Br Heart _' 1993;70:106-1 10

REVIEW

Current approaches and future developments inautomatic tachycardia detection and diagnosis

P S Astridge, G C Kaye, E J Perrins

Implantable automatic cardioverter/defibrilla-tors are now commonly used to treat malig-nant ventricular arrhythmias.' Early modelssimply detected a rapid heart rate and deliv-ered a defibrillation shock, but more modemdevices can deliver treatment as a tiered strat-egy of antitachycardia pacing, synchronisedlow energy cardioversion, and high energydefibrillation.2' Accurate detection and diag-nosis of arrhythmias is essential to avoid inap-propriate intervention: unnecessary shockscause battery depletion, increase patient mor-bidity, and may cause degeneration from astable to a poorly tolerated rhythm.45 An idealsystem should treat haemodynamically unsta-ble arrhythmias with a defibrillation shock,attempt pacing termination of stable ventricu-lar tachycardia, and remain silent during sta-ble atrial arrhythmias. Atrial fibrillation iscommon in patients with ventricular arrhyth-

6mias, and a rapid ventricular response maysuggest a ventricular arrhythmia to an unso-phisticated detection system. No currentlyavailable automatic detection system can reli-ably distinguish these situations.

Automatic tachycardia recognition byimplanted antitachycardia devices depends oncorrect sensing and interpretation of physio-logical changes. Ideally, the sensed variableshould alter rapidly with changes in rhythmand recover promptly after normal rhythm isrestored. We describe the currently availablemethods for automatic tachycardia recogni-tion and review future trends.The table shows the variables currently

proposed for automatic tachycardia detection.Many are under preliminary investigationonly, and few will achieve practical usage inimplantable devices.

ventricular tachycardia, there may be overlapin individual subjects.'0 Rate stability has beenused as an additional marker of a sustainedarrhythmia, but may also be unreliable."Systems employing rate criteria show long-term reliability and require relatively littlecomputing power but they are not diagnosticand offer no information about haemodyna-mic state.

ANALYSIS OF THE VENTRICULAR ELECTROGRAMThe ventricular endocardial electrogramrecorded from the right ventricular apex rep-resents the majority of ventricular activation.'2Unipolar and bipolar sensing seem equallysuccessful experimentally" though withunipolar sensing there is a greater risk of inap-propriate sensing of extraneous electricalnoise. Simple comparison of electrogramamplitude allows discrimination betweensinus rhythm and ventricular fibrillation'4 butneither the peak amplitude nor the maximumslew rate reliably detect ventricular tachy-cardia.'5

PROBABILITY DENSITY FUNCTIONProbability density function was used in theearliest implantable defibrillators'6: the func-tion calculates the time spent by the sensedelectrogram close to the baseline. In organisedrhythms, electrogram analysis results in a welldefined peak representing the baseline. Inventricular fibrillation, however, there willappear a random scatter of points, with nopeak around the baseline amplitude.'6 If usedin isolation, this sensing system is indepen-dent of rate, and will only diagnose arrhyth-mias similar in configuration to ventricularfibrillation.

Department ofCardiology, LeedsGeneral InfirmaryP S AstridgeG C KayeE J PerrinsCorrespondence to:Dr P S Astridge,Departinent of Cardiology,Leeds General Infirmary,Great George Street,Leeds LS1 3EX,West Yorkshire.

Electrical sensing in the right ventricleThe simplest method of tachycardia detectionis merely to sense an increase in heart ratemanifested by a series of short RR intervals.This technique does not discriminate betweensinus and atrial tachycardias and ventriculararrhythmias,7' and various refinements havebeen added to increase specificity. The onsetof arrhythmia is usually more abrupt than thatof sinus tachycardia and this may be used toaid detection of a pathological tachycardia.9However, when the sinus response to suddenmaximal effort is compared with the onset of

Template matchingTemplate matching involves comparison ofelectrogram configuration during arrhythmiaswith a standard template derived during sinusrhythm.

