US005176137A
`t:5)
`United States Patent
`5,176,137
`[11] Patent Number:
`Erickson et al.
`
`[45] Date of Patent: |. Jan. 5, 1993
`
`CACTI EAA
`
`[54] APPARATUS FOR DISCRIMINATION OF
`STABLE AND UNSTABLE VENTRICULAR
`TACHYCARDIA AND FOR TREATMENT
`THEREOF
`
`[75]
`
`[73]
`
`[21]
`
`[22]
`
`[51]
`[52]
`[58]
`
`[56]
`
`Inventors: Mark K. Erickson, Minneapolis; Tom
`D. Bennett, Shoreview, both of
`Minn.
`
`Assignee: Medtronic, Inc., Minneapolis, Minn.
`
`Appl. No.: 663,985
`
`Mar. 1, 1991
`Filed:
`Int, CUS oo eeececsccsceesesesseeesesereessecenees AGIN 1/39
`
`S.C).
`cccccsescssestccscssceesssersarsaesseceeness 128/419 D
`Field of Search ou... 128/419 D, 419 PG
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3/1987
`Re. 27,757
`6/1973
`3,737,579
`9/1978
`4.114.628
`4,240,442 12/1980
`4,291,699
`9/1981
`4,375,817
`3/1983
`4,384.585
`5/1983
`4,523,595
`6/1985
`4,548.209 10/1985
`4.577,633
`3/1986
`4,587,970
`53/1986
`4,693,253
`9/1987
`4,726.380
`2/1988
`4,750,495
`6/1988
`4.774.950 10/1988
`4.800.883
`1/1989
`4.817.634
`4/1989
`4,819,643
`4/1989
`4,830,006
`5/1989
`4,880,004 11/1989
`4,880,005 11/1989
`4,949,719
`8/1990
`4.953.551
`9/1990
`4,967.748 11/1990
`4,967,749 11/1990
`
`.
`
`.
`
`.
`
`.
`
`.
`
`.
`
`.
`
`Mirowski
`Bolduc .
`Rizk .
`Andresen et al.
`Geddeset al.
`.
`Engle et al.
`.
`Zipes .
`.
`Zibell
`Wielders et al.
`Berkovits et al.
`Holleyet al.
`.
`Adams .
`Vollmann et al.
`Moore et al.
`.
`Cohen .
`Winstrom .
`Holleman et al.
`Menken .
`.
`Haluska et al.
`Baker. Jr. et al.
`Pless et al.
`.
`Pless et al.
`.
`Mehra et al.
`Cohen .
`Cohen .
`
`.
`
`matic and Completely Implanted System”, Transac-
`tions American Society for Artificial Internal Organs,
`16:207 1970, by Schuderet al.
`“Automatic
`Implantable Cardioverter-Defibrillator
`Structural Characteristics”, PACE, vol. 7, Nov.-Dec.
`1984, Part II, pp. 1331-1334, by Moweretal.
`“Reliable R-Wave Detection from Ambulatory Sub-
`jects”, by Thakor, Biomed Sci Instrum 14:67-72, 1978.
`“Relationship Between Right Atrial and Mixed Venous
`Oxygen Saturation and Heart Rate During Exercise in
`NormalSubjects and Patients with Cardiac Disease” by
`a Frenchetal.
`
`Primary Examiner—William E. Kamm
`Attorney, Agent, or Firm—Reed A. Duthler,; Harold R.
`Patton
`
`ABSTRACT
`[57]
`A method and apparatus for discriminating between
`stable and unstable ventricular tachycardias based on a
`measurement of oxygen saturation. Reference values
`for average oxygen saturation level and of the pulsatile
`characteristic of the oxygen saturation level are taken
`while a patient is in a resting condition. Measurements
`of oxygen saturation are also taken in response to the
`detection of a high heart rate, and are compared to the
`reference measurements to discriminate between stable
`and unstable ventricular tachyarrhythmias. An unstable
`tachyarrhythmia is diagnosed if a tachycardia is diag-
`nosed and the current oxygen saturation level is similar
`to the reference average oxygen saturation level or if
`the oxygen saturation as presently measured displays a
`decreased pulsatile characteristic as compared to the
`reference measurements. The discriminator is intended
`for use in conjunction with or as part of an automated
`cardioverter of the type capable of delivering differing
`therapies for termination of stable and unstable ventric-
`ular tachyarrhythmias, such as antitachycardia pacing,
`cardioversion and defibrillation.
