United States Patent
`Fain et al. .
`
`[191
`
`[11] Patent Number:
`
`5,230,336
`
`[45] Date of Patent:
`
`Jul. 27, 1993
`
`llllll||||||I|llllllllllllllllllllllllllllllllllllllllllllllllllll|||l|||||
`USOOSZ30336A
`
`[54] METHOD AND APPARATUS FOR
`IMPEDANCE BASED AUTOMATIC PULSE
`DURATION ADJUSTMENT FOR
`DEFIBRILLATION SHOCK DELIVERY
`
`[75]
`
`Inventors:
`
`Eric Fain; Benjamin Pless, both of
`Menlo Park, Ca1if.; Michael Haulage,
`Kingwood, Tex.
`[73] Assignee: Ventritex, Inc., Sunnyvale, Calif.
`[21] Appl. No.: 746,430
`[22] Filed:
`Aug. 16, 1991
`[51]
`Int. Cl.5 ............................................... A61N 1/39
`[52]
`
`[58]
`[56]
`References Cited
`U.S. PATENT DOCUMENTS
`3.706,313 12/1972 Milani et al.
`................... 128/419 D
`4,574,810 3/1986 Lerrnan .......................... 128/419 D
`FOREIGN PATENT DOCUMENTS
`
`5/1989 European Pat. Off.
`0315368
`7/1991 European Pat. Off.
`0437104
`2070282 9/1981 United Kingdom .
`
`.
`.
`
`OTHER PUBLICATIONS
`
`“Transthoracic Ventricular Defibrillation in the 100 kg
`Calf with Symmetrical One—Cycle Bidirectional Rect-
`angular Wave Stimuli" (Schuder, et a1) IEEE Transac-
`tions on Biomedical Engineering, vol. BME—30, No. 7,
`Jul. 1983.
`
`Primary Examiner—William E. Kamm
`Attorney, Agent, or Firm—Steven M. Mitchell; Mark J.
`Meltzer
`
`ABSTRACT
`[57]
`An antitachycardia device, either implanted or external,
`which delivers a fixed pulse width truncated exponen-
`tial waveform defibrillation shock and automatically
`adjusts the pulse duration based upon the impedance
`measured or calculated following a delivered shock.
`The apparatus operates by measuring or calculating the
`high voltage system impedance, selecting a pulse width
`for that impedance value and using a pulse width de-
`rived from the selected pulse width for the next defibril—
`lation shock.
`
`22 Claims, 6 Drawing Sheets
`
`ARRHYTHMIA
`DTXN
`
`
`
`
`102
`
`103
`
`108
`
`106
`
`L|FECOR454-1007
`
`DEFIB THERAPY DELIVERY
`@ HV PULSE WIDTH
`
`IMPEDANCE MEASUREMENT
`
`
`
`107
`
`SHOCK
`VOLTAGE
`
`
`
`
`> 22.50 V
`
`SUGG PULSE WIDTH
`
`(BASED ON IMPEDANCE)
`
`
`
`
`105
`
`HV PULSE WIDTH =
`
`
`SUGG PULSE_W1DTH
`
`
`
`
`1
`
`LIFECOR454-1007
`
`

`

`US. Patent
`
`July 27, 1993
`
`Sheet 1 of 6
`
`5,230,336
`
`r..__._____.___._____
`
`knR
`
`i
`405200
`
`25...
`
`mN
`
`.8
`
`E
`
`
`
`mm5<z<Econ—NE;
`
`oz<wzfimeoxm
`
`8E?
`
`..
`
`>mmt.<m
`
`2
`
`
`
`
`

`

`US. Patent
`
`July 27, 1993
`
`Sheet 2 of 6
`
`5,230,336
`
`100
`
`ARRHYTHMIA
`
`DTXN
`
`DEFIB THERAPY DELIVERY
`@ HV__PULSE_WIDTH
`
`IMPEDANCE MEASUREMENT
`
`SUGG__PULSE_WIDTH
`
`(BASED ON IMPEDANCE)
`
`102
`
`103
`
`104
`
`HV__PULSE_W|DTH = SUGG_PULSE_WIDTH
`
`1 05
`
`@ 106
`
`Figure 2
`
`3
`
`

