 4,1997
`
`_ Sheet 1 of4
`
`5,607,454
`
`
`
`
`VOLTAGE
`
`l 1
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`VOLTAGE
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`US. Patent
`
`Mar. 4, 1997
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`Sheet 2 of 4
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`5,607,454
`
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`v.0."—
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`US. Patent
`
`Mar. 4,1997
`
`Sheet 3 of 4
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`5,607,454
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`5,607,454
`
`1
`ELECTROTHERAPY METHOD AND
`APPARATUS
`
`CROSS REFERENCE TO RELATED
`APPLICATION
`
`'
`
`This application is a continuation-impart of U.S. patent
`application Ser. No. 08/103,837 filed Aug. 6, 1993, the
`disclosure of which is incorporated herein by reference.
`
`BACKGROUND OF THE INVENTION
`
`This invention relates generally to an electrotherapy
`method and apparatus for delivering an electrical pulse to a
`patient’s heart. In particular,
`this invention relates to a
`method and apparatus for shaping the electrical waveform
`delivered by the defibrillator based on an electrical param-
`eter measured during delivery of the waveform The inven-
`tion also relates to a defibrillator design meeting certain
`threshold size and weight requirements.
`Sudden cardiac death is the leading cause of death in the
`United States. Most sudden cardiac death is caused by
`ventricular fibrillation, in which the heart’s muscle fibers
`contract without coordination, thereby interrupting normal
`blood flow to the body. The only efiective treatment for
`ventricular
`fibrillation is electrical defibrillation. which
`applies an electrical shock to the patient’s heart.
`To be efl‘ective, the defibrillation shock must he delivered
`to the patient within minutes of the onset of ventricular
`fibrillation. Studies have shown that defibrillation shocks
`delivered within one minute afier ventricular fibrillation
`begins achieve up to 100% survival rate. The survival rate
`falls to approximately 30% if 6 minutes elapse before the
`shock is administered. Beyond 12 minutes, the survival rate
`approaches zero.
`One way of delivering rapid defibrillation shocks is
`through the use of implantable defibrillators. 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. Implantable defibrillators are expensive.
`however, and are used on only a small fraction of the total
`population at risk for sudden cardiac death.
`to the
`External defibrillators
`send electrical pulses
`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 on short notice. The
`advantage 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
`electrodrerapeutic pulses to the patient’s heart indirectly
`(i.e., from the surface of the patient’s skin rather titan
`directly to the heart), they must operate at higher energies,
`voltages and/or eta-rents than implanted defibrillators. These
`high energy. voltage and current, requirements have made
`existing external defibrillators large, heavy and expensive,
`particularly due to thelarge size of the capacitors or other
`energy storage media required by these prior art devices. The
`size and weight of prior art external defibrillators have
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`limited their utility for rapid response by emergency medical
`response [381115.
`Defibrillator waveforms, i.e., time plots of the delivered
`current or volmge pulses, 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 an implantable
`defibrillators, on the other hand, use truncated exponential,
`biphasic waveforms. Examples of biphasic implantable
`defibrillators may he found in U.S. Pat No. 4,82l,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; U.S. Pat. No. 4,953,551 to Mehra et al.; and
`U.S. Pat. No. 5,230,336 to Fain 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 efi'ec-
`Lively titrated to the physiology of the patient to optimize the
`defibrillator’s efi‘ectiveness. 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
`amOunt of energy or to achieve a desired start and end
`voltage differential (i.e., a constant tilt). Even when an
`implanted defibrillator has the ability to change its operating
`parameters to compensate for changes in the impedance of
`the defibrillators leads and/or the patient’s heart (as dis-
`cussed in the Fain patent), the range of potential impedance
`changes for a single implantation in a single patient is
`relatively small.
`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 eifective 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
`patients heart for a given initial pulse amplitude and dura-
`tion. Pulse amplitudes and durations effective to treat low
`impedance patients do not necessarily deliver effective and
`energy efiiclent treatments to high impedance patients.
`External defibrillators may be subjected to extreme load
`conditions which could potentially damage the waveform
`generator circuits. For example, improperly applied defibril-
`lator electrodes may create a very low impedance current
`path during the shock delivery, which could result in execs»
`sively high current within the waveform circuit, Thus, an
`external defibrillator has an additional design requirement to
`limit the peak Current to safe levels in the waveform circuit,
`which is not normally a concern for implanted defibrillators.
`Prior art defibrillators have not fully addressed the patient
`variability problem. One prior an approach to this problem
`was to provide an external defibrillator with multiple energy
`settings that could be selected by the user. A common
`protocol for using such a defibrillator was to attempt
`. defibrillata'on at an initial energy setting suitable for defibril-
`lating a patient of average impedance, then raise the energy
`setting for subsequent defibrillation attempts in the event
`that the initial setting failed. The repeated defibrillation
`attempts require additional energy and add to patient risk.
`Some prior art defibrillators measure the patient imped-
`ance, or a pararueter related to patient impedance, and alter
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`BRIEF DESCRIPTION or THE DRAWINGS
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`3
`the shape of a subsequent defibrillation shock based on the
`earlier measurement. For example, the implanted defibril-
`lator described in the Fain patent delivers a defibrillation
`shock of predetermined shape to the patient's heart
`in
`response to a detected arrhythmia. The Fain device measures
`the system impedance during delivery of that shock and uses
`the measured impedance to alter the shape of a subsequently
`delivered shock.
`Another example of the measurement and use of patient
`impedance information in prior art defibrillators is described
`in an article written by R. E. Kerber, et at, “Energy. current,
`and success in defibrillation and cardioversion." Circulation
`(May 1988). The authors describe an external defibrillator
`that administers a test pulse to the patient prior to adminis»
`tering the defibrillation shock. The test pulse is used to
`measure patient
`impedance;
`the defibrillator adjusts the
`amount of energy delivered by the shock in response to the
`measured patient impedance. The shape of the delivered
`waveform is a damped sinusoid.