`
`
`
`OTHER PUBLICATIONS
`20 Claims, 7 Drawing Sheets oo“Experimental Ventricular Defibrillation with an Auto-
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`U.S. Patent
`
`Jan. 5, 1993
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`Sheet 1 of 7
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`5,176,137
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`ECG
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`OoSAT
`
`FIG.|
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`2
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`U.S. Patent
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`Jan. 5, 1993
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`Sheet 2 of 7
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`5,176,137
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`3
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`4
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`U.S. Patent
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`Jan. 5, 1993
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`Sheet 4 of 7
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`5,176,137
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`U.S. Patent -—Sheet 5 of 7Jan. 5,1993 5,176,137
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`
`
`RST VT COUNT
`
`COMPUTE
`AVO2, AVPO2
`
`VT THERAPY
`
`6
`
`

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`U.S. Patent
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`Jan, 5, 1993
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`Sheet 6 of 7
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`5,176,137
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`REF
`UPDATE
`TIMEOUT
`
`
`
`
`
`COMPUTE
`REFO2,REFPO2
`
`RESET REF
`UPDATE TIMER
`
`
`
`FIG.6
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`7
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`

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`U.S. Patent
`
`Sheet 7 of 7
`
`Jan. 5, 1993
`
`5,176,137
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`8
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`

`

`1
`
`5,176,137
`
`APPARATUS FOR DISCRIMINATION OF STABLE
`AND UNSTABLE VENTRICULAR TACHYCARDIA
`AND FOR TREATMENT THEREOF
`
`2
`a preselected numberof beats. As stated above, other
`researchers had suggested the rate of changeofrate or
`suddenness of onset, rate stability and sustained high
`rate as additional criteria to distinguish among various
`types of tachyarrhythmias.
`Very generally, the systems that depend upon the
`aforementioned rate criteria are capable of discriminat-
`ing tachycardia in greater or lesser degree from normal
`heart activity but can have difficulty in discriminating
`ventricular tachycardias from supraventricular tachy-
`cardias in somecasesorin discriminating stable tachy-
`cardias from unstable tachyarrhythmias in which car-
`diac function is compromised. These difficulties may
`result in delivery of inappropriate antitachycardia pac-
`ing, cardioversion or defibrillation therapies.
`Very recently, the concept of employing a physio-
`logic sensor in conjunction with ECG analysis as a
`method of tachyarrhythmia detection andidentification
`has been revived. Use of pressure sensors is addressed in
`U.S. Pat. Nos. 4,744,950 and 4,967,749, both issued to
`Cohen. The use of an oxygen saturation sensor is ad-
`dressed in U.S. Pat. No. 4,967,748, also issued to Cohen.
`
`BACKGROUNDOF THE INVENTION
`This invention relates to implantable stimulators gen-
`erally and more particularly to implantable cardiover-
`ters and defibrillators.
`Early automatic tachycardia detection systems for
`automatic implantable cardioverter/defibrillators relied
`upon the presenceor absence ofelectrical and mechani-
`cal heart activity (such as intramyocardial pressure,
`blood pressure,
`impedance, stroke volume or heart
`movement) andor the rate of the electrocardiogram to
`detect hemodynamically compromising ventricular
`tachycardia orfibrillation.
`For example,
`the 1961 publication by Dr. Fred
`Zacouto, Paris, France, entitled, “Traitement D’Ur-
`gence des Differents Types de Syncopes Cardiaques du
`Syndrome de Morgangni-Adams-Stokes” (National
`Library of Medicine, Bethesda, MD) describes an auto-
`matic pacemaker and defibrillator responsive to the
`presence or absence of the patient's blood pressure in
`conjunction with therate of the patient’s electrocardio-
`In the context of an automatic implantable device for
`gram to diagnose and automatically treat brady and
`treating bradyarrhythmias, tachyarrhythmias and fibril-
`tachyarrhythmias. Later detection algorithms proposed
`lation, the present invention comprises a method and
`by Satinsky, ‘Heart Monitor Automatically Activates
`apparatus for reliable discrimination of stable ventricu-
`Defibrillator”, Medical Tribune, 9, No. 91:3, Nov. 11,
`lar tachycardias from unstable ventricular tachycardias
`1968, and Shuderet al ‘Experimental Ventricular Defi-
`and fibrillation. The discriminator of the present inven-
`brillation with an Automatic and Completely Implanted
`tion employs an oxygen saturation sensor.
