United States Patent
`Gliner et a].
`
`[19]
`
`[11] Patent Number:
`
`5,735,879
`
`[45] Date of Patent:
`
`Apr. 7, 1998
`
`U8005735879A
`
`[54] ELECTROTHERAPY METHOD FOR
`EXTERNAL DEFIBRILLATORS
`
`W0 9509673
`W0 95/132020
`
`4/1995 WIPO .
`11/1995 MPG -
`
`[75]
`
`Inventors; Bradford E. Gliner. Bellevue; Thomas
`LaflgrbBotlgll; ghnton‘SéCl-rltle. B
`M '1 , be? 1- , :73”? i ‘0“ '
`ofovlgsil?
`of Barn n ge Is and. all
`'
`
`: H rtst
`3 A -
`ea
`[7 ]
`ssrgnee
`..
`,
`.
`[21] Appl No- 803 094
`[22] Filed:
`Feb. 20, 1997
`
`ream,
`
`In . S ttl . W h.
`c.
`ea
`e
`as
`
`Related U.s. Application Data
`
`[63] Continuation ofSer: No. 103,837,Aug. 6, 1993, abandoned.
`
`[51]
`Int. CL6 ..........................
`A61N 1139
`
`...........................
`[52] us. CI.
`607/7; 607/74; 607/5;
`58
`Field 1' S rch
`6372/8; 27,3692
`0
`ea
`607140424648 50 5,85 62‘ 7A:
`’
`’
`‘
`’
`”
`‘
`
`]
`
`[
`
`5
`[ 6]
`
`C'
`Ref
`erencesV "Cd
`U.S. PATENT DOCUMENTS
`
`32117154 10/1965 Becker et a" '
`gggizg 131332 $31,“,”d et a].
`3,862,636
`1/1975 Bell a a1
`3,886,950
`6/1975 Ukkestad‘et a1
`'
`’
`
`-
`
`‘
`'
`(List continued on next page.)
`
`FOREIGN PATENT DOCUMENTS
`0353341
`2,1990 European Pat. 01ft .
`0437104
`7/1991
`European pat ofi" .
`0457604 A 11/1991
`European Pat. 0E. .
`0491649A 6/1992 European Pat. 011‘”
`0507504 10/1992 EuropeanPat OE. .
`2070435
`9/1931 UH!“ nugdom -
`9233:233
`3:33:
`[$11113 _ngd°m '
`94/21327
`9/1994 WLPO .
`9432530 10/1994 WIPO .
`WO 95/05215
`2/1995 WEPO .
`
`OTHER PUBLICATIONS
`Saksena et a1.. “Developments for future implantable car-
`dioveiters and defibrillators." PACE. 10:1342—1353 (Nov.—
`Dec. 1987).
`Schuder “The role of an engineering oriented medical
`research groupin developing improved methods and devices
`for achieving ventricular defibrillation: The University of
`Missouri experience,” PACE. 16:95—124 (Jan. 1993).
`(List continued on next page.)
`
`Pn'mwy Emminrr-Wflfim E W
`Assistant Examiner—Kennedy J. Schaetzle
`n
`£30325); Agent, or Fine—James R Shay: Cecily Anne
`y
`[57]
`ABSTRACT
`This invention provides an external defibrilla‘tor and defibril-
`lation method that automatically compensates for patient-
`to-patient impedance diiferences 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 patients In a third aspect of the invention,
`the first phase ends when fire 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-
`enees by changing the nature of the delivered elecnothera-
`peutic pulse. resulting in a smaller, more efficient and less
`eans‘vc defibnflm‘m
`
`18 Claims, 7 Drawing Sheets
`
`
`
`" ~ ":\\ m t
`\ V1:W:VWr——
`
`L E ,
`
`“‘1
`
`«arm
`
`
`
`”\ RESUIEDSCMRGE
`“with?“
`2‘\::
`
`
`
`L|FECOR427-1011
`
`1
`
`LIFECOR427-1011
`
`

