Clinical Overview

Bundle branch block (BBB) is delayed or interrupted conduction below the bundle of His, in either the right or the left branch of the ventricular conduction (His-Purkinje) system, so that the two ventricles no longer depolarize together (Merck Manual Professional Edition, 2024). This dataset carries a bare “Bundle Branch Block” label as a general entry alongside separate right-sided labels — RBBB, and its complete and incomplete grades CRBBB and IRBBB — and separate left-sided labels — this dataset’s LFBBB entry, which despite its literal name is the complete left bundle branch block finding rather than a fascicular one, and ILBBB for the incomplete left-sided form. [CLINICAL REVIEW NEEDED: the dataset does not document how it decided to file a given record under this bare BBB label rather than under one of the side- and grade-specific labels, so a strip carrying this label should not be read as asserting which side or grade is involved.] This page covers the concept spanning both branches — which side is blocked, and how completely — while the side-specific criteria live on the family pages linked in the differential below.

Mechanistically, the bundle of His divides into a right bundle branch, which runs down the right side of the interventricular septum to the right ventricle, and a left bundle branch, which activates the left ventricle (and itself divides further into anterior and posterior fascicles, a separate and narrower finding covered on its own pages, not this one). When either branch is blocked, the ventricle it serves is not activated through the fast conduction system at all; instead, the depolarization wavefront reaches it late, spreading cell to cell across the septum from the side that is still conducting normally (StatPearls, Left Bundle Branch Block, 2024; Merck Manual Professional Edition, 2024). The direction of that delay is what separates the two families: a right-sided block delays right ventricular activation, producing a normal-timed first half of the QRS followed by a late terminal force aimed at the right ventricle; a left-sided block reverses the normal left-to-right septal activation sequence entirely, so the normal small septal q wave in the lateral leads disappears and is replaced by a broad, monophasic R wave for the length of the QRS (StatPearls, 2024; LITFL, Left Bundle Branch Block, 2024). Both sides share the same grading logic: a QRS reaching 120 ms is complete, and a QRS that is widened with the same morphology but stays below that threshold is incomplete, per the AHA/ACCF/HRS 2009 intraventricular conduction disturbance standardization statement and confirmed by current sources on both sides (Surawicz et al., Circulation, 2009; StatPearls, 2024; Merck Manual Professional Edition, 2024). [CLINICAL REVIEW NEEDED: sources do not agree on exactly where the incomplete band’s lower boundary sits — some put it at 100 ms, others above 110 ms — so a QRS in that narrow range should be reported as measured rather than forced into a grade.]

The two sides carry meaningfully different clinical weight, which is the main reason this dataset — and this page’s own differential — keeps them as separate labels rather than one. Right bundle branch block often occurs without any heart disease and, in isolation, does not appear to raise cardiovascular risk on its own; left bundle branch block rarely occurs without an identifiable underlying disorder — aortic stenosis, ischemic heart disease, hypertension, dilated cardiomyopathy, or degenerative disease of the conduction system — and carries higher associated cardiovascular risk (Merck Manual Professional Edition, 2024). Left bundle branch block is present in roughly a third of patients with heart failure, and left bundle branch block accompanying a reduced ejection fraction — new or longstanding — is itself an indication for cardiac resynchronization therapy or conduction-system pacing once the ejection fraction and QRS duration cross guideline thresholds (LVEF 35% or less, QRS 150 ms or more, NYHA class II-IV symptoms on optimized medical therapy) (StatPearls, 2024; Chung et al., 2023 HRS/APHRS/LAHRS guideline on cardiac physiologic pacing, Heart Rhythm, 2023). New left bundle branch block after transcatheter aortic valve replacement — a recognized procedural complication rather than an incidental finding — was independently associated with higher cardiovascular mortality (hazard ratio 2.31) in a 2025 single-center registry (Baranov et al., Brazilian Journal of Cardiovascular Surgery, 2025). New left bundle branch block with chest pain also changes how the ECG is read: the block’s own secondary repolarization changes can mimic or mask the changes of a myocardial infarction, so acute coronary syndrome in that setting is assessed with the modified Sgarbossa criteria rather than by treating the new block itself as an automatic STEMI-equivalent (LITFL, Sgarbossa Criteria, 2025; LITFL, Left Bundle Branch Block, 2024). Right bundle branch block does not obscure the same criteria to nearly the same degree.

