Bundle Branch Block

01Pathophysiology

The heart's ventricular conduction system begins at the bundle of His, which divides into the right bundle branch and the left bundle branch at the top of the interventricular septum. The left bundle branch further divides into the left anterior fascicle and the left posterior fascicle. These branches deliver electrical impulses rapidly to the ventricular myocardium, enabling synchronized contraction.

In a normal heartbeat, both ventricles depolarize nearly simultaneously because impulses travel through both bundle branches at the same speed. This produces a narrow QRS complex (< 120 ms). When one bundle branch is damaged or blocked, the impulse must first depolarize the ventricle connected to the intact branch, then spread cell-to-cell through the myocardium (rather than through the fast-conducting Purkinje fibers) to activate the opposite ventricle. This sequential, slower activation is what widens the QRS complex to $\ge 120\backslash geq\; 120$ ms and produces the characteristic morphologic changes on ECG.

In RBBB, normal septal depolarization occurs first (left to right, producing the initial small r wave in V1), then the left ventricle depolarizes normally. Finally, the right ventricle depolarizes late via slow myocardial conduction, producing a second R wave (R') in V1 and a late, broad S wave in the lateral leads (I, V6). Because the right ventricle finishes depolarizing after the left ventricle, the pulmonic valve closes later than the aortic valve, resulting in persistent (wide) splitting of S2 that does not disappear with expiration.

In LBBB, septal depolarization is reversed: it now proceeds right to left instead of the normal left-to-right direction. This reversal eliminates the normal septal Q waves in the lateral leads (I, aVL, V5-V6), which is a key diagnostic finding. The right ventricle depolarizes first via the intact right bundle, and then the impulse slowly crawls across the septum to activate the left ventricle late. This produces a broad, notched R wave in lateral leads and a deep S wave in V1-V2. Because the left ventricle finishes depolarizing after the right ventricle, the aortic valve closes after the pulmonic valve, producing paradoxical splitting of S2 (split on expiration, single on inspiration).

The clinical significance of BBB depends on context. Isolated RBBB can be a normal variant in healthy individuals because the right bundle branch is a thin, long, superficial structure vulnerable to stretch. LBBB, by contrast, is almost never benign and nearly always indicates structural heart disease, because the left bundle is a broad, thick structure that requires significant pathology to disrupt.

02Classification and Clinical Manifestation

ECG Classification

Clinical Manifestation by Etiology

The WiLLiaM / MaRRoW Mnemonic

This mnemonic helps you quickly recall the QRS morphology in leads V1 and V6:

03Diagnostic Workup

The 12-lead ECG is the only test needed to diagnose bundle branch block. The diagnosis is entirely electrocardiographic. You identify it by confirming a QRS duration of $\ge 120\backslash geq\; 120$ ms and then examining the QRS morphology in the right precordial leads (V1-V2) and lateral leads (I, V6) to classify it as RBBB or LBBB. For incomplete forms, the QRS falls between 100 and 119 ms with the same morphologic features. Once the BBB is identified, your next job on the exam is to determine whether it is new or old and whether it reflects underlying structural disease.

An echocardiogram is indicated in virtually all patients with newly discovered LBBB and in patients with RBBB who have symptoms or clinical suspicion for structural heart disease. The echo evaluates for left ventricular systolic dysfunction, regional wall motion abnormalities (suggesting prior infarction), valvular pathology, right ventricular dilation, and septal defects. In the setting of LBBB, the echo may also show interventricular mechanical dyssynchrony, which becomes relevant when considering CRT.

Serum potassium should be checked because severe hyperkalemia ($K^{+}>6.5K^+\; >\; 6.5$ mEq/L) can progressively widen the QRS and produce patterns that closely mimic a true BBB. This is a critical "do not miss" diagnosis because the treatment is urgent potassium correction, not pacemaker placement.

When a patient presents with new LBBB and chest pain, the traditional teaching was to treat this as a STEMI equivalent and activate the catheterization lab. Current guidelines have evolved: new LBBB alone is no longer an automatic STEMI equivalent, but if clinical suspicion for acute MI is high (ischemic symptoms, hemodynamic instability, markedly elevated troponin), emergent angiography is still indicated. In this setting, you apply the Sgarbossa criteria to identify MI in the presence of LBBB:

A score of $\ge 3\backslash geq\; 3$ is highly suggestive of acute MI. The modified Sgarbossa (Smith) criteria replace the third criterion with a ratio: discordant ST elevation / S wave amplitude $\ge 0.25\backslash geq\; 0.25$, which improves sensitivity.