`System”, Transactions American Society for Artificial
`The inventors of the present application have deter-
`Internal Organs, 16:207, 1970, automatically detected
`mined that an oxygen saturation sensor maybe benefi-
`and triggered defibrillation when the amplitude of the
`cially employed in two fashions to distinguish stable
`R-waveof the electrocardiogram fell belowa predeter-
`ventricular tachycardia during which the heart is not
`mined threshold over a predetermined period oftime.
`hemodynamically compromised from unstable ventric-
`Theinitial system proposed by Mirowskiet al in U.S.
`ular tachycardia and fibrillation,
`in which hemody-
`Pat. No. Re 27,757, which relied upon the decrease in
`namic functioning is either absent or severely reduced.
`the amplitude of a pulsatile right ventricular pressure
`The inventors of the present application have deter-
`signal belowa threshold over a predetermined period of
`mined by continuous oxygen saturation measurements
`time, was abandoned by Mirowskiet al in favor of the
`that the oxygen saturation of venous return blood as
`rate and/or probability density function morphology
`measured within the right ventricle of the heart displays
`discrimination as described in Moweret al, “Automatic
`a pulsatile characteristic when normal hemodynamic
`
`Implantable Cardioverter-Defibrillator|Structural
`45
`functioning is present. In the absence of normal hemo-
`Characteristics”, PACE, Vol. 7, November-December
`dynamic functioning, the oxygen saturation level ceases
`1984, Part II, pp. 1331-1334.
`to display this pulsatile characteristic, and this change
`Sensing of the mechanical pumping action of the
`heart in conjunction with loss of ability to pace the
`can be used to diagnose the presence of an unstable
`heart as a method offibrillation detection is taught in
`ventricular tachycardia orfibrillation, and for trigger-
`U.S. Pat. No. 4,144,628, issued to Rizk. Monitoring the
`ing delivery of a defibrillation pulse. In addition, the
`intracardiac impedance to detect pumping action in
`inventors have determined that while the measured
`conjunction with ECGanalysis is taught as a method of
`oxygen saturation level in venous return blood within
`fibrillation detection in U.S. Pat. No. 4,291,699, issued
`the right ventricle of the heart drops substantially in the
`to Geddesetal.
`presenceofa sinus tachycardia or other stable ventricu-
`Morerecently, others have suggested the use of high
`lar tachycardia in which hemodynamic functioning
`rate plus acceleration of rate or “onset” (U.S. Pat. No.
`persists, the presenceofa stable, invariant oxygen satu-
`4,384,585) with sustained high rate and rate stability
`ration level is to be expected during periods of hemody-
`(U.S.Pat. No. 4,523,595) to distinguish among tachyarr-
`namic compromise. This characteristic too can be used
`hythmias, As stated in the article “Automatic Tachy-
`alone, or in conjunction with EKGanalysis to detect
`cardia Recognition”, by R. Arzbaecher et al, PACE,
`the presence of a tachyarrhythmia requiring delivery of
`May-June 1984, pp. 541-547, anti-tachycardia pace-
`a defibrillation pulse. Conversely,
`in the absence of
`makers that were undergoing clinical studies prior to
`either of these two characteristics, delivery of an-
`the publication of that article detected tachycardia by
`titachycardia pacing or cardioversion pulses may be
`indicated.
`sensing a high rate in the chamber to be paced. The
`specific criteria to be met before attempting tachyarr-
`hythmia termination by pacing involved a comparison
`of the detected heart rate to a preset threshold, such as
`150 beats per minute (400 millisecond cycle length) for
`
`SUMMARYOF THE INVENTION
`
`20
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`25
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`30
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`35
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`40
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`50
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`55
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`65
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`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The above andstill further objects, features and ad-
`vantagesof the present invention will become apparent
`
`9
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`

`

`5,176,137
`
`3
`from the following detailed description of a presently
`preferred embodiment, taken in conjunction with the
`accompanying drawings, and, in which:
`FIG. 1 is an ECGtracing, illustrating the theory
`underlying the present invention.
