Bifascicular block

# Overview
Bifascicular block is a physiologic conduction interruption in the heart, in the main fascicles of the His-Purkinje system and below the atrioventricular nodes.

# Historical Perspective
- There is no historical perspective for bifascicular block in the literature.

# Classification
- Bifascicular block may present in 3 types based on the location of the block:
The block in right bundle branch and left anterior fascicle (more common type)
The block in right bundle branch and left posterior fascicle
Complete left bundle branch block 
- The block in right bundle branch and left anterior fascicle (more common type)
- The block in right bundle branch and left posterior fascicle
- Complete left bundle branch block 

# Pathophysiology
- The bifascicular block is due to a coronary blood supply occlusion or mechanotrauma to the fascicle. Because of a single coronary artery blood supply to the anterior fascicle or it's relationship with left ventricular outflow tract, the involvement of the left anterior fascicle is more common than left posterior fascicle. The block of two fascicles, the heart's electrical impulse is conducted through one fascicle. 

# Differentiating bifascicular block from other Diseases
- Bifascicular block must be differentiated from other diseases that cause similar ECG findings, such as:
- Ventricular tachycardia: Ventricular tachycardia is associated with atrioventricular dissociation which makes it different from the supraventricular rhythm with bifascicular block.
- Accelerated idioventricular rhythm: Accelerated idioventricular rhythm is associated with atrioventricular dissociation which makes it different from the supraventricular rhythm with bifascicular block.
- Wolff-Parkinson-White syndrome: The short PR interval which is not typically seen in bifascicular block can help in differentiating between bifascicular block and ventricular pacing.
- Right ventricular and biventricular pacing: The presence of pacemaker spikes in ventricular pacing can help in differentiating between bifascicular block and ventricular pacing. 

# Epidemiology and Demographics
- The bifascicular block occurs in approximately 1 to 2 % of adult population.
- Patients of all age groups may develop bifascicular block.
- Bifascicular block is more commonly observed among elderly patients. The highest incidence of bifascicular block was observed in males aged 70-74 years and females older than 80 years . 

# Causes:
- Common causes of bifascicular block development include :
- Ischemic hear disease
- Hypertension
- Anterior MI
- Hyperkalemia
- Degeneration of conduction system in Lev's disease
- Congenital heart disease
- Structural heart disease
- Aortic valve disease (especially aortic stenosis)
- Digoxin toxicity 

# Natural History, Complications and Prognosis
- The majority of patients remain asymptomatic until the progression of bradycardia due to atrioventricular block.
- The bifascicular block may progress to atrioventricular block in 1 to 4% of individuals and in 17% of symptomatic individuals annually. The risk of the complete atrioventricular block is increased in patients with a first-degree atrioventricular block.
- Common complications of bifascicular block include ventricular tachycardia and complete heart block.
- The long-term prognosis in patients with symptomatic bifascicular block is poor. The mortality of patients with bifascicular block is ranged between 2% to 15% with a 9% risk of sudden death. A higher mortality rate (29-38%) was reported in patients with syncope in the setting of structural heart disease and low left ventricular ejection fraction. However, the progression of bifascicular block to complete heart block is infrequent in asymptomatic patients. 
- The predictors of sudden cardiac death in chronic bifascicular block are atrial fibrillation, left ventricular ejection fraction < 35%, renal failure, NYHA  II, and structural heart disease .

# Diagnosis

## Diagnostic Criteria
- Bifascicular block is diagnosed on ECG.
- Findings on ECG include: 1) right bundle branch block and left anterior fascicular block, or 2) right bundle branch block and left posterior fascicular block, or 3) left anterior fascicular block and left posterior fascicular block.
- The ECG findings in the right bundle branch block include: 1) supraventricular rhythm, 2) QRS complex  120 ms, 3) slurred S-wave in lead I, 4) Terminal R-wave in lead V1.
- The ECG findings in the left anterior fascicular block include: 1) left axis deviation, 2) presence of rS complexes in inferior leads, 3) qR complexes in high lateral leads, 4) widening of QRS complexes
- The ECG findings in the left posterior fascicular block include: 1) right axis deviation, 2) qR complexes in inferior leads, 3) rS complexes in high lateral leads, 4) widening of QRS complexes 

## Symptoms
- Most of the patients with bifascicular block are asymptomatic.
- Symptoms of bifascicular block may include the following:
- Pre-syncope
- Syncope
- Sudden death 

## Physical Examination
- Patients with bifascicular block do not have any specific signs in the physical examination.
- Bradycardia may be present.

## Laboratory Findings
- Hyperkalemia may cause development of bifascicular block. Potassium level should be checked in patients with bifascicular block .

## X-ray
There are no x-ray findings associated with bifascicular block.

## Imaging Findings
- There are no imaging findings associated with bifascicular block.

## Other Diagnostic Studies
- Electrophysiology studies are needed in patients with syncope and bifascicular block  .

# Treatment
- Patients with asymptomatic bifascicular block do not need any treatment .
- Patients with acute bifascicular block may need a temporary pacemaker due to the possibility of complete heart block development.
- In patients with chronic bifascicular block, pacemaker implantation is needed in symptomatic patients, particularly syncope. It is also indicated in asymptomatic patients with intermittent third-degree, type II second-degree AV block, or alternating bundle branch block. Asymptomatic patients who undergo electrophysiologic study and have an incidental finding of prolonged HV interval (> 100 ms) or block below the His at long cycle length may need permanent pacing. Another indication for pacemaker therapy is the presence of neuromuscular disease (myotonic muscular dystrophy, Kearns-Sayre syndrome, peroneal muscular dystrophy, Erb's dystrophy) regardless of the presence of symptoms. 

## Prevention
- In patients with syncope and bifascicular block, the use of dual chamber permanent pacing can significantly prevent the syncopal episodes  .

# 2012 ACC/AHA/HRS Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities (DO NOT EDIT)

## Permanent Pacing in Chronic Bifascicular Block (DO NOT EDIT)

# Sources
- The ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities