llllllllllllllllllllll||l|lIlllllllllIllllllllllllllIllllllllllllllllllllll
`USOOS352239A
`[11] Patent Number:
`5,352,239
`[19]
`United States Patent
`
`
`Pless Oct. 4, 1994M[45] Date of Patent:
`
`[54] APPARATUS FOR PRODUCING
`CONFIGURABLE BIPHASTIC
`DEFIBRILLATION WAVEFORMS
`
`[75]
`
`.
`Inventor: Benjamin Pless, Menlo Park, Calif.
`
`[73] Assignee: Ventritex, Inc., Sunnyval6, Calif.
`
`[21] Appl. No‘: 37,482
`.
`I
`9
`[22] Filed-
`Mar 24 1993
`
`2
`ti
`1'
`R lat d US.
`App 1ca on D ta
`8
`e
`one .
`c(liontilnuation of Ser. No. 629,252, Dec. 18, 1990, aban-
`
`[63]
`
`Int. Cl.5 ............................................... A61N 1/39
`[51]
`[52] US. Cl. ...........
`
`[58] Field of Search
`........... 607/5
`
`240
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`128/419 D
`4,708,145 11/1987 Tacker, Jr. et a].
`4,996,984
`3/1991 Sweene
`......................... 128/419 D
`y
`Primary Examiner~William E. Kamm
`Assistant Examiner—Scott M. Getzow
`Attorney, Agent, or Firm—Steven M. Mitchell; Mark J.
`Meltzer
`ABSTRACT
`[57]
`A programmable implantable medical device utilizable
`for delivering a configurable defibrillation waveform to
`a heart. The device includes defibrillation electrode
`rneans adapted to be connected to the heart for deliver-
`mg a multiphasn: defibrillation waveform thereto. A
`programmable waveform generator connected to the
`heart generates the multiphasic waveform such that at
`least one phase of the defibrillation waveform has pro—
`grammed constant tilt.
`
`2 Claims, 3 Drawing Sheets
`
`200
`
`21 0
`
`HV
`CONVERTER ‘—
`
`212
`“SELECT
`HV'l
`
`'EOC“
`
` BIPHASIC
`
`OUTPUT
`STAGE
`
`214
`
`l+ / _
`SELECTII
`21 3
`
`'TIME /
`VOLTAGE
`SELECT'
`215
`
`
`
`252
`
`232
`
`1
`
`L|FECOR427-1006
`
`1
`
`LIFECOR427-1006
`
`

`

`US. Patent
`
`Oct. 4, 1994
`
`Sheet 1 of 3
`
`5,352,239
`
`( PRIOR ART)
`214 I+ ,—
`
`(PRIOR ART)
`
`240
`
`
`BIPHASIC
`' OUTPUT
`
`
`
`SELECT"
`213
`
`FIG.
`
`1
`
`200
`
`210
`
`HV
`CONVERTER 4—-
`
`212
`'SELECT
`HV'
`
`“E00"
`
`'EOP"
`
`'TlME /
`VOLTAGE
`SELECT'
`21 5
`'TRAILING
`TRAILING
`VOLTAGE <— VOLTAGE 222
`DETECTOR
`
`220
`
`
`
`2
`
`

`

`US. Patent
`
`Oct. 4, 1994
`
`Sheet 2 of 3
`
`5,352,239
`
`DELIVER
`HIGH VOLTAGE
`
`SELECT HV
`
`300
`
`3‘! 0
`
`HEAD E00
`
`312
`
`w 314
`
`YES
`
`. 316
`
`SELECT VOLTAGE MUX
`SELECT +PW MUX
`
`318
`
`SET TRAILING EDGE VOLTAGE
`( +PW STARTS )
`
`330
`
`320
`
`READ EOP
`
`READ EOP
`
`NO
`
`322
`
`YE
`
`3
`
`3
`
`24
`
`326
`
`332
`
`390
`
`YES
`
`m
`
`3
`
`

