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`U5005275157A
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`5,275,157
`[11] Patent Number:
`[19]
`Unlted States Patent
`
`Morgan et a1. Jan. 4, 1994 [45] Date of Patent:
`
`
`[54] PULSE FORMING CIRCUITS
`
`perature effects, and triggering behavior, Journal of Ap—
`plied Physics 61(6), Mar. 15 1987, pp. 2381—2386.
`R. Ford et al., Application of Non—linear Resistors to
`Inductive Switching, IEEE Transactions on Electrical
`Insulation, vol. E1-20 No. 1, Feb. 1985, pp. 29—37.
`
`Primary Examiner—Lee S. Cohen
`Assistant Examiner—Samuel G. Gilbert
`Attorney. Agent, or Firm—Christensen, O‘Connor,
`Johnson & Kindness
`
`[57]
`
`ABSTRACT
`
`Circuits for controlling the current flow of an energy
`pulse as a function of the temperature of a resistive
`element in the circuit so that the current flow varies
`over time in accordance with a predetermined wave-
`form. The circuits include at least one negative temper-
`ature coefficient thermistor connected between an en-
`ergy storage device and connectors for delivering en-
`ergy stored in the storage source to an external load. In
`one embodiment of the invention the circuit includes a
`second thermistor for shunting a residual portion of the
`current delivered by an energy pulse away from the
`external load. In another embodiment of the circuit, a
`small inductive device is used for adjusting the shape of
`the predetermined waveform. In yet another embodi-
`ment of the device, a plurality of thermistors arranged
`in a bridge-like configuration are used to control the
`current of the energy pulse so that its waveform is bi-
`phasic. Heat sinks may be attached to the thermistors
`for cooling the latter so as to increase the rate at which
`energy pulses may be delivered by the circuits.
`
`[56]
`
`[75]
`
`[73] Assignee:
`
`Inventors: Carlton B. Morgan, Bainbridge
`Island; Daniel Yerkovich; Donald C.
`Meier, both of Seattle, all of Wash.
`Physio-Control Corporation,
`Redmond, Wash.
`[2]] Appl. No.: 685,132
`[22] Filed:
`Apr. 12, 1991
`A61N 1/00
`Int. Cl.5 ......................
`[51]
`
`[52] US. Cl. ...................... 607/6
`
`
`.. 128/419; 307/540, 545,
`[58] Field of Search
`307/547, 549, 631; 338/22; 307/310, 264
`References Cited
`U,S. PATENT DOCUMENTS
`
`3,078,850 2/1963 Schein et al.
`128/419 D
`. 307/310 X
`3,219,942 11/1965 Bell
`..............
`
`128/419 D
`3,241,555
`3/1966 Caywood et a1.
`128/419 D
`3,359,984 12/1967 Daniher et a1.
`.
`
`128/419 D
`3,706,313 12/1972 Milani et a1.
`
`128/419 D
`3,866,615
`2/1975 Hewson
`
`
`128/419 D
`3,886,950 6/1975 Ukkestad e
`
`4,155,017
`5/1979 Gaule et 31.
`307/268
`.
`
`338/22 R X
`4,387,291
`6/1983 Keppel
`
`..... 307/106
`4,463,268
`7/1984 Levinson .
`3/1985 Suzuki et al.
`4,504,773
`128/419 D
`
`4,566,457
`1/1986 Stemple
`128/419 D
`4,574,810 3/1986 Lennan
`128/419D
`
`FOREIGN PATENT DOCUMENTS
`1586973
`3/1931 United Kingdom ........... 128/419 D
`OTHER PUBLICATIONS
`
`R. Ford et a1.,Positive temperature coefficient resistor: as
`highvpawer pulse switches: performance limitations, tem-
`
`23 Claims, 6 Drawing Sheets
`
`
`
`L|FECOR212-1008
`
`1
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`LIFECOR212-1008
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`5,275,157
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`1
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`PULSE FORMING CIRCUITS
`
`FIELD OF THE INVENTION
`
`2
`used in the various embodiments of the present inven-
`tion;
`FIG. 8 is a front elevation view of the heat sink and
`thermistor illustrated in FIG. 7;
`FIG. 9 is a schematic illustration of a fifth embodi-
`ment of the invention;
`FIG. 10 is a drawing illustrating typical variation in
`the current against time of the pulse provided by the
`circuit shown in FIG. 10; and
`FIG. 11 is a schematic illustration of a defibrillator
`designed to incorporate a selected one of the various
`embodiments of the circuit of the present invention.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`FIG. 1 illustrates a first embodiment of the circuit of
`the present invention in schematic form. This circuit
`includes a DC source 2 having its positive terminal
`connected through a first normally opened switch 4 to
`one side of a capacitor 6, the other side of which is
`connected to the negative terminal of the source 2.