`

`5,735,879
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`treatment of life threatening
`Mirowski et al.. “Clinical
`ventricular tachyarrhythmias with the automatic implantable
`defibrillator." American Heart Journal, 102(2):265—270
`(1981).
`‘Termination of malignant ventricular
`Mirowski et al..
`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 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,” Medlines, pp. 1—2 (Jun.—
`lul. 1984).
`Sakscna et al., “A prospective evaluation of single and dual
`current pathways for transvenous cardioversion in rapid
`ventricular tachycardia." PACE, 10:1130—1140 (Sep.—Oct.
`1987).
`Alferness et al.. ‘The influence of shock waveforms on
`defibrillation efficacy.” IEEE Engineering in Medicine and
`Biology, pp. 25—27 (Jun. 1990).
`Blilie et aL. “Predicting and validating cardiothoracic cur-
`rent flow using finite element modeling," PACE, 15:563.
`abstract 219 (Apr. 1992).
`Chapman et al., “Non—thoracotomy internal dcflbrillation:
`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,"
`Cimubztion. 84(4):612, abstract No. 2433 (1991).
`Cooper et al.. ‘The effect of phase separation on biphasic
`waveform defibrillation.“ PACE, 16:471—482 (Mar. 1993).
`Cooper et al., "The effect of temporal separation of phases
`on biphasic waveform defibrillation eflicacy.” The Annual
`International Conference of the IEEE Engineering in Medi-
`cine and Biology Society,
`l3(2):0766—0767 (1991).
`Crampton et al.. “Low—energy ventricular defibrillation and
`miniature defibrillators,” JAMA, 235(21):2284 (1976).
`Dahlbiick et al., "Ventricular defrillation 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." Circu-
`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..
`“Defibrillator waveshape optimization,”
`Devices and Tech. Meeting, NIH (1982).
`Ker-bet. et al.. “Energy. current. and success in deflbrillation
`and cardioversion: clinical studies using an automated
`impedance-based method of energy adjustment." Circula-
`tion. 77(5):1038-1046 (1988); and.
`Der-man, et al.. “Current—Based Versus Energy—Based Ven-
`tricular Defibrillation: A Prospective Study."
`JACC,
`12(5):1259—1264 (1988).
`
`.
`
`.
`
`5/1977 Pannidge et a].
`4,023,573
`8/1983 Money.
`4,399,818
`4,419,998 12/1983 Hath.
`4,494,552
`“1935 Heath.
`4,595,009
`6/1986 Lenders .
`4,619,265 10/1986 Morgan et al.
`4,745,923
`5/1988 Winstrom .
`4,848,345
`7/1989 Zenkich.
`.
`5,078,134
`1/1992 Hellman et a].
`5,097,833
`3/1992 Campos .................................... 607/46
`5,222,492
`6/1993 Morgan et a1.
`.
`5,249,573 10/1993 Fincke et al. .
`5,334,219
`8/1994 Kroll.
`.................................. 607/7
`5,334,430
`8/1994 Berg et a1.
`5,370,664 12/1994 Morgan et a1.
`.
`5,411,526
`5/1995 Kroll et al.
`5,413,591
`5/1995 Kroll et a1.
`................................. 60m
`5,431,686
`7/1995 Kroll et al.
`5,441,513
`3/1995 Adams et al.
`.
`OFHER PUBLICATIONS
`
`607/5
`
`.
`
`a1. “Comparison of effectiveness of relay—
`Schuder et
`switched. one—cycle quasisinusoidal waveform with criti-
`cally damped sinusoid waveform in transthoracic defibril-
`lation of loo-kilogram calves," Medical Instrumentation
`22(6):281—285(1988).
`Schuder et al.. “A multielectrode—time sequential laboratory
`defibrillator for the study of implanted electrode systems.”
`ArnenSocAm'flInLOrganr, XVII/:5 14—5 19 (1972).
`Schuder et al.. “Development of automatic implant/ed
`defibrillator.” 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.. “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 defilm'llation,” PACE,
`15:563. abstract 218 (Apr. 1992).
`Wathen et al.. “Improved defibrillation efficacy using four
`nonthoracotomy leads for sequential pulse deflbrillation.”
`PACE, 15:563. abstract 220 (Apr. 1992).
`Winkle "I'he implantable defibrillator in ventricular arrhyth-
`mias,” Hospital Practice, pp. 149—165 (Mar. 1983).
`Zipes, “Sudden cardiac death.” Circubtizm, 85(1):160—166
`(1992).
`Jones et al.. “Reduced excitation threshold in potassium
`depolarized myocardial cells with symmetrical biphasic
`waveforms.” J. Mol. Cell. Cardiol, 17(39):XXV]], abstract
`No. 39 (1985).
`Jude et al.. “Fundamentals of Cardiopulmonary Resuscita-
`tion,” EA. Davis Company. Philadelphia PA, pp. 98—104
`(1965).
`Knickerbocker et al.. “A portable defibrillator.” IEEE Trans.
`on Power and Apparatus Systems, 69:1089—1093 (1963).
`Kouwenhoven. "I‘he development of the defibrillator.”
`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 pac-
`ing and high-energy bi—directional shock algorithm for
`transvenous cardioversion in patients with ventricular tachy-
`cardia." Circulation, 76(3):601—609 (1987).
`
`2
`
`

