Analysis of the Morphology of the Unipolar Endocardial Paced Evoked Response

Analysis of the Morphology of the UnipolarEndocardial Paced Evoked Response

JAN BROUWER, DICK NAGELKERKE, MIKE J.L. DE JONGSTE,WIM BOUTE,* PETER DEN HEIJER, and K.I. LIE

From the Department of Cardiology, Thoraxcenter. University Hospital Groningen,The Netherlands and *Vitatron Medical B.V., Dieren, The Netherlands

BROUWER, J., ET AL.: Analysis of the Morphology of the Unipolar Endocardial Paced Evoked Response.In a retrospective study we analyzed the unipolar endocardia] evoked response signal (ERS) of 103 patientsprior to pacemaker implantation. The objective of this study was to give a complete description of the ERSmorphology and to evaluate influences on this morphology 0/ both various electrode characteristics andpacing rate. In addition, spontaneous endocardial signals were studied. The results demonstrate that acuteleads had both higher fl wave and T wave amplitudes and a /aster downslope of the T wave. In the acuteleads those with porous titanium carbon coated tips showed a more pronounced T wave. Pacing rateinfluences the R wave amplitude and the stimulus to T wave interval. Both stimulus to maximum andstimulus to minimum T wave interval show an exponential correlation with the stimulus interval. Theinterval between maximum and minimum of the T wave and the absolute amplitude of the T wave are notinfluenced by rate. Although there were significant correlations of the spontaneous endocardial signal withthe ERS, the predictive value 0/ the spontaneous signal for the ERS morpology is low. Prospective studieswill be necessary to conjirm the findings in this study. (PACE. Vol. 13, March I990J

evoked response, morphology, electrode characteristics, stimulus T interval, T wave sensing, rate respon-sive pacing

Introduction

In 1973, Preston and Bowers reported the do-sign of a threshold tracking system that senses theevoked response within 50 msec of a pacingpulse,' In 1978, Rickards et al.^ were the first todevelop a pacing circuit using a conventional leadcapable of direct recording of the evoked responsewithin 5 msec of the stimulus. This was achievedby using a modified pulse waveform and an auto-matic postpulse compensation to reduce polariza-tion and discharge effects. The autodiagnosticpacemaker of Auerbach et al.^ detects "failure tocapture" by distinguishing the normal evoked bi-phasic cardiac response from the monophasic

Address for reprints: Peter den Heijer, M.D., Department ofCardiology, Thoraxcenter, University Hospital Groningen,Oostersingel 59, 9713 EZ Groningen, The Netherlands,

Received August 15, 1989: revision November 30, 1989; ac-cepted november 30, 1989.

pseudoresponse generated by electrotonic spreadofa subthreshold stimulating current.

The first morphological description of theevoked response registrated from a conventionalunipolar lead is given by Rickards et al.̂ in theirpublication in 1981 "The evoked response is seenas a negative QRS complex which reaches a max-imum amplitude approximately 45 msec after thepacing pulse and is followed by a clearly definedpositive T wave. Both QRS and T wave durationtend to be shorter than those observed on thesurface electrocardiogram but are of higher am-plitude than spontaneous complexes sensed bythe pacing lead."

The first evoked T wave tracking pacemakerwas implanted in 1981.'* In 1982, Thalen et al.^reported on an evoked response sensing pace-maker with an automatic control of the stimulusintensity. Since then several investigators havereported on new developments and improve-ments of the QT driven pacemakers.^•^''''^•^^ The

302 March 1990 PACE. Vol. 13

ANALYSIS OF THE EVOKED RESPONSE

QT driven pacemaker nowadays is an establishedtherapy for bradyarrhythmia treatment. Alter-ation of the T wave sensing filter has improved Twave sensing remarkably [the T wave undersenserate decreased from 6.9% to < 1% in a fewyears'*'^), but QT driven pacemakers still havesensing problems, for example in the period im-mediately after the implantation,^^

We performed a retrospective study to char-acterize the morphology of the unipolar evokedresponse signal (ERS), To the best of our knowl-edge only parts of the ERS are described in litera-ture.̂ •̂ "̂ •̂ •̂ "̂̂ ^ Knowledge of factors that influencethe ERS morphology might be of use in improvingQT sensing, [since this is a parameter derivedfrom the ERS) and in the development of bipolarERS sensing systems."^''

Methods and Materials

The unipolar endocardial evoked responsesignal [ERS) was recorded during pacemaker im-plantation in 103 consecutive patients (53 male

and 50 female, mean age 71.3 ± 10.4 years). Sev-enty-three patients received a new pacemaker aswell a new (= acute) lead; in the remaining 30patients the device was replaced while the oldlead [^ chronic) was reused. The mean age ofthese leads was 79.1 ± 38.2 months (mean ± SD).Table I shows the characteristics of the leads thatwere used.

