CA2153338A1

CA2153338A1 - Method and apparatus for detection and treatment of tachycardia and fibrillation

Info

Publication number: CA2153338A1
Application number: CA002153338A
Authority: CA
Inventors: John G. Keimel
Original assignee: Individual
Current assignee: Medtronic Inc
Priority date: 1993-01-29
Filing date: 1993-12-15
Publication date: 1994-08-04
Also published as: US5354316A; AU670996B2; DE69326623D1; DE69326623T2; EP0681498B1; AU5849994A; WO1994016767A1; EP0681498A1; JPH08505793A

Abstract

An apparatus for detecting identifying and treating tach-yarrhythmias. Tachyarrhythmias are detected and identified by the use of overlapping ranges of intervals. Provisional identifica-tion of tachyarrhythmia is accomplished by measuring and track-ing intervals within two overlapping or adjacent interval ranges.
Further classification and identification of tachyarrhythmias is accomplished by determining the relative numbers of intervals within a preceding series falling within a third interval range overlapping one or both of the other interval ranges. In response to identification of the tachyarrhythmia, an appropriate therapy is selected and delivered.

Description

METHOD AND APPARATUS FOR DETECllON AND TRE~TMENT
OF TACHYCARDL~ AND FIBRILL~TION

Ba~l.Y~ d of th~ ~nvention This invention relates to devices which detect and/or treat tacl~ hlllias (rapid heart lL~l~s), and more specifi~lly, to mechA~ ...c to ~l;c~ ..;ch among various tacL~ L~rthll~ias and to provide ap~ iate lLelal,ies to h eat the idçntified tacL~ lh l ias.
Early A~,lo..AI;c tach~ ia detection systems for ~.JIo...Al;c cardioverter/~lefihrill~tors relied upon the presence or ~hsenre of electrical and m~ch~nic~l heart activity (such as i~ aL~l~ocardial ~ress~re, blood pressure, impedance, stroke volume or heart movement) and/or the rate of the electrocardiogram to detect hemodynamically col..~ru..~ g ventricular tac_ycardia or fibrill~hon In pAcçm~lrer/cardioverter/defibrillators presently in clinical cv~ tion~
fibrillAhon is ~;Cl;l~ -Lche~1 from ventricular tac~cardia using rate based criteria, In such devices, it is common to specify the rate or interval ranges that characterize a tachyarrhythmia as opposed to fibrillation. Ho..~er, some p~tien5C may suffe- from venh icular tacl~car~ia and ventricular fibrill~tion which have similar or overlapping rates, m~kine it lifficult to lictin~lich low rate fibrill~tion from high rate tacllycardia.
In addition, ventricular fibrill~tion rnay display R-R intervals which may vary considerably, resulti~ in intervals that may fall within both the tachycardia and fibrill~tion rate or interval ranges, or outside both.
Presently available p~cem~ker-cardioverter- defibrillator allhylhlllia control devices, such as the Model 7216A and 7217IB PCD devices available fro;n Medtronic, InC., employ progr~mm~ble fibrill~tion interval ranges and tachycardia detection interval ranges which are adjacent to one another but do not overlap. In the Medtronic devices in particular, the interval range desi~te~l as indicative of fibrill~tion con~icting of intervals less than a pro~l,...,.n~ble interval ~FDI) and the interval range desi~ted as indicative of ventricular tachycardia consisting of 2~s3338 PCIIUS93/1222~

intervals less than a ~ ble interval (IDI) and ~eatel than or equal to FDI.
R-R intervals falling within these ranges are measured and counted to provide a count (VTEC) of R-R intervals falling within the tac~caldia intenal range and a count (VF~C) of the number intervals, out of a preceding series of a predetermined number (FEB) of intervals, which fall within the fibrillation interval range. VTEC
is incremlq-nted in response to R-R intervals that are greater than or equal to FDI but shorter than TDI, is reset to zero in respol~se to intervals greater than or equal to TDI and is inc.o.-c;~ to intervals less than FDI. VTEC is comr~red to a lJro~"""rA value (VTNID) and VF~C is co...l~red to a col.e~onding pro~;~ ble value (VFNID). When one of the counts equals its co~lcs~onding ogr~ ble value, the device Ai~gnoses the presence of the corresponding arrhythmia, i.e. fibr~ hon or tacl~caldia and delivers an ~plopliate therapy, e.g.
anti-ta~h~ardia pacing, a cardioversion pulse or a defibrillation pulse. In addition, the physician may optionally require that the measured R-R intervals meet a rapid onset criterion before VTEC can be incrementeA and can also optionally require that should a rate stability criterion fail to be met, VTEC will be reset to zero. This detection system has proven effective in distin~liching between fibrillation andventricular ta~hyc~dia so that a~ opliate therapies may be delivered. However, in rare in~t~n~eC~ the detection metho~lology may require a sequence of a greater number of rapid heart beats than might optimally be desired to determine whetherthe rapid rhytbm is due to fibrill~tion or tacl~caldia Moreover, an i~pl~ved level of accuracy in cl~ir~hlg rl~ll~s having intervals close to FDI is also believed desirable. In addition, the ability to provide a se~ar~te therapy set for fast tachycardias as opposed to slower ta~:hy~fdiac or fibrill~tion is also desirable.

Summary of the Invention One object of the present invention is to Aicli"~,.ich fibrillation from t~c~ardia at similar rates, using the ...i.-i...ll,,, number of detected heart depolari7~tionc conc;ctent with available accuracy. An additional object of the invention in its preferred embodiments i to di tin~lich between slow tachycardia WO 94/16767 ' ~ ~; ? 1 5 3 3 3 8 PCTtUS93tl2221 -fast ta~ llycardia and fihrill~tion In its prcfellcd embo~limPntc, the device takes the form of an imrl~nt~ble p~cçm~ker/cardioverter/~çfihr~ tor~ and the invention in these embo~ .L~. also has the object of providing ~her~ies a~r~liate to the ~lPtecte~ tac~alll~llll.-ia The licclosed embo~limP-ntc sense the rhythm of theventricle and provide therapy for the ventricle, but the invention is also believed to be of value in detecting and treating atrial fibrill~fion and tachycaldias.
In accor~ ance with the present invention, it is re~li7e~1 that because of the r~n~lomnecs of sensed intervals betwveen depolari7~fionc (e.g., R-waves) during fibrill~tinn or bec~nse of unccl~;.-lies related to a patient's rhythms, sensed cardiac depolarization intervals during fibrill~tion may have durations which overlap those observed during tacl~cardias. From the pel~c~ive of a device which ~ gnoses all~rll~iasbasedon..~P-~c.~-cdintervals,intervals~çfine~lasin~3ic~tiveofventricular tachycardia, for example, may in fact be occurring during ventricular fibrill~tion The present irlvention provides a method and ap~,a~alus for quiclcly and accurately classi~yiu~ the nature of a tach-y~,l,ylh.llia with intervals near the border between the interval ranges associated with tacl~dia and fibrill~ti(!n The lir~losecl embo~limpntc of the invention operate in the ventricle of the heart and ~cc~n~rlishes idçntific~tion of such lhy~ns using a methodology which ~l-ofines three overlapping interval or rate ranges. Two of the ranges, colle~ollding generally to tacLycaldia and fibrillation are adjacent to or overlap one another. A
third interval range, colle~,onding to fast ventricular tachycardia overlaps one or both of the other two interval ranges. Following the provisional detection or identi~cation of ventricular ta~l,ycardia or fibrillation, the immediately preceding intervals are e,~An.;~rsl to determine how many fall within this third interval range.
If a predetermined number or percent of the immefli~tely preceding series of intervals fall within this third interval range. fast ventric~ r tachycardia is detected or identified and the therapy desi~n~ted for fast ventricular tachycardias is delivered.
If less than the predeterrnined number or percenl fall within the fast ventricular tac~ardia range, the initially detected a ll~ia (fibr~ tion or ventricular WO 94/16767 2 1s3338 PCI/US93/1222~

tac~cardia) i5 c5~ "r~1~ and the progr~mme~l therapy col~esponding to the co~ ...e~l, det~Pcted al~ }~ia is delivered.
In one r~ Qsed ennhotlimPnt of the invention, initial speed of detection or idertific~hQn of fibr~ tion or tac~cardia is increased by employing a combined count of all ~P-~culed intervals falling within the intenal ranges in~lie~tive of fibrill~tion or tacL~cardia and then detel...i~-;ng whether fibrillation or tachycardia is present by ~ ;-~..;..i.~g the l,.opolùons or numbers of recent intervals falling within the ta~:l~c~dia and fibr~ tion interval ranges to provide provisional detection or idPnhfic~tion of fibrill~tion or tacl~cardia. This aspect of the present invention can sub~ lly reduce the ~ cr of intervals required to detect fibrillation or tacl~caldia at rates near the border between the interval ranges associated withtac~ Jia and fibrillation. This provisional detection or identification of fibrillation or tacl~caldia may then be further processed as rlic~lcced above to dictinFlich between fast tac~ca,dia and fibrillation or between slow and fast ventricular tachycardia Brief Descr~tion of thP Dla~
The above and still further objects, fealu,es and advantages of the present invention will bfCo~f a~ e"l from the following ~let~iled description of a presently ~refe"ed emboAim~P-nt, taken in conjlm~tion vith the ~cco...~ g drawings, and, in which:
Figure la is an illustration of the detection interval ranges employed in the iirst ~refelled embo~liment of the present invention;
Figure lb is an illustration of the detection interval ranges employed in the second prcfelled embo-lim~nt of the present invention;
Figure 2 is a simplified block diagram illustrating the components of a device within which the method and appa~al~ls of the present invention may be implemente-l Figures 3a and 3b are simplified flow chart diagrams illustrating the functionaloperation of the ~ieferled embo~limentc of the present invention;

WO 94/16767 :, 21S-3338 PCI/US93/12221 Det~iled n~.ct r~tinn of the r~efe~lcd Embodiment Figure la is an illustration of the overlapping rate ranges which are employed in a plefelled embo~liment of the present invention. The range of intervals which are taken as inrlic~tive of sinus rhythm are those which are greater than or equal to TDI. The range of intervals taken as indicative of tacL~cardia incllldes intervals less than TDL but greater than or equal to FDI and greater than the device's bl~nkinginterval. The range of intervals taken as indicative of fibrillation in~ des intervals less than FDI. In the first embo~l;...enl of the invention, FTDIma,~ is by definitiQn greater than or equal to FDI and FTDImjn is by dçfinition less than or equal to FDI.
If ventric~ r ta~Ly~dia is proviciQn~lly tletecterl intervals less than FTDI~ are taken as in~ tive of fast ventricular tachycardia. If ventricular fibrillation is provision~lly detecte~l, intervals glcater than or equal to FlVIm"~ are taken asindicative of fast ventricular tachycardia.
Figure lb is an illustration of the overlapping rate ranges which are employed in second p,efe~lcd embo!liment of the present invention. The range of intervalswhich are taken as in~lic~tive of sinus rhythm are those which are greater than or equal to TDI. FTDI",~ is by tlefinition greater than or equal to ~ n. The range of intervals taken as in~ tive of tach~ca,dia incl~ldes intervals less than TDI and less than or equal to FTDI,njn. The range of intervals taken as indicative of fibrillation in~ des intervals less than ~ lVI,~ and greater than the device's bl~nking interval. As in the first embo~lim~nt, if ventricular tachycardia is provisionally ~letecte~l intervals less than FTDI,~,~ are taken as in~lic~tive of fast ventricular ta~ycardia~ If ventricular fibrill~tion is provisionally detected, intervals greater than or equal to FTDI",~ are taken as inAic~tive of fast ventricular tachycardia.
In the first embo~liment of the invention licc~lcce~l below, using interval ranges colles~onding to Figure la, the tachycardia and fiblillation detection criteria discussed above in conjunction with the Medtronic Model 7216 and Model 7217 implantable p~cçm~lrer/cardioverter/defibrillators are retained, and used as one set of criteria for provisional detection of tachycardia or fibrill~tion WO 94/16767 PCT/US93/1222~

In the ~ct l,refel,ed embotlimP!nt of the invention, in addition to the fibrill~tion and tachycardia detection criteria tliccllcced above in connecition with the Medtronic Model 7216 and Model 7217, (i.e. VF~C = VFNID or VTEC = VTNID), provLcional tletection of tacl,ycald~a or fibrill~tion detection may also be S zlrCQmrlichçd using a combined count of all intervals intlic~tive of tachycardia or fibr~ tiQn This c~mbine~l count (VF~C + VTEC) is co~ arcd to a combined count threshold (CNID). Lf VTEC ~ VFEC is equal or greater than CNID, the device checks to see whether VF~C is at leact a predetermined number (e.g. 6). If so, the device checks to determine how many of a number (e.g. 8) of the im_ediately prece~ling intervals are greater or equal to FDI. If a predeter_ined number (e.g. 8) are greater than or equal to FDI, tachycal-lia is provicion~lly detected, otherwise ventricular fihrill~tion is provicion~lly ~letected The immetli~tely precelling mP~cnred intervals are then e~ d as tlic~lcsed below to dete~ ine whether the initial ~letection of fibrill~tion or ta~hycardia should be co.~li....ed or ~mended to intlic~te detection of fast ventricular tachycardia.
In the second prefelled embo!limt~nt, provisional detection of ta~l~cardia and fibri tion is arcomrlished using overlapping interval ranges as diccucsed in conjnn~tion with Figure lb, above, but otherw-ise re~ ~ing the basic detection methodology of the Model 7216 and 7217 products referred to above. In this second embo~im~ont, VTEC is incremtonte-l by intervals less than TDI and greater than or equal to F'IDImjn, reset by intervals greater than or equal to TDI and unaffected by intervals less than ~ lVI,,Ijn. VFEC is indicative of the number of intervals out of a prece-ling series of predetermined number (FEB) of intervals which are less thanFID~
The ple,5e,lll invention, in both ~iicclosed embo&ents, is practiced by adding specific new fe~ es to the underlying detection methodology of eYicting prior devices. Howcver, the value of the present invention is not limited to the context of the specific detection criteria ~ closetl~ but is believed workable and valuable in the cont~Yt of any devices which tiictin~lich between tachycardia and fibrillation using rate or interval based criteria.