`FIG. 2 illustrates one embodiment of a lead system
`for cardiac pacing, cardioversion and defibrillation
`which may be used in conjunction with the present
`invention.
`
`20
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`25
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`30
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`35
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`40
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`45
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`50
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`FIG. 3 illustrates another embodimentofa lead sys-
`tem for cardiac pacing, cardioversion and defibrillation
`which may be used in conjunction with the present
`invention.
`FIG.4 is a schematic block diagram illustrating the
`structure of one embodimentof an implantable pacema-
`ker/cardioverter/defibrillator in which the present in-
`vention may be embodied.
`FIGS. 5, 6 and 7 are functional flow chartsillustrat-
`ing methods of discrimination between unstable and
`stable ventricular tachycardia provided bythe present
`invention, andillustrating the operation of the discrimi-
`nator of the present invention as embodied in a micro-
`processor based device.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`FIG. 1 is a simulated ECG strip and associated oxy-
`gen saturation tracingillustrating the theory underlying
`the present invention. The ECGtracing illustrates two
`R waves, and the interval therebetween,
`labeled “1”
`can be measured to determine the cycle length of the
`cardiac rhythm. This correspondsto the primary mech-
`anism for monitoring heart rate used in prior art im-
`plantable cardioverters and defibrillators. The oxygen
`saturation tracing, labeled O2SATillustrates the analog
`output of a two wavelength, reflectance oximeter as
`disclosed in U.S. Pat. No. 4,750,495, issued to Mooreet
`al located within the right ventricle of the heart, and
`operating continuously to measure oxygen saturation.
`Ascan beseen in this figure, during normal heart func-
`tioning, the oxygen saturation signal is pulsatile, having
`a minimum saturation percentage 2 and a maximum
`saturation percentage 3 during each cardiac cycle. The
`difference 4 between the minimum and maximum satu-
`tation percentages may be measured and used in con-
`junction with the present invention to detect compro-
`mise of the hemodynamic pumping efficiency of the
`heart,
`in conjunction with high rate tachycardia and
`tachyarrhythmias. For purposes of the present applica-
`tion,
`this measured difference 4 is referred to as the
`oxygen saturation pulse amplitude. In the presence of
`hemodynamic compromise, the pulsatile characteristic
`of the O2SATlevel is diminished or disappears alto-
`gether, as flow of blood through the heart ceases.
`In the present invention, the O2SAT signal may be
`monitored and averaged over various time intervals.
`Measurements of the average O2SAT level are taken
`during normal heart rhythm at a rate great enough to
`inhibit
`the cardiac pacemaking function of the im-
`planted pacemaker/cardioverter/defibrillator, but at a
`rate less than that which would indicate either a tachy-
`cardia or strenuous physical exercise. This base, resting
`measurement of oxygen saturation may be used to de-
`fine an average oxygen saturation reference value
`(REFO2) and an average oxygen saturation pulse am-
`plitude (REFPO2). REFO2 may be compared against
`the current average level of oxygen saturation (AVQ2).
`If the presently measured level of oxygen saturation is
`
`4
`not significantly less than this reference level, and the
`sensed cycle length is such that presence of a tachycar-
`dia is indicated. the device of the present invention may
`determine that the tachycardia is unstable, and trigger
`delivery of a defibrillation pulse.
`Alternatively or additionally, REFPO2 may be com-
`pared against
`the current average oxygen saturation
`pulse amplitude (AVPO2). If the presently measured
`oxygen saturation pulse amplitude is significantly less
`than this reference level, and the sensed cycle length is
`such that presence of a tachycardia is indicated, the
`device of the present invention may determine that the
`tachycardia is unstable, and trigger delivery of a defi-
`brillation pulse. Conversely,if the current oxygen satu-
`ration amplitude is significantly less than the reference
`level and/or the current oxygen saturation pulse ampli-
`tude is not significantly less than the reference pulse
`amplitude, the device determines that the detected tach-
`ycardia is stable and triggers delivery of an antitachy-
`cardia pacing therapy or a cardioversion pulse.
