[191
`United States Patent
`[11] Patent Number:
`5,431,686
`Jul. 11, 1995
`.Kroll et al.
`[45] Date of Patent:
`
`||||||||||||||Ill|||||||l|l||l|||||||||||||||||||||||||||l||||||||||||||||l
`USOOS431686A
`
`[75]
`
`[73] Assignee:
`
`[54] METHOD FOR OPTIMAL PULSE
`DEFEBRJILATION USING AN
`IMPLANTABLE DEFIBRILLATOR
`Inventors: Mark W. Kroll; Charles U. Smith,
`both of Minnetonka, Minn.
`Angeion Corporation, Plymouth,
`Minn.
`[21] App]. No.: 835 836
`[22] Filed:
`[51]
`Int. Cl.6 ............................................... A61N 1/00
`
`[52] US. Cl.
`.........................
`. 607/7; 607/6
`
`[58] Field of Search ........................................ 607/6, 7
`[56]
`References Cited
`U.S. PATENT DOCUMENTS
`
`4/1989 Baker, Jr. ct a].
`4,821,723
`4,850,357 7/1989 Bach, Jr.
`.
`OTHER PUBLICATIONS
`
`.
`
`J L Prevost and F Batelli, “Sur quelques effets des
`descharges electriques sur le couer des mammifers,”
`Comptes rendus hebdomadaires des seances de l’Academie
`des sciences, vol. 129, pp. 1267, 1899.
`A C Guyton and J Satterfield, “Factors concerned in
`defibrillation of the heart, particularly through the un-
`opened chest,” Am J of Physiology, vol. 167, pp. 81,
`1951.
`J C Schuder, G A Rahmoeller, and H Stoecklc, “Trans-
`thoracic ventricular defibrillation with triangular and
`trapezoidal waveforms,” Circ Res, vol. 19, pp. 689-694,
`Oct. 1966.
`W A Tacker, L A Geddes, J McFarlane, et a1, “Opti-
`mum current duration for capacitor-discharge defibril-
`lation of canine ventricles,” JApplied Physiology, vol. 27
`#4, pp. 480-483, Oct. 1969.
`J C Schuder, H Stoeckle, J A Wes, et al, “Transthoracic
`ventricular defibrillation in the dog with truncated and
`untruncated exponential stimuli," IEEE Trans. Biom.
`Eng, vol. BME—18 #6, pp. 410—415, Nov. 1971.
`J D Bourland, W A Tacker, L A Geddes, et al, “Com-
`parative efficacy of damped sine wave and square wave
`current for transchest ventricular defibrillation in ani-
`mals,” Medical Instrum, vol. 12 #1, pp. 38-41, 1978.
`G Weiss, “Sur la possibilite’ de rendre comparable entre
`eux les appareils survant a l’excitation electrique,” Arch.
`Ital de Biol, vol. 35, pp. 413—446, 1901.
`J D Bourland, W A Tacker, and L A Geddes, “Strength
`duration curves for trapezoidal waveforms of various
`
`tilts for transchest defibrillation in animals,” Med. Instn,
`v01. 12 #1, pp. 38—41, 1978.
`L Lapicque, “Definition experimentalle de l’excitabi-
`lite’,” Proc. Soc. de Biol, vol. 77, pp. 280—285, 1909.
`P S Chen, P D Wolf, and F J Claydon, “The potential
`gradient field created by epicardial defibrillation elec-
`trodes in dogs,” Circulation, vol. 74, pp. 626—635, Sep.
`1986.
`M Mirowski, M M Mower, W S Staewen, et a1.,
`“Standby automatic defibrillator,” Arch Int. Med, vol.
`126, pp. 158—161, Jul. 1970.
`J C Schuder, H Stoeckle, J A West, et a1., “Ventricular
`defibrillation in the dog with a bielectrode intravascular
`catheter,” Arch. Int. Meal, vol. 132, pp. 286-290, Aug.
`1973.
`M Mirowski, M M Mower, V L Gott, et a1, “Feasibility
`and effectiveness of low—energy catheter defibrillation
`in man,” Circulation, vol. 47, pp. 79-85, Jan. 1973.
`Primary Examiner—William E. Kamm
`Assistant Examiner~Scott M. Getzow
`Attorney, Agent, or Firm—Patterson & Keough
`
`ABSTRACT
`[57]
`The model that is developed in the present invention is
`based upon the pioneering neurophysiological models
`of Lapicque and Weiss. The present model determines
`mathematically the optimum pulse duration, dp, for a
`truncated capacitor—discharge waveform employed for
`defibrillation. The model comprehends the system time
`constant, RC, where R is tissue resistance and C is the
`value of the capacitor being discharged, and also the
`chronaxie time, dc, defined by Lapicque, which is a
`characteristic time associated with the heart. The pres»
`ent model and analysis find the optimum pulse duration
`to be dp=(0.58)(RC+dc). Taking the best estimate of
`the chronaxie value from the literature to be 2.7 ms,
`permits one to rewrite the optimum pulse duration as
`dp=(0.5 8)RC+ 1.6 ms. The present invention makes use
`of the mathematical definition of optimum pulse dura-
`tion by storing in the control circuitry of the defibrilla-
`tion system the actual measured value of the particular
`capacitor incorporated in the system. The optimized—
`pulse prescription of this invention can be applied to a
`monophasic waveform, or to either or both of the pha-
`ses of a biphasic waveform.
`
`12 Claims, 5 Drawing Sheets
`
`
`
`L|FECOR427-1007
`
`1
`
`LIFECOR427-1007
`
`

