ECG INTERPRETATION PART 2: DETERMINATION

OF BUNDLE BRANCH AND FASCICULAR BLOCKS

Authors: Louise Rose, RN, DipNurs, ICUCert, AdultTeachCert, BN, MN, PhD, and Lisa Kuhn, RN, DipAppSci(Nurs),EmergCert, GradDip(Nurs), MHlthSci(Nurs), Toronto, ON, Canada, and Victoria, Australia

Earn Up to 7 CE Hours. See page 176.

Recognition of the ECG characteristics associatedwith bundle branch and fascicular blocks is impor-tant for identification of new or worsening cardiac

disease. These characteristics often herald an increased riskof mortality for patients with acute coronary syndrome (ACS).Clinicians need to have an understanding of the ECG changesassociated with bundle branch blocks in order to distinguishbetween these changes and those associated with ST-segmentelevation myocardial infarction (STEMI).

Intraventricular conduction defects result from a delayor obstruction in an area of the conduction system belowthe bundle of His.1 Conduction defects may be due to acomplete obstruction of either the right or left bundlebranches or may be located further down the conductionsystem in either the left anterior or left posterior fascicle.These regions of the conduction system are responsible foractivation of the ventricles. Therefore, ECG changes as-sociated with intraventricular conduction defects are mostlyseen in the QRS complex. The ability of clinicians to iden-tify newly acquired intraventricular conduction defects suchas bundle branch blocks is important because their pres-ence in ACS has implications for patient prognosis and ur-gency of treatment aimed at revascularization.2,3

This article is the second in a 2-part series on ECGinterpretation. In Part 1, concepts required for an under-

standing of mean electrical axis were explored. In Part 2,the mechanisms responsible for the ECG characteristicsof bundle branch and fascicular blocks are examined.

QRS Complex: Normal Morphology

The duration of the QRS complex is determined by thebeginning and end of ventricular depolarization. Normally,the right and left ventricles depolarize simultaneously. Theright bundle branch depolarizes the right ventricle, whereasbecause of its larger muscle mass, the left bundle branchsplits into 2 fascicles to enable synchronized depolarizationof the left ventricle.

In any given lead of the 12-lead ECG, the Q, R, or Swave may be present.4 A Q wave is the first initial deflec-tion below the isoelectric line, whereas the R wave is thefirst positive deflection above the isoelectric line. The Swave is the first negative deflection that extends belowthe isoelectric line after the R wave. The initial sectionof the QRS complex represents depolarization of the sep-tum. The septum depolarizes from left to right, generatingsmall nonpathologic q waves in the leads with leftward-facing vectors (leads I, aVL, V5, and V6). In the leads thathave a rightward-facing vector (aVR and V1), septal depo-larization is depicted as a small r wave.

The next part of the QRS complex reflects the depo-larization of the large muscle mass of the left ventricle.Right ventricular depolarization is hidden because of thesmaller electrical force generated by its smaller musclemass. The second deflection of the QRS complex is repre-sented differently according to the lead from which it isviewed. Leads with a leftward-facing vector will have a largepositive deflection following the septal q wave (qR),whereas leads with a rightward-facing vector will have anegative deflection after the initial r wave (rS).

In adults, the duration of the QRS complex is 0.05 to0.10 seconds.1 The QRS should be measured from thebeginning of the Q wave (or R wave in absence of a Qwave) to the end of the last wave of the complex or J point(Figure 1, B). The QRS duration should be measured in thelead that it is at its widest—usually one of the precordial leads,

Louise Rose is Lawrence S. Bloomberg Limited Tenure Professor in CriticalCare Nursing, Lawrence S. Bloomberg Faculty of Nursing, University ofToronto, Toronto, ON, Canada.

Lisa Kuhn, is Research Assistant and PhD Student, St Vincent’s/ACUNational Centre for Nursing Research, Australian Catholic University (St.Patrick’s Campus), Melbourne, Australia.

For correspondence, write: Lisa Kuhn, RN, DipAppSci(Nurs), EmergCert,GradDip(Nurs), MHlthSci(Nurs), St. Vincent’s/ACU National Centre forNursing Research, St. Vincent’s Public Hospital, Aikenhead Bldg, Level 7,PO Box 2900 Fitzroy, VIC 3065, Australia; E-mail: l.kuhn@bigpond.net.au.

J Emerg Nurs 2009;35:123-6.

Available online 15 July 2008.

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Copyright © 2009 by the Emergency Nurses Association. Published byElsevier Inc. All rights reserved.

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V2 or V3.4 The duration of the QRS complex also can be

analyzed in terms of the ventricular activation time (VAT)and the slope of the intrinsicoid deflection. The intrinsicoiddeflection is the downward slope from the peak of the Rwave to the J point or tip of S wave. The VAT (also referredto as the time of onset to the intrinsicoid deflection) is measuredfrom the QRS onset to the peak of the last R wave, and rep-resents the time taken for depolarization of the septum; andthe ventricle under the facing lead from endocardium toepicardium.5 VAT is normally <0.035 seconds in the pre-cordial leads V1 and V2 and <0.055 seconds in V5 and V6.