`

`US. Patent
`
`Oct. 4, 1994
`
`Sheet 3 of 3
`
`5,352,239
`
`DELIVER
`HIGH VOLTAGE
`
`400
`
`SELECT HV
`
`READ E00
`
`412
`
`® 414
`
`YES
`
`416
`
`SELECT VOLTAGE MUX
`SELECT +PW MUX
`
`418
`
`434
`
`SET PW TIMER
`
`SET TRAlLING EDGE VOLTAGE
`( +PW STARTS )
`
`44o
`
`'
`
`a442
`
`' 490
`
`YES
`
`m
`
`g 422
`8
`
`READ PULSE WIDTH
`COUNTER
`
`YE
`
`430
`
`432
`
`SELECT TIME MUX
`SELECT — PW MUX
`
`FIG' 4
`
`4
`
`

`

`1
`
`5,352,239
`
`APPARATUS FOR PRODUCING CONFIGURABLE
`BIPHASTIC DEFIBRILLATION WAVEFORMS
`
`5
`
`10
`
`15
`
`20
`
`25
`
`3O
`
`35
`
`This is a continuation of application Ser. No.
`07/629,252, filed on Dec. 18, 1990, now abandoned.
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`
`The present invention relates to implantable medical
`devices and, in particular, to a programmable defibrilla-
`tor capable cf delivering a configurable biphasic wave—
`form.
`2. Discussion of the Prior Art
`Implantable defibrillators use truncated exponential
`waveforms to defibrillate the heart. The earliest devices
`used monophasic waveforms. More recent clinical in-
`vestigations have evaluated the increased effectiveness
`of biphasic waveforms. See Troup, Implantable Cardio-
`verters and Defibrillators, Current Problems in Cardiol-
`ogy, Volume XIV, Number 12, December 1989, pages
`729—744. Some investigators have even recommended
`the use of triphasic waveforms as the most effective
`waveform for defibrillating a heart. See U.S. Pat. No.
`4,637,397 issued to Jones and Jones on Jan. 20, 1987.
`As described by Troup, monophasic waveforms are
`typically produced using silicon controlled rectifier
`(SCR) technology that truncates the pulse by “dump-
`ing” the energy on the defibrillator capacitor. This
`leaves no energy available on the capacitor for produc-
`ing multiphasic waveforms.
`As further described by Troup, there have been two
`methods available for truncation of a monophasic defi—
`brillation waveform. According to one method, pulse
`truncation is accomplished by comparing the capacitor
`voltage to a reference voltage which is usually chosen
`as a function of the waveform leading edge voltage.
`The result is a defibrillation pulse with a constant ratio
`of trailing edge to leading edge voltage, or a “constant
`tilt” pulse.
`Defibrillation pulse “tilt”, described as percent tilt, is
`defined as follows:
`
`% ri1r=1oo[1—(Vf/'Vi>h
`
`where VfiS the trailing edge voltage of the pulse and V,-
`is the leading edge voltage.
`According to the second method, the defibrillation
`pulse is truncated by a timing circuit so that the pulse
`duration is constant.
`Biphasic waveform generators have used MOS
`switches to produce the defibrillator output. The MOS
`switch technique is better suited to multiphasic wave-
`forms since the defibrillator capacitor does not need to
`be “dumped” to truncate the pulse.
`Prior art biphasic waveforms have been programma-
`ble in terms of pulse duration. The disadvantage of
`programming biphasic waveforms in terms of duration
`can be seen in FIG. 1. Panel 1 of FIG. 1 shows a con-
`ventional biphasic waveform with a 50 ohm load. Panel
`2 shows a conventional biphasic waveform with the
`same duration of phases with a 25 ohm load. With a 50
`ohm load, there is adequate residual voltage to produce
`an effective negative phase of the biphasic waveform.
`However, at the same pulse durations, with a 25 ohm
`load, the voltage during the positive phase has decayed
`to the point where very little is left for the negative
`phase.
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`While it is possible to select optimal pulse durations
`for a given patient impedance, the patient impedance
`may change. In particular, for higher defibrillation volt-
`ages, the patient impedance is lower. In addition, over
`time, the lead impedance may increase due to the build-
`up of scar tissue.
`im~
`Due to their small size and battery operation,
`plantable defibrillators have limited output energy ca-
`pability. It is not unusual for an implantable defibrillator
`to have only slightly more output capability than is
`required to defibrillate a patient. This lack of safety
`margin makes it all the more important that the output
`energy that is available is used in the most effective
`manner. While biphasic waveforms are a step in the
`right direction, the optimal settings for the positive and
`negative phase durations have not been addressed in the
`prior art.
`U.S. Pat. No. 4,850,357 issued to Stanley M. Bach, Jr.
`on Jul. 25, 1989, discloses a circuit for generating a
`biphasic defibrillation waveform wherein both the posi-
`tive and negative phases have constant tilt. However,
`the Bach, Jr. defibrillator generates a biphasic wave-
`form having fixed characteristics. That is, only a single
`type of waveform can be delivered that has a first posi-
`tive pulse having a specified constant tilt and a second
`negative pulse also having a specified constant tilt
`Thus, the Back defibrillator circuit provides none of the
`therapeutic flexibility that
`is desirable in restoring
`rhythm to a fibrillating heart.
`SUMMARY OF THE INVENTION
`
`invention provides a microprocessor
`The present
`controlled output stage that allows for greater flexibil-
`ity than has been available in defining a biphasic defi-
`brillation waveform. In accordance with the invention,
`the biphasic waveform generator may be programmed
`to provide either positive and negative phases having
`selected constant tilt or a positive phase having a se-
`lected constant tilt and a negative phase having a dura-
`tion that is related to the duration of the positive phase.
`The disclosed apparatus can also produce conventional
`multiphasic waveforms, if desired.
`A better understanding of the features and advan-
`tages of the present invention will be obtained by refer
`ence to the following detailed description and accompa-
`nying drawings which set forth an illustrative embodi-
`ment in which the principles of the invention are uti-
`lized.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 provides a comparison between prior art bi-
`phasic waveforms and configurable biphasic waveforms
`generated in accordance with the present invention.
`FIG. 2 is a block diagram illustrating an embodiment
`of an apparatus for generating a configurable, biphasic
`waveform in accordance with the present invention.
`FIG. 3 is a flow chart of a method for producing a
`biphasic waveform having selected constant positive
`and negative tilts.
`FIG. 4 is a flow chart of a method for producing a
`biphasic waveform with a selected constant tilt positive
`phase and a negative phase the duration of which is
`related to the duration of the measured positive phase
`duration.
`
`5
`
`