`Connected in parallel with the capacitor 6 is a second
`normally opened switch 8, a thermistor 10, and a load
`represented by a resistive element 12. Thermistor 10 is a
`self-heating thermally active resistive device. Thermis-
`tor 10 is selected to have a negative temperature coeffi-
`cient so that its resistance will decrease as its tempera—
`ture rises. The load represented by element 12 typically
`does not form part of the circuit of the present inven-
`tion, and is shown in the FIGURES merely to illustrate
`the manner in which the present circuit is coupled with
`a resistive load.
`In an exemplary embodiment of the invention, three
`series-connected thermistors of the type manufactured
`by Keystone Carbon Company of St. Mary's, Pa. 15857
`and identified by model No. CL-70 may be satisfactorily
`used as thermistor 10. Also in this embodiment, capaci-
`tor 6 is a 37.5 mF capacitor and about a 4,200 volt
`charge is stored in the capacitor. In connection with the
`selection of these circuit elements, patient impedance is
`assumed to be about 50 ohms.
`Referring to FIGS. 1 and 2, the circuit illustrated in
`FIG. 1 may be operated as follows: the switch 4 is
`closed causing the capacitor 6 to be charged to a value
`determined by the voltage of the source and the dura-
`tion of the switch closure. The switch 4 is then opened
`and switch 8 closed. When the switch 8 is closed the
`charge on the capacitor 6 is applied to the series circuit
`of thermistor 10 and load 12. Initially, the resistance of
`the thermistor 10 is relatively high so that the current to
`the load 12 is relatively low. However, the initial cur-
`rent flow causes the temperature of the thermistor to
`rapidly increase,
`thereby lowering its resistance and
`causing the current flow through the thermistor to in-
`crease. The temperature of the thermistor continues to
`rise with increases over time in current flow. The rate of
`change of resistance of the thermistor, and hence the
`change in magnitude of the current delivered by the
`pulse, illustrated by waveform K in FIG. 2, is deter—
`mined by the temperature/resistance characteristics of
`the selected thermistor and by the heat capacity of the
`thermistor selected. The temperature of the thermistor
`will continue to rise until a maximum value of current
`flow is reached, which value is determined by the
`amount of the charge on the capacitor 6. As the capaci-
`tor continues to discharge through the thermistor and
`load 12, the current through these elements is reduced.
`
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`This invention relates generally to circuits for form-
`ing electrical pulses. More specifically,
`it relates to
`circuits for forming such pulses with a determined am-
`plitude, duration, and shape.
`BACKGROUND OF THE INVENTION
`
`Electrical pulses have traditionally been shaped using
`resistors, capacitors, and inductors, so-called RLC cir-
`cuits, to form pulses having desired rise and fall pat»
`terns, amplitudes, and duration. One disadvantage of
`such prior art circuits, particularly when pulses of rela-
`tively large voltages or currents are required, has been
`the size and weight of the components required. As a
`consequence of the size and weight of such prior art
`RLC circuits, the latter are not adapted for use in rela-
`tively small pieces of equipment or where the total
`weight of the piece of equipment must be minimized.
`For instance, to the extent the weight and size of the
`pulse-forming circuits in a defibrillator can be reduced,
`its portability and hence utility will be increased.
`Therefore, a strong need exists for novel wave-shap-
`ing circuits utilizing solid state devices of relatively
`small size and weight which are reliable and can pro-
`duce pulses having desired current wave form on a
`repeated basis.