To evaluate the influence of pacing rate onthe ERS we made registrations at pacing intervalsdecreasing from 1200 msec to 400 msec (in steps of100 msec) in five patients. We measured four ERScomplexes at each pacing interval. In order toevaluate the relative changes the intervals andamplitudes were normalized by dividing themeasured value by the mean value.

Registrations of the spontaneous endocardia!signal were made in 77 patients. This subgroupdid not differ significantly in baseline character-istics from tho total group.

Measurements were performed with the pa-tients at rest in supine position. The ERS registra-tions were performed after final placement of the

Lead Nr.

431-04*"

412S"

5 0 6 1 " "

helifix-12*

4/15/18/289*"

MIP-2147LOE*

Epicardial 324-6 5 1 " " '

Table 1.

Characteristics of the Used Leads

Material TipElectrode

Carbon coatedporoustitanium

Activatedvitreouscarbon

Piatrnizedplatinumalloy

Platinum/Iridium(90/10%)

Elgiloy

Platinum/Iridium(90/10%)

Elgiloy

Area

11 n

1 2 n

8.4 n

12 rr

12 rr

7.6 n-

15rT

Tip Shape Tip

im^ Blunt

im^ hemisphere

lm^ Circumferentialgroved tip

im^ Helifix

im^ Hemisphere

lm^ Ring

lm^ 6 mm longspiral

Fixation TypeTip of Lead

Tines

Tines

Tines

Helifix

Balloon

Wedge

Fixed pad (suture)

Number

Ac

9

36

14

7

—

—

Chr

_

5

—

8

5

5

Manufacturers: 'Vitatron; "Siemens-Elema; ""Intermedics; ""Medtronic; *"**Cordis.

PACE, Vol. 13 March 1990 303

BROUWER, ET AL.

St-R-

Figure 1. Example of an evoked response registration.

Figure 2. Measured parameters evoked response.

304 March 1990 PACE, Vol. 13


lead tip (acute situation] and after a stabilizationtime of 5 minutes with a pacemaker rate 15 to 20ppm above the spontaneous rate (min 65 ppni).

Detection of the ERS was possible by meansof a specially developed external pacemaker (Vi-tatron Medical BV. Dieren, The Netherlands) witha dual fast recharge pulse, that reduces the polar-ization caused by the stimulus, This polarizationis < 5 mV (depending on the tip of the electrode)and is extinguished within 10 msec. The dual fastrecharge technique consists of two recharge pe-riods, one immediately before and one immedi-ately after the stimulus. The mean pulse ampli-tude is 2.5 V, the pulse duration varies from 0.5msec to 1.0 msec. The input impedance of theexternal ERS pacemaker is floating during thesensing of the ERS. it is approximately 100 kohm:this is similar to implantable ERS sensing rate re-sponse pacemakers. The potential of the signalwas measured between the pacemaker lead andan indifferent plate electrode in the pocket. Regis-trations were performed on an analog signal re-corder (Mingolog 803 inkwriter, Siemens-Elema.Solna, Sweden), with cutoff frequency DC-1,250Hz and paper speed of 100 mm/sec. Figure 1shows an example of such a registration. The ERSstarts with a very fast deflection caused by thepacemaker pulse. After a short interval a maxi-

mal negative peak (R-] is reached. From R- thesignal shifts upward and reaches a positive peak(R+). The positive peak following R+ is consid-ered as the maximal deflection of the evoked Twave (T+). After T+ the signal rapidly deflectsdownward and reaches a negative peak (T—). Thesignal then slowly drifts back to the isoelectricline.