`

`3
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`5,352,239
`
`The invention is directed to programmable control
`circuitry for an implantable defibrillator output stage
`that generates biphasic defibrillation waveforms having
`selected constant tilt. In the disclosed embodiment, the
`defibrillator has an on-board microprocessor and the
`control circuitry acts as a peripheral
`to the micro-
`processor.
`With a biphasic waveform, where both phases have
`constant tilt, sufficient voltage for the negative phase is
`assured, as shown in panels 3 and 4 of FIG. 1. Panel 3
`shows a constant tilt biphasic waveform with a 50 ohm
`load. Panel 4 shows a biphasic waveform with the same
`constant tilt with a 25 ohm load. The initial voltage on
`the biphasic waveform generated by the apparatus of
`the invention is the same in both cases. With the inven~
`tive apparatus, the amount of tilt in each phase is inde-
`pendently programmable. Since J=0.5*C(V,2—Vfi);
`constant tilt can also be expressed as constant energy
`where the energy is independent, to some extent, from
`the initial voltage.
`,
`With a multiphase constant tilt defibrillation wave-
`form, the duration of each phase of the waveform is
`dependant upon the patient impedance. Some studies
`(Tang, et a1, Ventricular Defibrillation Using Biphasic
`Waveforms: The Importance of Phasic Duration,
`JACC Vol. 13, No. ], January 1989) support the idea
`that the relative durations of the phases of a biphasic
`waveform are important in determining its efficacy.
`Therefore, it is desirable to be able to measure the dura-
`tion of the first, constant tilt phase of a biphasic wave-
`form and then set the negative phase duration to some
`percentage of the measured positive phase duration.
`This is a further capability of the disclosed apparatus,
`thus providing the ability to optimize multiphasic wave-
`form durations.
`Referring to FIG. 2, in the illustrated embodiment of
`the invention, a control system is used which comprises
`functional modules and addresses that the microproces-
`sor can read or write.
`
`FIG. 3 is a flow diagram that will be used in conjunc-
`tion with the FIG. 2 block diagram to describe how a
`biphasic waveform having selected constant tilt positive
`and negative pulses can be generated.
`Referring to FIGS. 2 and 3 at step 300 of the FIG. 3
`flowchart, the microprocessor decides that a defibrilla-
`tion output is necessary. This could be due to the auto-
`matic detection of fibrillation by the device, or due to an
`external command from the physician, or due to any
`other reason.
`
`5
`
`10
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`15
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`20
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`25
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`30
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`35
`
`4O
`
`45
`
`50
`
`Before a pulse can be delivered, energy must be
`stored on the defibrillation capacitor 200 (FIG. 2),
`which typically has a value of about 150 microfarads.
`At step 310, the microprocessor addresses the high
`voltage converter 210 to command it to start charging
`the defibrillation capacitor 200 to a selected voltage
`(address “select HV” 212).
`At step 312, the microprocessor starts a polling loop
`by reading “EOC” 214. “EOC” is the “end—of-convert”
`signal from the high voltage converter 210 and signifies
`that the converter has finished charging the capacitor
`200 to the selected initial voltage. After reading “EOC”
`at step 312, the microprocessor determines if the initial
`high voltage is ready at step 314.
`If the initial high voltage is not ready, then the micro-