`
`SUMMARY OF THE INVENTION
`
`in its various embodiments, the invention
`Briefly,
`utilizes a temperature sensitive resistor, such as a therm-
`istor, connected between or across a switched source
`and a load to shape the electrical pulse supplied to the
`load. The temperature/resistance characteristics of the
`resistor are selected so that the shape of the pulse varies
`in a predetermined manner as a function of the tempera-
`ture of the resistor. In one particular implementation of
`the invention, the load can be the transthoracic impe-
`dance of a person requiring defibrillation.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The invention itself is set forth in the claims appended
`hereto and forming a part of this specification. How-
`ever, an understanding of the structure and operation of
`various embodiments of the invention may be had by
`reference to the detailed description taken in conjunc-
`tion with the drawings in which:
`FIG. ] is a schematic drawing of one embodiment of
`the invention;
`FIG. 2 is a drawing illustrating typical variation in
`the current against time of the pulse produced by the
`circuit shown in FIG. 1;
`FIG. 3 is a schematic illustration of a second embodi-
`ment of the invention;
`FIG. 3a is a schematic illustration of a third embodi-
`ment of the invention;
`FIG. 4 is a drawing illustrating typical variation in
`the current against time of the pulse produced by the
`circuit shown in FIG. 3;
`FIG. 5 is a schematic illustration of a fourth embodi-
`ment of the invention;
`FIG. 6 is a drawing illustrating typical variation in
`the current against time of the pulse provided by the
`circuit shown in FIG. 5;
`FIG. 7 is a side elevation view of a heat sink attached
`to front and back surfaces of a thermistor of the type
`
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`This effect of thermistor 10 on current flow delivered
`by the pulse is illustrated graphically in FIG. 2, where
`the current through the load is plotted against time.
`Immediately upon the closure of switch 8, the current
`through the load 12 jumps to a finite value that depends
`on the initial resistance of thermistor 10 and load 12. As
`the switch 8 remains closed the current continues to
`increase until a maximum value is reached. This in-
`crease is due to the decreasing resistance of thermistor
`10. At the same time the charge on the capacitor 6 is
`decreasing and at a certain point the current through
`the load reaches a maximum value and then decreases.
`The resultant current waveform identified at K in FIG.
`2, may be described approximately as a damped sinusoi-
`dal wave.
`It is believed to be desirable in the case of defibrilla-
`tors, and possibly in other applications, to reduce the
`load current (the patient current in the case of defibrilla-
`tors) to zero very shortly after the peak value of the
`current is achieved. To achieve such rapid decrease in
`the load current, the embodiment of the present inven-
`tion illustrated in FIG. 3 was developed. The circuit
`illustrated in FIG. 3 is identical to the circuit illustrated
`in FIG. 1 and described above, except that a second
`thermistor 14 is connected in parallel with the load. The
`thermistor 14 has a negative coefficient of resistance so
`that the current through it will increase as the tempera—
`ture of the thermistor increases. In an exemplary em-
`bodiment of the circuit illustrated in FIG. 3, three series
`30
`connected thermistors of the type manufactured by
`Keystone Carbon Company of St. Mary's, Pa. and iden- ~
`tified by model No. CL-7O may be satisfactorily used as
`thermistor 14.
`The effect of the circuit illustrated in FIG. 3 on cur-
`rent ilow delivered by the pulse is illustrated in FIG. 4,
`where once again current through the load is plotted
`against time. The interaction of thermistor 10 and the
`load occurs in substantially the same manner as with the
`circuit
`illustrated in FIG. 1.
`Initially,
`the value of
`thermistor 14 is much larger than the load resistance, so
`that most of the current flows through the load during
`initial portions of the pulse, as illustrated in current
`' waveform L. However, as the pulse continues, thermis-
`tor 14 heats, its resistance decreases, and the thermistor
`begins to shunt current away from the load. By the end
`of the pulse, the resistance of thermistor 14 is much less
`than the resistance of the load, and as a result the therm-
`istor shunts away from the load almost all of the remain-
`ing energy in the capacitor.
`In some cases, it may be desirable to employ thermis-
`tor 14 in a conventional RLC circuit to cause the magni-
`tude of the current delivered by the circuit to decrease
`more rapidly with respect to time than would otherwise
`occur. That is, thermistor 14 may be employed in a
`conventional RLC circuit to perform the shunting func-
`tion described above in connection with the circuit
`illustrated in FIG. 3. FIG. 30 illustrates a conventional
`RLC circuit in which thermistor 14 is connected in
`parallel with the load so as to shunt residual portions of
`the energy discharged by capacitor 6 away from the
`load, as described above with respect to the circuit
`illustrated in FIG. 3. The circuit illustrated in FIG. 3a
`differs from the circuit illustrated in FIG. 3 only in that
`inductor 15 is used in place of thermistor 10. The cur-
`rent flow of the pulse delivered by the circuit illustrated
`in FIG. 3a will rise and fall substantially in accordance
`with waveform L illustrated in FIG. 4, except that the
`current will rise from a lower initial magnitude.
