United States Patent
`[19]
`[11] Patent Number:
`5,607,454
`
`Cameron et a].
`[45] Date of Patent:
`Mar. 4, 1997
`
`llllllllllllllllllllll|||||Illlllllllllllllllll|||||l|||lIIIIIIIIIIIIIIIIII
`USOOS607454A
`
`[54] ELECTROTHERAPY METHOD AND
`APPARATUS
`
`9/1994 WIPO .
`WO94/21327
`W094/22530 10/1994 WIPO .
`
`[75]
`
`Inventors: David Cameron, Seattle; Thomas D.
`Lyster, BOthCH; Daniel .1. Powers,
`Bainbfi'dge Island; Bradford E- Gliner,
`Bellevue; Clinton S. Cole, Seattle;
`Carlton B~ Morgan, Bainbridge Island,
`all Of Wash.
`
`.
`73] Asmgneei Heartstream, Int-t Seattle, Wash.
`
`OTHER PUBLICATIONS
`Alferncss et 211., “The influence of shock waveforms on
`dcfibrillation eflicacy," IEEE Engineering in Medicine and
`Biology, pp. 25—27 (Jun. 1990).
`Anderson et al., “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 ExamineriMarvin M. Lateef
`Assistant rExamineriKennedy J. Schaetzle
`Attorney, Agent, or Firm—~Monison & Foerster
`
`[57]
`
`ABSTRACT
`
`_
`,
`A“ ‘T’lemflerapy math“ and appam‘“ for dellveni‘g *1
`multiphaSic waveform from an energy source to apatieiit.
`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—dependent 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; a connect-
`irig mechanism forming an electrical circuit with the energy
`source and the electrodes when the electrodes are attached to
`
`_
`
`
`
`r
`
`2]] App1‘ N0" 227’553
`22] Elm.
`Apr. 14’ 1994
`Related US. Application Data
`
`63] Continuation in part of Ser. No. 103,897, Aug. 6, 1993.
`
`Int. Cl.6 ....................................... A61N 1/39
`51]
`
`.. 607/5; 607/7; 607/6; 607/74;
`52] us. (:1.
`......
`607/62
`_
`58] Field of Search .................................... 607/2, 4, 5—7,
`607/62, 74
`
`.
`References Cited
`
`56]
`
`3,211,154
`3,241,555
`3,706,313
`3,782,389
`3,860,009
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`3.336950
`
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`10/1955 Becker et a] _
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`,
`.
`12/1972 Milani et a1.
`
`1/1974 Bell
`............
`.. 607/8
`1/1975 Bell et a1.
`..
`1/ 1975 Bell Cl 31.
`6/ 1975 Ukkesmd 61 31-
`
`
`
`--
`
`(List continued on next
`
`a e )
`p g ’
`FOREIGN PATENT DOCUMENTS
`
`EUYDPBan Pal- 011‘
`9/1983
`0281219
`European Pal- 01f.
`-
`5/1989
`0315368
`2/1990 European Pfil- Off ~
`0353341
`uropean a.
`.
`.
`3:33:31 1333: European :1:- 8% ~
`2070435
`9/1981 United Kingdom .
`2083363
`3/1982 United Kingdom .
`WO93/16759
`9/1993 WIPO .
`
`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 approxrmately 141 cu. in.
`59 Claims, 4 Drawing Sheets
`
`
`
`L|FECOR212-1006
`
`1
`
`LIFECOR212-1006
`
`