`FIG. 2 is a cutaway view of the heart and a plan view
`of an implantable pacing. cardioversion and defibrilla-
`tion lead system. A ventricular defibrillation lead 8
`carrying a bipolar electrode pair located at the right
`ventricular apex. The electrode pair includesa tip elec-
`trode 10, which takes the form of a helical electrode
`screwed into the right ventricular myocardium and a
`ring electrode 12. The lead also includes an elongated
`coiled defibrillation electrode 14. The illustrated lead
`corresponds generally to the ventricular lead described
`in U.S. Pat. No. 5,014,696 by Mehra for an Endocardial
`Defibrillation Electrode System,
`incorporated herein
`by referencein its entirety. but other defibrillation leads
`mayalso be employed.
`The lead system also comprises a coronarysinus lead
`6 and mayoptionally include a subcutaneouslead, not
`illustrated. The coronary sinus lead 6 is provided with
`an elongated electrode located in the coronary sinus
`and great veinin the regionillustrated by broken outline
`at 16, extending around the heart until approximately
`the point at which the great vein turns downward,
`toward the apex of the heart. Lead 6 may also corre-
`spond to the coronary sinus lead disclosed in U.S. Pat.
`No. 5,014,696 by Mehra, cited above. If a subcutaneous
`electrode is also used, it generally will be implanted in
`the left chest and may correspondto the electrodeillus-
`trated in U.S. Pat. No. 5,044,374, by Lindemansetal.
`for a Medical Electrical Lead, filed July 7, 1989 and
`incorporated herein by referencein its entirety.
`Also included is an oxygen saturation sensing lead 18,
`which carries a two wavelength reflectance oximeter
`20, mounted adjacenta pliant distal end member22. For
`purposes of the present invention, the sensor is prefera-
`bly located in the right ventricle. The oxygen sensor
`maycorrespondto the sensor disclosed in U.S. Pat. No.
`4,750,495, issued June 14, 1988 to Mooreetal, incorpo-
`rated herein by reference in its entirety. This lead is
`provided with two mutually insulated conductors,illus-
`trated schematically at 24, 26. The sensor 20 includes an
`oscillator which sequentially activates red and infrared
`diodes. The duty cycle ofthe oscillator is regulated by
`the relative amounts of red and infrared light reflected
`by the blood. The duty cycle of the oscillator is re-
`flected by modulation of the current drawnbythe oscil-
`lator over conductors 24,26, and thus providesa signal
`which can be demodulated to provide a measurement of
`oxygen saturation. While sensor 20 is shown mounted
`on a separate lead, it may also be incorporated on a lead
`
`10
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`5,176,137
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`35
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`5
`6
`such as lead 8, mounted distal to defibrillation electrode
`Electrodes 500 and 502 are coupled to the R-wave
`14. Alternatively, sensor lead 18 may be provided with
`detector circuit, comprising bandpass filter circuit 514,
`an electrode or electrodesatits distal end which may be
`and automatic gain control circuit 516 for providing an
`used for pacing and sensing in the ventricle.
`adjustable sensing threshold as a function of the mea-
`FIG. 3 is a view of the heart and a plan view of an
`sured R-wave amplitude and a comparator 518. A signal
`alternative implantable pacing, cardioversion and defi-
`is generated on R-out
`line 564 whenever the signal
`sensed between electrodes 500 and 502 exceeds the
`brillation head system which may be employed in con-
`junction with the present invention. This figure illus-
`present sensing threshold defined by the automatic
`threshold adjustment circuit 516. Asillustrated, the gain
`trates an epicardial electrode system of the sort that
`on the band pass amplifier 514 is also adjustable by
`may be present in patients who previously have had
`means of a signal from the pacer timing and control
`implantable cardioverter/defibrillators implanted. The
`circuitry 520 on GAIN ADJline 566.