For patients with bifascicular block and unexplained syncope, an electrophysiology study may be performed to measure the HV interval (conduction time from the His bundle to the ventricle). An HV interval $\ge 70\backslash geq\; 70$ ms or infra-Hisian block during atrial pacing indicates high risk for progression to complete heart block and supports pacemaker implantation.

04Management and Treatment

The fundamental principle is that bundle branch block is a conduction abnormality, not a primary disease. Management is therefore directed at the underlying etiology. An isolated, incidentally discovered RBBB in a young, otherwise healthy patient with a normal echocardiogram requires no treatment at all and carries an excellent prognosis.

For new LBBB without acute symptoms, the next best step is an echocardiogram to evaluate ventricular function and structural integrity. Common treatable causes include uncontrolled hypertension (optimize antihypertensive therapy), aortic stenosis (valve intervention if severe and symptomatic), and ischemic cardiomyopathy (revascularization and guideline-directed medical therapy for heart failure).

In the acute MI setting with new LBBB, management follows standard STEMI protocols: dual antiplatelet therapy (aspirin 325 mg loading + a P2Y12 inhibitor such as ticagrelor 180 mg or clopidogrel 600 mg loading), anticoagulation with heparin, and emergent percutaneous coronary intervention (PCI). Do not delay reperfusion while waiting for serial troponins if clinical suspicion is high and Sgarbossa criteria are met.

Permanent pacemaker indications in the context of BBB include: symptomatic bifascicular or trifascicular block with intermittent third-degree or type II second-degree AV block, alternating BBB on serial ECGs, and bifascicular block with syncope when other causes of syncope are excluded. Isolated first-degree AV block with bifascicular block (so-called "trifascicular block") does not automatically require pacing unless the patient is symptomatic.

Cardiac Resynchronization Therapy (CRT) is one of the most tested topics linked to LBBB. CRT involves placing a biventricular pacemaker that paces both ventricles simultaneously, restoring synchronous contraction. The strongest evidence supports CRT in patients with:

• NYHA Class II, III, or ambulatory IV heart failure

• LVEF $\le 35\mathrm{\%}\backslash leq\; 35\backslash \%$

• LBBB morphology with QRS $\ge 150\backslash geq\; 150$ ms

• Optimal guideline-directed medical therapy already in place

• Sinus rhythm

Patients with non-LBBB patterns (RBBB or nonspecific intraventricular conduction delay) derive less benefit from CRT, and the evidence is weaker for QRS durations between 120 and 149 ms. This distinction is frequently tested.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students lose points on BBB questions is by failing to recognize LBBB as a condition that masks the ECG diagnosis of acute MI. In a normal ECG, ST elevation localizes the infarction. But in LBBB, the baseline ST-T segment is already altered (discordant to the QRS), so traditional STEMI criteria are unreliable. Students either miss the MI entirely because they do not apply Sgarbossa criteria, or they call everything a STEMI equivalent simply because LBBB is present. The tested skill is knowing that you must use Sgarbossa criteria to detect concordant ST changes, and that clinical context (symptoms, hemodynamics, troponin) guides the decision.

A second frequent trap involves confusing VT with BBB. When a vignette presents a wide-complex tachycardia, students sometimes default to SVT with aberrant conduction (BBB). The safe rule on exams: a wide-complex tachycardia is VT until proven otherwise, especially in a patient with known structural heart disease or prior MI. Look for AV dissociation, fusion beats, and extreme axis deviation to confirm VT.

Third, students often forget that RBBB does not interfere with MI diagnosis. Because septal depolarization is preserved in RBBB, the initial forces of the QRS are unchanged, and ST elevation from an acute MI remains interpretable. Only LBBB obscures the picture. If a vignette shows RBBB with ST elevation, diagnose and treat the MI normally.

Finally, for CRT questions, the discriminator that exam writers test is the QRS morphology and duration. CRT provides the strongest benefit in true LBBB with QRS $\ge 150\backslash geq\; 150$ ms. A vignette describing a heart failure patient with RBBB and QRS of 130 ms is a distractor designed to trick students into selecting CRT. The answer in that scenario is to optimize medical therapy, not implant a biventricular device.

The core competency being tested across all BBB questions is pattern recognition on ECG combined with clinical reasoning about what the BBB implies and what it obscures. You must be able to identify the BBB morphology, determine its clinical significance in context, and select the correct next step in management.