Bundle branch and fascicular blocks
Bundle branch and fascicular blocks are delayed conduction in the right bundle, the left bundle, or one fascicle of the left bundle, diagnosed on the 12-lead ECG by a QRS of 120 ms or longer with typical V1 and V6 morphology, or by axis deviation for a fascicular block. Stable isolated or bifascicular block needs no pacing; alternating bundle branch block, syncope from intermittent complete block, a new LBBB with ischemic symptoms, and LBBB with an LVEF of 35% or less each change management.
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What this article covers
- Overview
Definition: a bundle branch block is delayed conduction in the right bundle (right bundle branch block, RBBB) or the left bundle (left bundle branch block, LBBB); a fascicular block (hemiblock) is…
- Epidemiology
Low progression rate: stable LBBB or RBBB, with or without a prolonged PR interval, progresses to complete heart block at about 1% to 3% per year.
- Risk factors
Age-related fibrosis and calcification of the AV node, His bundle, and bundle branches, the usual degenerative substrate.
- Pathophysiology
Asynchronous ventricular activation: when one bundle fails, the affected ventricle depolarizes late by slow myocyte-to-myocyte conduction spreading from the unaffected ventricle, which widens the QRS to 120 ms or longer.
- Clinical features
Usually an asymptomatic ECG finding: isolated block causes no symptoms; symptoms point to coexisting AV block or structural disease.
- Differential diagnosis
Ventricular tachycardia: wide-complex tachycardia is ventricular in more than 85% of cases; concordance, AV dissociation, fusion or capture beats, and atypical BBB morphology favor VT over aberrancy (ventricular tachycardia).
- Diagnosis
Establish the diagnosis with the 12-lead ECG criteria: a bundle branch block requires a QRS of 120 ms or longer (normal is below 120 ms) with the defining V1 and V6 morphology, and a fascicular block…
- Management
First, decide whether the patient is unstable: for hypotension, altered mentation, or ischemic chest pain from bradycardia, give atropine 1 mg IV every 3 to 5 minutes to a maximum total of 3 mg…
- Complications
Sudden death or syncope from intermittent complete heart block, the main threat in alternating and bifascicular block.
- Prognosis
Stable isolated block is benign: stable RBBB or LBBB progresses to complete heart block at only 1% to 3% per year, and asymptomatic bifascicular block may never progress in most patients.
- Prevention
Avoid drugs that worsen conduction: tricyclic antidepressants are contraindicated with bundle branch or AV block, and donepezil, rivastigmine, and galantamine are avoided in documented conduction disease.
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