According to Figure 2 measurements of timeintervals (in msec) and signal amplitudes (in mV)were performed manually. T'-T was defined asthe interval between the maximum and mini-mum deflection of the T wave, and AmpTH— asthe total T wave amplitude (i.e., the sum ofAmpT+ and AmpT-). We calculated the maxi-mal slew rate of the signal at the maximal upwarddeflection of the R wave (Slr-R, in V/s] and themaximal downward deflection of the T wave(Slr-T, in V/s) by determining the slope of thetangent of the signal (angle a and /5). The maximalfrequency content of the T wave was calculatedas 1000 msec/(2 X interval T^-T~ in msec).

Of the spontaneous signals the correspondingparameters were measured. Time intervals weremeasured starting at the initial deflection of thissignal. The maximal slew rate of the R wave wascalculated at the downward deflection of the Rwave.

st-stSt-R-St-R+St-T*St-T"T-^-T-AmpRAmpR'AmpT'AmpT"AmpT'S!r-RSlr-T

API(n

752.130,9

165.7284,5429.1144.6

14,417.005.533.138,660,4050.103

Table II.

Average Parameters of All

Leads= 103)

± 120.6± 15.4± 28.1± 39.3± 38.3± 30.7± 4.81± 3.69± 2.36± 1,76± 3.15± 0,275± 0.041

Acute< n -

Leads73)

753.6 ± 131,827.4 j

160.9 H277.2 d428.6 d151.4 d14,12 d7,72 d6,27 H3.50 d9.77 d0.396 H0.113d

b 12,9t 26,7t 39.2b 37.1t 27.91 4.86: 2.95: 2.15I 1.82: 2.75: 0.209: 0,039

Leads

Chronic Leads( n -

748,340.0

177.4302.2430.4128.315,165,263,712,245,950,4260.079

30)

± 88.1± 17.8± 28,5±33.8+ 41.7±31.5± 4.69± 4.67± 1.83± 1.20± 2,32± 0,397± 0,033

P-valueAcute

VersusChronic

NS<0.001<0.01<o.aoi

NS<a.ooi

NS<0.01<0.001<0.001£0.001

NS<0.001


BROUWER. ET AL.

AH

Acute

Chronic

-15 -

-20O 200 400 600 800

time (ms)Figure 3. Approximated evoked response of al). acnte. and chronic leads.

The measurement of time intervals was per-formed with an accuracy of 2.5 msec, the mea-surement of ampUtudes with an accuracy of 0.1mV. The accuracy of the determination of thetangent was 0.0005 V/s. The measurements wereperformed hy two individuals, the inter-ohservervariance was not significant [mean 1.105%). TheStudent's t- test was used to calculate the signifi-cance of the differences between groups of param-eters.

ResultsThe average results of the parameters are

given in Table II. Printed are the mean values± SD. Intervals are given in msec, amplitudes inmV, and slew rates in V/s. Figure 3 shows anapproximation of the mean signal hy connectingthe plotted values for R-, R-h, T-h, and T- .

Acute Versus Chronic LeadsTahle II also shows the average parameters

when tbe population is divided according to the

age of tbe lead, We found significant differencesbetween the acute and the chronic leads. Acuteleads had a higher amplitude of the R+ wave(AmpR-l-, difference 2.46 mV) and T wave(AmpT+, AmpT— and AmpT-l--, differences of2.56, 1.26 and 3.82 mV, respectively). T^-T" was23.1 msec longer and the slew rate of the Twave(Slr-T) was 0.034 V/s higher. The major fre-quency content of the T wave was 3.3 Hz acuteand 3.9 Hz chronic. Figure 3 shows the approxi-mation of the mean signal acute and chronic.

Influence of Type of Lead

Subdivision of the acute and chronic leadsaccording to the type of lead results in the ap-proximated signals as shown in Figures 4 and 5.

In the group of acute leads, lead 431-04,wbicb has a porous titanium carbon coated tip,shows significantly different parameters whencompared with the other leads: AmpT— is 1.58mV higher (P < 0.01), the T wave amplitude,

306 March 1990 PACE, Vol, 13


10

-15 -

-20 L

o

431-04

4 12S

5061

hELIFIX

200 400 600 800

time (ms)Figure 4. Approximated evoked response oj Ihe individual Qcuie leads.

AmpT+-, is 1.83 mV higher [P < 0.05), T-'-T" is19.5 msec shorter (P < 0.05) and the Slr-T is 0.037V/s (i.e.. 30.1%) higher (P < 0.01). The other pa-rameters do not show significant differences.

