British Heart Journal, 1977, 39, 502-505

Primary ST changesDiagnostic aid in paced patients withacute myocardial infarctionVICTOR NIREMBERG, SHLOMO AMIKAM, NATHAN ROGUIN,BENYAMIN PELLED, AND EGON RISS

From the Department of Cardiology, Rambam Medical Center, Aba Khoushy School of Medicine,Haifa, Israel

In 34 out of 36 patients with apical right ventricular endocardial pacing, primary ischaemic ST alterationswere observed during the early stage of acute myocardial infarction. These ST changes, indicating acuteinjury, were detected in the paced beats in inferior and in anterior infarct. The primary ST changes wereconsistent only during the early stages of acute myocardial infarction and were not detected when the electrodetip was not in the apex of the right ventricle. It is suggested that the primary ST changes should be used todiagnose acute myocardial infarction in paced patients.

The diagnosis of acute myocardial infarction inpatients with implanted pacemakers is difficult andhas remained an electrocardiographic diagnosticproblem. Previous attempts to resolve this difficultywere based on the following concepts:(1) Analysis of escape beats or of the instrinsic

rhythm by means of continuous monitoring orby various electrical interventions in pacemakerfunction (Center et al., 1971; Bathen andAbrahamsen, 1973).

(2) Applying the same criteria used to diagnosemyocardial infarction in the presence ofcompleteleft bundle-branch block (Sodi-Pallares et al.,1963) to the paced QRS complexes (Cardenaset al., 1972; Castellanos et al., 1973). However,it was found that the changes in the paced QRSwere not consistent, especially in cases ofinferior myocardial infarction (Barold et al.,1976).

Rothfeld et al. (1973) described ischaemic ST-Talterations in paced days in whom myocardialinfarction was induced by ligation of coronaryarteries. They noticed large ST vectors orientedtoward the site of infarction, reflecting an injurypattern similar to those seen in non-paced patientswith or without intraventricular conduction de-fects. These ST-T changes were present in allpaced dogs with infarction.The purpose of this study was to measure the

incidence of the ischaemic ST alterations in pacedReceived for publication 31 August 1976

patients during the acute phase of myocardialinfarction, in order to assess their reliability as amethod of diagnosing acute myocardial infarctionin patients with permanent pacing.

Subjects and methods

Studies were made on 45 patients with acute myo-cardial infarction who were admitted to the coronarycare unit. The diagnosis of acute myocardialinfarction was confirmed in all patients by clinical,electrocardiographic, and enzymatic criteria. Atemporary pacemaker was inserted in all thesepatients because of the development of 2nd and3rd degree atrioventricular block or trifascicularblock during the acute phase. The transvenousbipolar electrode was introduced into the rightventricle under fluoroscopic control through theantecubital or jugular veins. The external pacemakerused was Medtronic model No. 5880. In 41 patientsthe tip of the electrode was placed in the apex ofthe right ventricle, in 2 patients it was found to bein a mid-ventricular position, and in another 2patients in the outflow tract. The site of infarctionwas as follows: anterior wall-15 patients, inferiorwall-16, inferior and true posterior-12, anteriorand inferior-2 patients. In cases with completeatrioventricular block, the electrocardiographicdiagnosis of acute myocardial infarction was alwaysmade before the development of this conductiondisturbance. Thus, in all the patients the informa-

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ST changes in paced patients

tion about the phase and the site of infarction wasobtained by the analysis of conducted sinus beats.A routine 12 lead electrocardiogram was obtainedtwice daily during pacing, and during interruptionof pacing, unless contraindicated.