`

`US. Patent
`
`Apr. 7, 1998
`
`Sheet 1 of 7
`
`5,735,879
`
`VOLTAGE
`VOLTAGE
`
`
`
`VOLTAGE
`
`A
`
`VTHRESH'
`
`
`
`E_—'1
`tTHRESH
`I
`
`Cy
`
`
`
`3
`
`

`

`US. Patent
`
`Apr. 7, 1998
`
`Sheet 2 of 7
`
`5,735,879
`
`INITIATE DISCHARGE
`IN FIRST POLARITY
`
`10
`
`
`
`
`
`NO
`
`
`14
`
`
`IS
`VOLTAGE < VTHHESH
`
`
`?
`
`16
`
`18
`
`YES
`
`STOP DISCHARGE
`IN FIRST PHASE
`
`WAIT FOR
`INTERIM TIME G
`
`
`
`CHANGE
`POLARITY
`
`
`
`
`20
`
`
`
`RESUME DISCHARGE
`FOR SECOND PHASE
`DURATION F
`
`
`
`
`
`24
`
`STOP DISCHARGE
`
`FIG. 3
`
`4
`
`

`

`US. Patent
`
`Apr. 7, 1998
`
`Sheet 3 of 7
`
`5,735,879
`
`VOLTAGE
`
`A
`
`VTHRESH‘
`
`VOLTAG E
`
`A
`
`VTHHESH‘
`
`tTHRESH
`
`VOLTAG E
`
`A
`
`V%HHESHi
`
`5
`
`

`

`US. Patent
`
`Apr. 7, 1998
`
`Sheet 4 of 7
`
`5,735,879
`
`INITIATE DISCHARGE
`IN FIRST POLARITY
`
`I
`
`
`
`
`
`
`
`
`58
`
`60
`
`62
`
`64
`
`STOP DISCHARGE
`IN FIRST PHASE
`
`WAIT FOR
`INTERIM TIME G
`
`I
`
`CHANGE
`POLARITY
`
`RESUME DISCHARGE
`FOR SECOND PHASE
`DURATION F
`
`
`
`
`
`
`STOP DISCHARGE
`
`FIG. 6
`
`6
`
`

`

`US. Patent
`
`Apr. 7, 1998
`
`Sheet 5 of 7
`
`5,735,879
`
`INITIATE DISCHARGE
`IN FIRST POLARITY
`
`90
`
`91
`
`NO
`
`
`
`
`
`VOLTAGE < VTHRESH
`
`FIG. 9
`
`STOP DISCHARGE
`OF FIRST PHASE
`
`WAIT FOR
`INTERIM TIME G
`
`CHANGE
`POLARITY
`
`RESUME DISCHARGE
`FOR SECOND PHASE
`DURATION F
`
`
`
`STOP DISCHARGE
`
`94
`
`95
`
`96
`
`97
`
`98
`
`7
`
`