Right Bundle Branch Block

The presence of a right bundle branch block (RBBB) fol-lowing ACS indicates greater damage to the myocardiumand increased risk of mortality.1 New onset RBBB maysuggest worsening coronary artery disease or congestiveheart failure.

The conduction defect is determined to be an incom-plete RBBB when the ECG changes are observed in the pres-ence of QRS duration of 0.08 to 0.11 seconds.1 IncompleteRBBB may be a normal variant or may be associated withright ventricular hypertrophy or right ventricular dilatation.4

Complete RBBB may occur in the absence of underly-ing heart disease; however, it is more commonly associatedwith diseases such as coronary artery disease (includingACS), hypertensive heart disease, pulmonary embolism,cardiomyopathy, myocarditis, cor pulmonale, and rheu-matic heart disease.5,6 Transient RBBB also may occur fol-lowing cardiac surgery due to edematous conduction cells.

The ECG criteria for complete RBBB are as follows:

• QRS duration ≥0.12 seconds• Delayed intrinsicoid deflection >0.05 in right precor-dial leads (V1, V2)

• Second R wave visible in right precordial leads (rSRcomplex)

• Wide terminal S wave in Leads I, aVL, V5, V6

• QRS axis unchanged• Discordant ST–T complex in right precordial leads

The right bundle branch is more easily blocked be-cause it is structurally thinner than the left bundle branchand has only one main blood supply.4 When a block occursin the right bundle branch, the left ventricle continues todepolarize normally via the left bundle branch and fascicles,whereas right ventricular depolarization is delayed as itcomes from the left ventricle in a left-to-right direction.This situation results in altered QRS morphology wherethe first part of the complex remains unchanged, but a latesecond R wave is seen in the right precordial leads resultingin an rSR pattern. The delayed activation of the right ven-tricle is also seen as a deep S wave in the left precordialleads (leads 1, V4, V5, and V6) (Figure 1, A). Altered ven-tricular depolarization results in altered ventricular repolar-ization, which can be viewed on the ECG as an alterationor distortion of the ST–T complex. The ST–T complex is aterm used by Harrigan et al (2003) to describe ST segmentand T wave morphology.4 Because the left ventricle depo-larizes first, it also repolarizes first, meaning that repolariza-tion occurs from left to right; this alters the appearance ofthe ST–T complex. The ST–T complex is discordant (op-posite) to the QRS complex in the right precordial leads,4

whereas in the left precordial leads, the ST–T complex isnormal and upright (Figure 1, A).

Left Bundle Branch Block

Patients in whom left bundle branch block (LBBB) de-velops in association with ACS are at increased risk of

F I G U R E 1

A, Right bundle branch block (RBBB), as reflected in V1 and V6 with an intactinterventricular septum. Labeling of first and second R waves indicated in V1. B,Left bundle branch block (LBBB), as reflected in V1 and V6 with an intactinterventricular septum. Ventricular activation time and intrinsicoid deflectiondepicted on V6.

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hemodynamic instability and of having complete atrio-ventricular block develop and have reduced long-term sur-vival.7 The ST–T complex changes associated with LBBBimitate those usually associated with STEMI, making ECGinterpretation difficult. Failure to diagnosis STEMI in thepresence of LBBB in a presenting ECG may result in delaysin revascularization. As with any ECG changes suggestiveof ACS, further assessment of the patient including pre-senting history, past ECG (if available), and cardiac enzymesis required to exclude ACS.

Incomplete LBBB is characterized by loss of septal qwaves and a slurred or notched R wave in the left precor-dial leads in the presence of a QRS duration of 0.10 to0.11 seconds.1,4 This ECG pattern also may be referredto as an intraventricular conduction defect.

Complete LBBB usually is associated with heart dis-ease, particularly previous myocardial infarction, hyperten-sive heart disease, and cardiomyopathy; however, it alsomay be idiopathic.

The ECG criteria for complete LBBB are as follows:

• QRS duration ≥0.12 seconds• Delayed intrinsicoid deflection >0.07 seconds in leftprecordial leads (V5, V6)

• Wide QS wave in right precordial leads (V1, V2)• Wide monophasic R wave in V5, V6, leads I and aVL• Absence of septal q waves in V5, V6, lead I and aVL• Absence of septal r wave in V1, V2, and V3

• Poor progression of R waves across right and mid pre-cordial leads

• QRS axis unchanged or left axis deviation• Discordant ST–T complex in right and left precor-dial leads

When the left bundle branch is blocked, normal acti-vation of the right ventricle occurs; however, late activationof the left ventricle takes place through depolarization acrossthe interventricular septum from right to left, hence allforces are directed right to left. This delayed depolarizationof the left ventricle results in a deep QS wave in the right pre-cordial leads (Figure 1, B). The dominant vector is directedleftward. Septal q waves normally seen in leads I, aVL, V5,and V6 are absent because of loss of normal septal activation.

In the presence of LBBB, repolarization occurs fromright to left as the right ventricle is activated first. TheST–T complexes are discordant to the QRS complex inboth the right and left precordial leads (Figure 1, B).