Results

Ischaemic ST-T changes were considered to bediagnostic if ST elevation was at least 2 mm witha convex shape. Such transient changes during theacute phase of myocardial infarction were found in34 patients out of the total group of 45 (Fig. 1 and2).During the study we realised that patients in

phase III of the electrocardiographic evolution ofacute myocardial infarction, or with the electrodetip not situated at the apex (Fig. 3) did not showthese typical ischaemic ST-T alterations. Ninesuch patients( 7 in phase III and 2 with the electrodetip not at the apex) were excluded from the totalnumber of patients. In order to determine the phaseof the electrocardiographic evolution of acutemyocardial infarction, we have used the followingaccepted classification which differentiates fourphases (Rushmer, 1955): Phase I is characterisedby a prominent ST elevation and a positive orbiphasic T wave and phase II by a pathological qwave, a less prominent ST elevation, and an invertedT wave. In phase III, the ST segment is iso-electric, the T wave is inverted and the q wave ismore pronounced. The electrocardiographic evo-lution might end at this phase or can proceed tophase IV, in which the T wave returns to normal.The time interval from phase I to phase II rangesbetween a few minutes and several hours and fromphase II to phase III between a few hours andseveral days. Thus, in 34 of the remaining 36patients (94 4%) ischaemic ST-T changes werediagnostic of acute myocardial infarction duringpacing. In only 2 patients were these changes notobserved though the tip of the electrode was at theapex and the electrocardiographic phase was I orII, and should, therefore, be considered falsenegatives. The ischaemic ST-T alterations duringpacing were always located in the same leads inwhich they appear on conducted sinus beats (Fig.1 and 2).

Discussion

Sodi-Pallares et al. (1963) were the first to describethe patterns of myocardial infarction in the presenceof complete right or left bundle-branch block. Byapplying the same criteria used by Sodi-Pallareset al. (1963) several electrocardiographic patterns

were recently described as diagnostic of myocardialinfarction in the presence of right ventricularpacing, based on the similarity between the morph-ology of complete left bundle-branch block and thepaced QRS. Cardenas et al. (1972) described aninitial R in aVR and V1 for inferior and trueposterior myocardial infarction, qR in V5 and V6for antero-septal myocardial infarction, and RSin V5 and V6 for lateral myocardial infarction.Castellanos et al. (1973) described a St-qR patternin lead I, aVL, V5, V6 for anteroseptal myocardialinfarction and pointed out that these changes inQRS might be overlooked or even absent when uni-polar stimulation is used. The distortion that largeunipolar spikes can produce might make it difficultto determine whether the finding is an artefact or atrue reflection of an alteration in ventricularactivation. Barold et al. (1976) did not agree withthe specificity ofCardenas's criteria for inferior myo-cardial infarction and suggested that a qR patternin the inferior leads is a more specific though raresign of inferior myocardial infarction during rightventricular pacing. In summary, the patternsdescribed are inconsistent and their significance iscontroversial.

Rothfeld et al. (1973) showed recently that in allpaced dogs with experimental acute myocardialinfarction, large ST vectors oriented toward the siteof infarction were observed even during pacemakerrhythm. In the present study we have shown thatprimary ischaemic ST elevation can be also seen ina high percentage of paced patients during theacute phase of infarction. During right ventricularendocardial pacing as in complete left bundle-branch block, it is the septal depolarisation thatdetermines the direction of the AQRS and thereforealso the direction of the AST and AT. Thus, theST changes seen during right ventricular pacingand in complete left bundle-branch block are ofthe secondary type, reflecting a disturbed de-polarisation. In the presence of acute myocardialinfarction, it is the site of the injured area (epicardialor endocardial) that determines the direction of theST vector and its shape. If both ST vectors(primary and secondary) coincide in direction, therewill be exaggeration of the ST displacement. On theother hand, if these vectors are opposed, the final STdisplacement will be minimal or even absent. Forexample, in paced patients with inferior acutemyocardial infarction, both ST vectors are orientedtoward the same direction, causing exaggeration ofthe convex ST elevation in leads II, III, and aVF(Fig. 1). The same findings can be observed inleads Vl to V4 in anteroseptal acute myocardialinfarction (Fig. 2). However, in anterolateral acutemyocardial infarction there is an opposition between

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Niremberg, Amikan, Roguin, Pelled, and Riss

Before pacing

During pacing

*J-i-f f----r] :- - :i-H--AL ---+ABefor- ..

Before pacing

Fig. 1 Acute inferior myocardialinfarctiont. lschaemic STelevation is present in the sameleads, before and during pacing.