`

`US. Patent
`
`July 11, 1995
`
`Sheet 1 of 5
`
`5,431,686
`
`655 VOLTS
`
`12/
`
`Fig. 1A
`
`
`
`
`.0
`
`/
`
`/‘I4
`
`3O JOULES
`
`486 VOLTS
`
`/ F
`
`ig.2
`
`32
`
`[/30
`
`34
`
`2
`
`

`

`US. Patent
`
`July 11, 1995
`
`Sheet 2 of 5
`
`5,431,686
`
`Fig. 3A
`
`750 VOLTS
`
`42
`
`
`
`so
`
`5'55
`‘
`669/0 TILT
`554
`(250 VOLT CUTOFF')
`
`750 VOLTS
`522/
`
`
`
`O
`
`F1g.3C
`/ 44% INTERMEDIATE TILT
`
`(420 VOLT CUTOFF)
`
`57
`
`750 VOLTS
`
`'
`
`5:
`
`59
`
`
`
`3
`
`

`

`US. Patent
`
`July 11, 1995
`
`Sheet 3 of 5
`
`5,431,686
`
`
`----------
`
`
`
`.fl-I-I-I-I
`
`
`4 .I...i-———
`2 ----------
`
`III-III..-
`
`20
`O
`2
`4
`6
`8
`l0
`l2
`l4
`I6
`I8
`PULSE DURATION (ms)
`
`'
`
`
`
`’
`
`Fig. 5
`
`70
`
`/
`
`DCSOURCE Dc
`
`YEAR ANIMAL (ELECTRODES)
`AUTHOR
`
`
`L197?
`
`2.WM
`
`3-mama-__m-
`4.__IEI
`
`
`5.—_mfi-
`6- [flag-MEI
`
`
`v.mum—man
`\72
`
`1.
`
`2.
`
`1H. GOLD, ct 8.1., CIRCULATION, VOL. 56, p.745, NOVEMBER 1977.
`
`74
`
`JD. BOURLAND, et a1., MED. INSIR, VOL. 92, p. 38, 1978.
`
`3. LL. WESSALE, et al., J. ELECTROCARDIOLOGY, VOL.13, p. 359, 1980.
`
`4. LL. JONES AND R.E. JONES, AM I PHYSIOL., VOL. 242, p. IH662, 1982.
`
`5. MJ. NIEBAUBR, et 31., CRIT. CARE MEDICINE, VOL. 11, p. 95, FEBRUARY 1983.
`
`6. LA. GEDDES, et 31., MED. BIOL. ENG. COMP., VOL. 23, p. 122, 1985.
`
`7. SA. FEESER, et a1., CIRCULATION, VOL. 82, p.2128, DECEMBER 1990.
`
`4
`
`

`

`US. Patent
`
`July 11, 1995
`
`Sheet 4 of 5
`
`5,431,686
`
`Fig. 6
`
`R
`
`80
`
`PHYSIOLOGICALLY EFFECTIVE CURRENT I pe ACHIEVED USING THREE
`
`DIFFERENT METHODS FOR SPECIFYING A MONOPHASIC WAVEFORM.
`
`88
`
`82
`
`86
`
`RESISTANCE
`
`65%TILT
`
`FIXED PULSE
`DURATION OF ems
`
`
`
`
`
`OPTIMUM DURATION
`
`25 ohms
`
`I0.70 A
`
`50
`
`100
`
`
`
`3.92
`
`4.20
`
`4. 23
`
`
`6.79
`6.94
`6.96
`
`
`5
`
`

