Normal Sinus Rhythm with Right Bundle Branch Block and Left Posterior Fascicular Block
This ECG pattern represents bifascicular block—a combination of complete right bundle branch block (RBBB) and left posterior fascicular block (LPFB)—which indicates extensive conduction system disease affecting two of the three major fascicles of the ventricular conduction system. 1
What This Pattern Means Anatomically
Bifascicular block indicates that electrical conduction is impaired in both the right bundle branch and the posterior fascicle of the left bundle branch, leaving only the left anterior fascicle functioning normally. 1 This represents more extensive conduction system disease than isolated fascicular blocks and carries implications for risk of progression to complete heart block. 2
ECG Diagnostic Criteria Present
The combination you describe meets the following specific criteria:
Right Bundle Branch Block (Complete): 1
- QRS duration ≥120 ms
- rsr′, rsR′, or rSR′ pattern in leads V1 or V2
- S wave duration greater than R wave or >40 ms in leads I and V6
- Normal R peak time in leads V5 and V6 but >50 ms in lead V1
Left Posterior Fascicular Block: 1
- QRS duration <120 ms (though this is modified by the concurrent RBBB)
- Frontal plane axis between 90° and 180° (right axis deviation)
- rS pattern (small r, deep S) in leads I and aVL
- qR pattern (small q, tall R) in leads III and aVF
Clinical Significance and Risk Assessment
The combination of RBBB with LPFB carries a risk of progression to complete (third-degree) atrioventricular block, though the rate of progression is generally slow. 1 However, this risk is significantly higher than with isolated bundle branch block alone. 2
Key Prognostic Considerations
Patients with bifascicular block who develop syncope have a high mortality rate and substantial incidence of sudden death, particularly if the syncope is due to intermittent complete heart block. 1 The presence of symptoms dramatically changes the clinical approach and urgency of evaluation. 1
In asymptomatic patients, bifascicular block alone does not predict high risk of death from future bradyarrhythmia, and the progression to third-degree AV block occurs slowly. 1 However, this does not eliminate the need for appropriate evaluation and monitoring. 2
Essential Diagnostic Workup Required
Immediate Structural Assessment
Transthoracic echocardiography should be performed to exclude structural heart disease, as bifascicular block is commonly associated with coronary atherosclerotic heart disease, hypertensive heart disease, infiltrative cardiomyopathies (sarcoidosis, amyloidosis), and other structural cardiac pathology. 2, 3, 4
If echocardiography is unrevealing but structural heart disease is suspected, advanced imaging with cardiac MRI should be considered, particularly to evaluate for infiltrative diseases that can cause conduction abnormalities before overt structural changes appear. 5, 2
Critical Monitoring Strategy
Ambulatory electrocardiographic monitoring should be strongly considered even in asymptomatic patients with bifascicular block to document suspected intermittent higher-degree AV block, as this may occur without symptoms. 2 This is particularly important because:
- Intermittent complete AV block can occur between clinic visits 6
- The longest pauses may only be captured with extended monitoring 2
- Detection of asymptomatic high-grade block changes management 2
Evaluation for Underlying Ischemia
Stress testing should be performed to exclude ischemic heart disease as an underlying cause, as coronary disease is a common etiology of bifascicular block. 2, 3, 4 The majority of patients with this pattern have significant coronary atherosclerotic disease, often involving multiple vessels. 4
Management Approach Based on Symptoms
If Patient Is Asymptomatic
In asymptomatic patients with bifascicular block and 1:1 atrioventricular conduction, permanent pacing is not indicated. 2 However, the following monitoring strategy is essential:
- Regular clinical and ECG follow-up, with frequency determined by underlying heart disease 2
- Patient education about symptoms of bradycardia or syncope that should prompt immediate evaluation 2
- Ambulatory monitoring to exclude intermittent high-grade block 2
- Periodic reassessment as conduction disease may progress over time 2
If Patient Has Syncope or Presyncope
If syncope occurs in the presence of bifascicular block and cannot be attributed to another cause with certainty, prophylactic permanent pacing is indicated, especially if syncope may have been due to transient third-degree AV block. 1 This is because:
- Syncope with bifascicular block is associated with increased incidence of sudden death when third-degree AV block is present 1
- Even if syncope is recurrent, pacing relieves neurological symptoms 1
- The cause of syncope cannot always be determined with certainty 1
For symptomatic patients where AV block is suspected, ambulatory electrocardiographic monitoring is recommended to document the arrhythmia. 2
If symptoms are suggestive of intermittent bradycardia but no AV block has been documented, electrophysiological study (EPS) is reasonable. 2 If EPS demonstrates an HV interval ≥70 ms or evidence of infranodal block, permanent pacing is recommended. 2
If Patient Develops Complete Heart Block
Permanent pacemaker implantation is indicated for third-degree and advanced second-degree AV block at any anatomic level associated with bradycardia with symptoms (including heart failure) or ventricular arrhythmias presumed to be due to AV block. 1
Permanent pacing is also indicated for third-degree AV block in awake, symptom-free patients with documented asystole ≥3.0 seconds or any escape rate less than 40 bpm, or with an escape rhythm below the AV node. 1
Special Clinical Scenarios
Alternating Bundle Branch Block Pattern
If ECG evidence shows alternating bundle branch block (RBBB with LPFB on one ECG and RBBB with left anterior fascicular block on another, or RBBB alternating with left bundle branch block), permanent pacing is recommended regardless of symptoms. 2 This pattern indicates disease in all three fascicles and represents trifascicular block. 6, 3
Infiltrative or Progressive Diseases
In patients with sarcoidosis, amyloidosis, or neuromuscular diseases presenting with bifascicular block, additional evaluation and potential permanent pacing should be considered even if the conduction abnormality appears stable, as these conditions may warrant pacemaker implantation due to the possibility of disease progression. 1, 2
Medication Considerations
If the patient requires medications that may exacerbate AV block (beta-blockers, calcium channel blockers, antiarrhythmics), prophylactic permanent pacing should be considered, particularly if syncope has occurred. 1
Critical Pitfalls to Avoid
Do not assume bifascicular block is benign simply because the patient is asymptomatic—intermittent complete heart block can occur without warning and may only be captured with ambulatory monitoring. 2, 6
Do not miss the diagnosis of left posterior fascicular block by failing to recognize the characteristic right axis deviation and specific lead morphologies, as LPFB is less common than left anterior fascicular block and may be overlooked. 1, 4
Do not delay evaluation for structural heart disease, particularly infiltrative cardiomyopathies, which may require specific therapies beyond pacing and can cause conduction abnormalities before obvious structural changes appear on standard echocardiography. 5, 2
Do not rely solely on PR interval or HV interval measurements to predict progression to complete heart block, as there is no reliable correlation between these measurements and the development of third-degree AV block or sudden death in bifascicular block. 1
Recognize that the prognosis of patients with this pattern is generally poor due to the high prevalence of underlying structural heart disease and the risk of progression to trifascicular block and complete AV block. 3