`

`U.S. Patent
`
`July 27, 1993
`
`Sheet 3 of 6
`
`5,230,336
`
`Measured
`Impedance
`(ohms)
`
`Monophasic Waveform
`Suggested Pulse Width
`(msec)
`
`Biphasic Waveform
`Suggested Pulse Width
`(msec)
`
`12.0/12.0
`
`3.0/3.0
`
`4.0/4.0
`
`5.0/5.0
`
`6.0/6.0
`
`7.0/7.0
`
`8.0/8.0
`
`9.0/9.0
`
`10.0/10.0
`
`11.0/1 1 .0
`
`‘
`
`~ Figure 3
`
`4
`
`

`

`US. Patent
`
`July 27, 1993
`
`Sheet 4 of 6
`
`5,230,336
`
`100
`
`ARRHYTHMIA
`
`DTXN
`
`DEFIB THERAPY DELIVERY
`@ HV_PULSE_W|DTH
`
`IMPEDANCE MEASUREMENT
`
`mi
`
`
`
`SHOCK
`VOLTAGE
`
`
`> 2:0 v
`
`
`102
`
`103
`
`108
`
`N
`
`_
`HV_PULSE_WIDTH _
`HV_PULSE_WIDTH
`
`106
`
`Y
`
`
`
`SUGG_PULSE_WIDTH
`(BASED ON IMPEDANCE)
`
`104
`
`105
`
`HV_PULSE_WIDTH =
`SUGG_PULSE_W|DTH
`
`106
`
`Figure 4
`
`5
`
`

`

`US. Patent
`
`July 27, 1993
`
`Sheet 5 of 6
`
`5,230,336
`
`100
`
`ARRHYTHMIA
`
`DTXN
`
`DEFIB THERAPY DELIVERY
`@ HV__PULSE_WIDTH
`
`IMPEDAN CE MEASUREMENT
`
`102
`
`1 03
`
`108
`
`106
`
`N
`
`HV_PULSE_W|DTH =
`HV_PULSE_WIDTH
`
`107
`
`
` SHOCK
`VOLTAGE
`
`
`> 250 V
`7
`
`
`Y
`
`SUGG_PULSE_W|DTH
`(BASED ON IMPEDANCE)
`
`104
`
`
`
`(HV_PULSE_WIDTH + SUGG_PULSE_WIDTH)/2
`
`HV_PULSE_WIDTH =
`
`109
`
`@ 106
`
`Figure 5
`
`6
`
`

`

`US. Patent
`
`July 27, 1993
`
`Sheet 6 of 6
`
`4 5,230,336
`
`100
`
`ARRHYTHMIA
`
`DTXN
`
`DEFIB THERAPY DELIVERY
`@ HV__PULSE__W|DTH
`
`IMPEDANCE MEASUREMENT
`
`102
`
`103
`
`108
`
`HV_PULSE_WIDTH =
`HV_PULSE_WIDTH
`
`‘06
`
`107
`
`N
`
`SHOCK
`
`VOLTAGE
`
`
`> 250 V
`
`
`7
`
`
`Y
`
`SUGG_PULSE_WIDTH
`(BASED ON IMPEDANCE)
`
`1 04
`
`110
`
`1 12
`
`
`
`THERAPY
`
`
`SUCCE?SSFUL ’
`
`
`'
`
`N
`
`HV__PULSE_WIDTH =
`SUGG_PULSE_WIDTH
`
`1 06
`
`Y
`
`111
`
`HV_PULSE_W|DTH =
`
`(HV_PULSE_WIDTH + SUGG_PULSE_WIDTH)/2
`
`@ 106
`
`Figure 6
`
`7
`
`