`electrode system includes two enlarged epicardial elec-
`The operation ofthis R-wave detection circuitry may
`trodes 50, 52, and a third such electrode located on the
`correspond to that disclosed in commonly assigned,
`backside of the heart, not visible in this view. These
`copending U.S. patent application Ser. No. 07/612,760,
`electrodes are coupled to the implantable cardiover-
`by Keimel, et al., filed Nov. 15, 1990 for an Apparatus
`ter/defibrillator by means of insulated conductors 54,
`for Monitoring Electrical Physiologic Signals, incorpo-
`56 and 58. The epicardial electrodes may correspond to
`rated herein by referencein its entirety. However,alter-
`those disclosed in U.S. Pat. No. 4,817,634,
`issued to
`native R-wave detection circuitry such as that
`illus-
`Hollemanet al. on Apr. 4, 1989, incorporated herein by
`trated in U.S. Pat. No. 4,819,643, issued to Menken on
`referencein its entirety. Also included are two epicar-
`Apr. 11, 1989, U.S. Pat. No. 4,880,004, issued to Baker
`dial pacing/sensing electrodes 60 and 62, mounted adja-
`et al on Nov. 14, 1989 and U.S. Pat. No. 4,240,442,
`cent the apex of the heart and coupled to the implant-
`issued to Andresen et al on Dec. 13, 1980, all incorpo-
`able cardioverter/defibrillator by means of insulated
`rated herein by reference in their entireties, may also
`conductors 64 and 66. These epicardial leads may corre-
`usefully be employed to practice the present invention.
`spond to the leads disclosed in U.S. Pat. No. 3,737,579,
`For purposes of the present application, it should be
`issued to Bolduc on June 5, 1973, also incorporated
`understood that the threshold adjustment circuit 516
`herein by reference in its entirety.
`sets a threshold corresponding to a predetermined per-
`An oxygen sensing lead 68 is also shown, with its
`centage of the amplitude of a sensed R-wave, which
`location indicated by broken line. Its distal
`tip 70 is
`threshold decays to a minimum threshold level over a
`located in the apex ofthe right ventricle, in this embodi-
`period of less than three seconds thereafter, similar to
`ment, and the oxygen sensor thereon is also located
`the automatic sensing threshold circuitry illustrated in
`within the right ventricle. The lead is provided with
`the article ‘Reliable R-Wave Detection from Ambula-
`two mutually insulated conductorsillustrated schemati-
`tory Subjects”, by Thakoret al, published in Biomedi-
`cally at 72 and 74 and otherwise corresponds exactly to
`cal Science Instrumentation, Vol. 4, pp 67-72, 1978,
`lead 18 illustrated in FIG. 2.
`incorporated herein by referencein its entirety. How-
`FIG. 4 is a functional schematic diagram of an im-
`ever, in the context of the present invention,it is prefer-
`plantable
` pacemaker/cardioverter/defibrillator
`in
`able that the threshold level not be adjusted in response
`which the present invention may usefully be practiced.
`to paced R-waves, but instead should continue to ap-
`This diagram should be taken as exemplary of the type
`proach the minimum threshold level following paced
`of device in which the invention may be embodied, and
`R-waves to enhance sensing of low level spontaneous
`not as limiting, as it is believed that the invention may
`R-waves associated with tachyarrhythmias. The time
`usefully be practiced in a wide variety of device imple-
`constant of the threshold circuit is also preferably suffi-
`mentations, including devices having functional organi-
`ciently short so that minimum sensing threshold may be
`zation similar to any of the implantable pacemaker/defi-
`reached within 1-3 seconds following adjustmentof the
`brillator/cardioverters presently being implanted for
`sensing threshold equal to 70-80% of the amplitude of a
`clinical evaluation in the United States. The invention is
`detected spontaneous R-wave. The invention mayalso
`also believed practicable in conjunction with implant-
`be practiced in conjunction with more traditional R-
`able pacemaker/cardioverters/defibrillators
`as
`dis-
`wavesensors of the type comprising a band pass ampli-
`closed in prior U.S. Pat. No. 4,548,209, issued to Wield-
`fier and a comparator circuit to determine when the
`ers, et al on Oct. 22, 1985, U.S. Pat. No. 4,693,253,
`bandpassed signal exceeds a predetermined,fixed sens-
`issued to Adamset al on Sept. 15, 1987, U.S. Pat. No.
`ing threshold.
`4,830,006, issued to Haluska et al on May 6, 1989 and
`Switch matrix 512 is used to select which ofthe avail-
`U.S. Pat. No. 4,949,730, issued to Pless et al on Aug.21,
`able electrodes make up a second electrodepair for use
`1990, all of which are incorporated herein by reference
`in sensing the electrical activity of the heart. The sec-
`in their entireties.