Correlation waveform analysis calculatesfrom beat to beat the mathematical relationbetween a normal template and the waveformunder analysis.'7'8 Such a system is indepen-dent of amplitude and baseline variation, butmay be confounded by the development ofparoxysmal bundle branch block.'9 Specificityfor uniform ventricular tachycardia can beincreased by comparison of the suspect wave-

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Current approaches andfuture developments in automatic tachycardia detection and diagnosis

Variables usedforautomatic tachycardiarecognition

Electrical sensingRate criteria:

Heart rateSustained high rateRate stabilityRate of onset

Activation sequenceDual chamber sensingElectrogram configuration:

Correlation waveformanalysis

Area of differenceanalysis

Gradient patterndetection

Frequency domainanalysis

Haemodynamic sensingRight ventricular pressure:

AbsoluteMeanPulse pressuredP/dt

Right atrial pressure:MeanWaveform

Right ventricularvolumetry

Mixed venous oxygensaturation

form and a template derived during previousepisodes of induced arrhythmia.20 Reliabilityis not affected by filtering at typical levels foran implantable device.2' The "bin areamethod" reduces the complexity of analysisbecause it compares corresponding areas or"bins" in the template and the waveformunder analysis by a simple error measure, andrequires a fraction of the calculations.22 Datacompression before correlation analysisreduces computational demands while stillreliably detecting ventricular tachycardia.2'

"Area of difference" analysis assesses thedifference between templates of sinus rhythmand ventricular arrhythmias by totalling theabsolute differences between individual sam-ple points on the waveform under investiga-tion and the normal template.5 18 Themethod is relatively efficient in terms of com-puting capacity required, but is affected bysignal amplitude and baseline fluctuations.Critical statistical comparison suggests thatalgorithms using correlation waveform andarea of difference analysis are similarly adeptin distinguishing ventricular tachycardia fromsinus rhythm or atrial fibrillation.24

So far the computing demands of rapidanalogue-digital conversion and filtering forthese complicated algorithms have preventedpractical application. There may be addition-al problems affecting a chronically implanteddevice; animal work has demonstrateddecreases of up to 18% in electrogram volt-ages on exercise,25 and use of chronic leadsmay be associated with a reduction in ampli-tude and slew rate of the recorded signal.2627

Gradient pattern detection involves calcu-lation of a first time derivative of the endocar-dial electrogram, in which the amplitude isproportional to the rate of change of the origi-nal electrogram. Abnormal rhythms and leftbundle branch block may be reliably distin-guished from normal sinus rhythm and theresults seem unaffected by changes in respira-tion and in posture of haemodynamically sta-ble patients.28 The system has also beenshown to function accurately for the identifi-cation of a single arrhythmia configuration inreal time,29 and, when a microprocessor ofsimilar power to implantable pacemakerprocessors is used, can distinguish multiplearrhythmias to which the computer had previ-ously been exposed, provided the configura-tions are not too similar.30 However, thenumbers studied have been small and furtherresearch is needed to assess the longer termprospects for this method.

Frequency domain analysisComparison of frequency spectra of cardiacelectrograms for the detection and diagnosisof arrhythmias has been relatively disappoint-ing. Fast Fourier transformation is used toconvert the raw signal to a plot of voltageversus frequency. This requires a large com-puting capacity. Appropriate filtering canhighlight differences between normal andpathological electrograms."'2 Significant dif-ferences between the centre frequency ofspectra derived during sinus rhythm and ven-

tricular tachycardia have been demon-strated," and fibrillatory and non-fibrillatoryrhythms may be discriminated.'4 However, ina study of open chest dogs there was overlapbetween values for sinus rhythm and ven-tricular beats in almost half the animalsstudied.2'

ATRIAL ELECTRICAL SENSINGSensing the relation between atrial and ven-tricular activation may aid tachycardia diag-nosis. However, sensing of ventricularfar-field signals may obscure a low amplitudeatrial signal, as in atrial fibrillation,'5 andrhythms involving a 1:1 atrioventricular rela-tion (including atrioventricular reentry andatrioventricular nodal tachycardias and ven-tricular tachycardia with 1:1 retrograde con-duction) remain difficult to discriminate. It ispossible to distinguish between anterogradeand retrograde atrial depolarisation36 37 andbetween atrial fibrillation and regular atrialrhythms both by atrial rate calculation andamplitude distribution and probability densityfunction generation.38 39

Simple ventricular rate criteria in combina-tion with atrial sensing might identify ventric-ular tachycardia correctly in almost everypatient.40 An experimental algorithm, whichmay be suitable for use in implantabledevices, incorporates dual chamber sensingand rate criteria and was capable of discrimi-nating atrial intrinsic and reentry tach-yarrhythmias and ventricular tachycardia withand without 1:1 retrograde conduction.4' Anattempt to discriminate sinus tachycardiafrom paroxysmal tachycardias with a 1:1 atri-oventricular relation by introducing a latediastolic atrial extrasystole is effective in thelaboratory: in sinus rhythm the subsequentventricular depolarisation will be correspond-ingly premature, but in paroxysmal tachy-arrhythmias the ventricular regularity will notbe disturbed.4142 Clearly, the arrhythmia mustbe absolutely regular for this technique towork.