wo 94/16767 21 S 3 3 3 8 PCT/US93/12221 As illu~llaled in Figures la and lb, both l,lerelred embo~limens-c of tbe present invention add a third, fast VT interval range. The third interval range inrlude.s intervals which are less than FTDI,~ if venh icular tachycardia is provisinn~lly tletecte~l, and incllldes intervals greater than or equal to ~-lVImin if venh~ r fibr~ hon is provic;on~lly detecte~l Such intervals are taken as indicative of the po rcibility that a fast ventricular tachycardia is oc~ lg. Both ~ lVI~ and FTDImjn are pro~.. ~hle values.
Following provisional detection of tacl~cal.lia or fibrillation using either thenon-overlapping interval ranges de_ned by TDI and FDI in the _rst embodiment or the uv~lla~hlg interval ranges of the second embo~liment the present invention es tbe most recent series of a predetermined number of R-R intervals (e.g.
the last 8 intervals) or of a predetç-mine~ duration to determine how many of tbese intervals fall within the fast venh icular ta~ cardia interval range. The number of intervals in the series may be set less tban or equal to VFEC or VTEC and will typically be less than CNID. If a predetermined number of or percell~age of sensed intervals within the series fall within the fast ventricular tachycardia range, the rhythm is lia~os~tl as fast ventricular tacl~cardia. The number of intervals required to rli~ose fast ventricular tacl~cal~dia may va~y depending on whether ventricular fihrill~tion or ventricular tael~cardia is provi~ion~lly detecte~l For purposes of the present invention, detel~g the number or percenlage of intervals within the series, which fall within the fast VT interval range may be ~ccomrlished by loohng to the intervals within the fast VT range, by looking to the intervals outside the fast VT range, or both.
For example, if _brillation was provisionally detected, the device may require that at least 7 or all 8 of the prece~ling 8 intervals fall within the fast ventricular tachycardia interval range (greater than or equal to FlvI",in) to detect fast ventricular ta~ dia. Equivalently, the device may require that none or no more than one of the prece~ling 8 intervals fall outside the fast ventricular tachycardia interval range (less than ~ l DI"""). Otherwise, detection of ventricular fibrillation is con~ed. If ventricular hchycardia is provisionally detected, the device may only WO 94tl6767 ~ PCT/US93/12221 require that at least 1 or 2 of the precedin~ 8 intervals fall within the fast ventricular tacL~carJia interval range (less than FIDImaX) in order to detect fast ventricular tac~cal.lia Equivalently, the device may require that all 8 or at least 7 of theprece-lin~ 8 intervals fall outside the fast ventricular tacl~cardia interval range S (~eater than or equal to ~-lVI~). Otherwise, detection of (slow) ventricular ta.L~ dia is c~ .--cd.
In the cont~Yt of the present invention, it is ~res.. ed that each of the possible ~etected al~Lyll~ias provided by the device will trigger a preset therapy, with the general a~es~ivGness of the therapies increasing from least agglessive if (slow) ventricular tachycardia is detected to most agglcssive if ventricular fibrillation is detected. For example, anti-tachycardia pacing may be employed in response to~etection of (slow) ventricular tachycardia, cardioversion may be employed if fast ventricular ta~Lycaldia is ~etecte~ and defibrill~tion may be employed if fibrillation is ~letecte~l Most cullelllly available devices of the type in which the present invention may be practiced provide for a menu of therapies for each type of detected ta~:hy~lLy~ ia, with sequenti~lly more agglcsiive therapies being applied after previous therapies fail to te..~ ale the a~.LylLLuia. In a device inco.~ofalillg such therapy menus and employing the present invention, the difference between fast and slow tac~ d;a therapies may lie in the rapidity with which therapies become moreag~essivG as previously tried therapies fail. For example, the menu of therapies for slow ventricular tachycardia may require three or more attempts at anti-tachycardia pacing prior to providing a high voltage cardioversion shock, the fast ventricular tachycardia menu may provide for only one attempt at anti-tachycardia pacing prior to high voltage cardioversion and the ventricular fibrill~tion menu may provide only defibrillation shocks of inc.easing m~itu~le~
Figure 2 is a filn~ion~l schematic diagram of an implantable pacem~ker/cardioverter/defibrillator in which the present invention may usefully be practiced. This diagram should be taken as exemplary of the type of device in which the invention may be embodied, and not as limiting, as it is believed that the Y~ ?`
invention may usefully be pr~ctice~l in a wide variety of device implementations, inrlll(line devices having fim~ tiQn~l or~ ion similar to any of the implantablep~cem~ker/~-fihr~ tor/cardioverters presently being jmrl~nted for clinical ev~ tinn in the United States. The invention is also believed practicable in conj--nction with ;~ ble p~ce-m~ker/cardioverters/ defibrillators as dicclosed in prior U.S. Patent No. 4,548,209, issued to Wielders, et al. on October 22, 1985, U.S.
Patent No. 4,693,253, issued to Adams et al. on September 15, 1987, U.S. Patent No.
4,830,006, issued to H~ll-C~ et al. on May 6, 1989 and U.S. Patent No. 4,949,730, issued to Pless et al. on August 21, 1990, all of which are incorporated herein by reference in their entireties.
The device is illustrated as being provided with SLY electrodes, 50Q 502, 504, 506, 508 and 510. Electrodes 500 and 502 may be a pair of endocardial electrodeslo~ted in the ventricle mollnte~ to a l,~cllous lead. Electrode 504 may colresl,ond to a remote, indi~ercnl electrode located on the housing of the impl~nt~h~e pacçm~ker/cardioverter/ defibrillator. Electrodes 506, 508 and 510 may collcspond to the large surface area defibrill~tion electrodes located on ventricular, coronau~ sinus, superior vena cava or subc~ nçol~c leads, to electrodes located on or part of the device housing or to epicardial defibrillation electrodes.
Electrodes 500 and 502 are shown as hard-wired to the R-wave detector circuit, CO~ iSillg band-pass filter circuit 514, auto threshold circuit 516 for providing an adjustable se-ncing threshold as a fimction of the mea~ Ired R-wave ~mplitllde and co.,.r~.ator 518. A signal is generated on R-out line 564 whenever the signal sensed between electrodes 500 and 502 PYree-lc the present se~Cing threshold defin~ by the auto threshold circuit 516. As illustrated, the gain on the band pass ~mplifier 514 is also adjustable by means of a signal from the pacer timing and control circuitry 520 on GAIN ADJ line 566.
The operation of this R-wave detection cil~ y may colle~olld to that dicclose~l in commQnly ~cci~n~d U.S. Patent No. 5,118,824, issued to Keimel and incorporated herein by reference in its entirety. However, alternative R-wave detection cil~;uilly such as that illustrated in U.S. Patent No. 4,819,643, issued to Menken on April 11, 1989 and U.S. Patent No. 4,880,004, issued to Baker et al. on November 14, 1989, both il,coll,ol~ted herein by reference in their entireties, may also usefully be employed to practice the present invention. T h e t h r e s h o l d adjllctmPnt circuit 516 sets a threshold collesl)onding to a predeter~ led percentage S of the ~mrlihl~le of a sensed R-wave, which threshold decays to a Illil~illl~llll threshold level over a period of less than three seconAc thereafter, similar to the ~ olll~tic sencin~ threshold .;ircuill~ illustrated in the article "Reliable R-Wave Detection from Ambulatory Subjectsn, by Thakor et al., published in Biomedical Science ...,Pns~tioll~ Vol. 4, pp. 67-72, 1978, incorporated herein by reference in its entirety.
It is preÇelable that the threshold level not be adjusted in response to paced R-waves, but incte~ll should con~ e to approach the "~i"i"~ threshold level following paced R-waves to enh~nre sencing of low level spontaneous R-waves associated with tachyarrhythmias. The time CQII!~ of the threshold circuit is also prefelably sllffi~ently short so that Illil~;llllllll sen.cing threshold may be reached within 1-3 secon~lc following adjuctment of the sencing threshold equal to 7~80% of the ~mplit~de of a ~letected s~ -Pous R-wave. The invention may also be practiced in conjunction with more traditional R-wave sensors of the type co~l~lisillg a band pass amplifier and a CQ~Ilr~.~tor circuit to determine when the band-passed signal ~Yree~lc a predetelll.ined, fiYed sencing threshold.
Switch matriY. 512 is used to select which of the available electrodes are coupled to band pass ~mplifi~or 534. Selection of which two electrodes are so coupled is controlled by the microprocessor 524 via data/address bus 540. Signals from the selected electrodes are passed through band-pass amplifier 534 and into multiplexer 532, where they are converted to multi-bit digital signals by A/D converter 530, for storage in random access memory 526 under control of direct memory address circuit 528. Microprocessor 524 analyzes the digitized EGM signal stored in random access memory 526 to determine the width of the stored R-wave or in conjunction with the ta~:~cal lia/ fibrillation discl;,.,i"~tion fimrtion discussed below.

