• Bundle branch blocks can occur idiopathically as people age and the conduction system wears out, but can also be the result of infarction, ischemia, HTN, cardiomyopathies, endocarditis with abscess, etc.
• Management centers around distinguishing old from new where possible, and parsing whether patients are symptomatic in order to guide treatment.
• Important to work up as they may be secondary to new pathology or reversible cause.
• They can be a precursor to complete heart block, and may be an indication that a pacemaker is needed.
• Clinical starting point is comparing to previous ECG where possible and taking a good history.
• Clinical endpoint depends on patient condition, and history. If asymptomatic discharge with follow up may be appropriate, whereas if they are symptomatic, progressing towards complete heart block, or showing signs underlying of ACS or other cause they should be treating accordingly.
• In normal conduction electrical impulses originate at the SA node, travel through the AV node, through the bundle of his, which then splits into the left and right bundles. The Left Bundle then further divides into the Left Anterior Fascicle, and the Left Posterior Fascicle (and in some cases there is a median fascicle as well).
• In Left Bundle Branch Block (LBBB) the conduction is interrupted to both the left anterior fascicle and the left posterior fascicle.
• In Right Bundle Branch Block (RBBB) the right bundle has conduction blocked.
• In fascicular blocks occur when the complete left bundle is not blocked, but rather only the left anterior or left posterior fascicles are blocked (LAFB, LPFB).
• In Trifascicular block can be complete or incomplete, and occurs when all three of the above conduction blocks occur together.
• In Incomplete block: some of the fascicles may be damaged but still intermittently conducting. These patients may present with only bifascicular block on ECG, but have episodes of syncope and/or bradycardia as they are intermittently in complete block.
• In Complete heart block: conduction is not able to pass through the ventricles from the AV node, leading to complete AV dissociation (3rd degree heart block) and a ventricular escape rhythm.
• Trifascicular and bifascicular with intermittent trifascicular block are both class I indications for pacemaker, and require cardiology to see.

• Ideally any patient presenting with a conduction block on ECG should have an old ECG of theirs for comparison to determine if this change is new. However this is not always possible. ECG criteria:
• LBBB:
  • QRS >120ms (3 small squares)
  • Large S waves in V1-V3 caused by depolarization from RV instead of Left bundle.
  • Large R waves (>60ms/1.5 small squares) in lateral leads often with characteristic ‘M’ shaped notch caused by depolarization from right bundle/ventricle instead of Left Bundle.
  • Often have left axis deviation observed because of depolarization from Right bundle/ventricle towards left rather than downward from Left Bundle.

*Note – In LBBB patients can have allowable “discordant” ST changes where the abnormal QRS will have appropriately abnormal depolarization. I.e., if the QRS has deep S waves some ST elevation is allowable, or if there is a large preceding R wave, some ST depression is allowable. General rule is that it is <25% of S or R wave it follows is allowable. In these cases Scarbossa’s criteria (https://www.mdcalc.com/modified-sgarbossas-criteria-mi-left-bundle-branch-block) can be used to distinguish this from worrisome concordant ST changes. In addition to making the diagnosis of Myocardial infarction and schema more difficult it also makes the diagnosis of left ventricular hypertrophy more difficult.

• RBBB
  • -QRS >120ms (3 small squares)
  • -slurred S wave in lateral leads: AvL, I, V5, V6 because depolarization is moving from the left bundle/ventricle towards the Right ventricle
  • characteristic RSR’ “bunny ears” with second R’ larger than the first in anterior/septal leads: V1, V2

*Note : Distinguishing LBBB from RBBB quickly – if the QRS is long look at V1 and the depolarization is positive (big R wave) its a RBBB. If the depolarization is negative (big S wave) its a LBBB.

• Fascicular blocks: Fascicular blocks occur when conduction to one of the divisions of the left bundle branches become blocked. Depolarization can still travel down the other fascicles of the left bundle, which is why the QRS is usually borderline prolonged, rather than obviously >120ms (3 small squares), and leads to axis deviation away from the blocked area.
• LAFB: Left anterior fascicular block (LAFB) is the most common fascicular block because it is thinner than the posterior fascicle. In LAFB depolarization can only travel through the posterior fascicle which supplies the lateral leads areas of AvL and I. Therefore on ECG LAFB will have:
  • Large R waves in AvL and I, and large S waves in the inferior leads (II, III, AvF) to which conduction is blocked.
  • Left axis deviation.
  • Borderline prolonged QRS (>120ms)

• LPFB: In left posterior fascicular block (LPFB) conduction is blocked to the posterior fascicle which supplies the lateral leads (AvL and I) but conduction travels freely to the inferior leads (II, III, AvF). Therefore on ECG LPFB will have:

• • Large R waves are seen in the inferior leads, and large S waves are seen in the lateral leads.
  • Right axis deviation.
  • Borderline prolonged QRS (>120ms)
• Trifascicular Block
  • Trifascicular block can be complete or incomplete, and occurs when all three of the above conduction blocks occur together.
  • Incomplete block: some of the fascicles may be damaged but still intermittently conducting. These patients may present with only bifascicular block on ECG, but have episodes of syncope and/or bradycardia as they are intermittently in complete block.
  • Complete heart block:conduction is not able to pass through the ventricles from the AV node, leading to complete AV dissociation (3rd degree heart block) and a ventricular escape rhythm.
  • Trifascicular and bifascicular with intermittent trifascicular block are both class I indications for pacemaker, and require cardiology to see.

• There is no hard and fast treatment algorithm/rule about treatment of bundle branch blocks. It all depends on whether the blocks are new, or symptomatic. If the history is compatible with ACS they should be worked up and treated accordingly. Scarbossa’s criteria may help to distinguish ST elevation from appropriate discordance.
• Heart blocks are often the result of degeneration of the conduction system over time, but in some cases can be secondary to neuromuscular conditions, ischemia or infarction or reversible causes.
• Patients with new fascicular blocks are at greater risk of future atrial fibrillation and heart failure, and should therefore be educated on symptoms to look out for and return in case of.
• Patients with new complete or incomplete trifascicular block may require transcutaneous or transvenous pacing in the ED for symptomatic bradycardia, and need to be seen by cardiology and an electrophysiologist for pacemaker implantation.
• Drugs
  • Do not give AV nodal blocking agents (ie. atropine) to patients with bifascicular block. These blocks are occurring distal to the AV node so AV nodal blockers will be of little use.

1. Sauer, W. (2019, July 1). Left anterior fascicular block. UpToDate.

   
   

2. Larkin, J., & Buttner, R. (2021, December 9). Left Posterior Fascicular Block (LPFB). Life in the Fast Lane.

   
   

3. Jones, T. (2015, July 13). Fascicular blocks. CanadiEM.

   
   

4. Rezaie, S. (2013, November 1). Bundle Branch Blocks: 101. REBELEM.

   
   

5. https://www.mdcalc.com/modified-sgarbossas-criteria-mi-left-bundle-branch-block