`processor loops back to step 312. In some implementa—
`
`55
`
`60
`
`65
`
`4
`tions it may be desirable for the microprocessor to at-
`tend to other tasks or to be disabled for periods to con—
`serve current during polling loops. If, at step 314, the
`microprocessor finds that the defibrillator capacitor 200
`is charged to the selected initial voltage, then the defi-
`brillation system is prepared for delivery of a positive
`pulse.
`The pulse width of the positive pulse is determined by
`the length of time required for the defibrillator capaci-
`tor to decay to a selected decay voltage. If, in this illus-
`trative example, the selected peak voltage is 500 volts,
`then 200 volts would be a reasonable decay voltage for
`the trailing edge voltage of the positive pulse to assure
`an effective negative phase (a trailing edge voltage of
`100 volts for the negative phase will be used for this
`example).
`At step 316, the microprocessor manipulates the con-
`trols of two multiplexers 212 and 214 to set the output
`stage to terminate the positive pulse when the selected
`trailing edge is detected on the defibrillation capacitor
`200. Multiplexer 212 selects the signal flow to either
`generate a positive pulse or a negative pulse.
`At step 316, the microprocessor addresses “+/ —
`select” 213 to choose a positive pulse. Multiplexer 214
`selects the signal flow to either produce a pulse with a
`selected time duration or a pulse which terminates
`when a selected decay voltage is detected on the defib-
`rillator capacitor 200 At step 316, the microprocessor
`addresses “time/voltage select” 215 to choose a pulse
`which terminates when a selected decay voltage is de-
`tected.
`
`The positive pulse is started by the microprocessor at
`step 318 by addressing “Trailing voltage select” 222 and
`setting the selected trailing edge voltage to 200 volts.
`Since the voltage on the defibrillator capacitor 200 is at
`500 volts, the output 221 of the trailing voltage detector
`220 goes high. This signal 221 goes through multiplexer
`214 to line 223, through multiplexer 212 to the positive
`pulse input 225 of the biphasic output stage 240 which
`generates a positive defibrillation output as long as posi-
`tive pulse input 225 is asserted.
`Once the positive pulse is started, the voltage on the
`defibrillator capacitor starts to decline as current flows
`into the patient’s heart 290. Trailing voltage detector
`220 maintains signal 221 high until the voltage on the
`defibrillator capacitor 200 has decayed to less than the
`trailing voltage selected by address 222. In this example,
`when the capacitor voltage decays to 200 volts, the
`trailing voltage detector 220 responds by forcing its
`output 221 low. This signal goes through multiplexer
`214, line 223, and multiplexer 212 to the positive pulse
`input control 225 of the biphasic output stage 240, ter-
`minating the positive pulse.
`While the positive pulse is being generated, the mi-
`croprocessor waits in a polling loop for the pulse to end.
`The microprocessor reads “EOP” at step 320. “EOP” is
`the “end-of—pulse” signal and is the same as line 223
`discussed above. As long as the pulse is being generated,
`“EOP” is high; when the pulse is over, “EOP” goes
`low. Having read “EOP” at step 320, the microproces-
`sor checks to see if the pulse is over at step 322. If the
`pulse is not over, the microprocessor loops back to step
`320. When the positive pulse ends, the microprocessor
`sets up the defibrillator system to produce the negative
`pulse.
`At step 324, the microprocessor addresses the “+/ -
`select” 213 of multiplexer 212 to select a negative pulse.
`The negative pulse is started by the microprocessor at
`
`6
`
`