When the chronic leads are compared, theepicardial leads have a significantly greaterAmpR- of 21.55 ± 1.34 mV (P < 0.001), whileAmpR+ is the largest in leads 412S and the epi-cardial leads (resp. 8.08 ± 3.40 mV and 7.32± 9.89mV]. The other parameters, among whichT+-T-. AmpT+- and Slr-T do not differ signifi-cantly.

Influence of Pacing Rale

Figure 6 shows the approximated signal atthe different pacing intervals. In Figure 7A thenormalized values of the time intervals are dis-played: St-T-f and St-T— show a strong correla-tion with the stimulus interval. Using the formula

of Sarma et al^^^ we found that the matching re-gression formulas were:

St-T-" - 419.1 - 389.1 X

St-T^ = 553.5 - 431.4 X

with correlation factors of 0.9986 resp. 0.9968 [P< 0.001). St-R- also correlates significantly (P< 0.001) with the stimulus interval, the variationin absolute figures being rather small (St-R- is39.0 ± 5.6 msec at St-St of 1,200 msec, and St-R- is33.0 ± 3.9 msec at St-St of 400 msec). St-R +and T^-T" do not correlate with the stimulusinterval.

Figure 7B displays the normalized values ofthe amplitudes. With shorter stimulus intervalAmpR-, AmpR+ and AmpT+ increase signifi-cantly (P < 0.001), while AmpT- decreases (P< 0.001). The T wave amplitude (AmpT+-), Slr-Rand Slr-T show no significant correlation with thepacing interval.


BROUWER, ET AL.

412S

SE 4.15.18289 balloon

- - - - - Epicardiat

o 200 600 800400

time (ms)Figure 5. Approximaled evoked response of (he individual chiomc Jeads.

The Spontaneous Endocardial Signal

In 11 of the 103 patients tbe spontaneous en-docardial signal in the right ventricle was alsomeasured. This spontaneous endocardial signalshows more variation in morphology than theERS. In 22 registrations after T - a second positivepeak was seen before fhe signal drifted to the iso-electric line. In 38 registrations, R+ and T+ couldnot clearly be disfinguished as separate peaks.The average results are shou^n in Table III.

The average amplitudes of fhe sponfaneoussignals are lower than those of fhe ERS: AmpR- is52.4% of fhe ERS AmpR- (P < 0.001), AmpR+ isonly 28.9% of fhe ERS AmpR+ (P < 0.001]. Thespontaneous T wave amplifude. AmpT+- is29.3% of fhe ERS AmpT+- (P < 0.001). We foundmoderate linear correlafions befween fhe ampli-fudes and slew rafe of fhe spontaneous signalscompared wifh the ERS (Table IV). No significanfcorrelations were found befween the spontaneoussignal and the ERS for AmpR-, AmpT+. and

AmpT—, neifher was fhere a correlation betweenfhe sponfaneous R wave amplitude [AmpR— orAmpR+) and fbe amplifude of fhe ERS T wave,AmpT+-.

Discussion

Average Evoked Response Morphologyof all Leads

Table V shows a review of data collected inprevious sfudies compared wifh our findings. Thestudies in men and dogs show comparable resultsfor fhe mentioned parameters, the study in rabbifhearfs shows evoked responses wifh quife smalleramplifudes.

Evoked Response Morphology inAcute Versus Chronic Leads

In our study, acute leads had a higher R+ andT wave amplifude. Boute et al.^ also found in fhe



- 8300 600 900 1200

Figure 6. Approximated evoked response at stimuJation intervals decreasing from 1,200 ms to400 ms in steps of 100 ms.

St-R-

St-R+

St-T+

St-T-

T+-T-

400 500 600 700 800 900 1000 1100 1200 1300Stimulus intervai (ms)

Figure 7A. NormaJized values o/the time inlervals af decreasing stimulus intervals.