`

`METHOD AND APPARATUS FOR IMPEDANCE
`BASED AUTOMATIC PULSE DURATION
`ADJUSTMENT FOR DEFIBRILLATION SHOCK
`DELIVERY
`.
`
`FIELD OF THE INVENTION
`
`This invention relates generally to implantable medi-
`cal devices and in particular to an implantable defibrilla-
`tor that automatically adjusts defibrillation pulse dura-
`tion based on measured impedance following a shock
`delivery.
`'
`BACKGROUND OF THE INVENTION
`
`5
`
`10
`
`15
`
`20
`
`25
`
`Ventricular tachyarrhythmias are electrical diseases
`of the heart which may result in “sudden death".
`In one type, ventricular tachccardia, the heart mus-
`cle, which comprises the ventricles, contracts rapidly in
`a coordinated fashion. In another type, ventricular fi-
`brillation, which may be a sequela to ventricular tachy-
`cardia, there is very rapid and uncoordinated contrac-
`tion of individual muscles fibers of the ventricles, These
`rapid heart rhythms result in inefficient, or in the case of
`ventricular fibrillation, no blood being pumped from the
`heart and may result in death unless an effective inter-
`vention is applied within minutes.
`Supraventricular tachyarrhythmias, including atrial
`fibrillation, atrial flutter, and supraventricular tachycar-
`dias, are generally nonlethal arrhythmias that also result 30
`in less efficient pumping of blood from the heart, and
`may result in symptoms of palpitations, pre-syncope and
`angina.
`It is well known in the field of cardiology that these
`atrial and ventricular tachyarrhythmias can be effec- 35
`tively treated by the application of a sufficiently strong
`electric shock. Such shocks may be delivered manually
`by medical personnel via electrodes placed outside the
`body on the chest wall, or directly on the heart during
`surgery. Recently, implantable antitachycardia devices 40
`have been developed which automatically monitor the
`heart’s rhythm and deliver an electric shock or rapid
`pacing pulses via implanted electrodes in response to a
`tachyarrhythmia episode. Likewise, external automatic
`devices can be used for in and out-of-hospital therapy 45
`for ventricular and supraventricular arrhythmias.
`Defibrillation output waveforms used by clinically
`available defibrillators are produced by capacitor dis-
`charge. Internal or implantable defibrillators, as well as
`some external or transthoracic defibrillators, utilize so
`truncated exponential defibrillation waveforms. The
`waveforms are produced by charging the capacitors to
`a selected initial voltage and then allong the capaci-
`tors to discharge for a period of time through defibrilla—
`tion leads placed in or on the body so that current flows 5;
`through the heart. The rate of capacitor discharge is
`dependent upon the impedance of the system.
`These truncated exponential waveforms can be de-
`signed to have either “fixed tilt” or “fixed pulse width".
`Fixed tilt defibrillators discharge the capacitors from 60
`the selected initial voltage until a predetermined final
`voltage is reached, the “tilt” being the percentage de—
`cline in capacitor voltage from its initial value; there-
`fore, the pulse duration varies directly with the system
`impedance. In contrast, fixed pulse width defibrillators 65
`discharge their capacitors for a preselected duration
`and, as a result, the tilt of the waveform varies inversely
`with the impedance of the system;
`low impedances
`
`1
`
`5,230,336
`
`2
`cause the waveform to have a high tilt, while high im-
`pedances result in low tilt.
