United States Patent
`Gliner et a1.
`
`{19]
`
`[11] Patent Number:
`
`5,593,427
`
`[451 Date of Patent:
`
`Jan. 14, 1997
`
`lllllllllllllllllIlllllllll|||||Illllllllll||||IIIIIIIIIHIIIIIIlllllllllll
`USOOSS93427A
`
`[54]
`
`[75]
`
`[73]
`
`[21]
`
`122]
`
`[621
`151]
`[521
`[581
`
`[56]
`
`ELECTROTHERAPY METHOD
`
`Inventors: Bradford E. Gliner, Bellevue; Thomas
`D. Lyster, Bothell; Clinton S. Cole,
`Kirkland; Daniel 1. Powers, Bainbridge
`Island; Carlton B. Morgan, Bainbridge
`Island, all of Wash.
`
`Assignee: Heartstream, Inc., Seattle, Wash.
`
`Appl. No.: 601,617
`Filed:
`Feb. 14, 1996
`
`Related US. Application Data
`
`Division of Ser. No. 103.837, Aug. 6, 1993.
`Int. Cl.6 ....................................................... A61N 1/39
`
`US. Cl.
`...................
`607/7; 607/5; 607/74
`
`Field of Search
`607/7, 5. 6, 4.
`607/74
`
`References Cited
`
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`Anderson, et al., “The Ellicacy of Trapezoidal Wave Forms
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`
`Primary ExaminerA—William E. Katnm
`Assistant Examiner—Kennedy l. Schaetzle
`Attorney, Agent. or Finn—Morrison & Foerstcr
`
`[57]
`
`ABSTRACT
`
`This invention provides an external defibrillator and defibril-
`lation method that automatically compensates for patient»
`to-patient impedance difierences in the delivery of electro»
`therapeutic pulses for defibrillation and cardiovcrsion. In a
`preferred embodiment, the defibrillator has an energy source
`that may be discharged through electrodes on the patient to
`provide a biphasic voltage or current pulse. In one aspect of
`the invention, the first and second phase duration and initial
`first phase amplitude are predetermined values, In a second
`aspect of the invention, the duration of the first phase of the
`pulse may be extended if the amplitude of the first phase of
`the pulse fails to fall to a threshold value by the end of the
`predetermined first phase duration, as might occur with a
`high impedance patient. In a third aspect of the invention,
`the first phase ends when the first phase amplitude drops
`below a threshold value or when the first phase duration
`reaches a threshold time value. whichever comes first, as
`might occur with a low to average impedance patient. This
`method and apparatus of altering the delivered biphasic
`pulse thereby compensates for patient impedance differ—
`ences by changing the nature of the delivered electrothera—
`peutic pulse, resulting in a smaller, more efficient and less
`expensive defibrillator.
`
`18 Claims, 7 Drawing Sheets
`
`5‘" verwznscumr l
`arns'wuuam ‘
`p,
`
`
`
`
` ST”JISK‘MRGE
`
`lin -\ WESWE BISCNAHGE .
`. FORstem]ms: 1
`titration r
`
`‘
`
`1
`
`L|FECOR427-1001
`
`1
`
`LIFECOR427-1001
`
`