`

`5,352,239
`
`5
`step 326 by addressing “Trailing voltage select” 222 and
`setting the selected trailing edge voltage to 100 volts (in
`this example). Since the voltage on the defibrillator
`capacitor 200 is at 200 volts, the output 221 of the trail-
`ing voltage detector 220 goes high. This signal 221 goes
`through multiplexer 214 to line 223, through multi-
`plexer 212 to the negative pulse input 226 of the bipha-
`sic output stage 240 which generates a negative defibril—
`lation output as long as negative pulse input 226 is as»
`serted.
`
`Once the negative pulse is started, the voltage on the
`defibrillator capacitor 200 starts to decay again as cur-
`rent flows into the patient’s heart 290. Trailing voltage
`detector 220 maintains signal 221 high until the voltage
`on the defibrillator capacitor 200 has decayed to less
`than the trailing voltage selected by address 222. In this
`example, when the capacitor voltage decays to 100
`volts,
`the trailing voltage detector 220 resPonds by
`forcing its output 221 low. This signal goes through
`multiplexer 214 to line 223 and multiplexer 212 to the
`negative pulse input control 226 of the biphasic output
`stage 240, terminating the negative pulse.
`While the negative pulse is being generated, the mi—
`croprocessor waits in a polling loop for the pulse to end.
`The microprocessor reads “BOP” at step 330. As long
`as the pulse is being generated, “EOP” is high; when the
`pulse is over, “EDP” goes low. Having read “EOP” at
`step 330, the microprocessor checks to see if the pulse is
`over at 332. If the pulse is not over, then the micro-
`processor loops back to step 330. When the negative
`pulse ends, the microprocessor exits the program flow
`at step 390.
`FIG. 4 is a flow diagram that will be used in conjunc-
`tion with the FIG. 2 block diagram to describe how a
`biphasic waveform having a positive pulse of selected
`constant tilt and a negative pulse duration related to the
`positive pulse duration can be generated. Generation of
`the positive pulse is accomplished in the same manner as
`described above in conjunction with FIG. 3, but is re-
`peated here for completeness.
`At step 400, the microprocessor decides that a defi-
`brillation output is necessary. Before a pulse can be
`delivered, however, energy must be stored on the defi-
`brillation capacitor 200, which typically has a value of
`about 150 microfarads. At step 410, the microprocessor
`addresses the high voltage converter 210 to command it
`to start charging the defibrillation capacitor 200 to the
`selected initial voltage (address “select HV” 212).
`At step 412, the microprocessor starts a polling loop
`by reading “EOC” 214. “EOC” is the end-of-convert
`signal from the high voltage converter 210 and signifies
`that the converter has finished charging the capacitor
`200 to the selected voltage. After reading “EOC” at
`step 412, the microprocessor determines if the high
`voltage is ready at 414.
`If the high voltage is not ready, then the microproces-
`sor loops back to step 412. In some implementations, it
`may be desirable for the microprocessor to attend to
`other tasks or to be disabled for periods to conserve
`current during polling loops. If, at step 414, the micro-
`processor finds that the defibrillator capacitor 200 is
`charged to the selected initial voltage, then the defibril-
`lator system is prepared for delivery of a positive pulse.
`The pulse width is determined by the length of time
`required for the defibrillator capacitor to decay to a
`selected decay voltage. If, in this illustrative example,
`the selected peak voltage is 500 volts, then 200 volts
`would be a reasonable target voltage for the trailing
`
`6
`edge voltage of the positive pulse to assure an effective