wos4/l6767 ~ ?/S33~ PCT/US93/12~21 .~mrlifier 534 may be a broad band pass amplifier, having a band pass ten~li~ for ~roY;..~tely 0.5 to 200 hert_. The filtered EGM signal from ~mrlifier 534 is passed through multiplexer 532, and di~iti7e~l in A-D collvel~er .;Ui~ 530. The ligiti7e~1 EGM data is stored in random access memory 526 under control of direct memory address cil.;uilly 528. ~,ably, a portion of random access memory 526 is confi~lred as a looping or buffer memory which stores at least the prece~in~ several secQnds of the EGM signal. The data stored in the buffer memory may be optionally employed to perform R-wave width ...~..ie...entc as disclosed in co pentling U.S. Patent Applicaffon Serial No.07/867,931, filed April 13, 1992 by Mader et al, incorporated herein by leferellce in its eulirely and/or toperform the ventricular fibrillation/ventricular tacllycardia dis~ tion function~ic~-lose~ in allowed U.S. Patent Applir7,tion Serial No. 07/750,679 iled August 27, 1991 by Bardy et al., also incorporated herein by reference in its e~lilcly. However, the present invention may also readily be practiced in devices which do not in~ de such fim~tio~l~c The oc-;urlel,ce of an R-wave detect signal on line 564 is c~--------niç~te~l tomicroprocessor 524 via data/address bus 540, and microprocessor 524 notes the time of its oc.;~lence. If the width me~c.., eluent fimction is activated, microprocessor 524 waits 100 milliceconds or other physician selected interval following the oc~ullence of the R-wave detect signaL and thereafter transfers the most recent 200 milliceconds or other physician selecte~ interval of (li~iti7ed EGM stored in the looping or buffer memory portion of the random access memory circuit 526 to a second memory loc~tion, where the contents may be digitally analyzed to determine the width of the stored R-wave. The tra~lled 200 millicecQn~lc of stored EGM cor~es~onds to a time window eYten~ling 100 milliceconds on either side of the R-wave detect signal.
Window sizes in any case should be sufficient to allow me~cllrement of the width of detected R-waves. Preferably, the window should expire during the bl~nking period following R-wave detection. For purposes of the present invention, a sampling rate of 256 Hz with a bandpass of 1.5 - 100 Hz should be sufficient. As discussed below, . 30 the width measurement function is intended to discrimin~te between high rate sinus l~s and ventricular tacl~ardias, and is prcfelably only applied to R-waves that define the endpoint of an R-R interval within the interval range indicative of tac~cardia Either as a criterion for provisional detection of tac~cardia or after co~med detection of (slow) tachycardia, the device determines whether a S predetell~ed number or proportion of a series of preceding R-waves, the widths of which have been me~c~lred~ are greater than a preset threshold value (e.g. at least 8 of the prece~lin~ 12 measured R-waves). If the width criterion is satisfied, provisional detection of tachy~dia or co~ll-ed detection of slow ventricular tachycardia may optionally occur. If the width criterion is not met, the rhythm is ~ gnosed as rapid sinus rhythrn and no thela~ is delivered.
Similar to the width me~Cu~elllcnt fimction, if the dis~ tor function is activated, microprocessor 524 waits 100 milliceconds or other physician selectedinterval following the oc~;ullence of the R-wave detect signal, and thereafter transfers the most recent 200 millicecQn~ls or other physician selected interval of digitized EGM stored in the looping or buffer memory portion of the random access memory circuit 526 to a second memory loc~tion, where the contents may be digitally analyzed. The microprocessor 524 i~çntifies the points in time at which the R-wave detect signal occurs and the point in time at which the 200 ms of stored ECG meets a predetermined criterion (e.g. peak slope). These two stored times, hereafter referred to as the first and second "fi~ l points". The c lmlll~tive variability of the time intervals separating the oc.;ullence of the first and second fid~ points over a series of beats is used to ~lictin~lich fibrill~tion from high rate ventricular tachycardia.
The tirne interval ô separating the two fid~ l points associated with a single detected depolarization wave-front is me~cllred and stored if the detected depolarization occurs at the end of an R-R interval within the interval range associated with fibrill~tion In the conteYt of the present invention, following detection of a rhythm which otherwise would be detected as fast VT, the c~lm~ tive variability of the value of ~ over a series of a predetermined number (e.g. 8) of such detected depolarizations is compared to a threshold value set by the physician based wo 94/16767 iS3338 on an eV~ tiQn of the p~tiPnt If the cllmnl~tive v~ri~bility eyr~e~lc the threshold, fibrillation is detecte~l Otherwise, detection of fast ventricular tachycardia is col.G,...ed.
The microproceCcor also nrd~tes counts related to the R-R intervals previously sçnce~ The counts, VFEC and VTEC, are increm~nte~l on the oc.;u-- _nce of a mC~cv~cd R-R intervals falling within the fibrill~ti~n and ventricular tacl~cardia ranges, respectively, as lic~lr~e~ above. These rate ranges may be ~fined by the ~ro~A~ stored in the RAM 526.
These counts, along with other stored i~o~ n reflective of the previous series of R-R intervals such as i~ol---~tion leg~dhlg the rapidity of onset of the detected short R-R intervals, the stability of the rletecte~ R-R intervals, the duration of co..~ ed detectiQn of short R-R intervals, the average R-R interval duration and inform~tion derived from analysis of stored EGM segrnentc are used to determine whether tia.:h~ ~ias are present and to ,~ ,.;ch between di~erent types of ta:l~.l~lh---;~c, as lic~lc~se~l above in cQnjlln~i~n with Figure 1. Other such~letectiQn algo-itl~s for reco~;~ tacl~cardias are described in the above cited U.S. Patent No. 4,726,380, issued to Vollm~nn, U.S. Patent No. 4,88Q005, issued to Pless et al. and U.S. Patent No. 4,830,006, issued to ~hlcl~ et al., incorporated by reference in their entireties herein. An ~d~litiQn?~l set of tachycardia recQ~ition methodologies is ~ic~losed in the article "Onset and .St~bility for Ventricular Tacl~-l~lh~ia Detection in an Tmpl~nt~ble Pacer-Cardioverter-Defibrillator" by Olson et al., published in Com~puters in (~rdioloey~ October 7-lQ 1986, IEEE
Computer Society Press, pages 167-17Q also inco.~olated by reference in its entirety herei~ However, other criteria may also be m~cllred and employed in conjunction with the present invention.
It is envici~ned that onset and stability requirement are optional in a device employing the present invention, and preferably are made available as progr~mm~ble options, which may be deleted by external programmer c~mm~n(l If incl~l~e-l, it is believed preferable that the onset criteria be required to met prior to initi~ting coul~ g of VTEC, and that once met, the criterion will remain s~tisfie~l until detec~ion of ta.~ rdia tel.~.;n~ n Thus, onset is not intende~ to be a ~etecti~ncriteria required for re-detection of tach~cardia~ following initial detection The width criterion, if used, should also be understood to preferably used only in initial ~letection of tacL~carclia. This reflects a pre~ .lylion that following initial ~letecti~n of ventricular tachycardia, absent a proven return to normal heart rhythm (terTin~ti~n detect), subsequent high ventricular rates should be ~lesu,l,ed to be ventricular in origi~. The stability criterion, on the other hand, is believed to be al)propliate for use both in initial detectio~ of tacL~wdia and in re~etec~iQn of tac~ca~d,;a.
The rem~inder of the cir-;uiL~ is ~ledic~tç~ to the provision of cardiac pacing,cardioversion and deffbr~ tiQn therapies. The pacer timing/control ~;h~;ulll~ 520 inrllldes pro~s--...~ble digital collnters which control the basic time intervals ~cco~ te~ with VVI mode cardiac pacing, in.~ in~ the pacing escape intervals, the refractory periods during which senced R-waves are ineffective to restart timing of the escape intervals and the pulse width of the pacing pulses. The durations of these intervals are determined by mi~r~locessor 524, and are co~ unic~ted to the pacing ~h.;~ 520via address/data bus 540. Pacer timing/control cilcui~ also determines the amplitude of the cardiac pacing pulses and the gain of band-pass amplifier, under control of microproce-~cor 524.
During VVI mode pacing, the escape interval colmter within pacer timing/control ~ir-;.ull~ 520 is reset upon sencing of an R-wave as in~lir~t~d by a signal on line 564, and on timeout triggers generation of a pacing puLce by pacer output c~ 522, which is coupled to electrodes 500 and 502. The escape interval counSer is also reset on generation of a pacing pulse, and thereby controls the basic timing of cardiac pacing functions, inclu-lin~ anti-tac~cardia pacing. The duration of the interval defined by the escape interval timer is determined 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 m~cllre the duration of R-R
intervals, to detect the presence of tachycardia and to determine whether the ... ;n;.. ~.. rate criteria are met for activation of the width m~curement function.