`

`5,593,427
`Page 2
`
`U.S. PATENT DOCUMENTS
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`Jones et al., “Reduced excitation threshold in potassium
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`12(5): 1259—1264 (1988).
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`ing and high—energy bi—directional shock algorithm for
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`Mirowski et al., “Clinical
`treatment of life threatening
`ventricular tachyarrhythmias with the automatic implantable
`defibrillator," American Heart Journal, 102(2):265—270
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`Mirowski et al., “Termination of malignant ventricular
`arrhythmias with an implanted automatic defibrillator in
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`Podolsky, “Keeping the beat alive," U.S. News & World
`Report (Jul. 22, 1991).
`Product Brochure for First Medic Semi—Automatic Defibril-
`lators (1994), Spaeelabs Medical Products, 15220 N.E. 40th
`Street, P.O. Box 97013, Redmond, WA 98073—9713.
`Product Brochure for the Shock Advisory System (1987),
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`97006, Redmond, WA 98073—9706.
`Redd (editor), “Defibrillation with biphasic waveform may
`increase
`safety,
`improve
`surviv ," 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,
`[021130—1141 (Sep.—Oct.
`1987).
`Saksena et al., “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).
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`.
`
`.
`
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`.
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`.
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`.
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`.
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`.
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`.
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`.
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`.
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`.
`
`.
`
`.
`
`.
`
`.
`
`.
`
`.
`
`.
`
`.
`
`.
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`OTHER PUBLICATIONS
`
`Chapman et al., “Non—thoracotomy internal defibrillation:
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`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, 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, 13(2):0766—0767 (1991).
`Crampton cta1., “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).
`Eeht et al., “Biphasic waveform is more efficacious than
`monophasie waveform for transthoraeic cardioversion,”
`PACE, 162914, 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, 141443—451 (Mar, 1991).
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`et
`al.,
`“Defibrillator waveshape optimization,”
`Devices and Tech. Meeting, NIH (1982).
`
`2
`
`

`

`5,593,427
`Page 3
`
`of
`effectiveness
`of
`“Comparison
`al.,
`et
`Schuder
`relay—switched, one-cycle quasisinusoidal waveform with
`critically damped sinusoid waveform in transthoracic
`defibrillation of IOO—kilogram calves," Medical Instrumeni
`ration, 22(6):2817285 (1988).
`Schuder et al., “A multielectrode—time sequential laboratory
`defibrillator for the study of implanted electrode systems,”
`Amer. Soc. Artif. Int. Organs, XVIII:514r-5]9 (1972).
`Schuder et
`21]., “Development of automatic implanted
`defibrillator,” Devices & Tech. Meeting NIH (1981).
`Schuder, et al., “0ne4ycle Bidirectional Rectangular Wave
`Shocks for Open Chest Defibrillation in the Calf,” Abs. Am.
`Soc. Aftif. Intern. Organs, 9:
`l6.
`Schudcr, ct a1., “Tranthoracic Ventricular Defibrillation with
`Square—wave Stimuli: One half Cycle,“ Cr'rc. Res,
`XV:258—264 (1964).
`Schuder, et al., ”Defibrillation of 100 kg calves with assy-
`metrical, bidirectional,
`retangular pulses,” Card. Res,
`18:419-426 (1984).
`Schuder, et 31., ‘Transthoracic Ventricular Defibrillation in
`the 100 kg Calf with Symetrical One—Cycle Bidirectional
`Rectangular Wave
`Stimuli,”
`IEEE
`Trans.
`BME,
`30(7):415—422 [1983).
`Schuder, et a1, “Ultrahigh—energy hydrogen thyraton/SCR
`Biderectional waveform defibrillator,” Med. & Biol. Eng. &
`Comput, 20:419—424 (1982).
`Schuder, et al., “Wave form Dependency in Defibrillating
`100 kg Calves," Devices & Tech. Meeting, NIH (1981).
`Schuder, or 31., “Waveform dependency in defibrillation,"
`Devices & Tech. Meeting NIH (1981).
`
`Stanton et al., “Relationship between defibrillation threshold
`and upper limit of vulnerability in humans,” PACE, 152563,
`abstract 221 (Apr. 1992).
`Tang et al., “Ventricular defibrillation using biphasic wave~
`forms: The
`importance of phasic duration,"
`JA CC,
`I3(l):207—214 (1989).
`Tang, et al., “Strength Duration Curve for Ventricular
`Defibrillation Using Biphasic Waveforms.“ PACE, abstract
`no. 49 (1987).
`Tang, et al., “Ventricular Defibrillation Using Biphasic
`Waveforms of Diflerent Phasic Duration," PACE, abstract
`no. 47 (1987).
`Walcott ct 3.1., “Comparison of monophasic, biphasic, and
`the edmark waveform for external defibrillation,” PACE,
`151563, abstract 218 (Apr. 1992).
`Wathen et al., “Improved defibrillation efficacy using four
`nonthoracotomy leads for sequential pulse defibrillation,"
`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?, ” Cirt.
`72:111—384, abstract no. 1536 (1985).
`Winkle “The implantable defibrillator in ventricular arrhyth-
`mias," Hospital Practice, pp. 149—165 (Mar. 1983).
`Winkle et al., “Improved Low Energy Defibrillation Efficacy
`in Man Using a Biphasic Truncated Exponential Wave.
`form," JACC, 9(2): 142A (1987).
`Zipes, “Sudden cardiac death,” Circulation. 85(1):160—166
`(1992).
`
`3
`
`

