Explain right bundle branch block on an electrocardiogram chart.

Right Bundle Branch Block (RBBB) on ECG

Diagnostic ECG Criteria

Complete RBBB is diagnosed when the QRS duration is ≥120 ms in adults, accompanied by an rSR' (or rsR', rSR) pattern in leads V1-V2, and S waves in leads I and V6 that are either wider than the R wave or exceed 40 ms duration. 1

Core Diagnostic Features

• QRS Duration: ≥120 ms in adults (>100 ms in children 4-16 years, >90 ms in children <4 years) 1
• Right Precordial Leads (V1-V2): Classic rSR' configuration with the R' deflection typically wider than the initial R wave 1
• Lateral Leads (I, V6): S wave duration exceeds R wave duration or is >40 ms 1
• R Peak Time: Normal in V5-V6 but prolonged >50 ms in V1 1

Morphological Variations on ECG

The classic RBBB pattern shows an rSR' configuration in right precordial leads (V1-V2), creating the characteristic "rabbit ears" or "M-shaped" appearance. 1 In some cases, you may see a wide, notched R wave in V1-V2 instead of the typical rSR' pattern. 1

Important ECG change: RBBB reduces the amplitude of the S wave in right precordial leads, which decreases the sensitivity of voltage criteria for detecting left ventricular hypertrophy. 1

Distinguishing Complete from Incomplete RBBB

• Complete RBBB: QRS ≥120 ms with characteristic morphology 1
• Incomplete RBBB (iRBBB): QRS duration 110-120 ms in adults with similar morphological features 1
• iRBBB is most often a benign normal variant, especially in children and athletes, but can signal underlying structural heart disease, RV strain, or congenital heart disease 2

ECG Patterns in Specific Clinical Contexts

RBBB with Left Ventricular Hypertrophy

When RBBB is present, standard voltage criteria for LVH become less reliable. Special criteria enhance LVH detection in RBBB: 1

• S wave in V1 >2 mm (0.2 mV)
• R wave in V5-V6 >15 mm (1.5 mV)
• QRS axis to the left of -30°
• Presence of left atrial abnormality

Bifascicular Block Pattern

Bifascicular block (RBBB + left anterior or posterior hemiblock) appears as RBBB morphology combined with axis deviation: 3

• RBBB + Left Anterior Fascicular Block: RBBB pattern with left axis deviation (typically -45° to -90°), larger R waves in leads I and aVL, deeper S waves in leads II, III, aVF 3
• RBBB + Left Posterior Fascicular Block: RBBB pattern with right axis deviation (>+90°), though this combination is rare 4

Critical pitfall: Bifascicular blocks reflect extensive conduction system involvement and carry increased risk of progression to complete AV block, warranting closer monitoring. 3

Masquerading RBBB

In rare cases with severe left ventricular disease and high-degree left anterior fascicular block, the characteristic slurred S wave in lead I may become smaller or disappear, mimicking left bundle branch block in standard leads. 5 This "masquerading RBBB" carries a poor prognosis as it always implies severe underlying heart disease. 5

Clinical Significance and Underlying Causes

Prevalence and Benign vs. Pathological

Complete RBBB occurs in approximately 1% of the general population, with prevalence of 0.5-2.5% in young adult athletes and 0.6% in males under 40 years. 1, 4 RBBB may represent either an idiopathic, isolated, clinically benign conduction delay through the right bundle branch, or it may signal serious underlying cardiovascular disease. 4

Pathological Causes Requiring Evaluation

Bundle branch block may develop from: 4

• Ischemic heart disease (particularly anterior infarction with persistent conduction disturbances, carrying unfavorable prognosis) 2
• Cardiomyopathies (dilated, hypertrophic, arrhythmogenic right ventricular cardiomyopathy) 4
• Myocarditis and infectious diseases (including Chagas' disease) 2
• Infiltrative processes (sarcoidosis, cardiac tumors, amyloidosis) 2
• Congenital heart disease (both unoperated and post-surgical, including atrial septal defects, Ebstein's anomaly) 4, 1
• Hypertensive heart disease 2
• Degenerative lesions of specialized conducting tissue 2
• Genetic conditions (Lenegre disease—autosomal dominant SCN5A mutations causing progressive conduction disease in young individuals) 4, 2