WOg4/16767 21~S3338 PCI/US93/12221 Microprocec~r 524 operates as an interrupt dri:ven device, and re.s~nAC to interrupts from pacer timing/control ~ y 520 colles~onding to the occ~ ence of sensed R-waves and collcs~ollding to the genela~ion of cardiac pacing pulses.These interrupts are provided via data/address bus 540. Any ..rc~sc~ m~th~m~tic~l S c~ tiollc to be performed by microproceccor 524 and any ~lpA~tin~ of the values or intervaLc controlled by pacer ti_ing/control cir.;.lilly 520 take place following such interrupts.
In the event that a tacl~ ~nia is ~etecte~l and an ~nfit~ k~lL~lhlllia pacing regim~n is desired, a~,l,r~.iate ti_ing intervals for controlling generation of anti-tac~cardia pacing therapies are loaded from microproce-~cor 524 into the pacer timing and control cil~.~ill~ 520, to control the operation of the escape interval colmter and to define refractory periods during which letection of an R-wave by the R-wave detection cI~ is ineffective to restart the escape interval colmter.
Si~larly, in the event that generation of a cardioversion or defibr~ tiQn pulse is required, _icroproceCcor 524 employs the co~mtcrs in timing and control cil~ y 520 to control ti_ing of such cardioversion and defibr~ tion pulses, as well as timing of ~ccoci~te~ refractory periods during which sensed R-waves are ineffective to reset the timing c ~
In response to the ~etectiQn of fihrill~tir~n or a tacl~cardia requiring a cardioversion pulse, _icroprocessor 524 activates cardioversion/defibrillqfion control 554, which initi~tes charging of the high voltage ~p~itQrs 556, 558, 560 and 562 via charging circuit 550, under control of high voltage char~g line 552. Thevoltage on the high voltage capacitors is mo~ ored via VCAP line 538, which is passed through multiplexer 532, and, in leSl~O~;e to re~rllin~ a pre~lete~ Pd value set by microproce-~cor 524, results in generation of a logic signal on CAP F ULL line 542, termin~tin~ charging. Thereafter, delivery of the timing of the defibrill~tion or cardioversion pulse is controlled by pacer timing/control C~;.~iLI~ 520. One emboAiment of an ~pro~l;ate system for delivery and syncl~-o~ tion of cardioversion and defibrillation pulses, and controlling the timing fimcti(!nc related to them is disclosed in more detail in allowed, commonly ~cci~ne~l U.S. Patent WO 94/16767 2~1 5 3 3 3 8 PCT/US93/1222 Applir~tion Serial No. 07/612,761, by Keimel, for an A~p~alus for Detectin~ and Treating a Tac~~ , ia, filed November 15, 1990 and incorporated herein by refe.encc in its e.llirel~. However, any known cardioversion or defibrill~tiQn pulse generation ch~-ui~ is believed usable in c~nilln~ion with the present invention. For example, cir~ controlling the timing and generation of cardioversion and defibrill~tion pulses as ~lir~losed in U.S. Patent No. 4,384,585, issued to Zipes on May 24,1983, in U.S. Patent No. 4949719 issued to Pless et al., cited above, and in U.S.
Patent No. 4,375,817, issued to Engle ét al., all incorporated herein by reference in their elllirelies may also be employed. Similarly,known cil-;uill~ for controlling the timing and generation of anti-ta~L~rdia pacing pulses as described in U.S. Patent No. 4,577,633, issued to Berl~ovi~ et al. on March 25, 1986, U.S. Patent No.
4,880,005, issued to Pless et al. on November 14, 1989, U.S. Patent No. 7,726,380, issued to Vollm~nn et al. on February 23, 1988 and U.S. Patent No. 4,587,970, issued to Holley et al. on May 13, 1986, all of which are incol~,olated herein by reference in their entireties may also be used.
In modern p:~em~Pr/cardio._l ler/defibrillators, the particular anti-tachycardia and defibrill~tiQn therapies are ~.o~ ..cd into the device ahead of time by the physician, and a menu of therapies is typically provided. For e~..ple, on initial detection of tacl~ dia, an anti-ta~ hdia pacing therapy may be selecte~l On re-detection of tac~cardia, a more a~essi~ anti-~:L~ardia pacing therapy may be schP~lllled If repeated attempts at anti-tacl~cardia pacing therapies fail, a higher level cardioversion pulse therapy may be selecte~ therearler. Prior art p~tentc illuslla~ g such pre-set therapy menus of ~ntit^~k~ll~lhmia therapies incl~lde the above-cited U.S. Patent No. 4,83Q006, issued to ~ln~ et al., U.S.
Patent No. 4,727,380, issued to Vollm~nn et al. and U.S. Patent No. 4,587,970, issued to Holley et al. The present invention is believed practicable in conj~ln~ion with any of the known anti-tachycardia pacing and cardioversion thelay;es, and it is believed most likely that the invention of the present appliration will be practiced in conjunction with a device in which the choice and order of delivered therapies is _W094/lC767 ?15.333~ PCr/US93/12221 pro~; ...~ ble by the physician, as in ~ enl impl~ns~ble p ^e~ rer/cardioverter/defibrillators.
In ~ lditi~m to varying the therapy delivered following a failed attempt to telll i, ate a tac~ l ia, it is also known that ~ Iju~l...rnt of detection criteria may be a~,lo~,liate. For example, ~j!~l.. rnt may co~ ise re~ ring the number of i~tervals required to detect a tac~ hmia to allow a more rapid re~et~cti~n or by cl-~ing the interval ranges to bias ~letectir)n to~ etecti~n of ventricular fibrill~tion, for example as disclosed in U.S. Patent No. 4,971,058, issued to Pless et al and incorporated herein by reference in its enlilel~.