PACE. Vol. 13 March 1990 309

BROUWER, ET AL,

1.3

1.2

1.1

1.0

0.9

0.8

0.7

\

Ai L

AmpR-

AmpR+

AmpT+

-•• AmpT—

— AmpT+-

400 500 600 700 800 900 1000 1100 1200 13OO

Stimulus interval (ms)Figure 7B. HoT\^\oX\7Jdd va/uus o/ (he unipiiludes fit decreasing slimuius intervals.

chronic leads a smaller T wave amplitude (fil-fered). They suggesfed fhe following explanalionfor this difference: In their patient population fheaverage elecfrode tip surface of the acute leadswas smaller fhan thaf of the chronic leads. Eur-thermore, acufe porous leads were present in48.2% of all acute leads and in only 22.7% of thechronic leads; the porous leads in their studyshowed superior T wave characferistics. In ourgroup, fhe lafter explanation is also valid, becausein the acufe leads fhere were 8.8% porous leadsand 0% in fhe chronic group. However, the meansurface area of fhe bofh groups did not differ(acufe 11.7 mm^, chronic 11.8 mm^). We considerfhe distribufion in carbon leads of significant im-porfance for fhe mentioned difference. The per-cenfage carbon leads is 49.3% in the acute leadsand only 16.7% in fhe chronic leads. In our com-parison, the acufe and chronic group are notmatched for the influence of the type of lead.However, when only the signals regisfrafed wifhlead 412S are compared (36 acufe and 5 chronicleads) fhere are also significant differences in

T'-T (27.1 msec longer in fhe acufe leads, P< 0.05), AmpT+ (2.02 mV higher acufe, P < 0.05)and fhe T wave amplitude (AmpT+- 2.95 mVhigher, P < 0.02). The difference in AmpR+ andfhe difference in Slr-T (0.023 V/s) are not signifi-cant. This is mainly due to the smaller number ofleads. Local lesion pofenfials in the immediafeposfoperafive phase are probably one factor re-sponsible for fhe menfioned differences betweenthe acufe and chronic leads, especially for thehigher AmpT+. Fibrotic tissue reactions, whichincrease the distance befween fhe elecfrode andfhe excitable fissue, form another factor fhatcan reduce the amplitude of evoked endocardialsignals affer fhe implant.'''^

Influences of Type of Lead on the ERSMorphology

In the acufe leads the lead wifh the poroustifanium carbon coated tip shows a more pro-nounced T wave, which is seen as a higherAmpT- and AmpT+-, a shorter T^-T" and a



Table III.

Spontaneous Endocardial Signals

All Leads(n - 77)

Acute Leads(n = 57)

Chronic Leads(n = 20)

St-stSt-RSt-R*St-T'St-T"T'̂ -TÂmpR"AmpRÂmpT*AmpTAmpT*"Slr-RSIr-T

1035.152.5

107.9242.3361.0118,7

7,552.020.651.892.541,6250,029

± 414,8+ 17.3± 30.7± 70.3± 72.9± 71.6± 3.84± 1,56± 0,79± 1.30± 1.33± 0.907± 0.015

1015.9 ±49.6 ±

101.1 ±245.5 ±368.5 ±123.0 ±

6.99 ±2.34 ±0.71 ±2,07 ±2.78 ±1,679 ±0.032 ±

401.612.823.875.072.873.9

3.751,410,821.171.370.9630.015

1088.861.3

127.3232.2339.8107,6

9.231.140.471.341.811.4730,021

± 456.1± 25.2± 39.7± 54,1± 70,4± 67.3± 3.67± 1.66± 0.71± 1.55± 0.93± 0,723± 0.012

higher maximal slope of the T wave. These find-ings cnnform with those in a semiquantitativestudy of Boute et al,,^ performed with a TAl Twave analyzer (Vitratron), They concluded thatporous electrodes and in particular porous carbonelectrodes showed superior T wave sensing char-acteristics.

Porous titanium carbon coated electrodesgive, due to large pore size, a very small foreigntissue reaction, thus keeping the distance be-tween the electrode surface and the excitable tis-sue to a minimum.^'' They exhibit, as a result ofthe considerable increase of the electrode surface,a very low poststimulus polarization and the im-pedance is low, particulary for low frequency sig-nals such as the evoked response.