`

`US. Patent
`
`Jan. 14, 1997
`
`Sheet 1 of 7
`
`5,593,427
`
`VOLTAGE
`
`VOLTAGE
`
`VTHRESHJ
`
`
` E —D
`
`tTHFlESH
`
`
`
`“ME
`
`VOLTAGE
`
`VA
`
`4
`
`

`

`US. Patent
`
`Jan. 14, 1997
`
`Sheet 2 of 7
`
`5,593,427
`
`10
`
`INITIATE DISCHARGE
`IN FIRST POLARITY
`
`VOLTAGE < VTHRESH
`
`YES
`
`STOP DISCHARGE
`IN FIRST PHASE
`
`
`
`
`
`
`
`16
`
`18
`
`20
`
`22
`
`24
`
`,
`
`WAIT FOR
`INTERIM TIME G V
`
`CHANGE
`POLARITY
`
`RESUME DISCHARGE
`FOR SECOND PHASE
`DURATION F
`
`
`
`STOP DISCHARGE
`
`FIG. 3
`
`5
`
`

`

`US. Patent
`
`Jan. 14, 1997
`
`Sheet 3 of 7
`
`5,593,427
`
`VOLTAGE
`
`1A
`
`VTHRESH HM
`
`tTHRESH
`
`VOLTAGE
`
`.
`
`I
`
`A
`
`Vnmfifl
`
`L»G+1
`tTHRESH
`
`VOLTAG E
`
`,
`A
`VTHRESHi
`
`Y
`
`6
`
`

`

`US. Patent
`
`Jan. 14, 1997
`
`Sheet 4 0f 7
`
`5,593,427
`
`5°
`
`
`
`INITIATE DISCHARGE
`IN FIRST POLARITY
`
`
`VOLTAGE < VITHRESH
`
`YES
`
`STOP DISCHARGE
`IN FIRST PHASE
`
`WAIT FOR
`INTERIM TIME G
`
`CHANGE
`POLARITY
`
`RESUME DISCHARGE
`FOR SECOND PHASE
`DURATION F
`
`
`
`
`
`
`
`
`60
`
`62
`
`64
`
`STOP DISCHARGE
`
`FIG. 6
`
`7
`
`

`

`U.S. Patent
`
`Jan. 14, 1997
`
`Sheet 5 of 7
`
`5,593,427
`
`INITIATE DISCHARGE
`IN FIRST POLARITY
`
`
`
`VOLTAGE SVTHRESH
`
`VOLTAGE S VTHRESH
`
`
`STOP DISCHARGE
`' OF FIRST PHASE
`
`WAIT FOR
`INTERIM TIME G
`
`FIG. 9
`
`
`
`
`95
`
`
`
`CHANGE
`POLARITY
`
`RESUME DISCHARGE
`FOR SECOND PHASE
`DURATION F
`
`STOP DISCHARGE
`
`96
`
`97
`
`98
`
`8
`
`

`

`US. Patent
`
`Jan. 14, 1997
`
`Sheet 6 of 7
`
`5,593,427
`
`
`
`FIG. 10
`
`9
`
`