`

`5,607,454
`Page 2
`
`U. S. PATENT DOCUMENTS
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`“Defibrillator waveshape optimization,”
`al.,
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`Jones
`Devices and Tech. Meeting, NIH (1982).
`Jones or al., “Improved defibrillator waveform safety factor
`with biphasic waveforms," Am. J. Physiol, 245:H60—65
`(1983).
`Jones et al., “Reduced excitation threshold in potassium
`depolarized myocardial cells with symmetrical biphasic
`waveforms," J. Mol. Cell. Cardiol, 17(39):XXVII, abstract
`no. 39 (1985).
`Jude et al., “Fundamentals of Cardiopulmonary Resuscita—
`tion,” F.A. Davis Company, Philadelphia PA, pp. 98—104
`(1965).
`Kerber et al., “Energy, current, and success in defibrillation
`and cardioversion: Clinical studies using an automated
`impedanceebased method of energy adjustment,” Circula-
`tion, 77(5):103841046 (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 of Internal Medicine, 71(3):4494158 (1969).
`Langer et al., “Considerations in the development of the
`automatic implantable defibrillator,” Medical Instrumenta—
`tion, 10(3)2163—167 (1976).
`Lerman et al. “Current—based versus energy—based ventricu-
`lar
`dcfibrillation:
`A
`prospective
`study,”
`JACC,
`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 automatic implantable
`defibrillator,” American Heart Journal, 102(2):265—270
`(1981).
`Mirowski ct al., “Termination of malignant ventricular
`arrhythmias with an implanted automatic defibrillator in
`human beings," The New England Journal of Medicine,
`303(6):322—324 (1980).
`Podolsky, “Keeping the beat alive,” U.S. News & World
`Report (Jul. 22, 1991).
`Product Brochure for First Medic Semi—Automatic Defibril-
`lators (1994), Spacelabs Medical Products, 15220 NE. 40th
`Street, PO, Box 97013, Redmond, WA 98073—9713.
`Product Brochure for the Shock Advisory System (1987),
`Physio—Control, 11811 Willows 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., “Developments for future implantable car—
`dioverters
`and
`defibrillators,”
`PACE,
`10:1342—1358
`(Nov.ADec. 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
`defibrillation of loo—kilogram calves,” Medical Instrumen-
`tation, 22(6):281—285 (1988).
`
`.
`
`.
`
`.
`
`.
`
`607/5
`
`5/1977 Pantn'dge eta].
`4,023,573
`5/1982 Charbonnier et a1.
`4,328,808
`12/1983 Heath.
`4,419,998
`9/ 1984 Angel .......................................... 607/6
`4,473,078
`1/1985 Heath.
`4,494,552
`3/1985 Suzuki et a1.
`4,504,773
`3/1986 Lei-man.
`4,574,810
`6/1986 Leinders.
`4,595,009
`9/ 1986 Morgan et al.
`4,610,254
`10/1986 Morgan et al.
`4,619,265
`1/ 1987 Jones ct a1.
`.
`4,637,397
`5/1988 Winstrom .
`4,745,923
`1/ 1989 Winstrom .
`4,800,883
`.
`4/ 1989 Baker, Jr. et a1,
`4,821,723
`6/ 1989 Charbonnier et a1.
`4,840,177
`7/1989 chkich.
`4,848,345
`7/1989 Bach, lr..
`4,850,357
`9/1990 Mehra et a1.
`4,953,551
`.
`3/1991 Bocchi et al,
`4,998,531
`1/1992 Heilman et al. .
`5,078,134
`1/1992 de Coriolis et a1.
`5,083,562
`4/1992 Ideker et a].
`.
`5,107,834
`5/1992 Charbonnier et a1.
`5,111,813
`5/1992 Pless et a1.
`.
`5,111,816
`5/1993 Adams et a1.
`5,207,219
`6/1993 Ostrotf.
`5,215,081
`.
`6/1993 Couche et a1.
`5,222,480
`.
`6/1993 Morgan et a1.
`5,222,492
`7/1993 Fain et a].
`................................... 607/7
`5,230,336
`8/1993 Morgan et a1.
`5,237,989
`
`10/1993 Fincke et all
`5,249,573
`l/1994 Morgan et a1.
`5,275,157
`4/1994 Kroll et a1.
`.
`5,306,291
`8/1994 Kroll.
`5,334,219
`5,352,239 10/1994 Pless ........................................... 607/5
`5,370,664 12/1994 Morgan et a1.
`.
`5,372,606 12/1994 Lang et al.
`.................................. 607/8
`OTHER PUBLICATIONS
`
`.
`
`.
`.
`
`.
`
`.
`
`.
`
`Chapman et al., “Non—thoracotomy internal defibrillation:
`Improved efficacy with biphasic shocks,” Circulation,
`76:312, 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 on biphasic
`waveform defibrillation,”PACE, 161471—482 (Mar. 1993).
`Cooper et al., “The eifect of temporal separation of phases
`on biphasic waveform defibrillation eflicacy,” The Annual
`International Conference of the IEEE Engineering in Medi—
`cine and Biology Society, 13(2):0766—O767 (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, 162914, abstract no. 256 (Apr. 1993).
`Feeser et al., “Strength~duration and probability of success
`curves for defibrillation with biphasic waveforms,” Circa—
`lation, 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., “Decreased defibrillator—induced dysfunction
`with biphasic rectangular waveforms,” Am. J. Physiol,
`247:H792—796 (1984).
`
`2
`
`

