Significance of High Degree Atrioventricular Block in AcutePosterior Myocardial Infarction

The Importance of Clinical Setting and Mechanism of Block

By MICHAEL ROTMAN, M.D., GALEN S. WAGNER, M.D., AND ROBERT A. WAUGH, M.D.

SUMMARYThis report evaluates the morbidity and mortality, during hospitalization and follow-up, of a sub-

group of patients with posterior or diaphragmatic myocardial infarction (PDMI) who developedhigh degree A-V block via a type I mechanism and in the absence of power failure (pulmonaryedema or cardiogenic shock). This subgroup was not at any higher risk of hospital morbidity, hospitalmortality, or 1-year mortality than three other groups: (a) patients with PDMI but neither highdegree A-V block nor initial power failure; (b) patients with other infarct sites who developed highdegree A-V block in the absence of power failure; and (c) patients with other infarct sites butneither high degree A-V block nor initial power failure. The significance of subgrouping patientswith high degree A-V block by the quantity of clinical heart failure is exemplified by a review ofthe literature and the present study.

Additional Indexing Words:Power failure Type I second-degree

HE IMPROVED survival of hospitalizedpatients with acute myocardial infarction has

resulted primarily from an increased understandingof cardiac arrhythmias and their successful manage-ment and prevention.1 2 The precipitating factors,natural history, and modes of therapy for manifes-tations of ventricular irritability in acute infarctionare well established.3-5 A comparable understand-ing of disturbances leading to bradyeardia, morespecifically high degree atrioventricular (A-V)block, has not yet been accomplished. One reasonfor this is that most previous studies haveconsidered the whole spectrum of high degree A-Vblock as an entity without characterizing homoge-neous clinical subgroups that are at high or low riskof specific target events. This report evaluates themorbidity and mortality, during hospitalization andfollow-up, of a subgroup of patients with posterior

From the Division of Cardiology of the Department ofMedicine, Duke University Medical Center, Durham, NorthCarolina.

Supported in part by grants HL-05736 and HL-4807, andcontract PH-43-NHLI-67-1440 from the U. S. Public HealthService.

Address for reprints: Galen S. Wagner, M.D., Box 3327Duke University Medical Center, Durham, North Carolina27710.

Received August 15, 1972; revision accepted forpublication October 5, 1972.

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Third-degree A-V block

or diaphragmatic myocardial infarction who devel-oped high degree A-V block via a type I mechanismand in the absence of power failure (pulmonaryedema or cardiogenic shock). This subgroup iscompared and contrasted to other subgroups withand without A-V block and power failure.

MethodsFive hundred thirty-nine consecutive patients with

acute myocardial infarction who had continuouselectrocardiographic monitoring on our coronary careunit were evaluated. The diagnosis of acute infarctionwas based on (1) a typical clinical history and (2)electrocardiographic changes consisting of either new Qwaves or typical evolutionary ST-T changes, and (3)characteristic serial changes in serum enzymes includ-ing CPK, SGOT, and LDH. Continuous electrocardio-graphic monitoring was carried out in all patientsduring their stay on the coronary care unit with the useof the Hewlett-Packard patient monitoring systems. Themonitoring equipment was supervised continuously bytrained ECG monitor attendants. Furthermore, dailyconventional 12-lead electrocardiograms were taken onall patients and additional electrocardiograms weretaken as necessary. The hospital course of each patientand the full composite of the electrocardiograms takenduring the hospitalization were carefully reviewed.High degree A-V block was defined as the presence ofeither second- or third-degree A-V block occurring via atype I mechanism. The presence or absence of powerfailure and its relation to the development of highdegree A-V block was noted. Power failure was definedas pulmonary edema (respiratory distress due to the