`negative phase.
`At step 416, the microprocessor manipulates the con-
`trols of two multiplexers to set the output stage to termi—
`nate the pulse when the selected trailing edge is de—
`tected on the defibrillation capacitor 200.
`Multiplexer 212 selects the signal flow to either gen-
`erate a positive pulse or a negative pulse, At step 416,
`the microprocessor addresses “+/—— select” 213 to
`choose a positive pulse. Multiplexer 214 selects the
`signal flow to either produce a pulse with a timed dura-
`tion or a pulse which terminates when a selected decay
`voltage is detected on the defibrillator capacitor 200. At
`step 416, the microprocessor addresses “time/voltage
`select” 215 to choose a pulse which terminates when a
`selected decay voltage is detected.
`The positive pulse is started by the microprocessor at
`step 418 by addressing “Trailing voltage select” 222 and
`setting the selected trailing edge voltage to 200 volts (in
`this example). Since the voltage on the defibrillator
`capacitor 200 is at 500 volts, the output 221 of the trail-
`ing voltage detector 220 goes high. This signal 221 goes
`through multiplexer 214 to line 223 and through multi-
`plexer 212 to the positive pulse input 225 of the biphasic
`output stage 240 which generates a positive defibrilla-
`tion output as long as positive pulse input 225 is as—
`serted.
`
`Once the positive pulse is started, the voltage on the
`defibrillator capacitor starts to decline as current flows
`into the patient’s heart 290. Trailing voltage detector
`220 maintains signal 221 high until the voltage on the
`defibrillator capacitor 200 has decayed to less than the
`trailing voltage selected by address 222. In this example,
`when the capacitor voltage decays to 200 volts, the
`trailing voltage detector 220 responds by forcing its
`output 221 low. This signal goes through 214, 223, and
`212 to the positive pulse input control 225 of the bipha—
`sic output stage 240, terminating the positive pulse.
`While the positive pulse is being generated, the mi—
`croprocessor waits in a polling loop for the pulse to end.
`The microprocessor reads “EOP” at step 420. “EOP” is
`the end-of-pulse signal and is the same as line 223 dis-
`cussed above. As long as the pulse is being generated,
`“EOP” is high; when the-pulse is over “BOP” goes
`low. Having read “EOP” at step 420, the microproces-
`sor checks to see if the pulse is over at step 422. If the
`pulse is not over, then the microprocessor loops back to
`step 420. When the positive pulse ends,
`the micro-
`processor sets up the hardware to produce the negative
`pulse which is to have a duration related to the positive
`pulse (in this example, the negative pulse will be set
`equal in duration to the positive pulse).
`Since the positive phase pulse was terminated by the
`capacitor 200 reaching a selected decay voltage (200
`volts in this example), the pulse duration is dependant
`upon the impedance of the patient’s heart. For example,
`a comparatively low impedance of 25 ohms would re-
`sult in a shorter pulse duration of about 3.4 milliseconds
`(for a 150 microfarad capacitor 200), while a 50 ohm
`patient impedance would result in a pulse duration of
`6.8 milliseconds.
`
`10
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`30
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`
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`65
`
`Once the positive pulse is over, at step 430 the micro-
`processor addresses the pulse width counter 230 (ad—
`dress “pulse width read” 232) to determine the positive
`phase pulse duration. The pulse width counter 230 mea-
`sures the duration of “EOP” 223. Thus, the address
`“pulse width read” 232 contains the duration of the
`
`7
`
`