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`Referring to FIGS. 5 and 6, under certain circum-'
`stances it is desired to mimic the damped sinusoidal
`waveform created by conventional RLC pulse genera-
`tion circuits of the type employed in known defibrilla-
`tors more closely than is possible with the circuits illus-
`trated in FIGS. 1 and 3. To this end, the circuit 30
`illustrated in FIG. 5 was developed. Circuit 30 is identi-
`cal to the circuit illustrated in FIG. 1, except that it
`includes an inductive element 32 connected in series
`between switch 8 and thermistor 10. In an exemplary
`embodiment of the present invention, inductive element
`32 is a 10 mI-I inductor.
`As illustrated by current waveform M in FIG. 6 the
`amplitude of the current delivered by the pulse gener-
`ated by circuit 30 increases more slowly than does the
`amplitude of the current delivered by the pulses gener-
`ated by the circuits illustrated in FIGS. 1 and 3. Under
`certain conditions, this slower increase in the amplitude
`of the current delivered by a pulse is desired. For in-
`stance, when circuit 30 is incorporated in a defibrillator
`and the pulse the circuit generates is to be delivered to
`a patient for the purpose of defibrillating the patient’s
`heart, for certain medical conditions it may be desirable
`that the current delivered by the pulse not increase too
`rapidly toward a peak value.
`Alternatively, inductive element 32 may be employed
`in the circuit illustrated in FIG. 3. In this case, the in-
`ductive element 32 is also connected in series between
`switch 8 and thermistor 10.
`Circuit 30 illustrated in FIG. 5 is slightly heavier and
`more expensive than the circuits illustrated in FIGS. 1
`and 3 due to the inclusion of inductive element 32.
`However, the circuit does not weigh as much or cost as
`much as conventional RLC pulse forming circuits
`which typically include at least one inductive element
`rated at between 20 mH and 50 mI-I. In certain circum-
`stances, an ideal balance between weight and cost sav-
`ings versus circuit performance may be achieved by
`using circuit 30 illustrated in FIG. 5 in place of the
`circuits illustrated in FIGS. 1 and 3 or in place of con-
`ventional RLC circuits.
`Referring to FIGS. 7 and 8, it is frequently desired to
`apply defibrillation or other pulses repeatedly in short
`succession to a patient or a load. Under certain condi-
`tions, thermistors 10 and 14 in the circuits illustrated in
`FIGS. 1, 3, and 5 and described above will retain suffi-
`cient residual heat between pulses as to prevent the
`circuits from generating pulses having the current
`waveform illustrated in FIGS. 2, 4, and 6 and described
`above. The extent to which the thermistors retain resid-
`ual heat will vary as a function of the frequency of the
`pulses, the temperature/resistance characteristics of the
`thermistors, and the environment in which the thermis-
`tors are located.
`To minimize this retention of heat, and thereby in-
`crease the frequency at which pulses may be applied,
`heat sinks 50 may be attached to thermistors 10 and 14.
`Each heat sink 50 includes a plurality of planar fin mem—
`bers 52 which are attached via integral base plate 53 to
`the front and back surfaces of the thermistor. Such
`attachment is achieved using a heat conductive bonding
`agent 54, such as a suitable epoxy. Heat sinks 50 are
`made from a material having a high thermal conductiv-
`ity, such as aluminum. By adding heat sinks 50 to therm—
`istors 10 and 14, the latter will cool rapidly, typically in
`less than about five seconds, thereby allowing rapid
`application of pulses.
`
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`Referring to FIGS. 9 and 10, recently experimental
`and clinical tests have been performed using defibrilla-
`tors designed to produce a defibrillation pulse having a
`biphasic or alternating polarity waveform. This'wave-
`form is characterized by the first portion of the pulse
`having a positive polarity and the second portion of the
`pulse having a negative polarity, much like the wave-
`form N illustrated in FIG. 10. A circuit for generating a
`defibrulation pulse having a biphasic waveform, which
`also possesses the size and weight saving features dis
`cussed above in connection with the circuits illustrated
`in FIGS. 1, 3, 30, and 5, is illustrated in FIG. 9 and
`identified at 60.