`

`US. Patent
`
`July 11, 1995
`
`Sheet 5 of 5
`
`5,431,686
`
`Fig. 7
`
`90
`
`START
`
`92
`
`SENSE A MYOCARDIAL
`DYSRHYTHMIA
`
`94
`
`96
`
`BEGIN DELIVERING
`ENERGY FROM
`CAPACITOR
`
`
`
`WAIT UNTIL CAPACITOR
`DISCHARGE VOLTAGE
`DECAYS GIVEN
`PERCENTAGE
`
`98
`
`DELAY A FIXED TIME
`PERIOD
`
`'102
`
`“0
`
`STOP
`
`100
`
`
`MULTIPLE
`
`PHASE
`
`PULSE?
`
`. YES
`
`104
`
`REVERSE PULSE
`POLARITY
`
`106
`
`STOP
`
`6
`
`

`

`1
`
`5,431,686
`
`METHOD FOR OPTINIAL PULSE
`DEFIBRILLATION USING AN MPLANTABLE
`DEFIBRILLATOR
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates generally to defibrilla-
`tion methods, and more particularly, to an optimum
`truncated capacitive-pulse duration that is based upon
`the time constants of the system and of the heart.
`2. Description of the Prior Art
`Defibrillation, or causing the cessation of chaotic and
`uncoordinated contraction of the ventricular myocar-
`dium by application of an electrical direct current and
`voltage, in its most primitive form, goes back to the last
`century. [1. L. Prevost and F. Batelli, “Sur Quelques
`Et‘fets des Descharges Electrriques sur le Couer des
`Mammifers”, Comptes Rendus Hebdomaa’aires des Se-
`ances de L’Acadmie des Sciences, Vol. 129, p. 1267,
`1899.] Because of the large currents required for defi-
`brillation, large-area electrodes are employed. [A. C.
`Guyton and J. Satterfield, “Factors Concerned in Defi-
`brillan'on of the Heart, Particularly Through the Un-
`opened Chest”, Am. J. of Physiology, Vol 167, p. 81,
`1951.]
`For reasons of simplicity and compactness, capacitor-
`discharge systems are almost universally used in defi-
`brillation. The discharge of a capacitor C through a
`resistance R results in a curve of voltage versus time
`(and hence, of current versus time as well) that is a
`declining exponential function, with a characteristic
`time given by the product RC. But it has also been
`recognized for some time that the long-duration, low-
`amplitude “tail” of the capacitor—discharge pulse is det-
`rimental.
`[.T. C. Schuder, G. A. Rahmoeller, and H.
`Stoeckle, “Transthoracic Ventricular Defibrillation
`with Triangular and Trapezoidal Waveforms”, Circ
`Res, Vol. 19, p. 689, October, 1966; W. A. Tacker, et
`al., “Optimum Current Duration for Capacitor-dis-
`charge Defibrillation of Canine Ventricles”, J. Applied
`Physiology, Vol 27, p. 480, October, 1969.] The exact
`reason for this detrimental effect is not known, although
`plausible speculations exist, with one possibility being
`that field heterogeneties cause arthythmias in signifi-
`cantly large regions of the heart. [P. S. Chen, et al.,
`“The Potential Gradient Field Created by Epicmrdial
`Defibrillation Electrodes in Dogs”, Circulation, Vol 74,
`p. 626, September, 1986.] A convenient way to elimi-
`nate the low-amplitude “tail” of a capacitor discharge is
`by switching, which is to say, simply opening the
`capacitor-load circuit after a predetermined time, or
`else when voltage has fallen to a particular value. For
`this reason, the time-truncated capacitor discharge has
`been extensively used after its effectiveness was first
`demonstrated.
`[.1. C. Schuder, et al., “Transthoracic
`Ventricular Defibrillation in the Dog with Truncated
`and Untruncated Exponential Stimuli”, IEEE Trans.
`Biom Eng, Vol. BME-IS, p. 410, November, 1971.]
`The defibrillation effectiveness of time-truncated
`capacitor .discharges can be convincingly shown by
`comparing an untruncated waveform and a truncated
`waveform of equal effectiveness. The full discharge
`waveform 10 of FIG. 1A was generated by charging a
`l40—NF capacitor to 455 V, for an energy delivery of 30
`J. But the truncated waveform 20 shown in FIG. 1B