`

`5,607,454
`Page 3
`
`Schuder et al., “A multielectrode—time sequential laboratory
`defibrillator for the study of implanted electrode systems,”
`Amer. Soc. Artifi 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 et al., “Development of automatic implanted
`defibrillator,” Devices & Tech. Meeting NIH (1981).
`Schuder et al., “Oneicycle bkdirectional rectangular wave
`shocks for open chest defibrillation in the calf,” Abs. Am.
`Soc. Artif. Intern. Organs, 9216.
`Schuder et al., “Transthoracic ventricular defibrillation in
`the 100 kg calf with symmetrical one—cycle bi—directional
`rectangular
`wave
`stimuli,”
`IEEE
`Trans.
`BME,
`30(7):415422 (1983).
`Schuder et al., “Transthoracic ventricular defibrillation with
`square—wave stimuli: One—half cycle, one—cycle, and mul-
`ticycle waveforms,” Circ. Res, XV:258—264 (1964).
`Schuder et al., “Ultrahigh—energy hydrogen thyratron/SCR
`bi—dircctional waveform defibrillator," Med. & Biol. Eng. &
`Comput, 20:419—424 (1982).
`Schuder et al., “Waveform dependency in dcfibrillating 100
`kg Calves,” Devices & Tech. Meeting NIH (1982).
`Schuder et al., “Waveform dependency in defibrillation,"
`Devices & Tech. Meeting NIH (1981).
`Stanton et al., “Relationship between defibrillation threshold
`and upper limit of vulnerability in humans,” PACE, 15:563.
`abstract 221 (Apr. 1992).
`Tang et al., “Strength duration curve for ventricular defibril-
`lation using biphasic waveforms,” PACE, 10: abstract no. 49
`(Aug. 1987).
`
`Tang et al., “Ventricular dcfrbrillation using biphasic wave-
`forms of different phasic duration," PACE, 10: abstract no.
`47 (Man—Apr. 1987).
`Tang ct al., “Ventricular defibrillation using biphasic wave-
`forms: The
`importance of phasie 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 ct al., “Improved defibrillation efficacy using four
`nonthoracotomy leads for sequential pulse dcfibrillation,”
`PACE, 15:563, abstract 220 (Apr. 1992).
`Wetherbee et al., “Subcutaneous patch electrode—A means
`to obviate thoracotomy for implantation of the automatic
`implantable
`cardioverter defibrillation system?” Cim,
`722384, abstract no. 1536 (1985).
`
`'Winklc “The implantable defibrillator in ventricular arrhyth—
`mias,” Hospital Practice, pp. 149—165 (Mar. 1983).
`
`Winkle et al., “Improved low energy defibrillation eflicacy
`in man using a biphasic truncated exponential waveform,”
`JACC, 9(2):]42A (1987).
`
`Zipes, “Sudden cardiac death,” Circulation, 85(1):l60—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.
`
`3
`
`

`

`US. Patent
`
`Mar. 4, 1997
`
`Sheet 1 of 4
`
`5,607,454
`
`VOLTAGE
`
`VOLTAGE
`
`
`
`
`l 1
`
`FIG. 3
`
`4
`
`

`

`US. Patent
`
`Mar. 4, 1997
`
`Sheet 2 of 4
`
`5,607,454
`
`
`
`'<mooLuLL.—c
`
`v.0."—
`
`5
`
`
`

`

`US. Patent
`
`Mar. 4, 1997
`
`Sheet 3 of 4
`
`5,607,454
`
`
`
`6
`
`

`

`US. Patent
`
`Mar. 4, 1997
`
`Sheet 4 of 4
`
`5,607,454
`
`96
`
`FIG.6
`
`7
`
`

`

`5,607,454
`
`1
`ELECTROTHERAPY METHOD AND
`APPARATUS
`
`CROSS REFERENCE TO RELATED
`APPLICATION
`
`7
`
`This application is a continuation-in—part of US. 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 effective, the defibrillation shock must be delivered
`to the patient within minutes of the onset of ventricular
`fibrillation. Studies have shown that defibrillation shocks
`delivered within one minute after 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 inra
`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 be 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 than
`directly to the heart), they must operate at higher energies,
`voltages and/or currents than implanted defibrillators. These
`high energy, voltage and current, requirements have made
`existing 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
`size and weight of prior art external defibrillators have
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`65
`
`2
`limited their utility for rapid response by emergency medical
`response teams.
`Defibrillator waveforms, i.e., time plots of the delivered
`cmrent or voltage 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 art
`implantable
`defibrillators, on the other hand, use truncated exponential,
`biphasic waveforms. Examples of biphasic implantable
`defibrillators may be found in US. Pat. No. 4,821,723 to
`Baker, Jr., ct al.; US. Pat. No. 5,083,562 to de Coriolis ct al.;
`US. Pat. No. 4,800,883 to Winstrom; US. Pat. No. 4,850,
`357 to Bach, In; US. Pat. No. 4,953,551 to Mehra et al.; and
`US. 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 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
`amount of energy or to achieve a desired start and end
`voltage diiferential (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 diflerences, exter—
`nal defibrillators must operate according to pulse amplitude
`and duration parameters that will he ellective 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 efiective to treat low
`impedance patients do not necessarily deliver effective and
`energy efficient 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 exces—
`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 art 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
`defibrillation 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 parameter related to patient impedance, and alter
`
`8
`
`