I Ut U!l aVR aVL aVF VI V2 V3 V4 VYS V

During pacingI Ul U!I YVR aVI aVF VI V2 V3 Y4 VS Yb

~~~~~~ t;;~~~~~~~~~~~~~~~~~~~; ,-f--- -4I

Fig. 2 Acute anterior myocardial infarction. Ischaemic ST elevation is present in conducted sinus beats and inthe paced beats. The diagnostic QRS criteria are also present (q wave in lead I, aVL, V4 to V6).

Before ppsinq

inin.....4 .. ... .. ..

t ....|....|...

apex

....

..... ........

._V.

Duri ngpocicnqmid-ventr.

-Ut

A-f*' :I::':i's p1-4-

outflowti + t-

^ ..

I

....... ..

..

-.k...I.i.^j

ff1:g :

Fig. 3 Acute inferior infarction.The electrode tip was transferredfrom the right ventricular apexto the mid-ventricle and outflotract regions. During apicalpacing, primary ST elevation isdetected in leads II and III, butat both other tip positions it ismasked by the shift to the rightof the AQRS.

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ST changes in paced patients

the 2 vectors and, therefore, the typical diagnosticST elevation can be detected only during theinitial stage of electrocardiographic evolution, whenthe disturbed electrophysiological activity of theinjured area is still predominant.The AQRS, and therefore the AST, is also

determined by the site of the electrical stimulationat the right ventricle. When the electrode tip is atthe apex, the AQRS is usually -60' or more to theleft. However, if the tip is located at mid-ventricularregion or at the outflow tract, the AQRS shifts to theright. This causes a secondary displacement of theST segment which is opposed to the direction ofthe primary ST vector, and, therefore, may maskthe typical injury pattern (Fig. 3).

In summary, we found in this study that primaryST changes, suggesting acute injury, are presentduring the early stage of infarction in alnost allpatients undergoing right ventricular apical pacing.We suggest the use of these ST changes as a diag-nostic aid for detection of the acute event in pacedpatients.

References

Barold, S. S., Ong, L. S., and Banner, R. L. (1976). Diagnosisof inferior wall myocardial infarction dui ing right ventri-cular apical pacing. Chest, 69, 232-235.

Bathen, J., and Abrahamsen, A. M. (1973). Electrocardio-graphic diagnosis of acute myocardial infarction in patientswith implanted pacemakers. British Heart Journal, 35,1336-1337.

Cardenas, M. L., Sanz, G., Linares, J. C., Zamora, C.,Medrano, G. A., and Estandfa, A. (1972). Diagnostico-electrocardiografico de infarto del miocardio en pacientescon estimulacion endocardica del ventriculo derecho pormarcapasos. Archivos del Instituto de Cardiologla Mexico,42, 345-357.

Castellanos, A., Zoble, R., Procacci, P. M., Myerburg, R. J.,and Berkovits, B. V. (1973). St-qR pattern: new sign fordiagnosis of anterior myocardial infarction during rightventricular pacing. British Heart Journal, 35, 1161-1165.

Center, S., Berger, R. A., and Tarian, P. I. (1971). Thediagnosis of acute myocardial infarction in patients withpermanent pacemakers. Archives of Internal Medicine,127, 932-933.

Rothfeld, E. L., Zucker, I. R., and Ahuja, U. (1973). Electricaldiagnosis of myocardial infarction in the paced dog heart.Journal of Electrocardiology, 6, 27-30.

Rushmer, R. F. (1955). Cardiac Diagnosis. A PhysiologicApproach, pp. 309-311. W. B. Saunders, Philadelphia andLondon.

Sodi-Pallares, D., Cisneros, F., Medrano, G. A., Bisteni, A.,Testelli, M. R., and De Micheli, A. (1963). Electrocardio-graphic diagnosis of myocardial infarction in presence ofbundle-branch block (right and left), ventricular prematurebeats, and Wolf-Parkinson-White syndrome. Progress inCardiovascular Diseases, 6, 107-136.

Requests for reprints to Dr. Shlomo Amikam,Department of Cardiology, Rambam MedicalCenter, Bat-Galim, Haifa, Israel.

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