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`5,593,427
`
`1
`ELECTROTHERAPY METHOD
`
`This application is a divisional of application Ser. No.
`08/103,837 filed Aug. 6, 1993.
`BACKGROUND OF THE INVENTION
`
`This invention relates generally to an electrotherapy
`method and apparatus for delivering a shock to a patient‘s
`heart. In particular, this invention relates to a method and
`apparatus for using an external defibrillator to deliver a
`biphasic defibrillation shock to a patient’s heart through
`electrodes attached to the patient.
`Defibrillators apply pulses of electricity to a patient's
`heart to convert ventricular arrhythmias, such as ventricular
`fibrillation and ventricular tachycardia,
`to normal heart
`rhythms through the processes of defibrillation and cardio-
`version, respectively. There are two main classifications of
`defibrillators: external and implanted. Implantable defibril-
`lators 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.
`to the
`send electrical pulses
`External defibrillators
`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 defibrilla-
`tors 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 defibril-
`lators, The high energy, voltage and current requirements
`have made current external defibrillators large, heavy and
`expensive, particularly due to the large size of the capacitors
`or other energy storage media required by these prior art
`devices.
`
`The time plot of the current or voltage pulse delivered by
`a defibrillator shows the defibrillator’s characteristic wave-
`form. Waveforms are characterized according to the shape,
`polarity, duration and number of pulse phases. Most current
`external defibrillators deliver monophasic current or voltage
`electrotherapeutic pulses, although some deliver biphasic
`sinusoidal pulses. Some prior art implantable defibrillators,
`on the other hand, use truncated exponential, biphasic wave-
`forms. Examples of biphasic implantable defibrillators may
`be found in U.S. Pat. No. 4,821,723 to Baker, In, et a1.; U.S.
`Pat. No. 5,083,562 to de Coriolis et 31.; 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 a1.
`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 that desired start and end
`voltage differential (i.e, a constant tilt).
`
`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 differences, exter-
`nal defibrillators must operate according to pulse amplitude
`and duration parameters that will be effective in most
`patients, no matter what
`the patient's physiology. For
`example, the impedance presented by the tissue between
`external defibrillator electrodes and the patient’s heart varies
`from patient to patient, thereby varying the intensity and
`waveform shape of the shock actually delivered to the
`patient's heart for a given initial pulse amplitude and dura-
`tion. Pulse amplitudes and durations efiective to treat low
`impedance patients do not necessarily deliver effective and
`energy efficient treatments to high impedance patients.
`Prior an 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 defibrillau'on at an initial energy setting suitable for
`defibrillating a patient of average impedance, then raise the
`energy setting for subsequent defibrillation attempts in the
`event that the initial setting failed. The repeated defibrilla—
`tion attempts require additional energy and add to patient
`risk. What is needed, therefore, is an external defibrillation
`method and apparatus that maximizes energy efliciency (to
`minimize the size of the required energy storage medium)
`and maximizes therapeutic efficacy across an entire popu—
`lation of patients.
`
`SUMMARY OF THE INVENTION
`
`This invention provides an external defibrillator and
`defibrillation method that automatically compensates for
`patienttepatient impedance ditferences in the delivery of
`electrotherapeutic pulses for defibrillation and cardiover—
`sion. In a preferred embodiment,
`the defibrillator has an
`energy source that may be discharged through electrodes on
`the patient to provide a biphasic voltage or current pulse. In
`one aspect of the invention,
`the first and second phase
`duration and initial first phase amplitude are predetermined
`values. In a second aspect of the invention, the duration of
`the first phase of the pulse may be extended if the amplitude
`of the first phase of the pulse fails to fall to a threshold value
`by the end of the predetermined first phase duration, as
`might occur with a high impedance patient. In a third aspect
`of the invention, the first phase ends when the first phase
`amplitude drops below a threshold value or when the first
`phase duration reaches a threshold time value, whichever
`comes first, as might occur with a low to average impedance
`patient. This method and apparatus of altering the delivered