`was equally effective for defibrillation in spite of having
`about only half the energy, and a lower initial voltage.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`4O
`
`45
`
`50
`
`55
`
`65
`
`2
`This demonstration was carried through for the case of
`dogs using a catheter electrode and a subcutaneous
`patch [M. Mirowski, et al., “Standby Automatic Defib-
`rillator”, Arch Int Med, Vol 126, p. 158, July, 1970], as
`well as with a dual-electrode intraventricular catheter.
`[J. C. Schuder, et al., “Ventricular Defibrillation in the
`Dog with a Bielectrode Intravascular Catheter”, Arch.
`Int. Med, Vol. 132, p. 286, August, 1973.]The latter
`electrode arrangement was also used to demonstrate the
`point for the case of man. [M. Mirowski, et al., “Feasi-
`bility and Effectiveness of Low-energy Catheter Defi-
`brillation in Man”, Circulation, Vol 47, p. 79, January,
`1973.] Such demonstrations that compact capacitor-
`storage systems could be used with effectiveness paved
`the way for implantable defibrillator system.
`In spite of the dramatic results obtained with time-
`truncated capacitor—discharge defibrillator systems, the
`waveform specifications have not been systematically
`optimized. For example, some manufacturers such as
`Medtronic (in their PCD product) simply specify pulse
`duration, although the physician can choose and adjust
`the value. A typical value might be a programmable
`duration of 6 ms. Other manufacturers such as Cardiac
`Pacemakers (in their Ventak product) specify the rela-
`tive amount of voltage decline at the time of truncation,
`with a typical value of the decline being 65% of the
`initial voltage. It has become customary to use the term
`“tilt” to describe the relative amount of such voltage
`decline, expressed either as a decimal fraction or a per-
`centage. In algebraic language:
`
`filt=(Vinian-VfinaI)/Yinizia1
`
`Eq- 1
`
`Both of the systems just cited employ the monophasic
`waveform. This means that it consists of a single-
`polarity single pulse,
`specifically a time-truncated
`capacitor-discharge waveform like that of FIG. 18.
`However, biphasic waveforms are also widely used. In
`this case capacitor discharge is also used, but instead of
`truncation, polarity reversal
`is accomplished (by
`switching once more), so that a second opposite-
`polarity pulse immediately follows the initial pulse, and
`is then itself trimcated. The result is illustrated in FIG.
`2.
`
`Prior art in waveform specification for biphasic sys-
`tems is parallel to that for monophasic systems. Specifi-
`cally, some systems simply specify initial-pulse dura-
`tion.
`[Baker, Intermedics, US. Pat. No. 4,821,732.]
`Other systems specify tilt. [Bach, Cardiac Pacemakers,
`US. Pat. No. 4,850,537.] The central focus of the pres-
`ent invention is to optimize pulse duration by using the
`model of this invention, which comprehends both the
`time constant of the system (capacitor and load resis-
`tance), and the natural time constant of the heart as
`explained below.
`It is worthwhile to examine specific examples of pri-
`or-art waveform specification, In FIG. 3A is shown the
`simple pulse-duration specification, applicable to either
`monophasic pulses or biphasic initial pulses. And in
`FIG. 3B is shown a tilt specification, also applicable to
`either monophasic pulses or biphasic initial pulses.
`The foundation for optimizing the time-truncated
`waveform is a family of mathematical neurophysiologi-
`cal models for tissue stimulation going back to the turn
`of the century, with the first important such model
`having been developed by Weiss. [G. Weiss, “Sur la
`Possibilite de Rendre Comparable entre Eux les Ap-
`
`7
`
`