`ond electrode pair may comprise electrode 502 or 500 in
`The device is illustrated as being provided with six
`conjunction with electrode 504, 506, 508 or 510, or may
`electrodes, 500, 502, 504, 506, 508 and 510. Electrodes
`comprise other combinations of the illustrated elec-
`500 and 502 maybea pair ofelectrodes located in or on
`trodes. Selection of electrodes are controlled by the
`the ventricle, for example, corresponding to electrodes
`microprocessor 524 via data/address bus 540. Signals
`10 and 12 in FIG. 2. Electrode 504 may correspond to
`from the selected electrodes are passed through band-
`a remote, indifferent electrode located on the housing of
`pass amplifier 534 and into multiplexer 532, where they
`the implantable pacemaker/cardioverter/defibrillator.
`may be converted to multibit digital signals by A/D
`Electrodes 506, 508 and 510 may correspond to the
`converter 530, for storage in random access memory
`large surface area electrodes located on the ventricular
`526. Preferably, a portion of random access memory
`and coronarysinus Jeads 6 and8illustrated in FIG. 2, in
`526 is configured as a looping or buffer memory which
`conjunction with a subcutaneouselectrode or may cor-
`can store at least the preceding several seconds of the
`respond to the epicardial electrodes of FIG. 3.
`ECGsignal. The occurrence of an R-wavedetectsignal
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`5,176,137
`
`7
`on line 564 is communicated to microprocessor 524 via
`data/address bus 540, and microprocessor 524 notes the
`time of its occurrence.
`If the second electrode pair is used, microprocessor
`524 waits 100 milliseconds following the occurrence of
`the R-wave detect signal, and thereafter transfers the
`most recent 200 millisecondsof digitized ECG stored in
`the looping or buffer memory portion of the random
`access memory circuit 526 to a second memory loca-
`tion, where the contents may be digitally analyzed
`under control of direct memory access circuit 528. Mi-
`croprocessor 524 may analyzethe digitized ECGsignal
`stored in random access memory 526 to identify prede-
`termined characteristics of the stored waveforms in
`conjunction with tachyarrhythmia detection and dis-
`crimination functions. For example, the point of maxi-
`mum slope, the width or the area under each recorded
`R-wave maybe determined.
`Most of the remainderof the circuitry is dedicated to
`the provision of cardiac pacing, cardioversion and defi-
`brillation therapies. The pacer timing/control circuitry
`520 includes programmable digital counters which con-
`trol the basic time intervals associated with VVI mode
`cardiac pacing, including the pacing escape intervals,
`the refractory periods during which sensed R-wavesare
`ineffective to restart timing of the escape intervals and
`the pulse widths of the pacing pulses. The durations of
`these intervals are determined by microprocessor 526,
`and are communicated to the pacing circuitry 520 via
`address/data bus 540. Pacer timing/control circuitry
`520 also determines the amplitude of the cardiac pacing
`pulses and the gain of bandpass amplifier, under control
`of microprocessor 524.
`the escape interval
`During VVI mode pacing,
`counter within pacer timing/control circuitry 520 is
`reset upon sensing of an R-waveas indicated bya signal
`on line 564, and on timeout triggers generation of a
`pacing pulse by pacer output circuitry 522, which is
`coupled to electrodes 500 and 502. The escape interval
`counteris also reset on generation of a pacing pulse, and
`thereby controls the basic timing ofall cardiac pacing
`functions, including anti-tachy pacing. The duration of
`the interval defined by the escape interval timer is deter-
`mined by microprocessor 524, via data/address bus 540.
`The value of the count present in the escape interval
`counter when reset by sensed R-waves may be used to
`measure the duration of R—R intervals, to detect the
`presence of tachycardia and to determine whether the
`minimum ratecriteria are met for activation of the dis-
`crimination function.
`Microprocessor 524 operates as an interrupt driven
`device, and is awakened by interrupts from pacerti-
`ming/control circuitry 520 corresponding to the occur-
`rence of sensed R-waves and correspondingto the gen-
`eration of cardiac pacing pulses. These interrupts are
`provided via data/address bus 540. Any necessary
`mathematical calculations to be performed by micro-
`processor 524 and any updating of the values orinter-
`vals controlled by pacer timing/control circuitry 520
`take place following such interrupts.