`biphasic pulse thereby compensates for patient impedance
`differences by changing the nature of the delivered electro-
`therapeutic pulse, resulting in a smaller, more efficient and
`less expensive defibrillator.
`
`BRIEF DESCRIPTION OF THE DRAMNGS
`
`FIG. 1 is a schematic representation of a low-tilt biphasic
`electrotherapcutic waveform according to a first aspect of
`this invention.
`
`FIG. 2 is a schematic representation of a high-tilt biphasic
`electrotherapeutic waveform according to the first aspect of
`this invention.
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`FIG. 3 Is a flow chart demonstrating part of an electro-
`therapy method according to a second aspect of this inven-
`tion.
`
`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 die second aspect of this inven—
`tion.
`FIG. 6 is a flow chart demonstrating part of an electro-
`therapy method according to a third aspect of this invention.
`FIG. 7 is a schematic representation of a biphasic wave—
`form delivered according to the third aspect of this inven-
`tion.
`
`FIG. 8 is a schematic representation of a biphasic wave-
`form delivered according to the third aspect of this inven—
`tion.
`
`FIG. 9 is a flow chart demonstrating part of an electro—
`therapy method according to a combination of the second
`and third aspects of this invention.
`FIG. 10 is a block diagram of a defibrillator system
`according to a preferred embodiment of this invention.
`FIG. 11 is a schematic circuit diagram of a defibrillator
`system according to a preferred embodiment of this inven-
`tion.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`FIGS. 1 and 2 illustrate the patient-to—patient differences
`that an external defibrillator design must take into account.
`These figures are schematic representations of truncated
`exponential biphasic waveforms delivered to two different
`patients from an external defibrillator according to the
`electrotherapy method of this invention for defibrillation or
`cardioversion. In these drawings, the vertical axis is voltage,
`and the 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:
`
`As shown in FIGS. I 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 differences in end voltages B and D reflect differences in
`patient impedance.
`Prior art disclosures of the use of tmncated 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 affect
`
`4
`the choice of switch and capacitor technologies. Total
`energy delivery requirements affect defibrillator battery and
`capacitor choices.
`We have determined that, for a given patient. extemally-
`applied truncated exponential biphasic waveforms defibril-
`late at lower voltages and at lower total delivered energies
`than externally-applied monophasic waveforms. In addition,
`we have determined that there is a complex relationship
`between total pulse duration, first to second phase duration
`ratio, initial voltage, total energy and total tilt.
`Up to a point, the more energy delivered to a patient in an
`electrotherapeutic pulse, the more likely the defibrillation
`attempt will succeed. Low-tilt biphasic waveforms achieve
`effective defibrillation rates with less delivered energy than
`high»tilt waveforms. However,
`low-tilt waveforms are
`energy inefficient, since much of the stored energy is not
`delivered to the patient. On the other hand, defibrillators
`delivering high-tilt biphasic waveforms deliver more of the
`stored energy to the patient than defibrillators delivering
`low-tilt waveforms while maintaining high 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
`efficient therapy. An optimum capacitor charged to a prede-
`termined voltage can be chosen to deliver an efiective and
`efficient waveform across a population of patients having a
`variety of physiological differences.
`This invention is a defibrillator and defibrillation method
`that takes advantage of this relationship between waveform
`tilt and total energy delivered in high and low impedance
`patients. In one aspect of the invention,
`the defibrillator
`operates in an open loop, i.e., without any feedback regard-
`ing patient impedance parameters and with preset pulse
`phase durations. The preset parameters of the waveforms
`shown in 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 dependent upon the physiological
`parameters of the patient and the physical connection
`between the electrodes and the patient.
`the total
`For example,
`if the patient
`impedance (i.e.,
`impedance between the two electrodes) is high, the amount
`of voltage drop (exponential decay) from the initial voltage
`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 efiective per unit of
`delivered energy, and low impedance patients receive a
`high—tilt waveform that delivers more of the stored energy