`

`3
`pareils Suivant a l’Excitation Electrique” Arch. Ital.
`deBioI., Vol. 35, p. 413, 1901.] He employed the ballis-
`tic-rheotome technique for pulse generation, wherein a
`rifle shot of known velocity is used to cut two wires in
`sequence, their spacing being set and measured. Cutting
`the first wire eliminated at short from a dc source, caus-
`ing current to flow through the tissue under test, and
`cutting the second wire opened the circuit, terminating
`the pulse applied. Converting the electrical data into
`charge delivered by the pulse, Weiss found that the
`charge Q needed for stimulation was linearly dependent
`on pulse duration, (1;. Specifically,
`
`10
`
`Q=K1+K2dp
`
`Eq. 2
`
`15
`
`Subsequently and similarly, the physiologist L. La—
`picque collected substantial amounts of data on the
`amount of current required for tissue stimulation, using
`constant-current pulses of various durations. [L. La-
`picque, “Definition Experimentelle de l’excitabilite,”
`Proc. Soc. deBiol., Vol 77, p. 280, 1909.] Lapicque estab-
`lished an empirical relationship between the current I
`and the pulse duration (11,, having the form:
`
`20
`
`5,431,686
`
`4
`The defibrillation chronaxie for the heart is usually
`between 2 ms and 4 ms, as can be seen in the chart of
`FIG. 5. (A journal citation for each entry is given below
`the chart.) In this synopsis of published data, chronaxie
`was inferred from a strength-duration curve such as that
`of FIG. 4 when such a curve was provided, and these
`cases are labeled “given”; in the case labeled “deter-
`mined”, chronaxie was calculated from discrete data
`provided. In the only other case (6. Geddes, et a1. ),
`curves were given for waveforms of various tilts, and
`these were averaged to arrive at 2.8 ms. For the overall
`chart, 2.7i0.9 ms is the average chronaxie value.
`SUMMARY OF THE INVENTION
`
`The inventors have developed an analytic method for
`waveform optimization. It builds upon the models of
`Lapicque and Weiss, the finding of Boarland, and the
`data summarized in FIG. 5. To do this one first defines
`a “sufficiency ratio’,’, the ratio of Bourland’s ruling aver-
`age current and the current needed for defibrillation
`according to the Lapicque model for a heart of a given
`K1, rheobase current, and a given K2, a charge. Alge-
`braically,
`
`I: Klara/(1,)
`
`Eq. 3
`
`25
`
`Sufficiency ratio =
`
`Eq.5
`
`(Note that multiplying this expression through by dp
`yields an expression in charge rather than current, iden-
`tically the equation given by Weiss. Thus, K1=l<1/dp
`and K2=kzdp.)
`Equation 3 of Lapicque shows that the necessary
`current and the pulse duration are related by a simple
`hyperbola, shifted away from the origin by the amount
`of the constant term K1. Hence the stimulating current
`required in a pulse of infinite duration is K1, a current
`value Lapicque termed the rheobase. Shortening the
`pulse required progressively more current, and the
`pulse length that required a doubling of current for
`excitation, or 2K1, he termed the chronaxie, dc. Substi-
`tuting 2K1 and dc into Eq. 3 in place of I and dp, respec-
`tively, yields:
`
`dc=K2/K1
`
`Eq. 4
`
`For the sake of specific illustration, assume a rheobase
`current of 3.7 amperes, and a chronaxie time of 6 milli-
`seconds. Then a plot of current strength required versus
`the pulse duration that must accompany it is as shown in
`FIG. 4.
`Lapicque’s model described cell stimulation, rather
`than defibrillation, but Bourland demonstrated that
`defibrillation thresholds in dogs and ponies followed the
`Lapicque model, provided average current is used in
`the exercise.
`[1. D. Bourland, W. Tacker, and L. A.
`Geddes, “Strength—Duration Curves for Trapezoidal
`Waveforms of Various Tilts for Transchest Defibrilla-
`tion in Animals”, Med Instr, Vol. 12, p. 38, 1978.] In a
`companion paper, the same workers showed that aver-
`age current is a useful and consistent measure of defi-
`brillation effectiveness for time-truncated pulses of a
`fixed duration through a substantial range of durations,
`from 2 to 20 milliseconds; in other words, so long as the
`exponential “tail” is eliminated, pulse effectiveness is
`not very dependent upon waveform details. [1. D. Bour-
`land, W. Tacker, and L. A. Geddes, “Comparative
`Efficacy of Damped Sine Waves and Square Wave
`Current for Transchest Defibrillation in Animals”, Med
`Instr., Vol. 12, p. 42, 1978]
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`It is simply the ratio of Bourland’s available therapeutic
`current (or the average current law) to the current re-
`quired according to the Lapicque formulation. Hence
`for a ratio of unity, the waveform of average current
`1m and duration dp will be able to defibrillate a heart
`characterized by K1 and K2.
`Multiplying Eq. 5 through by the rheobase current
`K1 yields an expression that of course has dimensions of
`amperes; noting from Eq. 4 that Kldc=K2, makes it
`possible to eliminate the heart-characterizing quantities
`K1 and K2 from this expression.
`
`
`lave
`Iave
`_
`
`K2
`'—
`11,;
`+ Kldp
`+ d,
`
`1
`
`Eq.6
`
`Thus, we have here an expression in the two pulse-
`characterizing quantities law and dp, but in only one
`heart-characterizing quantity, dc, the chronaxie time.
`Note that for an infinite pulse duration, this current
`simply equals the average current 1m, but for a pulse of
`finite duration, it will be smaller than lave. This current,
`therefore, measures the effectiveness of a particular
`waveform in defibrillating a particular heart. For this
`reason, the inventors have named it the Physiologically
`effective current, or Ipe, which can then be further ma-
`nipulated in straightforward fashion.
`
`I _ Iavedp
`Fe _ dc + 11,,
`
`EQ- 7
`
`Note further that 199 would be the same as 1m if one had
`a zero value of chronaxie time, dc. In this sense, Eq. 7
`constitutes a correction from actual average current
`necessitated by the chronaxie phenomenon. Since,
`—a'p/RC
`
`Eq. 3
`
`V}: Vie
`
`it follows that
`
`8
`
`