The block itself produces no symptoms on either side. It is a conduction finding, not a rhythm disturbance, and most people with an isolated bundle branch block have no idea it is present until it turns up on a routine ECG (Cleveland Clinic, 2026; StatPearls, 2024). Any dizziness, fatigue, palpitations, presyncope, syncope, or breathlessness a patient reports comes from the underlying cause or, occasionally, from block severe enough to also impair AV conduction — not from the delayed ventricular activation on its own (Cleveland Clinic, 2026).

Causes span both sides but are not evenly distributed between them. Shared causes include age-related fibrodegenerative change in the conduction system (Lenègre’s or Lev’s disease), ischemic heart disease (particularly anterior myocardial infarction), cardiomyopathy, myocarditis, and iatrogenic injury from cardiac surgery or transcatheter aortic valve replacement, which produces new left bundle branch block in roughly 30% to 50% of patients (StatPearls, 2024; Merck Manual Professional Edition, 2024). Aortic stenosis and hypertensive heart disease weigh more heavily toward the left side; right-heart pressure or volume overload — pulmonary embolism, pulmonary hypertension, chronic obstructive pulmonary disease — is a right-sided cause with no left-sided counterpart (Merck Manual Professional Edition, 2024; Cleveland Clinic, 2026). Either side can occur with no identifiable structural cause, though this is more common on the right (Merck Manual Professional Edition, 2024).

Interpretation Guide

Key Features:

• Rate: not a defining feature — bundle branch block is a conduction finding superimposed on whatever the underlying rate happens to be
• Rhythm: not a defining feature — the block describes how the impulse reaches the ventricles, not where the rhythm originates
• P waves: within normal limits. The delay sits below the AV node, so nothing about atrial depolarization changes
• PR interval: within normal limits (0.12-0.20 s) unless a separate, coexisting AV conduction disturbance is present
• QRS complex: the defining feature, and the one that assigns both the side and the grade. Measure the duration first: 120 ms or more is a complete block, and a QRS that is widened with the same morphology but stays below that threshold is an incomplete one. Then read the morphology to assign the side. A right-sided block shows an rSR’ or rsR’ (“M-shaped”) pattern in V1-V2 with a wide, slurred terminal S wave in the lateral leads (I, aVL, V5-V6) — the criteria detailed on this dataset’s Right Bundle Branch Block family pages. A left-sided block shows the opposite picture: a dominant, broad, often notched or slurred monophasic R wave in the lateral leads with no septal q wave there, and a QS or rS complex in V1 — the criteria detailed on this dataset’s Left Front Bundle Branch Block entry, its label for the complete left-sided form
• ST segment: appropriately discordant ST depression or elevation relative to the delayed terminal QRS forces, on either side — a secondary consequence of the altered depolarization sequence, not by itself a primary ischemic finding. A right-sided block does not significantly interfere with standard ischemia criteria; a left-sided block can, which is why new ischemic symptoms with a new left-sided block are read against the modified Sgarbossa criteria rather than the standard ST criteria alone
• T waves: appropriately discordant T wave inversion following the same depolarization sequence as the ST change above; confirmatory rather than diagnostic, and it does not indicate which side or grade is present
• QT interval: not independently diagnostic. Widening the QRS lengthens the measured QT mechanically, so a QT that looks prolonged on a strip carrying either form of this block may be reporting nothing beyond the QRS width already recorded
• Other findings: check the frontal-plane axis before calling a right-sided finding isolated — a marked axis shift alongside an RBBB pattern indicates an accompanying fascicular block, a different and higher-risk bifascicular finding (see the differential below). Note whether the pattern is fixed or appears only at faster rates; a pattern present only on early or fast beats is more likely rate-related aberrant conduction than a permanent block