`and is therefore more energy efficient.
`Another feature of biphasic waveforms is that waveforms
`with relatively longer first phases have better conversion
`properties than waveforms with equal or shorter first phases,
`provided the total duration exceeds a critical minimum.
`Therefore, in the case of high impedance patients, it may be
`desirable to extend the first phase of the biphasic waveform
`(while the second phase duration is kept constant) to
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`increase the overall efficacy of the electrotherapy by deliv-
`ering a more efficacious waveform and to increase the total
`amount of energy delivered. FIGS. 3—5 demonstrate a
`dcfibrillation method according to this second aspect of the
`invention in which information related to patient impedance
`is fed back to the defibrillator to change the parameters of
`the delivered electrotherapeutic pulse.
`FIG. 3 is a flow chart showing the method steps following
`the decision (by an operator or by the defibrillator itself) to
`apply an electrotherapeutic shock to the patient through
`electrodes attached to the patient and charging of the energy
`source, e.g., the defibrillator’s capacitor or capacitor bank, to
`the initial first phase voltage A. Block 10 represents initia-
`tion of the first phase of the pulse in a first polarity.
`Discharge may be initiated manually by the user or auto-
`matically in response to patient heart activity measurements
`(cg, ECG signals) received by the defibrillator through the
`electrodes and analyzed by the defibrillator controller in a
`manner known in the art.
`least a
`Discharge of the first phase continues for at
`threshold time Imus”, 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 VTHREsr-n first phase dis-
`charge continues, as shown in block 14 of FIG. 3. For high
`impedance patients, this situation results in an extension of
`the first phase duration beyond tTHRESH, as shown in FIG. 4,
`until
`the measured voltage drops below the threshold
`Vnmssu- Discharge then ends to complete the first phase, as
`represented by block 16 of FIG. 3. If, on the other hand, the
`patient has low impedance, the voltage will have dropped
`below VTHRESH when the time threshold is reached, result-
`ing in a waveform like the one shown in FIG. 5.
`At the end of the first phase. and after a predetermined
`interim period G, the polarity of die energy source connec-
`tion to the electrodes is switched, as represented by blocks
`18 and 20 of FIG. 3. Discharge of the second phase of the
`biphasic pulse then commences and continues for a prede-
`termined second phase duration F, as represented by block
`26 of FIG. 3, then ceases. This compensating clectrotherapy
`method ensures that the energy is delivered by the defibril-
`lator in the most efficacious manner by providing for a
`minimum waveform tilt and by extending the first phase
`duration to meet the requirements of a particular patient.
`Because this method increases the waveform tilt for high
`impedance patients and delivers more of the energy from the
`energy source than a method without compensation,
`the
`defibrillator’s energy source can be smaller than in prior art
`external defibrillators, thereby minimizing defibrillator size,
`weight and expense. It should be noted that the waveforms
`shown in FIGS. 4 and 5 could be expressed in terms of
`current versus time using a predetermined current threshold
`value without departing from the scope of the invention.
`FIGS. 6—8 illustrate a third aspect of this invention that
`prevents the delivered waveform from exceeding a maxi~
`mum tilt (i .e., maximum delivered energy) in low impedance
`patients. As shown by blocks 52 and 54 in FIG. 6, the first
`phase discharge stops either at the end of a predetermined
`time trauma or when the first phase voltage drops below
`V‘THRESH. The second phase begins after an interim period
`G and continues for a preset period F as in the second aspect
`of the invention. Thus, in high impedance patients, the first
`phase ends at time tmmsg, even if the voltage has not yet
`fallen below V'rmrasm 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 trams”, as shown in FIG. 8.
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`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-
`form will end if the voltage reaches a first voltage threshold
`V'THRESH prior to the first phase duration threshold imam”.
`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 17mins” the voltage has not
`fallen below V'THRESH, the duration of the first phase is
`extended beyond tmflm until the voltage measured across
`the electrodes reaches a second voltage threshold angsm
`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 or 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
`bipha