`

`US. Patent
`
`Apr. 7, 1998
`
`Sheet 6 of 7
`
`5,735,879
`
`
`
`FIG. 10
`
`8
`
`

`

`US. Patent
`
`Apr. 7, 1998
`
`Sheet 7 of 7
`
`5,735,879
`
`mmfiifi
`
`10:26
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`
`
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`
`

`

`5,735,879
`
`1
`ELECTROTHERAPY METHOD FOR
`EXTERNAL DEFIBRILLATORS
`
`This application is a CONTINUATION of application
`Ser. No. 08/103.837. filed 6 Aug. 1993. now abandoned.
`BACKGROUND OF THE INVENTION
`
`This invention relates generally to an electrotherapy
`method and apparatus for delivering a shock to a patient's
`heart. In particular. this invention relates to a method and
`apparatus for using an external defibrillator to deliver a
`biphasic defilxillation shock to a patient’s heart through
`electrodes attached to the patient.
`Defibrillators apply pulses of electricity to a patient’s
`heart to convert ventricular arrhythmias. such as ventricular
`fibrillation and ventricular tachycardia.
`to normal heart
`rhythms through the processes of defibrillation and
`cardioversion. respectively. There are two main classifica-
`tions of defibrillators: external and implanted. Implantable
`defibrillators are surgically implanted in patients who have
`a high likelihood of needing electrotherapy in the future.
`Implanted defibrillators typically monitor the patient‘s heart
`activity and automatically supply electrotherapeutic pulses
`directly to the patient’s heart when indicated. Thus,
`implanted defibrillators permit the patient to function in a
`somewhat normal fashion away from the watchful eye of
`medical personnel.
`External defibrillators send electrical pulses to the
`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 electrodierapy to a patient on short notice. The
`advantage of external defibrillators is that they may be used
`on a patient as needed. then subsequently moved to be used
`with another patient. However. because external defibrilla-
`tors deliver their electrotherapeutic pulses to the patient’s
`heart indirectly (i.e.. from the surface of the patient’s skin
`rather than diredly to the heart). they must operate at higher
`energies. voltages and/or eta-rents 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. din-alien and number of pulse phases. Most arr-rent
`external defibrillators deliver monophasic current or voltage
`electrothcrapeutic pulses. although some deliver biphasic
`sinusoidal pulses. Some prior art implantable defibrillators.
`on the other hand. use truncated exponential. biphasic waver
`forms. Examples of biphasic implantable defibrillators may
`be found in U.S. Pat. No. 4.821.723 to Baker. Jr., et al.; U.S.
`Pat. No. 5.083.562 to de Coriolis et al.; U.S. Pat. No.
`4.800.883 to Winstrom; U.S. Pat. No. 4.850.357 to Bach. Jr.;
`and U.S. Pat. No. 4.953.551 to Mehra et al.
`Because each implanted defibrillator is dedicated to a
`single patient. its operating parameters. such as electrical
`pulse amplitudes and total energy delivered. may be efiec—
`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 that desired start and end
`voltage differential (Le. a constant tilt).
`
`10
`
`15
`
`2
`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 difi'erences. exter-
`nal defibrillators must operate according to pulse amplitude