`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-
`lator that will achieve acceptable defibrillation or cardiover-
`siort
`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-
`ponents. In particular, operating voltage requirements aflcct
`the choice of switch and capacitor technologies. Total
`energy delivery requirements affect defibrillator battery and
`capacitor choices. Thus. even if an implantable defibrillator
`and an external defibrillator both deliver waveforms of
`similar shape, albeit with different waveform amplitudes, the
`actual designs of the two defibrillators would he radically
`different.
`
`SUMMARY OF THE INVENTION
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`FIG. 1 is a schematic representation of a low~tilt biphasic
`cleatrotherapcutic waveform.
`FIG. 2 is a schematic representation of a high-tilt biphasic
`electrotherapeutie waveform.
`FIG. 3 is a block diagram of a defibrillator system
`according to a preferred embodiment of the invention.
`FIG. 4 is a schematic circuit diagram of a defibrillator
`system according to a preferred embodiment of this inven-
`tion.
`FIG. 5 is an external View of a defibrillator according to
`a preferred embodiment of this invention.
`FIG. 6 is a partial cutaway view of a defibrillator accord,
`ing to a preferred embodiment of this invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`For any given patient and for any given defibrillator
`system design, whether implantable or external, there is an
`optimal biphasie waveform for treating a particular kind of
`arrhythmia. This principle is used when implanting defibril-
`lators; as noted ab0ve, implanted defibrillators are titrated to
`the patient at the time of implant. External defibrillators, on
`the other hand, must he designed to be efi'ective in a wide
`population of patients.
`For example. FIGS. 1 and 2 illustrate the patient-to-
`patient differences that an external defibrillator design must
`take into account. These figures are schematic representa-
`tions of truncated exponential biphasic waveforms delivered
`to two difierent patients from an external defibrillator
`according to the electrotherapy method of this invention for
`defibrillation or cardioversion. In these drawings, the verti-
`cal 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.
`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:
`
`iAi—iDi
`“tn—“W
`
`As shown in FIGS. 1 and 2, A is the initial first phase voltage
`and D 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 tannins] 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 patient
`dilferences.
`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 extemally-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 in the delivery
`of an effective eardioversion waveform. Thus, it is possible
`to design a defibrillator and defibrillation method that is
`eifective not only for a single patient (as in most prior art
`implantable defibrillators) but is also efl'eetive for a broad
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`This invention provides a defibrillator and defibrillation
`method that automatically compensates for patient-to-pa-
`tient diferences in the delivery of electrotherapeutic pulses
`for defibrillation and cardioversion. The defibrillator has an
`energy source that may be discharged through electrodes to
`administer a truncated exponential biphasic voltage or cur-
`rent puise to a patient.
`The preferred embodiment of the method comprises the
`steps of charging the energy source to an initial
`level;
`discharging the energy source across the electrodes to
`deliver electrical energy to the patient in a multiphasie
`waveform; monitoring apatient-dependent electrical param-
`eter during the discharging step; shaping the waveform of 50
`the delivered electrical energy based on a value of the
`monitored electrical parameter. wherein the relative duration
`of the phases of the multiphasie waveform is dependent on
`the value of the monitored electrical parameter.
`The preferred apparatus comprises an energy source; two
`electrodes adapted to make electrical contact with a patient:
`a connecting mechanism forming an electrical circuit with
`the energy source and the electrodes when the electrodes are
`attached to a patient; and a controller operating the connect-
`ing mechanism to deliver electrical energy from the energy
`source to the elecuodes in a multiphasic waveform the
`relative phase durations of which are based on an electrical
`parameter monitored during delivery of the electrical energy.
`The preferred defibrillator apparatus weighs less than 4
`pounds and has a volume less than 150 cubic inches, and
`most preferably, weighs approximately three pounds or less
`and has a volume of approximately 141 cu. in.
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`5
`population of patients. In addition, it is also possible to meet
`external defibrillator design requirements regarding the size,
`weight and capacity of the defibrillator energy source while
`still meeting the needs of a wide patient population.
`Up to a point, the more energy delivered to a patient in an
`electrodterapeutie pulse, the more likely the defibrillation
`attempt will succeed. Low-tilt biphasic waveforms achieve
`effective defibrillation rates with less delivered energy titan
`high-tilt waveforms. However,
`low-tilt waveforms are
`energy inefiicient, 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 efiieacy up to a
`certain critical tilt value. Thus, for a given capacitor, is given
`initial voltage and fixed phase durations, high impedance
`patients receive a waveform with less total energy and lower
`peel: currents but better conversion properties per 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 efi'ective and efii-
`cient therapy from an external defibrillator. An optimum
`capacitor charged to a predetermined voltage can be chosen
`to deliver an eifeetive and efficient waveform across a
`population of patients having a variety of physiological
`differences. For example, the defibrillator may operate in an
`open loop,
`i.e., without any feedback regarding patient
`parameters and with preset pulse phase durations which will
`be effective for a certain range of patients. The preset
`parameters of the waveforms shown in FIG. 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 depen-
`dent upon the physiological parameters of the patient and the
`physical connection 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 (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 etfective 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
`In order to ensure that the delivered shock will be within
`the optimum tilt range for an extended range ofpatients. this
`invention provides a defibrillator method and apparatus for
`adjusting the characteristics of the defibrillator waveform in
`response to a real-time measurement of a patient-dependent
`electrical parameter. FIG. 3 is a block diagram showing a
`preferred embodiment of the defibrillator system.
`The defibrillator system 30 comprises an energy source 32
`to provide a voltage or current pulse. In one p