`Circuit 60 is similar to the circuit illustrated in FIG. 1
`in that it comprises voltage source 2, switches 4 and 8,
`and capacitor 6. Additionally, the resistive load with
`which circuit 60 is designed to be connected is repre-
`sented by resistive element 12. One side of resistive
`element 12 is identified by reference numeral 66 and the
`other side of the resistive element is identified by refer~
`ence numeral 68. Circuit 60 differs from the circuit
`illustrated in FIG. 1 in that it includes thermistors 72,
`74, 76 and 78 in place of thermistor 10. Thermistors 72
`and 78 are negative temperature coefficient thermistors
`and thermistors 74 and 76 are positive temperature coef-
`ficient thermistors. Thermistors 72—78 are coupled with
`resistive element 12 in a bridge-like configuration, with
`thermistor 72 being connected between switch 8 and
`side 66 of resistive element 12, thermistor 74 being con-
`nected between switch 8 and side 68 of resistive element
`12, thermistor 76 being connected between side 66 of
`resistive element 12 and the negative polarity side of
`voltage source 2, and thermistor 78 being connected
`between side 68 of resistive element 12 and the negative
`polarity side of voltage source 2. Thermistors 72—78 are
`selected so that the resistance of thermistor 74 is less
`than the resistance of thermistor 72 when the thermis-
`tors are at ambient temperature, e. g., 25° C., and so that
`the resistance of thermistor 76 is less than the resistance
`of thermistor 78 when these thermistors are at ambient
`temperature. In addition, the thermistors are selected so
`that thermistors 74 and 76 have nearly identical resis-
`tance across substantially the entire temperature spec-
`trum at which they will operate, and so that thermistors
`72 and 78 have nearly identical resistance across sub-
`stantially the entire temperature spectrum at which they
`will operate.
`As a consequence of the bridge-like configuration
`and the above-discussed thermal/resistive characteris-
`tics of thermistors 72—78, when the latter are at ambient
`temperature, and when switch 8 is closed, current will
`flow from capacitor 6 along current path in (FIG. 9)
`which flows through thermistor 74, resistive element 12
`(Le, the resistive load), and then thermistor 76. This
`current flow through thermistors 74 and 76 causes the
`thermistors to heat rapidly with the result that their
`resistance increases rapidly. As a consequence of this
`increase in resistance, the amplitude of the current de-
`livered through thermistors 74 and 76 to the resistive
`load decreases with time, as illustrated in FIG. 10. This
`increase in resistance and decrease in current amplitude
`continues until the resistance of thermistor 74 exceeds
`that of thermistor 72 and the resistance of thermistor 76
`exceeds that of thermistor 78. When this occurs, the
`current delivered by capacitor 6 will follow path in
`(FIG. 9) which extends through thermistor 72, in the
`opposite direction through resistive load 12, and
`through thermistor 78. Thus, at the instant the resis-
`
`6
`tance of the positive temperature coefficient thermistors
`74 and 76 exceeds the resistance of the negative temper-
`ature coefficient thermistors 72 and 78, the direction of
`current flow through resistive element 12 changes, i.e.,
`the current flow through the load becomes negative. As
`the residual portion of the charge on capacitor 6 is
`discharged,
`the current amplitude of the waveform
`increases exponentially approaching the zero amplitude
`level, i.e., the current amplitude becomes less negative
`over time.
`Thus, by proper selection of the thermal/resistive
`coefficients of thermistors 72-78 used in circuit 70, the
`biphasic waveform N illustrated in FIG. 10 may be
`generated. The specific configuration of waveform N
`may thus be precisely tailored to the application in
`which circuit 60 is to be used by appropriate selection of
`thermistors 72-78.
`In one embodiment of circuit 70, capacitor 6 was a 50
`microfarad capacitor which was changed to 4,320 volts,
`the resistive load was 50 ohms, thermistors 74 and 76
`had a resistance of 500 ohms at 25' C. and a heat capac-
`ity of 0.5 joules/' C., and thermistors 72 and 78 had a
`resistance of 2 ohms at 25° C., a heat capacity of 0.6
`joulesl‘ C. and a transition temperature of 65° C. The
`load had a resistance of about 50 ohms. The waveform
`created by this embodiment of circuit 70 is illustrated in
`FIG. 10, with the current at the beginning of the pulse
`(i.e., at t=0) being about 78 amps, the polarity of the
`pulse changing at about 0.75 milliseconds (i.e.,
`time
`x=0.75 ms), and the maximum amplitude of the nega-
`tive polarity of the pulse being equal to about —32
`amps.