In the present invention, selGction of thc particular electrode confi~ration fordelivery of the cardioversion or defibrill~h~n puLses is controlled via output circuit 548, under control of cardioversion/defibrill~tinrl control cil~;uill~ 554 via control bus 546. Output circuit 548 detelll,ines which of the high voltage electrodes 506,508 and 510 will be employed in delivering the defibrill~ti~n or cardioversion pulse re~mPn, and may also be used to specify a multi-electrode, ~iml~lt~neolls pulse re~ym~on or a multi-electrode sequential pulse re~en. Monoph~cic or biph~cic pulses may be generated. One ~ ~l)le of ~ which may be used to perform this flm~i~ n is set forth in U.S. Patent No. 5,163,427, issued to Keim~Pl incol~lated herein by reference in its e lllirely. However, output control ~cuill~ as disclosed in U.S. Patent No. 4,953,551, is ued to Mehra et al. on Seplellll~r 4, 1990 or U.S. Patent No.
4,800,883, issued to Wi~l~on~ on January 31, 1989 both incolp~l~.ted herein by reference in their enlirelies, may also be used in the cQ .~ of the l,reselll invention.
Alternatively single monophasic pulse re~mPnc employing only a single electrode pair a~rding to any of the above cited references which lic~ se implantable cardioverters or defibrillators may also be used.
Figures 3a and 3b are a functional flow chart illucl~atillg the operation of thefirst preferred embo~3imP-nt of the present invention, as embodied in the conteYt of a device illustrated in Figure 2. In Figure 3a, it should be understood that the device may be in general operating as a ~3em~n~1 pa~P.m~lrer, and that the analysis undertaken takes place in res~ollse to the oc~;ullellce of sensed or paced ~53338 WO 94116767 PCI'IUS93/1222 depolari7~tionc of the heart. At 10, the device is awaiting the o~u-lel-ce of the next subsequent R-wave. Upon oc.;u..ence of the R-wave, the processes and f~lnction.cdescribed above related to storing the time of oc.;ullellce of the R-wave, measuring the R-R interval precerling the R-wave and, if a~r~,iate, analyzing the stored digital wave-form ~cc4~ted with the R-wave are all undertaken at 11. At 12, the VTEC and VFEC counts are ~p~l~te~l At 13, device checks to determine whether VFEC equals VFNID. If so, fibrillation ic provi.cion~lly dçtecte~l If not, the sum of VFEC and VTEC is co~ ared to CNID at 14. If their sum is equal to or greater than CNID, the device checks at 16 to determine whether VFEC is greater than a predetermined number, for example 5. If so, the device checks at 18 to see whether a predetermined number X of the prece~1ing Y R-R intervals (e.g. at least 7 or all 8 out of the prece~ 8 intervals) are greater than or equal to FDI. If so, tac~dia i. provicion~lly ~etçcte~l if not, fibrill~tion is provicion~lly ~letecte~l In the event that either the sum of VFEC + VTEC is less than CNID at 14 or VFEC is less than or equal to A at 16, the device checks at 20 to determine whether VTEC equaL VINID. If so, ventricular ta~ dia i provicion~lly detected, if not, the device checks at 22 to determine whether a ta.:hy~.l yll----ia hac previously been detected and whether the previous series of R-waves indicate te.. -.;n~lion of the previously detecte~ tacl-y~.l-ylhmia Detection of termination of tachy.;aldia or fibrill~ti~n may be s cc~nrlished by means of detection of a predetermined number (e.g. 8) of se~uçnti~l R-R intervals in~ tive of normal heart rate. Normal heart rate may be ~çfine~ as R-R intervals greater than or equal toTDI. If te~ ;on is detecte~l, detçction criteria and anti-arrhythmia therapy menus are reset at 24, as described above in conjlln~ ti~n with Figure 2. If not, the device simply waits for the next R-wave at 10.
In the event that the second preferred embodiment is to be practiced, the flow chart of Figure 3a remains applicable, with the exception that the cQrnbin~d count provisional detection fim~ n of blocks 14, 16 and 18 is not used. In this case, in response to a failure to provi~ion~lly detect fibrill~on at 13, the device proceeds WO94/16767 21~53338 PCI/US93/12221 directly to block 20, to determine whether ventricular tacl~cardia can be provicion~lly ~letecte~
In the event that tach~c~dia ic provicionally ~etecte~1 at 18 or 2Q the device checLc at 26 (Fig 3b) to determine whether a predetermined mlmber M of the S prece~li~ N intervals (e.g., 7 or 8 of the precedir~ 8) are greater than or equal to FIDI~ and thus olltcide the fact VT interval range (i.e. only one or none of theintervals are within the fact VT range). If not, a fact ventricular tachycardia is ~letecte~ and fast ventricular ta.;l~cardia therapy is delivered at 38. If so, the devioe may simply proceed to deliver slow VT therapy at 36. Ho~.e.er, as illustrated, the width criteria may optionally be applied at 30. In this case, appli~tir~n of the width criteria is directed tow~rds tlictin~lichi~ a slow ventricular tacl~ dia from a rapid sinus ll~l~. If the width criterion is met, a. liccllcr~1 above, a slow VT therapy is delivered at 36. If not, the device detects a rapid sinus ll~lhl.l and no therapy is delivered at 34.
In the event that fibrill~tic n is provicion~lly ~etecte~ at 13 or 18, the device checks at 28 to determine whether a predetermined number P out of the prece~lingQ intervals (e.g., 7 or 8 of the prece~ 8) are greater than or equal to ~-l ~I,~ and thus within the fast VT interval range. If not~ the device co~ ...c the ~l~tection of fibrill~tion and proceeds dirc~ ~ to deliver ventricular fibrill~tion therapy at 40. If so, the device may detect fast VT and proceed dilc~lly to deliver the fact ventricular tachycardia therapy at 38. However, as illustrated, the device may also optionally check at 32 to determine whether the criterion established by the ventricular tacl~cardia/ ventricular fibr~ tiQn dis~;-;--.;..sli--n f~lnrtinn is met. If the criterion is not met, ventricular fibrill~tion is ~etected and fibrill?tinn ~elapy is delivered at 40. If the criterion is met, fast ventricular ta~ calJia is co~med and fast VT
