Normal Sinus Rhythm with Right Bundle Branch Block and Left Posterior Fascicular Block
This ECG pattern represents a bifascicular block that requires immediate evaluation for symptoms (syncope, presyncope, dizziness) and consideration for electrophysiology study if symptomatic, as it carries significant risk for progression to complete heart block. 1
What This ECG Pattern Means
This combination indicates impaired conduction through two of the three fascicles of your ventricular conduction system:
- Right bundle branch block (RBBB): Delayed conduction through the right bundle branch, producing a QRS duration ≥120 ms with characteristic rSR' pattern in leads V1-V2 2
- Left posterior fascicular block (LPFB): Impaired conduction through the posterior fascicle of the left bundle branch, showing frontal plane axis between 90° and 180° with rS pattern in leads I and aVL 2
This specific combination (RBBB + LPFB) has a worse prognosis than RBBB with left anterior fascicular block, reflecting more extensive involvement of the specialized conduction system 1, 3
Immediate Clinical Assessment Required
Symptom Evaluation (Critical First Step)
You must be evaluated immediately for:
- Syncope or presyncope (sudden loss of consciousness or near-fainting) 1, 4
- Dizziness or lightheadedness 4
- Exercise intolerance or fatigue 4
- Palpitations 2
If ANY of these symptoms are present, you require an electrophysiology study urgently 1
Mandatory Testing
- 12-lead ECG documentation to confirm the diagnosis 1, 4
- Transthoracic echocardiogram to exclude structural heart disease (Class I recommendation) 2, 4
- Ambulatory ECG monitoring if symptoms suggest intermittent conduction abnormalities 4, 3
Risk Stratification and Management Algorithm
If You Have Symptoms (Syncope, Presyncope, Dizziness):
- Proceed directly to electrophysiology study 1
- Permanent pacemaker implantation is indicated (Class I) if:
- Risk of progression: With HV interval 70-99 ms, there is 12% risk of progression to AV block at 4 years; with HV ≥100 ms, risk increases to 24% 1
If You Are Asymptomatic:
- No pacemaker indicated (Class III: Harm recommendation for asymptomatic isolated bundle branch block) 4
- Close monitoring required with periodic ECG to detect progression 1, 4
- Watch carefully for development of symptoms including syncope, presyncope, or exercise intolerance 4
Underlying Causes to Investigate
The most common etiologies include:
- Coronary artery disease (most common, especially with acute myocardial infarction) 5, 6
- Hypertensive heart disease 6
- Cardiomyopathy 3
- Degenerative conduction system disease 2
- Heavy calcification of cardiac skeleton (in chronic cases) 5
LPFB is a rare finding and usually indicates more severe underlying pathology than left anterior fascicular block 3, 5
Prognostic Implications
- Risk of complete heart block: The combination of RBBB and LPFB carries increased risk of progression to third-degree AV block, particularly if symptoms are present 1, 6
- 10-year risk increases progressively with this conduction pattern 1
- Poor prognosis overall: Patients with this pattern, especially when associated with coronary disease, have poor outcomes and may require pacemaker implantation 6
Critical Caveats and Pitfalls
- Do not dismiss as benign: Unlike isolated RBBB which may be benign, bifascicular block with LPFB requires thorough evaluation 1, 3
- Medication caution: Avoid drugs that further impair cardiac conduction (beta-blockers, calcium channel blockers, antiarrhythmics) without cardiology consultation 4
- If first-degree AV block develops: This suggests trifascicular involvement and dramatically increases risk, warranting immediate EP study 3, 6
- Alternating bundle branch block: If ECGs show alternating LBBB and RBBB patterns, this is Class I indication for immediate pacemaker implantation 4