`While not disclosed in detail herein, it is of course
`possible to employ the measurement of oxygen satura-
`tion to regulate the ventricular escape interval to pro-
`vide rate responsive pacing. In such case, the micro-
`processor 224 could activate sensor 582 oncein re-
`sponse to each sensed R-wave and each ventricular
`pacing pulse, except during delivery of antitachyarr-
`hythmia therapies. The measurementsso obtained could
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`8
`be used to calculate the WVIR escape interval as dis-
`closed in the above cited Mooreet al patent.
`Wheninterrupted in responseto delivery ofa ventric-
`ular pacing pulse or in response to sensing of a ventricu-
`lar contraction, microprocessor 524 stores the duration
`of the measured R—Rinterval in random access mem-
`ory 526. Preferably, a portion of memory 526is config-
`ured as a recirculating buffer capable of storing the
`R-—R intervals measured over the preceding 5 to 30
`minutes. These stored R—-R intervals may be used for
`tachycardia detection. Alternatively, the measured du-
`rations of the R—R intervals may be divided into rate
`classes and counts kept of the relative numbers of the
`preceding sequenceofintervals which fall into the vari-
`ous rate classes, and these counts may be used for ta-
`chyarrhythmia detection andclassification.
`Detection of tachycardia and fibrillation, may corre-
`spond to any tachycardia andfibrillation detection algo-
`rithms knownto the art. For example, presence of tach-
`ycardia may be confirmed by means of a measurement
`of averagerate,sustainedrate, rapid onset, rate stability,
`or a numberof other factors knownto the art. Detec-
`tion algorithms for recognizing fibrillation and tachy-
`cardias are described in the above cited U.S. Pat. No.
`4,726,380, issued to Vollmann, U.S. Pat. No. 4,880,005,
`issued to Pless et al and U.S. Pat. No. 4,830,006, issued
`to Haluska et al. An additional set of tachycardia and
`fibrillation recognition methodologies is disclosed in the
`article “Onset and Stability for Ventricular Tachyarr-
`hythmia Detection in an Implantable Pacer-Cardiovert-
`er-Defibrillator” by Oison et al., published in Computers
`in Cardiology. Oct. 7-10, 1986, IEEE ComputerSociety
`Press, pages 167-170. However, one of the advantages
`of the present inventionis that it is believed practicable
`in conjunction with virtually any prior art tachycardia
`detection algorithm.
`In the event that a tachyarrhythmia is detected, and
`an antitachyarrhythmia pacing regimen is desired, ap-
`propriate timing intervals for controlling generation of
`antitachy pacing therapies are loaded from micro-
`processor 524 into the pacer timing and controlcir-
`cuitry 520, to control the operation of the escape inter-
`val counter therein and to define refractory periods
`during which detection of an R-wave by the R-wave
`detection circuitry is ineffective to restart the escape
`interval counter. Similarly, in the event that generation
`of a cardioversion or defibrillation pulses required, mi-
`croprocessor 524 employs the escape interval counter
`to control timing of such cardioversion and defibrilla-
`tion pulses, as well as associated refractory periods
`during which sensed R-wavesare ineffective to reset
`the timing circuitry.
`In response to the detection of either fibrillation or a
`tachycardia requiring a cardioversion pulse, micro-
`processor 524 activates cardioversion/defibrillation
`control circuitry 584, which initiates charging of the
`high voltage capacitors 556, 558, 560 and 562 via charg-
`ing circuit 550, under control of high voltage charging
`line 552. The voltage on the high voltage capacitorsis
`monitored via VCAP line 538, which is past through
`multiplexer 532, and, in response to reaching a predeter-
`mined value set by microprocessor 524,results in gener-
`ation of a logic signal on CAP FULLline 542,terminat-
`ing charging. Thereafter, the timing of the defibrillation
`or cardioversion pulse is controlled by pacer timing-
`/control circuitry 520.
`One embodiment of an appropriate system for deliv-
`ery and synchronization of cardioversion and defibrilla-
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`5,176,137
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`9
`tion pulses, and controlling the timing functions related
`to them is disclosed in moredetail i