therapy is delivered at 38. After delivery of therapy at 36, 38 or 40 or following inhibition of anti-tacl~cardia therapy at 34, the therapy menus and detection criteria are reset at 42 to reflect the prece-ling detecti~n of tac~ lhmia and delivery of tachy~lL~rl~ia therapy, as tliccllcce~l above in conjnnrtion with Figure 2. The device then relul~c to block 10, awaiting the next sllc~essive R-wave, so that it may WO 94/16767 2 1 5 3 3 3 8 PCr/US93/1222' determine whether the ta~ hlllia has been te~ t,erl, ~,l:~iSI:~, or has ch~nge~
to another type of ta~l~ll~lhlllia The above lir~l~sed embo~iim~nt illustrates the case in which all functions of the preferred embo~lim~-nt of the present invention are activated. However, it is to S be eYrected that in c~mmercially rele~ed devices, the physician will be provided with the op~llullilr to selectively enable and disable individual portions of the tachy~ll~l~ia detection and ~ ccifir~tion f mctiQnC illustrated. For example, the physician _ay wish to disable the detection of fast ventricular ta.:l~cardia from a provisional ~letectior of ventricular fibrill~tiQn, a provisional ~letectir~n of ventricular tachycardia or both. The physician m ay l,rog~ the device to allow detection of fast VT only following provisional detecti~n of ventricular fibrill~tir~n- In such case, FTDI,~ may be st equal to FDI. In respol.se to provisional detection of ventricular tachycaldia, the device would either proceed di~cc11y to delver a slow ventricular tacl,ycardia therapy 36 following provisional ventricular tachycardia lS detection, or might optionally apply the width criteria at 30 as prerequisite to delivery of slow ventricular tachy~dia therapy. Alternatively, the physician may ~)iO~ll the device to allow ~letectinn of fast V r only following provisional detectiQn of ventricular tachycardia In such case, F IDI,Djn may be set equal to FDI. In respollse to a provisional detection of ventricular fibrillation, the device would proceed directly to delivery of ventricular fibrill~tion the~a~ at 40 in Figure 3b.
Disabling all fast ve-ntri~ r tachycardia ~3etection and setting both F'IDI,~,~
and F-ll)ImjD to be equal FDI in effect returns the interval ranges to those that would be present in the Model 7216 and Model 7217, t~ lcced above, and elimin~tes the fast ventricular tachycardia interval range entirely. However, increased speed of 2S detection of ventricular fibrill~tion or tachycardia would still be provided by the combined count detectinn methodolof~y di~lcced above.
As such, the invention as illustrated provides for a wide flexibility in its use, and allows the physician to tailor the detection criteria to the needs of the specific patient in whom the device is to be implanted. In conjllnction with this, it should also be kept in mind that all numerical variables and counts illustrated in Figure 3b WO 9411C767 ~ ~ 3338 PCT/US93/12221 may also be subject to pro~; ~ and control by the physician, in q~lition to the boundaries of the various interval ranges.
While the ~refelled embo~lim~nt of the device takes the form of a microprocessor controlled device as illustrated in Figure 2, in which the various ~mrtinnql steps illustrated in Figures 3a and 3b would be implemP-nted in the form of sorlware, the invention may equally well be practiced in the form of a de~lic-q-te~l~
full ~lctQm digitJ integrated circuit or, even in the form of an analog circuit,employing analog values as sllbstit~ltes for the digital values ~ cl(!sed in conjlmctil~n with the above specification.
In q~ltliti~-n, while the l~lefelled embo~iment ~;~closed above takes the form of a p-q-cemqlr~r/cardioverter/ defibrillator, the enh-q-nce~ ability to ~ictin~lich between various tac~ h~r~ias and the al.~roved speed of detectinn provided by the present invention are also valuable and applicable to devices which are onlycapable of pelÇol~g a subset of the various therapies ~ sed above in conj~lnrti~ n with Figure 2. For example, the ability to acc~ately d;clin~ ch between slow and fast ventri~ q-r ta,.:~rlias would be valuable in an anti-tacl~ dia p ~C~m-q-lcer~ with or without the cardioversion pulse generator, to select between anti-tachycardia pacing therapies or between anti-ta~l~cardia pacing and cardioversion therapies. Similarly, the ability to ~ i~,.;ch between a fast ventricular tacllycatdia and ventricular fibrillqtion is valuable in an i-.~ le cardioverter defibrillator, even if the cardiac pacing ~lnrtion is omitte~l~ for example, as in the ~lllelltly available CPI AICD implantable cardioverter defibrillators. It should further be kept in mind that while the therapies described for delivery in respol~se to detection of the various alll~ ias licc~l~sed are all dicrlnsed in the CO..I~ ~ of electrical therapies, it is possible that the invention may be embodied in the forrn of an irnplantable drug dispenser, wherein one or more of the anti-tacl~cardia ll~elal)ies takes the form of injection of a drug locally into the heart or systemir-q-lly to treat the detected allh~ ia As such, the above licrlos~lre should be taken merely as an example ofan embodiment of the present invention, rather than limitin~, when reading the claims which follow.