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presence of pulmonary congestion) or cardiogenic shock(a systolic blood pressure less than 90 mm Hg, urineoutput less than 20 cc per hour, peripheral vascularconstriction, and an altered sensorium). The site of oldand acute infarction was evaluated and definedaccording to criteria as outlined by Lipman and Massie6and McConahay et al.7 Posterior myocardial infarctionincludes patients with characteristics of a diaphragmat-ic and/or posterior infarction (PDMI). Anteriorinfarction includes patients with anterior and/or lateralinfarction. Anterior-posterior infarction includes pa-tients with electrocardiographic characteristics of bothanterior and posterior infarction, and a "nontransmural"infarct was identified when there was a compatiblehistory accompanied by serial enzyme and ST-Tchanges but without the appearance of new QRSchanges. An indeterminate site of myocardial infarctionwas noted in those cases where there was enzymaticevidence of necrosis occurring in the presence of leftbundle-branch block. A careful review of all availableelectrocardiograms was carried out in order todetermine the mechanism of development of highdegree A-V block. The width of the QRS complex inthe setting of complete heart block was determined.Type I second-degree A-V block was defined by achanging P-R relationship in the presence of a changingR-P interval occurring on successive beats until a Pwave failed to reach the ventricles.8-10 Type II second-degree block was diagnosed by the presence of aconstant P-R interval despite a changing R-P intervalpreceding the failure of conduction to the ventri-cles.'1, 12 Complete or third-degree A-V block wasdiagnosed when none of the P waves propagated to theventricles, and the ventricular rate was maintained byan escape mechanism in either the junction or theventricle at a rate less thani 60 beats/min.9' 10, 13

During the period of this study, patients had atemporary pacemaker inserted on development of highdegree A-V block. A no. 5 or 6 bipolar (USCI) pacingcatheter was inserted into an antecubital vein,advanced to the right ventricular apex, and connectedto a Medtronic 5880A demand external pacemaker. InalI patients, the pacemaker was set at low ma (<2.0ma) and at a low rate (50 beats/min) on the demandmode. A need for an increase in rate was determined bythe patient's clinical state.During the period of hospitalization, all patients were

followed to time of discharge to determine morbidity,i.e., a worsening clinical state, and hospital mortality.After discharge, all patients were followed to a periodof 1 year. In those patients who expired, a carefulevaluation of the cause of death was made bycommunication with the patient's personal physician,family or, when available, by review of the autopsymaterial.

ResultsThe overall hospital mortality in the 539 patients

was 21% (111 patients). In the group with PDMIthere were 181 patients with 32 hospital deaths(18%). Within this group with PDMI, 45 patientsdeveloped high degree A-V block. The mean age of

the patients in this subgroup was 62.0 ± 8.0 years,and one or more previous electrocardiographicallydocumented myocardial infarctions was present in23 patients (45%). The overall mortality in these 45patients was 24% ( 11 patients) .

In the subgroup of 45 patients with high degreeA-V block, 13 developed type I second-degree A-Vblock as the highest degree of block, and 32developed third-degree block via a type I mecha-nism. In the group of patients with third-degreeA-V block:

(1) Thirteen progressed from first-degree to typeI second-degree to third-degree A-V block(all 13 subsequently returned to normal A-Vconduction).

(2) Four progressed from type I second-degreeto third-degree A-V block (all except onepatient in this group returned to normal A-Vconduction).

(3) Thirteen were admitted with third-degreeA-V block with a narrow QRS (<0.12 sec) andregressed through type I second-degree tofirst-degree A-V block to normal A-V conduc-tion.

(4) In two patients, third-degree A-V block wasnoted on admission with a narrow QRS, andboth returned to normal A-V conductiondirectly without passing through lesser de-grees of A-V block.