`

`5
`
`delivered charge=C(Vi—Vf)=CV,(1—e—dP/RQ
`and hence
`.
`
`cm _ e—dP/RC)
`41,, + dc
`
`IP‘ =
`
`5,431,686
`
`Eq.9
`
`Eq. 10
`
`6
`stant t and the heart’s characteristic time (1;. In other
`words, the optimum pulse duration is a compromise
`between the two characteristic times involved.
`Equation 19 for optimal pulse duration can be rewrit-
`ten as:
`
`dp=(0.58)(RC+dc)
`
`Eq. 20
`
`Thus, the optimum pulse duration is most naturally
`specified as a fraction of the sum of two characteristic
`times: that for the system, RC, and that for the heart, the
`chronaxie time dc. Letting dc=2.7 ms, this becomes:
`
`dp=(0.58)RC+1.6 ms
`
`Eq. 21
`
`10
`
`15
`
`It is clear that 11,: vanishes at both extremes of dp, so that
`intermediate extremum must be a maximum, defining
`explicitly the optimum waveform that can be achieved
`by varying pulse duration with a particular average
`current. To determine this optimum pulse duration, let
`RC=t, and set
`
`41,,
`4d,,
`
`
`(d, + dAe’d/Vt» _ (1 — e—d”)
`(dp + 4492
`
`=0
`
`Eq. 11
`
`Hence, using the system time constant (t=RC) for nor-
`malization yields
`
`20
`
`Because the sensing of tilt is easy, using tilt as a crite-
`rion for pulse termination is straightforward and conve-
`nient. Therefore, we define for present purposes a new
`term, intermediate tilt, to be the ratio (usually expressed
`as a percentage) of the voltage at some intermediate
`part of the pulse to the initial voltage. Next, convert
`(0.58)RC into intermediate tilt:
`
`Voltage ratio — (0.58)RC/RC
`= l _ e—O.58
`= I —- 0.56 = 0.44
`
`Eq. 22
`
`Eq. 13
`
`Eq. 14
`
`25
`
`Thus,
`
`30
`
`intermediate tilt=44% Eq. 23
`
`Eq. 15
`
`_ z
`2 _ d,
`
`and
`
`:1,
`a. = --2
`
`Using these definitions,
`
`(Z+a+l)e—z——l=0
`
`Next, multiply through by ——e—1 to obtain the simplified
`equation whose root is sought.
`
`ez—z—u—lzo
`
`Eq. 16
`
`Because the equation is transcendental, it cannot be
`solved in close form, so define the function on the left-
`hand side as f(z) and the first approximation for its root
`as 20. The Newton-Raphson method gives an approxi-
`mate value for the root as
`
`21:20-
`
`
`flZo)
`f(Zo)
`
`Eq. 17
`
`Experience shows that waveforms with a tilt of about
`65% are effective, and this corresponds to dp=t, or
`20: 1. Hence an appropriate approximate root is:
`
`Zl_1_e—l«a—l =_lfl
`e—l
`e—l
`
`Eq.lB
`
`Denormalization yields:
`
`d_ 1+JC/t=t+dc
`P_t e—l
`2—1
`
`£4.19
`
`for the approximate optimum value of pulse duration dp
`as a function of chronaxie dc and system time constant t.
`Carrying through the optimization numerically shows
`that this estimate is valid within 0.2% for typical values
`of R, C, and dc. Even for extreme values of these system
`and heart parameters, the approximate value of opti-
`mum duration produces a value for the current IPe that
`is within 2% of the optimum. Since (e—1)=l.72=2, the
`optimum pulse duration is approximately (and some—
`what larger than) the average of the system’s time con-
`
`so that the convenient alternate method for expressing
`optimum pulse duration is
`
`35
`
`dp={44% intermediate tilt}=l.6 ms Eq. 24-
`
`where the symbols {} are taken to mean “time interval
`for”. This new method for specifying an optimal pulse is
`illustrated in FIG. 3C, for comparison with the prior art
`method illustrated in FIGS. 3A and SB.
`It is evident that the optimum-duration criterion of
`the present invention could be translated into an equiva-
`lent tilt specification if the value of R were well-known,
`stable over time and constant from patient to patient.
`But typical resistance variation is from some 25 ohms to
`100 ohms. Furthermore, variations in capacitance exist,
`with 10% tolerances being typically encountered. Be—
`cause the new criterion takes account of such varia—
`tions, it is .clearly superior to both a fixed-tilt and a
`fixed-duration specification. To illustrate this superior-
`ity we calculate physiologically effective current I” in
`the face of resistance variations. The higher the Ipe, the
`better the criterion. In FIG. 6 is shown the result of
`such calculations for the popular fixed-tilt (65%) and
`fixed-duration (6 ms) criteria, as compared to the op-
`timum-duration criterion of the present invention. Note
`that the optimum-duration criterion yields a higher Ipe
`than either of the prior—art criteria through the full
`resistance range.
`The method of the present invention can be applied in
`the biphasic case to the first pulse or phase, to the sec—
`ond phase, or to both. If the first phase is chosen for
`optimum duration, there are a number of other ways to
`specify the second phase. For example,
`the second
`phase can be permitted to decay to an 80% overall tilt,
`or 80% charge removal
`from the capacitor. This
`method has the advantage of delivering fixed energy to
`the defibrillation path, irrespective of the kinds of varia-
`
`45
`
`50
`
`55
`
`65
`
`9
`
`