In the chronic leads especially the epicardialleads show a larger R wave amplitude [AmpR-and AmpR+); we found no differences betweenthe leads in T wave characteristics. In our group,registration of an evoked response with adequate

R wave amplitude and adequate T wave charac-teristics was therefore possible with epicardiac

Correlation

AmpR*AmpT'SIr-T

Table IV.

of Spontaneous Signals

R-value

0,47650,41050.3301

with the ERS

P-value

<0.001<0.001<0.01

Influence of Pacing Rate on the ERS Morphology

We found strong correlations between thepacing rate and St-T+ and between the pacingrate and St-T-. Bazett^^ was the first to describe acorrelation between the heart rate and QT inter-val. In 1981, Rickards et al.^ described a [linear)relation between pacing interval and QT interval.Later on the nonlinearity ofthe relation was rec-ognized."• '̂̂ •^^•^^ The relationship we foundmatches the one described by Sarma et al.̂ ** St-R+and (in absolute figures) St-R- do not vary withheart rate, which is conform the findings of Furu-kawa et al.^' With decreasing stimulus intervalthe R wave amplitude (AmpR- and AmpR+) in-creases, and the T wave shifts upwards": AmpT+increases, AmpT— decreases and AmpTH— doesnot change significantly, neither does T^-T ,

At higher pacing rate the positive peak of theR wave and that of the T wave tend to fuse, mak-ing their discrimination more difficult. Thechanges in morphology caused by the pacing ratemight be responsible for problems like the doublesensing of the T wave in QT pacemakers. Thisphenomenon occurs mainly at lower pacing ratesand less at higher rates, which might be explainedby the fusion of R+ and T+ at higher pacing rates.


BROUWER, ET AL.

Study

Rickards et ai.^'^Boute et al.^^Furukawa et al.̂ Êconomides et al.^'Brouweret al.

Table V.

Review ERS Parameters of

AmplitudeR~ Wave

(mV)

16.1 ±4.411.5 ±3.5

—

4.86 ± 0.9914.41 ±4.81

Different Studies

AmplitudeT^ Wave

(mV)

7.0 ±3.37.7 ±3.07.6 ± 2.31.47 ± 0.305.53 ± 2.36

Population

12 men50 men20 dogs20 rabbits103 men

The Spontaneous Endocardial Signal

The spontaneous signal is less uniform thanthe ERS and has amplitudes of 1/4 to 1/2 of theERS amplitudes. Correlations between the spon-taneous endocardial signal and the ERS werefound of AmpR+, AmpT+-, and Slr-T. Thesecorrelations are significant, the correlation factorshowever are rather moderate. This might be ex-plained by the difference in spread of depolariza-tion. No correlation was seen between the sponta-neous R wave amplitude and the ERS T waveamplitude. The predictive value of the morphol-ogy of the spontaneous signal for the ERS mor-phology is therefore low.

Boute et al,̂ ^ did not find a correlation be-tween the amplitude of the spontaneous R waveand either the evoked R or T wave, The absenceof a correlation between the spontaneous andevoked R wave is in contradiction with our find-ings.

Conclusion

From this study, that aims to characterize themorphology of the unipolar endocardial paced

evoked response, we conclude that the ERS mor-phology is markedly influenced by the type andage of lead and by the pacing rate:

The ERS morphology in acute leads showshigher amplitudes of the R and T wave. Inchronic leads, the T wave amplitude was39.1% lower, T wave slew rate was 30.1%lower and the T wave frequency content was18.2% higher.Porous titanium carbon coated leads show amore pronounced T wave.In our group adequate ERS sensing was possi-ble with epicardial leads.The pacing interval correlates exponentiallywith the stimulus to T wave intervals but hasno influence on the stimulus to R wave in-tervals. At higher pacing rate the R and Twave of the ERS tend to fuse.The spontaneous endocardial signal has a lowpredictive value for the ERS morphology.

Prospective studies will be necessary to con-firm the findings in this study and to determinethe predictive value ofthe ERS morphology at thetime of implantation of a new lead for the chronicsituation.

References

3.

Preston TA. Bowers DL. Report of a continuousthreshold tracking system. In Thalen HJTh (ed.):Cardiac Pacing. Assen. Van Gorcum. 1973. p. 295.Rickards AF, Norman }. Relation between QT in-terval and heart rate. New design of physiologi-cally adapting cardiac pacemaker. Br Heart} 1981;45:56-61.Auerhach AA, Furman S. The autodiagnosticpacemaker, PACE 1979; 2:58.

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5. Thaten HJTh, Rickards AF. Witkampf FHM, et al.Evoked response Sensing (ERS) as automatic con-trol of the pacemaker output. In GA Feruglio (ed.):Cardiac Pacing, Electrophysiology and PacemakerTechnology. Padova. Piccin Medical Books, 1982,pp. 1229-1234.



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Analysis of the Morphology of the Unipolar Endocardial Paced Evoked Response