`Previous studies (Gold et al., Am Heart J 1979, 98;
`207—212; Wessale et al., J Electrocardiology 1980, 13:
`359-366; Schuder et al., IEEE Trans Biomed Eng 1983,
`BME—30: 415—422; Chapman et al., PACE 1988, 11:
`1045—1050; Feeser
`et
`al., Circulation
`1990,
`82:
`2128—2141) have shown that there is a relationship be-
`tween the minimum energy or current required for
`successful defibrillation and the duration of the defibril-
`lation pulse. These experiments demonstrated that the
`pulse width could be optimized for a given defibrillation
`waveform and lead configuration. Shorter pulse dura—
`tions require higher energy to adequately depolarize the
`myocardium, while longer pulse widths are probably
`less effective because of their ability to refibrillate the
`heart.
`Some prior an external defibrillators describe adjust-
`ing the defibrillation shock based upon impedance (Ler-
`man et al., J Am Cardiol 1988, 12: 1259—1264; Kerber et
`al., Circulation 1988, 77: 1038—1046). However, these
`devices do not alter the waveform’s pulse duration in
`response to a previous impedance measurement. The
`defibrillators delivered damped sinusoidal waveform
`shocks and either the energy or peak current was ad-
`justed for a transthoracic impedance that was predicted
`in advance of any shock by passing high frequency
`alternating current between the defibrillation elec-
`trodes. In addition, it is not feasible to use this type of
`defibrillation waveform or method of predicting inter—
`electrode impedance in an implantable device.
`As explained above, fixed pulse width truncated ex-
`ponential waveforms will have differing tilts depending
`upon the impedance of the system. Therefore, the most
`effective pulse width for a defibrillation waveform will
`change as the impedance of the system changes. This is
`particularly true when a biphasic waveform is em-
`ployed. With a biphasic waveform produced from a'
`single capacitor discharge, the initial voltage of the
`second negative phase is dependent upon the final volt-
`age remaining on the capacitors at the end of the first
`phase. If the pulse duration of the first phase is too long
`for a given system impedance, then the tilt of the first
`phase will be high, resulting in little voltage remaining
`on the capacitors and a very low energy and less effec-
`tive negative phase. If the biphasic waveform’s pulse
`duration-impedance mismatch is large enough, it can
`result in the delivery of a waveform that is effectively
`monophasic.
`Investigations have shown that, for implantable de-
`fibrillator systems, the high voltage lead impedance can
`change dramatically from that measured at the time of
`implantation. Typically, the impedance decreases ini-
`tially, reaching its nadir during the first one to two
`weeks after implantation, and then gradually increases
`and stabilizes. In addition, the impedance may change
`significantly with changes in the patient’s clinical
`course, such as a new myocardial infarction, scarring,
`worsening or improving heart failure, pericarditis or
`pericardial effusion. Changes in the defibrillation lead
`system, including shifting of position, dislodgment or
`damage, may also cause a large impedance change.
`These changes in impedance could result in delivery of
`a preselected fixed pulse width defibrillation waveform
`which is unable to successfully terminate a tachyrhyth-
`mia episode.
`It would, therefore, be highly desirable to have avail-
`able a method of automatically adjusting the pulse dura—
`
`8
`
`

