What are the causes of an incomplete right bundle branch block (iRBBB)?

Causes of Incomplete Right Bundle Branch Block

Incomplete right bundle branch block (iRBBB) is most commonly a benign normal variant, particularly in children and athletes, but can also reflect underlying structural heart disease, right ventricular strain, congenital heart disease, or represent a technical artifact from electrode misplacement. 1

Benign and Physiological Causes

Normal Variant in Healthy Individuals

• iRBBB may be present in the complete absence of heart disease, particularly when lead V1 is recorded higher than or to the right of normal position and the terminal r wave is less than 20 ms. 1
• In children, an rsr' pattern in V1 and V2 with a normal QRS duration is a normal variant. 1
• The crista supraventricularis (CSV) pattern—defined as RSR' with QRS width below 100 ms and S wave <40 ms in leads I or V6—represents late activation of the right ventricular crest and is a benign finding. 2, 3

Athletic Heart Adaptation

• iRBBB is common in athletes (prevalence 13.3% in young athletes), often representing physiological cardiac remodeling from training rather than pathology. 3
• In athletes, iRBBB may result from right ventricular dilation with resultant QRS prolongation, representing a spectrum of structural and physiological cardiac remodeling. 1
• Isolated iRBBB in asymptomatic young athletes without family history of sudden cardiac death or symptoms may represent idiopathic, clinically benign conduction delay. 4

Technical and Anatomical Factors

• Higher placement of electrodes V1 and V2 can produce an iRBBB pattern artifactually. 2
• Pectus excavatum can cause an iRBBB appearance on ECG (typically with negative P wave in V1). 2

Pathological Causes Requiring Evaluation

Congenital Heart Disease

• Atrial septal defect (particularly ostium secundum type) is a classic association with iRBBB; clinicians should be alert to splitting of the second heart sound. 2
• Unoperated and operated congenital heart disease commonly presents with RBBB patterns. 1, 4
• Congenital defects with underdevelopment of the right ventricle (such as tricuspid atresia) can cause conduction abnormalities. 1

Right Ventricular Pathology

• Right ventricular enlargement or strain from any cause can manifest as iRBBB. 2
• Pulmonary hypertension causing right ventricular pressure overload may present with iRBBB. 5
• Arrhythmogenic right ventricular cardiomyopathy (ARVC) can present with localized QRS prolongation in right precordial leads (V1-V3), often with epsilon waves. 1, 4

Structural Heart Disease

• Ischemic heart disease, particularly anterior infarction with persistent intraventricular conduction disturbances. 6, 4
• Cardiomyopathies of various types (dilated, hypertrophic, restrictive). 6, 4
• Hypertensive heart disease. 6, 4

Degenerative and Infiltrative Processes

• Primary degenerative lesions of the specialized conducting tissue. 4
• Sarcoidosis, cardiac tumors, and other infiltrative processes. 4
• Lenegre disease (progressive cardiac conduction disease)—an autosomal dominant condition linked to SCN5A gene mutations presenting with various conduction defects in young individuals. 1, 4

Infectious and Inflammatory

• Myocarditis and Chagas' disease. 4

Metabolic and Other Causes

• Hyperkalemia can mimic iRBBB patterns. 2
• Channelopathies affecting cardiac conduction. 4

Critical Diagnostic Pitfalls to Avoid

Dangerous Mimics Requiring Immediate Action

• iRBBB pattern with ST-elevation in V1-V3 may represent Brugada syndrome (Type 2 pattern) and requires immediate specialized evaluation due to sudden cardiac death risk. 6, 2
• Ventricular preexcitation (Wolff-Parkinson-White syndrome) can mimic iRBBB. 2

High-Risk Features Requiring Further Evaluation

• Syncope, presyncope, palpitations, or exertional symptoms suggest arrhythmic etiology and require urgent evaluation including echocardiography. 6, 4
• Family history of sudden cardiac death warrants genetic evaluation and comprehensive cardiac assessment. 6, 4
• iRBBB in the context of left ventricular hypertrophy or other ECG abnormalities should not be considered benign. 2
• Associated conduction abnormalities (left anterior or posterior hemiblock, first-degree AV block) increase risk of progression to complete AV block. 6, 4

When to Investigate Further

• iRBBB should not be routinely regarded as harmless in high-risk individuals—it may carry clinical and prognostic significance. 5
• iRBBB may be associated with predisposition to atrial fibrillation in selected populations. 5
• In athletes, if abnormalities are found on clinical exam or there is concerning family history, heart disease should be excluded with echocardiography. 2

Recommended Evaluation Approach

Initial Assessment

• Assess for symptoms including syncope, presyncope, dizziness, fatigue, exercise intolerance, or palpitations. 6, 4
• Evaluate for family history of sudden cardiac death, cardiomyopathy, or conduction disease. 6, 4
• Perform careful physical examination including auscultation for fixed splitting of S2 (suggesting ASD). 2

Diagnostic Testing When Indicated

• Transthoracic echocardiography is first-line evaluation when structural heart disease is suspected, assessing right ventricular size and function, atrial septal defects, pulmonary hypertension, valvular abnormalities, and left ventricular function. 6
• Cardiac MRI may be needed if ARVC is suspected or echocardiography is inconclusive. 4
• Consider stress testing when ischemic heart disease is suspected. 4