Activation sequenceThe different sequence of myocardial depo-larisation during sinus rhythm and ventriculartachycardia may be detected by multipleendocardial electrodes43 and the addition ofan atrial lead may allow differentiation ofsinus and supraventricular tachycardias.44The number of electrodes required may bereduced by use of an electrode in the coro-nary sinus.45 It is not possible to differentiatebetween rhythms with 1:1 atrioventricularrelation on timing alone.

Haemodynamic sensingWhile the electrical sensing algorithmsdescribed are effective in detecting a patho-logical arrhythmia they give no informationon haemodynamic instability. In manyarrhythmias rapidity of heart rate is not thesole determinant of cardiovascular instabilityand there is no correlation between electro-gram configuration and hypotension.46

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Therefore, sensing of a haemodynamic vari-able, in combination with electrical events, isdesirable. Because of the thromboembolicrisk associated with chronic instrumentationof the left heart, studies have concentrated onthe response to arrhythmias in the right heart.

PHYSIOLOGICAL RESPONSES TO TACHYCARDIAIn a thorough evaluation of the haemody-namic consequences of arrhythmias, Nakanoshowed that both atrial and ventricular tachy-cardias are associated with a decrease inmean arterial pressure, stroke volume, andcardiac output, and consequently an increasein atrial pressure bilaterally.47 The interatrialpressure gradient increases in proportion toheart rate acceleration. Increased pulmonaryartery pressure and total pulmonary resis-tance may largely be due to the increases inleft atrial pressure. After onset of tachycardiathere is partial haemodynamic recovery andincreased myocardial contractility caused bycatecholamine activity.47 The magnitude ofhaemodynamic change is greater for ventricu-lar arrhythmias because of abnormal patternsof ventricular contraction and periodic mitralregurgitation, caused by atrioventricular dis-sociation, that are associated with variableventricular systolic and end diastolic pres-sures.47 However, echocardiography duringarrhythmias has failed to demonstrate anincreased incidence or severity of mitralregurgitation during stable ventricular tachy-cardia in humans, and impaired cardiac per-formance was believed to reflect reducedventricular diastolic filling.48 Large atrialwaves during arrhythmias may also representcannon waves as the atria contract againstclosed atrioventricular valves.49

In sinus tachycardia in response to exer-cise, mixed venous oxygen saturation fallspromptly, in line with the increase in heartrate, with changes achieved in less than 10seconds. Stroke volume also responds quicklyto exercise but does not reflect the level ofexertion. Maximal right ventricular pressureand dP/dt and mean right atrial pressureincrease rapidly (over 10 seconds) with exer-cise, and the changes observed correlate wellwith workload.50

RIGHT VENTRICULAR PRESSURE MEASUREMENTRight ventricular pressure was sensed by thefirst laboratory model for the implantabledefibrillator. Abolition of the phasic nature ofthis pressure triggered charging of the capaci-tor discharge circuit.5" During rapid right ven-tricular pacing to simulate ventriculartachycardia, mean right ventricular pressureshowed a significant (mean increase 45%)and sustained increase in dogs.52 However, inhumans rapid ventricular pacing at a compa-rable rate did not cause significant fluctuationin right ventricular pressure.5 Conversely, arapid fall in right ventricular systolic andpulse pressures at the onset of ventriculararrhythmias has also been demonstrated.54Changes in right ventricular pressureoccurred within two seconds of tachycardiaonset. By estimating the ratio of right ventric-

ular pulse pressure during tachycardia and atbaseline, it was possible to discriminatebetween stable and unstable ventriculartachycardia, though in stable cases drift backto baseline over 30 seconds was noted.Evidence of some correlation between the fallin right ventricular pulse pressure and theassociated fall in systemic arterial pressurehas been noted, but with poor correlationwith tachycardia cycle length.55 Because thereis large intra-group variability changes in rightventricular pulse pressure do not allow sinustachycardia to be distinguished from haemo-dynamically stable or unstable ventricular orsupraventricular tachycardia.56 Changes indP/dt distinguished between unstable ventric-ular tachycardia and sinus tachycardia, butagain there was extensive overlap betweenarrhythmia groups.56 An algorithm based ontime intervals derived from dynamic rightventricular pressures may help to identifyarrhythmias, but this was derived from a lim-ited number of animal experiments in whichpacing was used to simulate ventriculararrhythmias.57

Right ventricular volumetryRight ventricular volume may be estimatedcontinuously from changes in intracardiacimpedance.58 This involves passing a constantsubthreshold alternating current across theheart, between an intracardiac bipole orbetween intracardiac and remote surfaceelectrodes, and measuring the varying voltagegenerated by this current as the intracardiacimpedance changes. Errors may arise becauseof electrode motion and sensing of myopoten-tials. There is a practical method to reducebattery drain that uses a standard pacing lead,and the technique has been proposed for usein rate adaptive pacing.59 Stroke volume maybe more directly measured by a Doppler sen-sor, suitable for chronic implantation, situa-ted in the superior vena cava and orientatedtowards the ascending aorta.60 This approachis in the early stages of development but ispromising.