`and duration parameters that will be effective in most
`patients. no matter what
`the patient’s physiology. For
`example. the impedance presented by the tissue between
`external defibrillator electrodes and the patient’s heart varies
`from patient to patient. thereby varying the intensity and
`waveform shape of the shock actually delivered to the
`patient’s heart for a given initial pulse amplitude and dura-
`tion. Pulse amplitudes and durations effective to treat low
`impedance patients do not necessarily deliver effective and
`energy eflicient treatments to high impedance patients.
`Prior art external defibrillators have not fully addressed
`the patient variability problem. One prior art approach to this
`problem was to provide the external defibrillator with mul—
`tiple energy settings that could be selected by the user. A
`common protocol for using such a defibrillator was to
`attempt defibrillation at an initial energy setting suitable for
`defibrillating a patient of average impedance. then raise the
`energy setting for subsequent defibrillation attempts in the
`event that the initial setting failed. The repeated defibrilla-
`tion attempts require additional energy and add to patient
`risk. What is needed. therefore. is an external deflbrillation
`method and apparatus that maximizes energy efficiency (to
`minimize the size of the required enagy storage medium)
`and maximizes therapeutic eflicacy across an entire popu-
`lation of patients.
`
`SUMMARY OF THE INVENTION
`
`35
`
`45
`
`50
`
`55
`
`This invention provides an external defibrillator and
`defibrillation method that automatically compensates for
`patient-to-patient impedance differences in the delivery of
`electrotherapeutic pulses for defibrillation and cardiover-
`sion. In a preferred embodiment. the defibrillator has an
`energy source that may be discharged through electrodes on
`the patient to provide a biphasic voltage or current pulse. In
`one aspect of the invention. the first and second phase
`duration and initial first phase amplitude are predetermined
`values. In a second aspect of the invention. the duration of
`the first phase of the pulse may be extended if the amplitude
`of the first phase of the pulse fails to fall to a threshold value
`by the end of the predetermined first phase duration. as
`might occur with a high impedance patient. In a third aspect
`of the invention. the first phase ends when the first phase
`amplinrde drops below a threshold value or when the first
`phase duration reaches a threshold time value. whichever
`comes first. as might occur with a low to average impedance
`patient This method and apparatus of altering the delivered
`biphasic pulse thereby compensates for patient impedance
`differences by changing the nature of the delivered electro—
`therapeutic pulse, resulting in a smaller. more efficient and
`less expensive defibrillator.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a schematic representation of a low-tilt biphasic
`electrotherapeutic waveform according to a first aspect of
`this invention.
`FIG. 2 is a schematic representation of a high-tilt biphasic
`electrotherapeutic waveform according to the first aspect of
`this invention.
`
`FIG. 3 is a flow chart demonstrating part of an electro~
`therapy method according to a second aspect of this inven-
`tion.
`
`10
`
`10
`
`