`Heat sinks 50, may also be attached to thermistors
`72—78 in the manner described above with respect to
`thermistors 10 and 14. Such use of heat sinks 50 is ap-
`propriate when it is desired to increase the rate at which
`the thermistors 72—78 cool down after heating.
`The circuits illustrated in FIGS. 1, 3, 34, 5, and 9 may
`be advantageously incorporated in a known manner in a
`conventional defibrillator 100 of the type disclosed in
`US. Pat. No. 3,814,105 to Howard et al. and illustrated
`schematically in FIG.
`1 1. With defibrillator 100,
`switches 4 and 8 of the circuits illustrated in FIGS. 1, 3,
`3a, 5, and 9 are coupled, respectively, with charge but-
`ton 102 and discharge button 104. Also, with defibrilla-
`tor 100, the resistance 12 of the circuits illustrated in
`FIGS. 1, 3, 3a, 5, and 9 is provided by a patient (not
`showu) connected to electrodes 106 and 108. It is to be
`understood that defibrillator 100 includes all the other
`controls and displays of a modem defibrillator.
`Since certain changes may be made in the devices
`described above without departing from the scope of
`the invention herein involved, it is intended that all
`matter contained in the above description or shown in
`the accompanying drawings shall be interpreted in an
`illustrative and not in a limiting sense.
`The embodiments of the invention in which an exclu-
`sive property or privilege is claimed are defined as
`follows:
`
`1. In a medical instrument for delivering an electrical
`energy pulse to a patient, wherein the medical instru-
`ment includes a source of energy, an energy storage
`means, a pair of electrodes that are attachable to the
`patient, and connecting means for connecting energy
`stored in the storage means to the pair of electrodes, the
`improvement comprising a current control means con-
`nected between the energy storage means and the pair
`of electrodes for varying over time the current wave.
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`form that can flow from the energy storage means to
`the pair of electrodes to produce a damped sinusoidal
`current waveform that varies as a function of the tem-
`perature of said current control means.
`1. A medical
`instrument according to claim 1,
`wherein said current control means comprises a self-
`heating resistive device, the resistance of which de-
`creases in proportion to increases in temperature of the
`device.
`3. A medical instrument according to claim 2, further
`comprising:
`(a) first switching means, connected between the
`source of energy and the energy storage means, for
`selectively coupling the source of energy with, and
`decoupling the source of energy from, the energy
`storage means; and
`(b) second switching means, connected between the
`energy storage mans and the connecting means, for
`selectively coupling the energy storage means
`with, and decoupling the energy storage means
`form, the pair of electrodes.
`4. A medical
`instrument according to claim 2,
`wherein said current control means further comprises
`cooling means coupled to said resistive device for dissi-
`pating heat from said resistive device.
`5. A medical
`instrument according to claim 4,
`wherein said cooling means comprises a plurality of fins
`made from aluminum.
`6. A medical
`instrument according to claim 1,
`wherein said current control means comprises a therm-
`istor having a negative temperature coefficient.
`7. A medical instrument according to claim 1, further
`comprising shunt means, connected to said connecting
`means, for shunting a residual portion of the energy
`delivered from said energy storage means away from
`the patient when the energy storage means is connected
`to the pair of electrodes, wherein said residual portion
`has a magnitude that is a function of a temperature of
`said shunt means,
`instrument according to claim 7,
`8. A medical
`wherein said shunt means comprises a resistive device,
`the resistance of which varies as a function of the tem-
`perature of the resistive device.
`9. A medical
`instrument according to claim 7,
`wherein said shunt means comprises a negative temper—
`ature coefficient thermistor.
`10. A medical instrument for generating and provid-
`ing a pulse of electrical energy to a pair of electrodes
`that are attachable to a patient, the circuit comprising:
`energy storage means, coupleable to an energy
`source, for storing and discharging a predeter-
`mined quantity of energy;
`connecting means, connected to said energy storage
`means and connectable to the pair of electrodes,
`such that energy stored in said energy storage
`means is delivered from said energy storage means
`to the pair of electrodes; and
`current control means, connected between said en-
`ergy storage means and said pair of electrodes, for
`producing over time as a function of the tempera-
`ture of said current control means a damped sinu-
`soidal current which flows from said energy stor-
`age means to the pair of electrodes upon connec-
`tion of the energy storage means to the pair of
`electrodes.