During tachyarrhythmias cardiac output isreduced by reduced ventricular filling andsystolic emptying. Right ventricular imped-ance during unstable ventricular tachycardiashows a significant reduction to less than30% of that during sinus rhythm, though thechanges during stable ventricular tachycardiaare less pronounced.6' The variability intranscardiac impedance measured betweenepicardial defibrillator patches in dogs provedto be a reliable detector of ventricular fibrilla-tion.62 Such a system removes the need foradditional implanted sensors and little extracircuitry is required, but it cannot detectother arrhythmias and is unlikely to be ofclinical value with increasing use of trans-venous defibrillator lead systems.

Right atrial pressure monitoringRight atrial pressure has been recommendedas a physiological variable that varies consis-tently with changes in myocardial perfor-mance.63 Though pressures are small (- 1 to

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+ 2 mm Hg at rest, rising to + 5 mm Hg withexercise), significant changes have beenshown during tachycardia. In anaesthetiseddogs, ventricular pacing to simulate ventricu-lar tachycardia resulted in a significant rise inmean right atrial pressure, which converselyremained stable during rapid atrial pacingimitating a supraventricular tachycardia.5'These changes have subsequently been con-firmed in humans,53 and mean right atrialpressure has been shown to reach a plateauapproximately 30 seconds after the onset ofventricular tachycardia, returning to the base-line level within 30 seconds of the restorationof sinus rhythm.64 This may be too slow formean right atrial pressure estimation to be ofpractical use in an implantable antitachycar-dia device. In addition, baseline drift affectinga chronically implanted sensor becomes espe-cially important in a low pressure chamber,and (though regular self-zeroing may partlyovercome this problem) a system not relianton absolute pressures would avoid it. Thepressure waveform in the right atrium hasbeen proposed as a variable aiding tachycar-dia recognition. High fidelity pressure record-ings clearly demonstrate the relation betweenatrial and ventricular contraction and charac-teristic patterns have been demonstratedassociated with specific atrial and ventriculararrhythmias.65 Mean right atrial pressureshowed an increase during all types ofarrhythmia, regardless of haemodynamic sta-bility, with a considerable degree of overlapbetween atrial and ventricular arrhythmias,though there was a preponderance of well tol-erated arrhythmias in the study.66 Develop-ment of a pattern-recognition algorithm and areliable high fidelity chronic pressure sensingcatheter may allow atrial pressure waveformsto become a realistic method for tachycardiadiagnosis.

Mixed venous oxygen saturationMixed venous oxygen saturation may be mea-sured chronically by reflectance oximetry sen-sors. During ventricular fibrillation in dogs,the pulsatile intra-beat variation in oxygensaturation decreased significantly andreturned to baseline within 30 seconds of therestitution of sinus rhythm.67 In humansreduction in central venous oxygen saturationwas greater and more rapid during exercisethan at the onset of haemodynamically tolera-ble ventricular tachycardia.68 During unstabletachyarrhythmias increased use of peripheraloxygen results in greater desaturation, unlessthere is circulatory arrest and no admixture inthe central veins.69 Though these changesmay be helpful in the automatic determina-tion of haemodynamic state, the considerablelag before changes occur would delay inter-vention.

ConclusionsOver a short period the implantable defibrilla-tor has developed from a simple device fortachycardia detection and delivering shocksinto a system in which sophisticated tiered

anti-tachycardia pacing and cardioversion isnow possible. To avoid spurious shocks andto optimise device performance a detectionsystem giving increased diagnostic accuracy isdesirable. This may necessitate a multisensorsystem, using both electrical and haemody-namic data. Dual chamber defibrillators willsoon be available and may help overcomesome problems in tachycardia diagnosis, par-ticularly the difficulties in the detection ofatrial fibrillation. The addition of a haemody-namic sensor might allow more appropriateapplication of tiered treatment. With furtheradvances in lead and microprocessor technol-ogy a detection system capable of diagnosingall arrhythmias and determining their haemo-dynamic state should become a practicalreality.

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