`

`5,735,879
`
`3
`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 remesentation 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 preferred embodiment of this invention.
`FIG. 11 is a schematic circuit diagram of a defibrillator
`system according to a preferred embodiment of this inven—
`tion.
`
`DETAJLED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`FIGS. 1 and 2 illustrate the patient-to—patient ditferences
`that an external defibrillator design must take into account.
`These figures are schematic representations of truncated
`exponential biphasic waveforms delivered to two different
`patients from an external defibrillator according to the
`electrotherapy method of this invention for defibrillation or
`cardioversion. In these drawings, the vertical axis is voltage.
`and the 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:
`
`tat—1D:
`H‘T 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 tmninal 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 diffaences in end voltages B and D reflect diflerences in
`patient impedance.
`Prior art disclosures of the use of truncated exponential
`biphasic waveforms in implantable defibrillators have pro-
`vided little guidance for the design of an external defibril-
`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 afiect the size, cost. weight and availability of com-
`ponents. In particular, operating voltage requirements sheet
`the choice of switch and capacitor technologies. Total
`energy delivery requirements alfect defibrillator battery and
`capacitor choices.
`We have determined that. for a given patient. externally-
`applied truncated exponential biphasic waveforms defibril-
`
`4
`late at lower voltages and at lower total delivered energies
`than externally-applied monophasic waveforms. In addition.
`we have determined that there is a complex relationship
`between total pulse duration. first to second phase duration
`ratio. initial voltage. total energy and total tilt.
`Up to a point. the more energy delivered to a patient in an
`electrotherapeutic pulse. the more likely the defibrillation
`attempt will succeed Low-tilt biphasic waveforms achieve
`efiective defibrillation rates with less delivered energy than
`high-tilt waveforms. However.
`low-tilt waveforms are
`energy ineflicient. since much of the stored energy is not
`delivered to the patient. 0n 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. 3 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 hith peak
`currents. There appears to be an optimmn tilt range in which
`high and low impedance patients will receive effective and
`efficient therapy. An optimum capacitor charged to a prede-
`termined voltage can be chosen to deliver an effective and
`eflicient waveform across a population of patients having a
`variety of physiological diiferences.
`This invention is a defibrillator and defibrillation method
`that takes advantage of this relationship between waveform
`tilt and total energy delivered in high and low impedance
`patients. In one aspect of the invention. the defibrillator
`operates in an open loop. i.e.. without any feedback regard-
`ing patient impedance parameters and with preset pulse
`phase durations. The preset parameters of the waveforms
`shown in FIGS. 1 and 2 are therefore the initial voltage 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
`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 dun'ng time B 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 defibrillafion and/or eardioversion eflicacy across a
`population of patients. Thus, high impedance patients
`receive a lowstilt waveform that is more reflective 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.
`Another feature of biphasic waveforms is that waveforms
`with relatively longer first phases have better conversion
`properties than waveforms with equal or shorter first phases.
`provided the total duration exceeds a critical minimum.
`Therefore. in the case of high impedance patients. it may be
`desirable to extend the first phase of the biphasic waveform
`(while the second phase duration is kept constant) to
`increase the overall eflicacy 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 aspea of the
`invention in which information related to patient impedance
`is fed back to the defibrillator to change the parameters of
`the delivered electrotherapeutic pulse.
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`FIG. 3 is a flow chart showing the method steps following
`the decision (by an operator or by the defibrillator itself) to
`apply an electrotherapeutic shock to the patient through
`electrodes attached to the patient and charging of the energy
`source. e.g.. the defibrillator’ s capacitor or capacitor bank. to
`the initial first phase voltage A. Block 10 represents initia-
`tion of the first phase of the pulse in a first polarity.
`Discharge may be initiated manually by the user or auto-
`matically in response to patient heart activity measurements
`(e.g.. ECG signals) received by the defibrillator through the
`electrodes and analyzed by the defibrillator controller in a
`manner known in the art.
`Discharge of the first phase continues for at least a
`threshold time tmRESH. as shown by block 12 of FIG. 3. If.
`at the end of time Imus» the voltage measured across the
`energy source has not dropped below the minimum first
`phase terminal voltage threshold mem first phase dis-
`charge continues. as shown in block 14 of FIG. 3. For high
`impedance patients. this situation results in an extension of
`the first phase duration beyond gyms". as shown in FIG. 4.
`until
`the measured voltage drops below the threshold
`meu. Discharge then ends to complete the first phase, as
`represented by block 16 of FIG. 3. If. on the other hand. the