In the 30 patients who initially developed type Isecond-degree A-V block, 17 (57%) were noted toprogress to third-degree A-V block. High degreeA-V block occurred on admission or within 24-72hours in over 80% of the patients. The mean durationof high degree A-V block was 3 days, with a rangeof 1-14 days. In one patient, third-degree A-V blockwas persistent with the need for permanent pacingat time of discharge.Within this subgroup of 45 patients with PDMI

and high degree A-V block, there were 36 whodeveloped their A-V block in the absence of powerfailure: 12 with type I second-degree A-V block asthe highest degree of block and 24 with third-degree A-V block. In eight (22%) of these 36patients, the clinical condition deteriorated duringthe hospital course; five developed mild-to-moder-ate failure and three developed cardiogenic shock.These last three patients accounted for the onlymortality (8%) in this subgroup, and all threepatients were from the group with third-degree A-Vblock. In follow-up of the 33 survivors at 1 year, four(12%) had expired: two following a recurrentmyocardial infarction with cardiogenic shock, and

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two suddenly of unknown cause. This subgroupwithout power failure was contrasted to theremaining nine patients with PDMI and type Isecond-degree or third-degree A-V block whichappeared following the onset of power failure. Inthese nine patients, one had type I second-degreeA-V block and eight had third-degree A-V block as

the highest degree of block. Eight of the ninepatients expired (89% mortality). The one patientwho recovered from his power failure and A-Vblock was alive on follow-up at 1 year.

High degree A-V block did not occur in theremaining group of 136 patients with PDMI. Themean age in this group was 58.0 + 11.0 years, andone or more previous documented myocardialinfarctions was present in 61 patients (47%). Theoverall mortality in this group was 16% (22patients) .

There were 120 patients with PDMI who were

not initially in power failure and who did not havehigh degree A-V block. In the clinical course ofthese patients, 19 (16%) deteriorated during hospi-talization. From this group of 120 patients, 10 died(9% mortality), six of power failure, two of a

rhythm disturbance, and two of unrelated causes.

All 110 survivors were followed for a period of 1year, during which 10 (9%) expired. Death was

sudden in six patients, secondary to power failure inthe setting of a recurrent myocardial infarction inthree, and unrelated to cardiovascular disease inone. Sixteen patients with PDMI and without highdegree A-V block were in power failure at the timeof hospital admission. Twelve (75%) expired duringtheir hospital course, all of power failure. Two of

the four patients who were discharged and followedfor 1 year were alive.When all infarct sites other than PDMI were

evaluated, 13 patients with second-degree or third-degree A-V block via a type I mechanism were

identified. The overall mortality in this group was

38% (five patients). Nine developed high degreeA-V block in the absence of power failure. Five ofthese nine patients had type I second-degree A-Vblock as the highest degree of block. Two of these(22%) developed power failure and expired duringtheir hospitalization. Of the seven survivors, threewere dead by 1 year: two suddenly and one ofpower failure with recurrent infarction. In theremaining four patients in this group of 13 in whomhigh degree A-V block appeared after the onset ofpower failure, three expired of power failure, and a

single patient survived but subsequently died ofpower failure with a recurrent infarction.Upon evaluation of infarct sites other than

PDMI, a group of 261 patients were noted who didnot initially have power failure or high degree A-Vblock. The clinical state of 44 (17%) patientsworsened and 33 (13%) died in the hospital: 20 ofpower failure, seven of a rhythm disturbance, threesuddenly, and three of unrelated causes. Of the 228survivors, 45 patients (20%) expired during theirfirst year: 15 of power failure with a -subsequentinfarct, 18 suddenly of unknown cause, and 13 ofnoncardiovascular causes.

Table 1 contrasts the morbidity and mortality inthe seven subgroups discussed by the descriptionsof site of infarction and the presence or absence ofhigh degree A-V block and power failure.