`

`5,431,686
`
`7
`tions discussed above. Another method lets the duration
`of the second pulse equal a fraction of that determined
`to be optimal for the first. Limited animal studies indi-
`cate a benefit with respect to having the second pulse
`less than or equal to the first in duration. Finally, the 5
`second pulse could be made to meet a fixed-tilt or fixed-
`duration specification, just as in the prior-art monopha-
`SIC cases.
`
`8
`Yet a further object of the present invention is to
`employ a defibrillation pulse of optimum duration in
`one phase of a biphasic waveform, with the other phase
`having an equal duration.
`A still further object of the present invention is to
`employ a defibrillation pulse of optimum duration in
`one phase of a biphasic waveform, with the other phase
`having a duration fixed by overall tilt.
`Yet a further object of the present invention is to
`achieve an increase of defibrillation effectiveness for a
`given capacitor volume and system volume.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Other objects of the present invention and many of
`the attendant advantages of the present invention will
`be readily appreciated as the same becomes better un-
`derstood by reference to the following detailed deserip~
`tion when considered in connection with the accompa-
`nying drawings, in which like reference numerals desig-
`nate like parts throughout
`the figures thereof and
`wherein:
`FIG. 1A illustrates a particular voltage-time wave-
`form of a capacitor discharged through a resistor;
`FIG. 13 illustrates a particular voltage-time wave-
`form of a time-truncated pulse, produced by discharg-
`ing a capacitor through a resistor and terminating the
`pulse by switching, with this pulse delivering half the
`energy of that in FIG. 1A;
`FIG. 2 illustrates a biphasic waveform for defibrilla-
`tion;
`FIG. 3A illustrates a monophasic waveform of a
`particular initial voltage, specified in terms of pulse
`duration;
`FIG. 3B illustrate: a monOphasie waveform of a par-
`ticular initial voltage, specified in terms of tilt;
`FIG. 3C illustrates a monophasic waveform of a par-
`ticular initial voltage specified by the method of the
`present invention;
`FIG. 4 illustrates a chart of average current strength
`required for defibrillation versus the duration of the
`pulse of that average current;
`FIG. 5 illustrates a chart of chronaxie values drawn
`from the literature and a listing of the reference cita-
`tions; and,
`FIG. 6 illustrates a chart of physiologically effective
`current values achieved by three waveform-specifying
`methods in the face of load-resistance variations.
`FIG. 7 is a flow chart showing the decisional steps of
`a preferred embodiment of the present invention.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`FIG. 1A illustrates a particular voltage-time wave-
`form 10 of a capacitor discharged through a resistor,
`incorporating a particular initial voltage 12, and a par-
`ticular energy 14 delivered from the capacitor to the
`resistor.
`FIG. 1B illustrates a particular voltage-time wave-
`form 20 of a capacitor discharged through a resistor,
`incorporating a particular initial voltage 22, and a par-
`ticular energy 24 delivered from the capacitor to the
`resistor, and also incorporating time-truncation of the
`pulse, produced by terminating the pulse through
`switching at the particular time 26, thus eliminating the
`tail 28 of the pulse, with this particular pulse delivering
`half the energy of the particular pulse in FIG. 1A.
`
`15
`
`30
`
`One significant aspect and feature of the present in-
`vention is a defibrillation waveform of optimum dura- 10
`tion
`Another significant aspect and feature of the present
`invention is a pulse duration that comprehends the sys-
`tem time constant, RC, and the heart’s characteristic
`time, the chronaxie, dc.
`Still another significant aspect and feature of the
`present invention is a pulse duration that is approxi-
`mately an average of the system time constant, RC, and
`the heart’s characteristic time, the chronaxie, dc.
`Still another significant aspect and feature of the 20
`present invention is a pulse duration that is equal to the
`sum of the time interval corresponding to an intermedi-
`ate tilt of 44% and 1.6 ms.
`Another significant aspect and feature of the present
`invention is the programming of actual values of R, C, 25
`and dc into the control circuitry of the defibrillation
`system.
`Still another significant aspect and feature of the
`present invention is the use of a defibrillation pulse of
`optimum duration in a monophasic waveform.
`Yet another significant aspect and feature of the pres-
`ent invention is the use of a defibrillation pulse of opti-
`mum duration in at least one phase of a biphasic wave-
`form.
`Another significant aspect and feature of the present 35
`invention is the use of a defibrillation pulse of optimum
`duration in one phase of a biphasic waveform, with the
`other phase having an equal duration.
`Still another significant aspect and feature of the
`present invention is the use of a defibrillation pulse of 40
`optimum duration in one phase of a biphasic waveform,
`with the other phase having a duration fixed by overall
`tilt.
`Yet another significant aspect and feature of the pres-
`ent invention is an increase of defibrillation effective- 45
`ness for a given capacitor volume and system volume.
`Having thus described embodiments of the present
`invention, it is a principal object of the present inven-
`tion to employ a defibrillation waveform of optimum
`duration.
`A further object of the present invention is to employ
`a pulse duration that comprehends the system time
`constant, RC, and the heart’s characteristic time, the
`chronaxie, dc.
`,
`A still further object of the present invention is to 55
`employ a pulse duration that is approximately an aver-
`age of the system time constant, RC, and the heart’s
`characteristic time, the chronaxie, dc.
`Still another object of the present invention is to
`employ a pulse duration that is a function of actual 60
`measurement of the system time constant, RC.
`Another object of the present invention is to use a
`pulse duration that is the sum of a time interval corre—
`sponding to an intermediate tilt of 44% and a fixed time
`interval of 1.6 ms.
`-
`A further object of the present invention is to employ
`a defibrillation pulse of optimum duration in at least one
`phase of a biphasic waveform.
`
`50
`
`65
`
`10
`
`10
`
`