`

`3
`tion of a subsequent fixed pulse width truncated expo-
`nential defibrillation waveform based upon the impe—
`dance measured or calculated following a delivered
`shock.
`
`SUMMARY OF THE INVENTION
`
`invention provides an antitachyarr-
`The present
`hythmia device, either implanted or used externally,
`which delivers a fixed pulse width truncated exponen-
`tial waveform defibrillation shock and automatically
`adjusts the pulse duration based upon the impedance
`measured or calculated following a delivered shock.
`This method is primarily intended for use with a device
`which monitors the heart’s rhythm and automatically
`delivers therapy upon diagnosing a tachyarrhythmia,
`but can also be used with a manually triggered device.
`It may be used with a variety of defibrillation wave-
`forms including monophasic, biphasic or triphasic,
`when delivering therapy for supraventricular or ven—
`tricular arrhythmias.
`In one embodiment, the method comprises the steps
`of: diagnosing an arrhythmia; delivering defibrillation
`therapy using the current pulse width setting; measur-
`ing or calculating the high voltage system impedance;
`selecting a suggested pulse width for that impedance
`value from a pulse width table; and using that suggested
`pulse width for the next defibrillation shock.
`The method may also include an additional step of
`determining whether or not the delivered defibrillation
`waveform had an initial voltage greater than a predeter-
`mined minimum value. In this case, the method com-
`prises the steps of: diagnosing an arrhythmia; delivering
`defibrillation therapy using the current pulse width
`setting; measuring or calculating the high voltage sys-
`tem impedance; determining whether or not the deliv-
`ered defibrillation waveform had an initial voltage
`greater than a predetermined minimum value; only
`allowing the pulse width to be adjusted if the initial
`voltage was greater than that minimum value; selecting
`a suggested pulse width for that impedance value from
`a pulse width table; and using that suggested pulse
`width for the next defibrillation shock. This method
`prevents the pulse width from being adjusted after low
`voltage shocks, in which the impedance measurement is
`less accurate or different from higher voltage shocks,
`where optimization is important.
`The method may also include an additional step of
`averaging the current pulse width with the suggested
`pulse width. This average may have either the current
`pulse width and the suggested pulse width weighted
`equally or unequally. In this case, the method comprises
`the steps of: diagnosing an arrhythmia; delivering defi-
`brillation therapy using the current pulse width setting;
`measuring or calculating the high voltage system impe-
`dance; selecting a suggested pulse width for that impe-
`dance value from a pulse value table; averaging the
`current and suggested pulse widths in a weighted or
`nonweighted manner; and using that averaged pulse
`width for the next deflbrillation shock. This decreases
`the reactivity of the pulse width adjustment in response
`to a single impedance measurement.
`The method may also include an additional step of
`adjusting the pulse width by differing degrees based
`upon whether or not the defibrillation shock was suc-
`cessful in terminating the arrhythmia. In this case, the
`method comprises the steps of: diagnosing an arrhyth-
`mia; delivering defibrillation therapy using the current
`pulse width setting; measuring or calculating the high
`
`5
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`20
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`30
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`35
`
`45
`
`50
`
`55.
`
`60
`
`65
`
`5,230,336
`
`4
`voltage system impedance; selecting a suggested pulse
`width for that impedance value from a pulse value table;
`determining whether or not the deflbrillation, shock
`was successful in terminating the tachyarrhythmia; ad-
`justing pulse width differently based upon whether or
`not the shock was successful or unsuccessful; and using
`that pulse width for the next defibrillation shock. This
`method changes the reactivity of the pulse width adjust-
`ment depending upon the outcome of the defibrillation
`therapy.
`A better understanding of the features and advan-
`tages of the present invention will be obtained by refer-
`ence to the following detailed description of the inven-
`tion and accompanying drawings which set forth illus-
`trative embodiments in which the principles of the in—
`vention are utilized.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram illustrating an implantable
`pacer/defibrillator system constructed in accordance
`with the principles of the present invention.
`FIG. 2 is a flow chart illustrating an impedance-based
`automatic pulse width adjustment algorithm utilizable
`with the device shown in FIG. 1.
`FIG. 3 is an example of a table of suggested pulse
`widths, based on a 150 microfarad source capacitance,
`for a range of measured impedance values utilizable
`with the device shown in FIG. 1.
`FIG. 4 is a flow chart illustrating an impedance-based
`automatic pulse width adjustment algorithm which
`includes an additional step of determining whether or
`not the delivered defibrillation waveform had an initial
`voltage greater than a predetermined minimum value.
`FIG. 5 is a flow chart illustrating an impedance-based
`automatic pulse width adjustment algorithm which
`includes an additional step of averaging the current
`pulse width with the suggested pulse width.
`FIG. 6 is a flow chart illustrating an impedance—based
`automatic pulse width adjustment algorithm which
`includes an additional step of adjusting the pulse width
`by differing degrees based upon whether or not the
`defibrillation shock was successful in terminating the
`arrhythmia.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`FIG. 1 shows a block diagram of an implantable
`defibrillator system 10 which integrates the functions of
`antitachycardia pacing, cardioversion, defibrillation
`and demand bradycardia pacing.
`The implantable defibrillator system 10 includes four
`integrated circuits (IC) and a set of high voltage dis-
`crete components.
`A battery 20 produces a positive voltage with respect
`to ground that varies from about 6.4 volts when new, to
`5.0 volts at the end of service. The battery 20 directly
`powers integrated circuit 30 and the high voltage dis-
`cretes 60.
`Integrated circuit 30 contains a band-gap reference
`circuit 31 that produces 1.235 volts and 3 volt regulator
`32 that powers a microprocessor 90, integrated circuit
`70, and an ECG storage RAM 77 through line 100. The
`3 volt regulator runs off of a switched capacitor V §
`battery voltage down converter 33 for improved effi-
`ciency. The microprocessor 90 communicates with
`integrated circuit 30 through a data and address bus 83
`and an on-chip interface 34 that contains chip-select,
`address decoding and data bus logic typically used with
`
`9
`
`