Fascicular Blocks

Fascicular blocks (also known as hemiblocks) occur either inthe anterior or posterior fascicles of the left bundle branch

(Figure 2). The right bundle branch does not have fascicles.The anterior fascicle is thin, has only one blood supply, andlies within the turbulent outflow tract, making it moreprone to blockage.1

LEFT ANTERIOR FASCICULAR BLOCK

In the presence of a left anterior fascicular block (LAFB),the wave of depolarization travels inferiorly via the left pos-terior fascicle, resulting in delayed conduction to the ante-rior and lateral walls of the left ventricle. This situationresults in a shift in the mean electrical axis toward the leftand superior aspect, producing a left axis deviation.4 Thedelayed conduction to parts of the left ventricle cause awidening of the QRS complex, but not to the same extentas found in the bundle branch blocks. Additional ECG cri-teria include an initial small q wave and positive QRS com-plex in lead I, negative QRS complex in lead II and aVR,and small r wave in lead III.5

LAFBs may be associated with prior myocardial infarc-tion, hypertensive heart disease, aortic valve disease, cardio-myopathy, and degenerative cardiovascular changes.

F I G U R E 2

Table of figures illustrating location of bundle branch and fascicular blocks andresultant vectors, QRS morphology and axes in right bundle branch block (RBBB)with left anterior fascicular block (LAFB) and RBBB with left posterior fascicularblock (LPFB). The right column provides normal diagrams for comparison.

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LEFT POSTERIOR FASCICULAR BLOCK

Left posterior fascicular blocks (LPFBs) occur infrequentlybecause of the thicker structure of the fascicle, dual bloodsupply, and location away from turbulent blood flow.4 Inthe presence of an LPFB, the anterosuperior wall of the leftventricle depolarizes before the posteroinferior wall. Thisaltered depolarization causes a shift in the mean electricalaxis toward the right and inferior aspect, causing a rightaxis deviation and a slight widening of the QRS. AdditionalECG criteria include a small q wave in lead III and small rwave in lead I.5 For ease of comparison, the Table illus-trates the ECG criteria for both LAFB and LPFB.

The development of an LPFB may occur in associationwith ACS. Generally it is accompanied by an RBBB andhas a poor prognosis.1

MULTI-FASCICULAR BLOCKS

Intraventricular conduction blocks may occur in more thanone region of the conduction pathway. Blocks in 2 regionsare referred to as bifascicular blocks, and blocks in 3 regionsof the conduction pathway are referred to as trifascicularblocks. Bifascicular blocks may be defined as RBBB withLAFB or RBBB with LPFB. According to this terminology,the presence of LAFB and LPFB in combination is also abifascicular block; however, this scenario usually is depictedon the ECG as LBBB.

The most common bifascicular block is the combina-tion of RBBB and LAFB. This block is recognized on theECG by the RBBB pattern in combination with a left axisdeviation. The combination of RBBB and LPFB is recog-nized by a right axis deviation combined with the ECGcriteria for RBBB (Figure 2).

Trifascicular block occurs when conduction is ob-structed in the right bundle branch and both fascicles ofthe left bundle branch. If the interventricular conduction

system is completely blocked, complete atrioventricularblock will result. However, if the block in one of the fasciclesis incomplete, the trifascicular block may be identified bythe presence of a first- or second-degree atrioventricularblock in combination with a bifascicular block.

Summary

This article has identified the defining characteristics ofbundle branch and fascicular blocks on the 12-lead ECG.The ability to recognize these ECG patterns is importantfor ED nurses. Accurate ECG interpretation will assist withappropriate triage and timely intervention for patients. Allpatients who present to the emergency department withany suspicion of cardiac disease should have an ECG re-cording taken as soon as practicable. Also, ED nursesshould be educated in ECG interpretation to optimize pa-tient outcomes.

REFERENCES1. Conover M. Understanding electrocardiography. 8th ed. St Louis:

Mosby; 2003.

2. National Heart Foundation of Australia, The Cardiac Society ofAustralia and New Zealand. Guidelines for the management ofacute coronary syndromes 2006. Med J Aust 2006;184:S1-30.

3. Wellens H, Conover M. The ECG in emergency decision mak-ing. St Louis: Elsevier; 2006.

4. Harrigan R, Pollack M, Chan T. Electrocardiographic manifesta-tions: bundle branch blocks and fascicular blocks. J Emerg Med2003;25:67-77.

5. Huszar R. Basic dysrhythmias: interpretation and management.3rd revised ed. St Louis: Elsevier; 2007.

6. Aehlert B. ECGs made easy. 3rd ed. St Louis: Elsevier; 2006.

7. Geiter H. Understanding bundle-branch blocks. Nursing 2003;33(4):1-6.

TABLEECG criteria to distinguish fascicular blocks

Anterior fascicular block Posterior fascicular block

Mean electrical axis Left (−30° to −90°) Right (+110° to +180°)QRS duration <0.10 seconds <0.10 secondsQ waves Small q wave in I, aVL Small q wave in II, III, aVF

Absent in II, III, aVF Absent in I, aVL, V5, V6

R waves Small r waves in II, III, aVF Small r waves I, aVL

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