Table 1

Contrasting Morbidity and Mortality in Subgroups of Patients Described by Site of Infarction and Presence orAbsence of High Degree A-V Block and Power Failure*

Posterior and/or diaphragmatic myocardial infarct site All other infarct sitesHigh degree High degree High degree High degree High degree High degree High degreeA-V block- A-V block- A-V block- A-V block- A-V block- A-V block- A-V block-present; power present; power absent; power absent; power present; power present; power absent; power

failure- failure- failure- failure- failure- failure- failure-Groutp descriptions absent present absent present absent present absent

N % N % N % N % N % N % N %

Patients 36 100 9 100 120 100 16 100 9 100 4 100 261 100Hospital morbidity 8 22 19 16 - 2 22 - 44 17Hospital mortality 3 8 8 89 10 9 12 75 2 22 3 75 33 13Follow-up mortality

at 1 year 33 12 1 0 110 9 4 50 7 43 1 100 228 20

*The incidence of hospital morbidity (development of clinical evidence of heart failure including cardiogenic shock), hospitalmortality, and follow-up mortality at 1 year is shown. Percentages describing hospital morbidity and mortality refer to per-centage of the total number of patients. Percentages of mortality at 1 year refer to the percentage of those patients dischargedfrom the hospital. It should be noted that patients with all other infarct sites and high degree A-V block absent but power failurepresent are not considered in this comparison.Circulation, Volume XLVII, February 1973

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DiscussionA recent review on the bradyarrhythmic distur-

bances14 occurring in the setting of an acutemyocardial infarction noted that the type andconsequences of high degree A-V block are highlyinfluenced by the relationship of the conductionsystem to the coronary circulation and the amountof muscle necrosis.15-1' In general, occlusion of theposterior circulation usually involves the rightcoronary artery and occasionally the left circumflex,and leads to inferior infarction and A-V block inthe A-V node.20 2 In contrast, occlusion of the an-terior circulation (anterior descending coronaryartery) leads to conduction disturbances which in-volve the bundle of His and the bundle-branchsystem."' 17, 20-25 Pathologic studies of the conduc-tion system in patients with A-V block occurring inthe setting of PDMI have revealed necrosis eitherwithin or adjacent to the proximal conductionsystem.'6' 18, 26 The extent of infarction in thesepatients is not necessarily great, and the necrosis ofthe proximal conduction system is not extensive.16-18In the setting of anterior infarction, pathologicstudies have shown that the extent of infarction ispronounced and necrosis of the distal conductionsystem prominent. 15-9, 25-27With PDMI, the development of high degree A-V

block, since it occurs in the proximal part of theconduction system, usually has a prodrome begin-ning with prolongation of the P-R interval withnormal QRS morphology and progressing to type Isecond-degree and subsequently to third-degree A-Vblock. The escape pacemaker below the site ofblock usually has a normal QRS morphology and arate of 45-60 beats/min."' 20-24 Disturbances ofconduction in the bundle-branch system or theoccurrence of type II second-degree block israre.20-24, 28 The progressive development of highdegree A-V block, its time of occurrence in thecourse of infarction, and the duration of block inour study are similar to those found in theliterature.24, 29, 30

It is clear from our data that the subgroup ofpatients with PDMI and high degree A-V blockoccurring in the absence of power failure was not atany higher risk of hospital morbidity, hospitalmortality, or 1-year mortality than three othergroups:

(1) Patients with PDMI but neither high degreeA-V block nor initial power failure.

(2) Patients with other infarct sites who devel-

oped high degree A-V block in the absence ofpower failure.

(3) Patients with other infarct sites but neitherhigh degree A-V block nor initial powerfailure.

The follow-up of patients with PDMI whodeveloped high degree A-V block in the absence ofpower failure during their hospitalization has notpreviously been recorded in the literature. In ourseries, this subgroup of patients was not at anyhigher risk for death during a 1-year period offollow-up than the other groups described.