High-Risk Clinical Scenarios

Acute Myocardial Infarction: New RBBB in the setting of acute MI increases in-hospital mortality by approximately 64% compared to AMI patients without bundle branch block. 3 These patients are frequently undertreated with evidence-based therapies despite worse outcomes. 3

Post-TAVR: Pre-existing RBBB is a strong predictor of permanent pacemaker requirement after transcatheter aortic valve replacement (occurring in 40.1% vs. 13.5% without pre-existing RBBB) and is associated with increased late all-cause and cardiac mortality. 3

Mandatory Evaluation Algorithm

Initial Assessment

All patients with newly detected complete RBBB require: 4, 2

1. Transthoracic echocardiography to exclude structural heart disease (cardiomyopathies, ischemic disease, hypertensive heart disease, congenital abnormalities) 4, 2
2. Symptom assessment: syncope, presyncope, dizziness, fatigue, palpitations, exercise intolerance 2
3. Family history: sudden cardiac death, inherited cardiac disease, conduction abnormalities 4

Additional Testing Based on Clinical Context

• Exercise stress testing: To assess for exercise-induced arrhythmias, conduction worsening, or ischemia when suspected 4, 2
• 24-hour Holter monitoring: To detect intermittent conduction abnormalities or arrhythmias 4
• Cardiac MRI: Helpful for diagnosing infiltrative processes (sarcoidosis, hemochromatosis, amyloidosis) and can detect subclinical cardiomyopathy even when echocardiography is normal 2
• Electrophysiology study: May provide diagnostic information in patients with syncope, though sensitivity is variable 2

Red Flags Requiring Urgent Specialized Evaluation

Immediate cardiology referral is mandatory for: 4, 2

• RBBB pattern with ST-elevation in V1-V3 (Brugada pattern—sudden cardiac death risk) 2
• Syncope or presyncope, especially with palpitations 4
• Family history of sudden cardiac death (warrants genetic evaluation) 4
• Bifascicular block (RBBB + left anterior or posterior hemiblock—increased risk of progression to complete AV block) 4, 3
• Symptoms suggesting ARVC (family history of sudden death, ventricular arrhythmias, localized QRS prolongation >110 ms in V1-V3 with epsilon waves) 4, 2

Special Considerations in Athletes

In asymptomatic young athletes, isolated RBBB may represent idiopathic, clinically benign conduction delay, though follow-up is warranted. 4 Athletes with complete RBBB demonstrate larger right ventricular dimensions and lower RV ejection fraction but preserved fractional area change compared to those with normal QRS complexes. 1 Incomplete RBBB in athletes often reflects physiological RV dilation from cardiac remodeling rather than pathology. 2

Common Pitfalls and Caveats

• Do not assume RBBB is benign without structural evaluation: Even asymptomatic RBBB requires echocardiography given potential association with serious underlying disease 4, 2
• Electrode misplacement can mimic iRBBB: When lead V1 is positioned higher or more to the right than usual with terminal r-wave duration <20 ms, this may represent artifact rather than true conduction abnormality 2
• RBBB obscures ST-segment analysis: In acute MI settings, RBBB can delay recognition of ST-elevation and reperfusion decisions 3
• Intermittent RBBB has the same significance as stable RBBB: Rate-dependent or intermittent RBBB carries the same clinical and prognostic implications 4
• Alternating bundle branch block is a Class I pacing indication: Switching between left and right bundle branch block on successive ECGs indicates block in all three fascicles and mandates permanent pacemaker implantation 3