Key Leads

• Lead V1 – The single most useful lead for telling the two families apart. A right-sided block shows the rSR’ (“M-shaped” or “rabbit ears”) pattern with a prolonged R wave peak time (greater than 50 ms); a left-sided block shows the opposite — a QS or rS complex, since the delayed left ventricular forces point away from this lead (StatPearls, 2024; LITFL, Left Bundle Branch Block and Right Bundle Branch Block, 2024; Surawicz et al., AHA/ACCF/HRS standardization statement, Circulation, 2009)
• Leads I, aVL, V5-V6 – Show the reciprocal, side-defining finding. A right-sided block produces a wide, slurred terminal S wave here, confirming the delayed forces are directed toward the right ventricle. A left-sided block produces the opposite: a broad, often notched monophasic R wave with no septal q wave and an R wave peak time greater than 60 ms, confirming delayed left ventricular activation (StatPearls, 2024; Merck Manual Professional Edition, 2024)
• Leads II, III, aVF read against I and aVL – Not needed to recognize either form of block on its own, but these are the leads that reveal the frontal-plane axis shift of an accompanying fascicular block, which turns an isolated right-sided block into a bifascicular one
• Whichever lead shows the widest QRS complex – Used to measure the duration that assigns the complete or incomplete grade on either side. Any single lead can under-read the QRS, since the earliest onset and latest offset of the complex are not both visible in one lead; the standard is a global measurement across all leads, which is by definition at least as long as any single-lead reading (Surawicz et al., AHA/ACCF/HRS standardization statement, Circulation, 2009)

Differential Diagnosis

• Right Bundle Branch Block (RBBB) — this dataset’s family/umbrella page for the right-sided form specifically, with the complete and incomplete grades detailed on their own linked pages. Distinguishing clue: the rSR’ pattern in V1-V2 with a wide, slurred S wave in the lateral leads, rather than the broad monophasic R wave a left-sided block produces there.
• Left Front Bundle Branch Block (LFBBB, this dataset’s label for complete left bundle branch block despite its literal fascicular-sounding name) — the complete left-sided form. Distinguishing clue: a QRS at or above 120 ms with a broad, notched or slurred monophasic R wave in I, aVL, and V5-V6, absent septal q waves there, and a QS or rS complex in V1.
• Left Anterior Fascicular Block (LAnFB) — a block of only one fascicle of the left bundle, not the whole branch. Distinguishing clue: the QRS stays normal or only mildly widened (well under the 120 ms complete-block threshold) and does not take on either bundle branch morphology described above; a marked left-axis deviation is the dominant finding instead.
• Intraventricular Block (IVB) — this dataset’s label for a non-specific intraventricular conduction delay. Distinguishing clue: that finding is defined by exclusion — a widened QRS whose morphology reproduces neither the right-sided rSR’ pattern nor the left-sided broad monophasic R pattern described above. If either classic morphology is present, the finding belongs on the corresponding side-specific page instead.

Treatment Brief

The block itself is not treated on either side. What a monitoring technician or nursing student owns here is measuring the strip correctly, working out which side and grade it is, and recognizing the contexts where either form stops being an incidental finding.

• Measure the QRS duration and read the V1 and lateral-lead morphology together — that pairing is what assigns the strip to a side and a grade, and reporting both is more useful than reporting a label the dataset may have applied loosely.
• Confirm lead placement and repeat the tracing if the pattern is new or unexpected; misplaced precordial electrodes can manufacture a right-precordial appearance that is not there.
• Compare against a prior ECG whenever one exists. A longstanding, unchanged pattern in an asymptomatic patient is reassuring; a new one — on either side — is not.
• Notify the provider promptly if a new block accompanies chest pain or other symptoms suggestive of acute coronary syndrome, particularly for a new left-sided block, since standard ST-segment criteria are read differently in that setting.
• Check the frontal-plane axis before calling a right-sided finding isolated. A right-sided block with a fascicular block is a bifascicular pattern with a real risk of progressing to complete heart block, and that risk rises further if the patient has had syncope — flag it rather than treating it as routine.
• For a patient with known reduced ejection fraction or heart failure, note a new left-sided block explicitly; it is directly relevant to that patient’s candidacy for resynchronization therapy or conduction-system pacing, a decision for the care team rather than something to act on at the bedside.
• Watch whether the pattern is constant or intermittent. A wide complex appearing only on early beats or only at faster rates is aberrant conduction, not a newly acquired fixed block, and should be described that way.