In a review of the literature, a low mortality(25%) was noted in patients with PDMI, third-degree A-V block, and a junctional escape rhythm(narrow QRS). This is in contrast to a 50% mortalityin patients with PDMI, third-degree A-V blockand a wide QRS and to 87% and 77% mortalityin patients with anterior infarction, third-degreeA-V block and narrow or wide QRS, respec-tively.25' 30-34 A composite experience of the litera-ture identifies 147 patients with the onset of highdegree A-V block in the absence of power failure.The overall mortality was 11% (16 patients).21' 23, 35--8Since only four patients with anterior infarctionwere found in this group, it may be assumed thatalmost all patients reported with little or no con-gestive heart failure had PDMI.The literature affords little insight into the

natural history of problems of impulse conductionoccurring in the setting of acute infarction, becausemost patients received therapy, and its effect on thepatient's course cannot be evaluated.On review of the reported therapeutic modalities

used in patients with PDMI and high degree A-Vblock, it is clear that, prior to coronary care unitsand pacing therapy, these patients did not do well.In the series of 68 patients reported by Cohen,Doctor, and Pick31 in 1958, there was a 37%mortality. A 40% mortality was noted in 18 patientsreported by Courter, Moffat, and Fowler in 1963.28In contrast with these results, Jackson and Bas-hour39 presented 34 patients with third-degree A-Vblock; none was paced, and 23 received no therapyaimed at reversal of the block. Many patients hadhemodynamic and tachyarrhythmic complications,but the mortality was only 20%. Some investigatorshave continued to observe patients with highdegree A-V block on coronary care units withoutpacemaker insertion. Norris21 in 1969 reported a 19%mortality in 26 patients with PDMI and with highdegree A-V block (only one was paced). Only nine

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of these patients had third-degree block. In 197040he reported a 38% mortality in 29 patients withPDMI and third-degree A-V block. Still, he states,"few were paced." The degree of heart failure at thetime of onset of high degree A-V block was notnoted in these series.

Six serieS30' 32, 34, 36, 41, 42 in the literature describethe use of pacemaker therapy for patients withPDMI and third-degree A-V block. In this group of88 patients, pacing therapy was instituted only afterthe development of complications, i.e., heart failure,syncope, or arrhythmias. The overall mortality was35%, with a range of 11-45%. More recently,"prophylactic" pacemaker insertion has been usedin patients with acute myocardial infarction, i.e.,prior to the development of any of the complica-tions of high degree A-V block. In reviewing threeseries in the literature33' 37 43 and including ourpatients, a total of 167 patients were found who fitinto this therapeutic category. The overall mortalityin these patients was 24% (40 patients) with anarrow range of 22-25%. Thus, when the total groupof patients with PDMI and high degree A-V blockis considered, the facilities provided by coronarycare units, including "prophylactic" pacemakerinsertion, seem to have significantly influencedprognosis.

In view of these findings, it seems important tosubgroup these patients by that most importantprognostic parameter: the quantity of clinical heartfailure. Though pathologic studies have revealedthat the extent of myocardial infarction in patientswith PDMI need not be extensive to cause highdegree A-V block, all patients with PDMI and highdegree A-V block certainly do not have smallinfarcts.16-18, 26 A first approximation of the size ofinfarction may be attained by observing thequantity of clinical heart failure.44 45

The importance of subgrouping patients withPDMI is exemplified by the remarkably lowmortality of 11% in the group of 143 patientsreviewed in the literature21' 23, 35-38 and the 8%mortality in our 36 patients, none of whom was inpower failure at the time of onset of high degreeA-V block. This review of the literature and ourstudy leaves a number of questions still unansweredregarding the natural history of patients with highdegree A-V block and the effect of pacing therapy.To improve the objectivity in the care of thesepatients, future studies must identify patientswithin homogeneous subgroups according to site ofinfarction, mechanism of development of highdegree A-V block, and degree of heart failure at theCirculation, Volume XLVII, February 1973

time of onset of block, and must evaluate clinicaltherapeutic trials within these groups.

AcknowledgmentThe authors gratefully acknowledge the helpful sugges-

tions made by Dr. Andrew G. Wallace.

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MICHAEL ROTMAN, GALEN S. WAGNER and ROBERT A. WAUGHThe Importance of Clinical Setting and Mechanism of Block

Significance of High Degree Atrioventricular Block in Acute Posterior Myocardial Infarction:

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