`11. A medical instrument according to claim 10, fur-
`ther comprising shunt means, connected to said con-
`necting means, for shunting a residual portion of the
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`65
`
`5,275,157
`
`8
`energy delivered from said energy storage means away
`form the pair of electrodes when said connecting means
`is connected to the pair of electrodes, wherein said
`residual portion has a magnitude that is a function of a
`temperature of the shunt means.
`12. A medical instrument according to claim 10, fur-
`ther comprising inductive means connected between
`said energy storage means and said pair of electrodes,
`for storing and releasing a predetermined quantity of
`energy delivered upon the connection of said energy
`storage means to the pair of electrodes so as to increase
`the amount of time required for the energy delivered to
`the pair of electrodes to reach a peak value.
`13. A medical instrument according to claim 10, fur-
`ther comprising cooling means coupled with said cur-
`rent control means fcr dissipating heat from said current
`control means.
`14. A wave-shaping device for use in a medical instru-
`ment that produces an energy pulse and delivers the
`energy pulse to a patient, the medical instrument includ-
`ing an energy source, energy storage means coupleable
`to the energy source for storing and discharging a pre-
`determined quantity of energy and a pair of electrodes
`attachable to the patient, the wave-shaping device com-
`prising:
`self-heating current control means, connectable to
`the energy storage mans, for producing a damped
`sinusoidal current waveform delivered from the
`energy storage means as a function of the tempera-
`ture of said self-heating current control means; and
`connecting means for connecting said self-heating
`current control means to the energy storage means
`and to the pair of electrodes.
`15. A defibillator for producing and delivering a
`damped sinusoidal defibrillation pulse to a patient com-
`prising:
`energy storage means for storing electrical energy;
`electrode means, coupled with said energy storage
`means, for delivering said defibrillation pulse to the
`patient; and
`-
`circuit means, coupled with said energy storage
`means and including a resistive device, for control-
`ling the flow of current delivered from the energy
`storage means as a function of the temperature of
`said resistive device so that the flow of current
`from the energy storage means will vary over time
`in accordance with a predetermined dampened
`sinusoidal waveform.
`'
`16. A defibrillator according to claim 15, wherein the
`resistance of said resistive device decreases as the tem-
`perature of the resistive device increases.
`17. A defibrillator according to claim 15, further
`comprising shunt means for shunting a residual portion
`of the current delivered from the energy storage means
`away from said electrode means wherein said residual
`portion has a magnitude that is function of a tempera-
`ture of the shunt means.
`18. A medical instrument including a pulse generation
`circuit for producing and delivering an energy pulse to
`a patient, comprising:
`energy storage means, coupleable to an energy
`source, for storing and discharging a predeter-
`mined quantity of energy;
`connecting means, coupled to said energy storage
`means for connecting the energy storage means to
`a pair of electrodes that are attachable to the pa-
`tient; and
`
`11
`
`11
`
`

`

`9
`shunt means, connected to said connecting means, for
`shunting a residual portion of the energy dis-
`charged by said energy storage means away from
`the pair of electrodes when the pair of electrodes is
`connected to said energy storage means, wherein
`the residual portion has a magnitude that varies as
`a function of a temperature of the shunt means.
`19. A medical
`instrument according to claim 18,
`wherein said shunt means comprises a resistive device,
`the resistance of which varies as a function of a temper-
`ature of the resistive device.
`20. A medical instrument for providing a pulse of
`energy having a dampened sinusoidal, biphasic current
`waveform to a pair of patient electrodes that are attach- is
`able to a patient, the circuit comprising:
`energy storage mans, coupleable to an energy source,
`for storing and discharging a pulse of energy;
`connecting means, connected to said energy storage
`means and connectable to the pair of patient elec- 20
`trodes, for carrying a pulse of energy discharged
`by said energy storage means to the pair of patient
`electrodes; and
`current control mans, connected between said energy
`storage means and said pair of patient electrodes,
`for shaping a pulse of energy discharged by said
`energy storage means as a function of a tempera-
`ture of said current control means so that the cur-
`rent delivered by the pulse of energy initially has a 30
`damped sinusoidal shape of one polarity and then
`after a predetermined period of time has a damped
`sinusoidal shape of an opposite polarity.
`
`10
`instrument according to claim 20,
`21. A medical
`wherein said current control means comprises first and
`second thermistors connectable to the pair of patient
`electrodes so as to define a first current path, and third
`and fourth thermistors connectable to the pair of patient
`electrodes so as to define a second current path, said
`first and second thermistors having a negat