`patient has low impedance, the voltage will have dropped
`below me” when the time threshold is reached, result-
`ing in a waveform like the one shown in FIG. 5.
`At the end of the first phase. and after a predetermined
`interim period G. the polarity of the energy source connec-
`tion to the electrodes is switched. as represented by blocks
`18 and 20 of FIG. 3. Discharge of the second phase of the
`biphasic pulse then commences and continues for a prede-
`termined second phase duration F. as represented by block
`22 of FIG. 3. then ceases. This compensating electrotherapy
`method ensures that the energy is delivered by the defibril-
`lator in the most efficacious manner by providing for a
`minimum waveform tilt and by extending the first phase
`duration to meet the requirements of a particular patient.
`Because this method increases the waveform tilt for high
`impedance patients and delivers more of the energy from the
`energy source than a method without compensation. the
`defibrillator's energy source can be smaller than in prior art
`external defibrillators. thereby minimizing defibrillator size,
`weight and expense. It should be noted that the waveforms
`shown in FIGS. 4 and 5 could be expressed in terms of
`current versus time using a predetermined current threshold
`value without departing from the scope of the invention.
`FIGS. 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 54in FIG. 6. the first
`phase discharge stops either at the end of a predetermined
`time tummy or when the first phase voltage drops below
`V'mtwsH- The second phase begins after an interim period
`G and continues for a preset period F as in the second aspect
`of the invention. Thus. in high impedance patients, the first
`phase ends at time tmmfl. even if the voltage has not yet
`fallen below V'mmy. as shown in FIG. 7. In low imped-
`ance patients. on the other hand.
`the first phase of the
`delivered waveform could be shorter in duration than the
`time tmRESH. as shown in FIG. 8.
`Once again. the waveforms shown in FIGS. 7 and 8 could
`be expressed in terms of current versus time using a prede-
`termined current threshold value without departing from the
`scope of the invention.
`FIG. 9 is a flow chart illustrating a combination of the
`defibrillation methods illustrated in FIGS. 3 and 6. In this
`combination method. the first phase of the biphasic wave-
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`form will end if the voltage reaches a first voltage threshold
`V'mRES” prior to the first phase duration threshold tmRESH.
`as shown by blocks 91 and 92. This defibrillator decision
`path delivers a waveform like that shown in FIG. 8 for low
`impedance patients. For high impedance patients. on the
`other hand. if at the expiration of tmm” the voltage has not
`fallen below v‘mmzsm the duration of the first phase is
`extended beyond tmREs” until the voltage measured across
`the electrodes reaches a second voltage threshold men.
`as shown in decision blocks 91 and 93. This defibrillator
`method path will deliver a waveform like that shown in FIG.
`4.
`
`In alternative embodiments of this invention. the second
`phase pulse could be a function of the first phase voltage.
`current a time instead of having a fixed time duration. In
`addition. any of the above embodiments could provide for
`alternating initial polarities in successive monophasic or
`biphasic pulses. In other words.
`if in the first biphasic
`waveform delivered by the system the first phase is a
`positive voltage or current pulse followed by a second phase
`negative voltage or current pulse. the second biphasic wave-
`form delivered by the system would be a negative first phase
`voltage or current pulse followed by a positive second phase
`voltage or current pulse. This arrangement would minimize
`electrode polarization. i.e.. build-up of charge on the elec-
`trodes.
`For each defibrillator method discussed above. the initial
`first phase voltage A may be the same for all patients or it
`may be selected automatically or by the defibrillator user.
`For example. the defibrillator may have a selection of initial
`voltage settings. one for an infant. a second for an adult. and
`a third for use in open heart surgery.
`FIG. 10 is a schematic block diagram of a defibrillator
`system according to a preferred embodiment of this inven-
`tion. The defibrillator system 30 comprises an energy source
`32 to provide the voltage or current pulses described above.
`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 energy source 32 to and from a pair of
`electrodes 36 electrically attached to a patient. represented
`here as a resistive load 37. The connections between the
`electrodes 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 timing informa-
`tion from a timer 40. and timer 40 receives electrical
`information from electrical sensor 42 connected across
`energy source 32. In some preferred embodiments. sensor 42
`is a voltage sensor; in other preferred embodiments. sensor
`42 is a current sensor.
`FIG. 11 is a schematic circuit diagram illustrating a device
`according to the preferred embodiments discussed above.
`Defibrillator controller 70 activates a high voltage power
`supply 72 to charge storage capacitor 74 via diode 76 to a
`predetermined voltage. During this period. switches SW1.
`SW2. SW3 and SW4 are turned off so that no voltage is
`applied to the patient (represented here as resistor 78)
`connected between electrodes 80 and 82. SW5 is turned on
`during this time.
`After charging the capacitor. controller 70 de-activates
`supply 72 and activates biphase switch timer 84. Timer 84
`initiates discharge of the first phase of the biphasic wave-
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`form through the patient in a first polarity by simul