Claims

CLAIMS:

1. An apparatus for detection and treatment of arrhythmias, comprising:
means for sensing depolarizations of a patient's heart;
means for measuring the intervals separating successive depolarizations of said patient's heart;
means for defining first and second overlapping interval ranges;
means for determining the numbers of said measured intervals falling within said first and second interval ranges, over a first series of measured intervals;
first means for detecting the occurrence of an arrhythmia when said number of intervals in said first interval range equals a first predetermined number;
first identifying means responsive to said first detecting means for identifyingthe arrhythmia detected by said detecting means, comprising means for determining the number of said measured intervals falling within said second interval range within a second series of said measured intervals preceding detection of said arrhythmia by said detecting means, said identifying means comprising means for identifying the occurrence of a first type of arrhythmia in response to the number of said measured intervals, occurring within said second series of said measured intervals and falling within said second interval range, meeting a first predetermined value;
means for delivering anti-arrhythmia therapy; and means responsive to said first identifying means for triggering delivery of saidarrhythmia therapy.

2. An apparatus according to claim 1 wherein said delivering means comprises means for delivering at least two anti-arrhythmia therapies; and means responsive to said identifying means for selecting one of said therapies for delivery.

3. An apparatus according to claim 1 or claim 2 wherein said identifying means further comprises means for identifying the occurrence of a second type ofarrhythmia in response to the number of said measured intervals, occurring within said second series of said measured intervals and falling within said second interval range, being less than said first predetermined value;

4. An apparatus according to claim 1 or claim 2 or claim 3 wherein said identifying means comprises:
means for identifying the occurrence of a slow tachycardia in response to the number of said measured intervals, occurring within said second series of said measured intervals and falling within said second interval range, being less than said first predetermined value; and means for identifying the occurrence of a fast tachycardia in response to the number of said measured intervals, occurring within said second series of said measured intervals and falling within said second interval range, meeting said second predetermined value.

5. An apparatus according to claim 1 or claim 2 or claim 3 wherein said identifying means comprises:
means for identifying the occurrence of a fast tachycardia in response to the number of said measured intervals, occurring within said second series of said measured intervals and falling within said second interval range, meeting said first predetermined value; and means for identifying the occurrence of fibrillation in response to a number of said measured intervals, occurring within said second series of said measuredintervals and falling within said second interval range, being less said second predetermined value.

6. An apparatus according to claim 1 or claim 2 or claim 3 wherein said second series comprises a series of a second predetermined number of measured intervals.

7. An apparatus according to claim 1 or claim 2 or claim 3, further comprising:
second means for detecting the occurrence of an arrhythmia when said number of intervals in said second interval range equals a second predetermined number; and second identifying means responsive to said second detecting means for identifying the arrhythmia detected by said second detecting means, comprising means for determining the numbers of said measured intervals falling within said first and second interval ranges within a third series of said measured intervals preceding detection of said arrhythmia by said second detecting means, said second identifying means comprising means for identifying the occurrence of said first type of arrhythmia in response to the number of said measured intervals, occurring within said third series of said measured intervals and falling within said first interval range, meeting a second predetermined value and means for identifying the occurrence ofa third type of arrhythmia in response to the number of said measured intervals,occurring within said second series of said measured intervals and falling within said first interval range, being less than said second predetermined value; and wherein said triggering means is responsive to said first and second identifyingmeans for triggering delivery of said arrhythmia therapy.

8. An apparatus according to claim 1 or claim 2 or claim 3 or claim 7 wherein said means for sensing depolarizations comprises means for sensing depolarizations of said patient's ventricle.

9. An apparatus according to claim 1 or claim 2 or claim 3 or claim 7 wherein said means for defining said first and second interval ranges comprises means for defining a first interval range including intervals exceeding a first interval duration and for defining a second interval range including intervals less than a second interval duration, said second interval duration greater than said first interval duration.

10. An apparatus according to claim 1 or claim 2 or claim 3 or claim 7 wherein said first predetermined value comprises a number of said measured intervals falling within said second interval range.

11. An apparatus according to claim 1 or claim 2 or claim 3 or claim 7 wherein said second predetermined value comprises a number of said measured intervals falling within said first interval range.

12. An apparatus according to claim 1 or claim 2 or claim 3 or claim 7 wherein said second series of said measured intervals is a series of less than said first predetermined number of intervals.

13. An apparatus according to claim 7, wherein said second predetermined value is greater than said first predetermined value.

14. An apparatus according to claim 7 wherein said second series comprises a series of a second predetermined number of measured intervals and said third series comprises a series of a third predetermined number of measured intervals.

15. An apparatus according to claim 7 wherein:
said first identifying comprises means responsive to said first detecting means for identifying the occurrence of a fast tachycardia in response to the number of said measured intervals, occurring within said second series of said measured intervals and falling within said second interval range, meeting said first predetermined value and means for identifying the occurrence of a slow tachycardia in response to the number of said measured intervals, occurring within said second series of said measuredintervals and falling within said first interval range, being less than said first predetermined value; and said second identifying means comprises means for identifying the occurrence of a fast tachycardia in response to the number of said measured intervals, occurring within said third series of said measured intervals and falling within said first interval range, meeting said second predetermined value and means for identifying the occurrence of fibrillation in response to the number of said measured intervals,occurring within said second series of said measured intervals and falling within said first interval range, being less than said second predetermined value.