`

`5,352,239
`
`8
`The microprocessor reads “BOP” at step 440. As long
`as the pulse is being generated, “EOP” is high; when the
`pulse is over, “EOP” goes low. Having read “EOP” at
`step 440, the microprocessor checks to see if the pulse is
`over at step 442. If the pulse is not over, then the micro—
`processor loops back to step 440. When the negative
`pulse ends, the microprocessor exits the program flow
`at step 490.
`As should be apparent, many combinations of se-
`lected constant tilt and selected constant duration (or
`related duration) multiphasic waveforms can be pro-
`duced under microprocessor control using the appara-
`tus disclosed above.
`Thus, it should be understood that various alterna-
`tives to the embodiments of the invention described
`herein may be employed in practicing the invention. It
`is intended that the following claims define the scope of
`the invention and that methods and apparatus within
`the scope of these claims and their equivalents to cov-
`ered thereby.
`What is claimed is:
`1. An implantable medical device utilizable for deliv-
`ering a configurable biphasic defibrillation waveform to
`a heart, the medical device comprising:
`(a) charge storage means;
`(b) charging means for charging the storage means to
`an initial selected voltage;
`(c) control means for initiating delivery of a first
`defibrillation pulse of a first polarity to the heart
`when the storage means stores the initial selected
`voltage and for initiating delivery of a second defi—
`brillation pulse of a second polarity to the heart
`when the voltage of the charge storage means de-
`cays to a programmed decay voltage, said control
`means including timer means for measuring the
`duration of the first defibrillation pulse and means
`for delivering said second defibrillation pulse hav-
`ing a duration related to the measured duration of
`the first defibrillation pulse; and
`(d) programmable disable means for terminating the
`first defibrillation pulse when the voltage of the
`charge storage means decays to the programmed
`decay voltage.
`2. An implantable medical device as in claim 1
`wherein the duration of the second defibrillation pulse is
`programmable.
`*
`it:
`*
`*
`*
`
`7
`positive pulse. The microprocessor stores the duration
`of the positive pulse width for future use.
`At step 432, the microprocessor manipulates the con—
`trols of two multiplexers 212 and 214 to set the output
`stage to produce a negative pulse with a timed duration.
`Multiplexer 212 selects the signal flow to either gener-
`ate a positive pulse or a negative pulse. At step 432, the
`microprocessor addresses “+/~— select” 213 to choose
`a negative pulse. Multiplexer 214 selects the signal flow
`to either produce a pulse with a timed duration or a
`pulse which terminates when a selected decay voltage is
`detected on the defibrillator capacitor 200. At step 432
`the microprocessor addresses “time/voltage select" 215
`to choose a pulse with a timed duration.
`The negative pulse is started by the microprocessor at
`step 434 by writing to the pulse width timer 250 at
`address “pulse width select” 252) The pulse width timer
`produces a pulse of a duration which the microproces-
`sor sets by writing a value to address “pulse width se-
`lect” 252. In this example, the microprocessor makes
`the duration of the negative phase the same as the dura-
`tion of the positive phase. To do this, the microproces-
`sor writes into the pulse width timer 250 the value of the
`positive phase duration which it read from “pulse width
`read” 232 and stored. If the microprocessor was to
`make the negative phase twice the duration of the posi-
`tive phase, then the microprocessor would multiply by
`two the positive phase duration (which it read from
`“pulse width read” 232 and stored) before writing it
`into the pulse width timer 250. As should be clear, the
`negative phase duration can be made related in any
`mathematical way to the positive phase duration by
`manipulating the data representation of the positive
`phase duration read from “pulse width read” 232.
`By writing to the pulse width timer 252, at step 434,
`the microprocessor starts the negative pulse. The pulse
`width timer 250 produces a pulse the duration of which
`is set by the data the microprocessor wrote to address
`“pulse width select” 252 (which is equal to the positive
`pulse duration read from address “pulse width read”
`232 in this example). The pulse from the pulse width
`timer 250 passes through multiplexer 214 and multi—
`plexer 212 to the negative pulse input 226 of the bipha-
`sic output stage 240. The biphasic output stage 240
`applies the negative phase output to the heart 290 for as
`long as its input 226 is asserted.
`While the negative pulse is being generated, the mi-
`croprocessor waits in a polling loop for the pulse to end.
`
`5
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`10
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`15
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`20
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`25
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`30
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`35
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`4O
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`45
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`50
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`55
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`60
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`65
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`8
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`