`

`5,230,336
`
`5
`microprocessor peripherals. The internal bus 35 of inte-
`grated circuit 30 allows the microprocessor 90 to con—
`trol a general purpose 12 bit analog—to—digital converter
`(ADC) 36, the atrial pace circuits 37, the ventricular
`pace circuits 38, and the HV control and regulate block
`39.
`The ADC 36 is used by the microprocessor 90 to
`measure the battery and other diagnostic voltages
`within the system 10. The atrial pace circuits 37 include
`a digita1~to-analog converter (DAC) that provides the
`ability to pace at regulated voltages. It communicates
`with the atrium of a heart 40 through two lines. One line
`41 is a switchable ground; the other line 42 is the pacing
`cathode and is also the input to the atrial sense amplifier,
`as will be described in greater detail below.
`The ventricular pace circuits 37 include a DAC that
`provides the ability to pace at regulated voltages. It
`communicates with the ventricle of the heart 40
`through two lines. One line 43 is a switchable ground;
`the other line 44 is the pacing cathode and is also the
`input to the ventricular sense amplifier, as will be de-
`scribed in greater detail below.
`Both the atrial and ventricular pace lines pass
`through high voltage protection circuits 45 to prevent
`the defibrillation voltages generated by the system 10
`from damaging the pacing circuits 37, 38.
`The high voltage (HV) control and regulate block 39
`on integrated circuit 30 is used by the microprocessor
`90 to charge a high voltage capacitor included in the
`HV charge block 46 to a regulated voltage, and then to
`deliver the defibrillation pulse to the heart 40 through
`the action of switches in the HV delivery b10ck47. An
`HV sense line 48 is used by the HV regulation circuits
`39 to monitor the defibrillation voltage during charging.
`An HV control bus 49 is used by the HV control cir-
`cuits 39 to control the switches in the HV delivery
`block 47 for delivering the defibrillation pulse to the
`electrodes 52, 53 through lines 50 and 51.
`Integrated circuit 70 is another microprocessor pe-
`ripheral
`that provides timing,
`interrupt,
`telemetry,
`ECG storage, and sensing functions. A dual channel
`electrogram sensing and waveform analysis section 71
`interfaces with the atrium and ventricle of the heart 40
`through lines 42 and 44, respectively. The sensed elec-
`trogram is amplified and digitized. The amplifiers con—
`tained in sensing/analysis section 71 have multiple gain
`settings that are under microprocessor control for mainv
`taining an automatic gain control (AGC). Features such
`as peak voltage and complex width are extracted by the
`waveform analysis circuits 71 for the microprocessor 90
`to use in discriminating arrhythmias from normal sinus
`rhythm. The voltage reference 31 from integrated cir-
`cuit 30 is used by the digitizer circuit 71 in the usual
`fashion, and is supplied by line 72.
`The digitized ECG is provided to the RAM control-
`ler 74 through a bus 73. The RAM controller sequences
`through the addresses of a static EKG storage RAM 77
`to maintain a pretrigger area, and produces a post trig-
`ger area upon command from the microprocessor 90.
`The crystal and monitor block 78 has a 100 KHz
`crystal oscillator that provides clocks to the entire sys-
`tem. The monitor is a conventional R-C oscillator that
`provides a back-up clock if the crystal should fail.
`The microprocessor 90 communicates with inte-
`grated circuit 70 through two buses, 83 and 84. One bus
`83 is a conventional data and address bus and goes to an
`on-chip interface 81 that contains chip select, address
`decoding and data bus drivers typically used with mi‘
`
`6
`croprocessor peripherals. The other bus 84 is a control
`bus. It allows the microprocessor 90 to set up a variety
`of maskable interrupts for events like timer timeouts and
`sense events. If an interrupt is not masked, and the cor-
`responding event occurs, an interrupt is sent from inte—
`grated circuit 70 to the microprocessor 90 to alert it of
`the occurrence. On integrated circuit 70, the micro—
`processor control and interrupt section 79 contains
`microprocessor controllable timers and interrupt logic.
`The system 10 can communicate with the outside
`world through a telemetry interface 80. A coil 105 is
`used in a conventional fashion to transmit and receive
`pulsed signals. The telemetry circuits 80 decode an
`incoming bit stream from an external coil 110 and hold
`the data for subsequent retrieval by the microprocessor
`90. When used for transmitting, the telemetry circuit 80
`receives data from the microprocessor 90, encodes it,
`and provides the timing to pulse the coil 105. The com-
`munication function is used to retrieve data from the
`implanted device and to change the modality of opera-
`tion if required.
`The microprocessor 90 is of conventional architec-
`ture comprising an algorithmic logic unit (ALU) 91, a
`ROM 92, a RAM 93 and interface circuits 94. The
`ROM 92 contains the program code that determines the
`operation of the device. The RAM 93 is used to modify
`the operating characteristics of the device as regards
`modality, pulse widths, pulse amplitudes, and so forth.
`Diagnostic data is also stored in the RAM 93 for subse-
`quent transmission to the outside world. The ALU 91