`

`5,607,454
`
`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. Kerbcr, et al., “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-
`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»
`ponents. In particular, operating voltage requirements afieet
`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 be radically
`dilI'erent.
`
`SUMMARY OF THE INVENTION
`
`This invention provides a defibrillator and defibrillation
`method that automatically compensates for patient-to—pa—
`tient difierences in the delivery of electrotherapcutie 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 pulse 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~dependent 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 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 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.
`
`10
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`15
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`20
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`25
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`30
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`35
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`4
`BRIEF DESCRIPTION on THE DRAWINGS
`
`FIG. 1 is a schematic representation of a low—tilt biphasic
`electrotherapeutic waveform.
`FIG. 2 is a schematic representation of a high-tilt biphasic
`electrothcrapeutic 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 biphasic waveform for treating a particular kind of
`arrhythmia. This principle is used when implanting defibril-
`lators; as noted above, implanted defibrillators are titrated to
`the patient at the time of implant. External defibrillators, on
`the other hand, must be designed to be effective in a wide
`population of patients.
`For example, FIGS. 1 and 2 illustrate the patient—to—
`patient difierences that an external defibrillator design must
`take into account. These figures are schematic representa-
`tions of truncated exponential biphasic waveforms delivered
`to two dilferent 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:
`
`lAl—lD:
`HzT x100
`
`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 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 difierences in end voltages B and D reflect patient
`differences.
`
`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-applicd 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 cardioversion waveform. Thus, it is possible
`to design a defibrillator and defibrillation method that is
`effective not only for a single patient (as in most prior an
`implantable defibrillators) but is also effective for a broad
`
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`5,607,454
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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
`electrotherapeutie 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 eflicacy up to a
`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
`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 effective and effi-
`cient therapy from an external defibrillator. An optimum
`capacitor charged to a predetermined voltage can be chosen
`to deliver an effective and eflicient 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 eflicacy 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 efiicient.
`In order to ensure that the delivered shock will be within
`the optimum tilt range for an extended range of patients, 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 preferred
`embodiment, energy source 32 is a single capacitor or a
`capacitor bank arranged to act as a single capacitor.
`A connecting mechanism 34 selectively connects and
`disconnects a pair of electrodes 36 electrically attached to a
`patient (represented here as a resistive load 37) to and from
`the energy source. The connections between the electrodes
`
`6
`and the energy source may be in either of two polarities with
`respect to positive and negative terminals on the energy
`source.
`
`The defibrillator system is controlled by a controller 38.
`Specifically, controller 38 operates the connecting mecha-
`nism 34 to connect energy source 32 with electrodes 36 in
`one of the two polarities or to disconnect energy source 32
`from electrodes 36. Controller 38 receives discharge infor-
`mation (such as current, charge and/or voltage) from the
`discharge circuit. Controller 38 may also receive timing
`information from a timer 40.
`Controller 38 uses information from the discharge circuit
`and/or the timer to control
`the shape of the waveform
`delivered to the patient in real time (i.e., during delivery of
`the waveform), such as by selecting appropriate waveform
`parameters from a memory location associated with the
`controller or by otherwise adjusting the duration of the
`phases of the biphasic waveform. By controlling the wave-
`form shape, the system controls the duration, tilt and total
`delivered energy of the waveform. For example, biphasic
`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 increase the duration of the first phase
`of the biphasic waveform relative to the duration of the
`second phase to increase the overall efficacy of the electro-
`therapy by delivering a more efficacious waveform and to
`increase the total amount of energy delivered.
`A preferred embodiment of a defibrillator system accord—
`ing to the invention is shown schematically in FIG. 4. In this
`diagram, the energy source is a capacitor 32 preferably
`having a size between 60 and 150 microfarads, most pref-
`erably 100 microfarads. The system also includes a charging
`mechanism (not shown) for charging the capacitor to an
`initial voltage.
`A controller 38 controls the operation