`

`5,431,686
`
`9
`FIG. 2 illustrates a biphasic voltage-time waveform
`30 for defibrillation, incorporating a first phase 32 and a
`second phase 34 of opposite polarity.
`FIG. 3A illustrates a monophasic voltage-time wave-
`form 40 of a particular initial voltage 42 specified in
`terms of a specific pulse duration 44 of 4.5 ms.
`FIG. 3B illustrates a monophasic voltage-time wave—
`form 50 of a particular initial voltage 52, with the wave-
`form specified in terms of a final voltage 54, with the
`ratio of the voltage decline at truncation to the initial
`voltage 52 commonly being described as a tilt percent-
`age 56.
`FIG. 3C illustrates a monophasic voltage-time wave-
`form 57 of a particular initial voltage 59 with the wave-
`form specified in terms of a time interval corresponding
`to an intermediate tilt 61 of 44%, plus a fixed time inter—
`val 65 of 1.6 ms.
`FIG. 4 illustrates a chart 60 that incorporates a curve
`62 of average current strength required for defibrilla-
`tion versus the duration of the pulse of that average
`current.
`FIG. 5 illustrates a chart 70 of chronaxie values 72,
`these data taken out of the literature and drawn from
`the list 74 of cited references.
`FIG. 6 illustrates a chart 80 of physiologically effec-
`tive current values achieved by two defibrillation wave-
`form-specifying methods 82 and 84 of the prior art, and
`from the waveform-specifying method 86 of the present
`invention, all in the face of variations in load-resistance
`values 8, illustrating the superiority of the method 86
`of the present invention.
`MODE OF OPERATION
`
`The present invention makes use of the mathematical
`definition of optimum pulse duration by storing in the
`control circuitry of the defibrillation system the actual
`value of the particular capacitor incorporated in the
`system.
`The optimized-pulse prescription of this invention
`can be applied to a monophasic waveform, or to either
`or both of the phases of a biphasic waveform. In the
`latter case, when it is applied to a single phase, the other
`can be specified to have equal duration. Another option
`specifies the opposite phase in a way that produces a
`particular overall tilt, or degree of discharging of the
`capacitor.
`Referring now to FIG. 7, the decisional steps of a
`preferred embodiment of the present invention will now
`be described. The method of operating a defibrillator
`begins by sensing a myocardial dysrhythmia in a human
`patient (steps 90, 92). After the myocardial dysrhythmia
`is sensed, capacitor energy begins to be delivered to the
`heart (step 94). Capacitor energy continues to be dis-
`charged until the capacitor discharge voltage decays a
`given percentage from the preselected amount of elec-
`trical energy stored in the capacitor (step 96). For ex-
`ample, with a decay of 44%,