`performs the logical operations directed by the program
`code in the ROM.
`FIGS. 2, 4, 5 and 6 depict four different embodiments
`of the present invention, which are provided for pur-
`poses of illustration only. One skilled in the art will
`readily recognize from the following discussion that
`alternative embodiments of these embodiments may be
`employed without departing from the principles of the
`invention.
`The flow diagram shown in FIG. 2 represents a
`method of automatically adjusting the pulse duration of
`a fixed pulse width truncated exponential waveform
`defibrillation shock based upon the impedance mea-
`sured or calculated following a delivered shock. The
`flow diagram depicts the use of the algorithm by a
`device which monitors the heart’s rhythm, detects and
`diagnoses the presence of an arrhythmia 10 and auto—
`matically delivers defibrillation therapy. The delivery
`of high voltage therapy 102 may instead be initiated
`manually. The tachyarrhythmia therapy 102 may have
`been delivered for either supraventricular tachycardia,
`atrial fibrillation, ventricular tachycardia or ventricular
`fibrillation. It should be appreciated that, as the term is
`used herein, “defibrillation” includes both high and low
`voltage/energy shocks for either supraventricular tach-
`ycardia, atrial fibrillation, ventricular tachycardia or
`ventricular fibrillation.
`After delivery of the defibrillation shock, the impe—
`dance of the high voltage system is measured 103. An
`optimal pulse width for this impedance value can then
`be chosen from a predetermined table of suggested
`pulse durations 104, and the pulse width for the subse-
`quent defibrillation shock changed to this value 105.
`An example of such a table is provided in FIG. 3. In
`this case, the values have been chosen so that the wave-
`form has a relatively constant tilt over a wide range of
`impedances, assuming a source capacitance of 150 mi-
`crofarads. In addition, at both very high and very low
`
`10
`
`15
`
`20
`
`25
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`30
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`35
`
`45
`
`50
`
`55.
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`65
`
`10
`
`10
`
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`8
`
`10
`
`15
`
`20
`
`30
`
`35
`
`7
`impedance values, the pulse width has been limited to
`maximum and minimum durations in order to maintain
`an effective waveform. Suggested pulse widths in the
`FIG. 3 table are given for a monophasic waveform, as
`well as for a biphasic waveform that has equal positive
`and negative phase durations. Other biphasic or multi-
`phasic waveforms, that have unequal phase durations,
`could also be used in such a table by, for example, keep-
`ing the ratio of the phase durations constant as the total
`pulse width varies with the measured impedance, or by
`defaulting to equal pulse durations if the pulse width is
`adjusted.
`FIG. 4 shows an alternative embodiment of the pres-
`ent invention such that an additional test is performed to
`determine whether or not the pulse width should be
`adjusted based upon the impedance measurement. This
`test 107 determines whether or not the delivered defi-
`brillation waveform had an initial voltage greater than a
`predetermined minimum value.
`Impedance is, in a non-linear manner, partially depen-
`dent upon the initial voltage of the shock. As a result,
`low voltage shocks may over-estimate the impedance
`for higher voltage shocks.
`In the FIG. 4 embodiment, a value of 250 volts is
`shown as an example of this minimum value, but of 25
`course this can vary with the type of device and defi—
`brillation lead system that is used. If the initial voltage
`of the defibrillation shock is determined to have been
`greater than this predetermined value, then the pulse
`width is adjusted as in the method described above in
`conjunction with FIG. 2. However, if this condition is
`not satisfied, then the pulse width is left unchanged 8.
`FIG. 5 shows a further alternative embodiment of the
`present invention wherein the defibrillation waveform’s
`pulse width is adjusted to an average of the current
`pulse width and the suggested pulse width 109, instead
`of being set to the suggested pulse width. This average
`may have either the current pulse width and the sug-
`gested pulse width weighted equally, as shown in FIG.
`5, or weighted unequally. This averaging may be used
`in a method independent of the use of step 107. This
`decreases the reactivity of the pulse width adjustment in
`response to a single impedance measurement.
`FIG. 6 shows a further alternative embodiment of the
`present
`invention which includes an additional step
`wherein the defibrillation waveform’s pulse width is
`adjusted differently based upon whether or not the
`defibrillation shock was successful in terminating the
`arrhythmia 110. As an example, FIG. 6 shows a method
`whereby the pulse width is set equal to the suggested
`pulse width if the shock is unsuccessful 112 (i.e., more
`reactive), while it is set to an equally weighted average
`of the current and suggested pulse widths if the therapy
`is successful 111 (i.e., less reactive). Another variation
`could be to only adjust the pulse width following unsuc-
`cessful shocks. This step may be used in a method inde-
`pendent of the use of step 107 or averaging of the cur-
`rent and suggested pulse widths.
`It should be understood that various alternatives to
`the embodiment of the invention described herein may
`be employed in practicing the invention. For example,
`while the invention is disclosed above in the context of
`an implantable device, the concepts of the invention are
`also applicable to manual delivery systems. It is in-
`tended that the following claims define the scope of the
`invention and that structures and methods within the
`scope of these claims and their equivalents be covered
`thereby.
`
`5,230,336
`What is claimed is:
`1. A method of delivering a preselected pulse width
`defibrillation shock to a heart and automatically adjust-
`ing the pulse width for a subsequent shock based upon
`the impedance of the heart, the method comprising the
`sequential steps of:
`(a) diagnosing an arrhythmia of the heart;
`(b) delivering a defibrillation shock to the heart using
`a current pulse width value;
`(c) determining the impedance of the heart;
`((1) selecting a pulse width corresponding to the impe-
`dance of the heart; and
`‘
`(e) delivering a subsequent defibrillation shock hav—
`ing a pulse width derived from the selected pulse
`width.
`2. A method of delivering a selected pulse width
`defibrillation shock to a heart based on the impedance
`of the heart, the method comprising:
`(a) determining the impedance of the heart;
`(b) comparing the impedance of the heart with a
`predetermined criteria to determine whether the
`impedance of the heart meets the predetermined
`criteria;
`(0) selecting a pulse width corresponding to the impe-
`dance of the heart only if the impedance of the
`heart meets the predetermined criteria; and
`(d) delivering a defibrillation shock having a pulse
`width derived from the selected pulse width.
`3. A method of delivering a selected pulse width
`defibrillation shock to a heart based on the impedance
`of the heart, the method comprising:
`(a) determining the impedance of the heart;
`(b) selecting a pulse width corresponding to the impe—
`dance of the heart;
`(c) averaging the selected pulse width and a previous-
`ly-used pulse width; and
`(d) delivering a defibrillation shock having a pulse
`width derived from the average pulse width.
`4. A method of delivering a preselected pulse width
`defibrillation shock to a heart and automatically adjust-
`ing the pulse width for a subsequent shock based upon
`the impedance of the heart, the method comprising the
`sequential steps of:
`(a) diagnosing an arrhythmia of the heart;
`(b) delivering a defibrillation shock to the heart, the
`shock having a preselected pulse width value;
`(c) determining the impedance of the heart;
`(d) selecting a pulse width corresponding to the impe-
`dance of the heart;
`(e) averaging the preselected pulse width and the
`selected pulse width; and
`(f) delivering a subsequent defibrillation shock having
`the average pulse width.
`5. A method as in claim 4 wherein said averaging step
`comprises the step of calculating a weighted average.
`6. A method of delivering a selected pulse width
`defibrillation shock to a heart based on the impedance
`of the heart, the method comprising:
`(a) determining the impedance of the heart;
`(b) selecting a pulse width corresponding to the impe-
`dance of the heart;
`(c) delivering a defibrillation shock having a pulse
`width derived from the selected pulse width;
`(d) determining whether or not
`the defibrillation
`shock was successful in terminating the arrhyth-
`mia;
`
`45
`
`50
`
`55.
`
`60
`
`65
`
`11
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`11
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`9
`(e) adjusting the pulse width differently based upon
`whether or not the defibrillation shock was suc-
`cessful; and
`(f) delivering a subsequent defibrillation shock having
`the adjusted pulse width.
`~
`7. A method of delivering a preselected pulse width
`defibrillation shock to a heart and automatically adjust-
`ing the pulse width for a subsequent shock based upon
`the impedance of the heart following a delivered defi-
`brillation shock, the method comprising:
`(a) diagnosing an arrhythmia;
`(b) delivering a defibrillation shock having the prese-
`lected pulse width;
`(0) determining the impedance of the heart;
`(d) selecting a pulse width corresponding to the impe-
`dance of the heart;
`the defibrillation
`(e) determining whether or not
`shock was successful in termination the arrhyth-
`mia;
`(f) adjusting the selected pulse width differently
`based upon whether or not the defibrillation shock
`was successful; and
`(g) delivering a subsequent defibrillation shock hav—
`ing the adjusted pulse width:
`8. Apparatus for delivering an adjustable pulse width
`defibrillation shock to a heart based on the impedance
`of the heart, the apparatus comprising:
`(a) means connected to the heart for determining the
`impedance of the heart;
`(b) selection means connected to the impedance de-
`termining means for selecting a pulse width corre-
`sponding to the impedance of the heart; and
`(c) means connected to the heart for delivering a
`defibrillation shock having a pulse width deri