What are the implications and management of an RSR (Right Bundle Branch Block) pattern in limb leads on an electrocardiogram (ECG)?

RSR Pattern in Limb Leads: Clinical Implications and Management

An RSR pattern in limb leads (rather than the typical precordial location) represents "masquerading bundle branch block"—a severe conduction system disease with poor prognosis that warrants urgent cardiac evaluation, exclusion of reversible causes, and consideration for permanent pacemaker implantation. 1, 2, 3

Understanding the Pattern

Masquerading bundle branch block is a rare but critical ECG finding where:

• Precordial leads (V1-V3) show a left bundle branch block pattern (wide QRS with broad, deep S waves) 1
• Limb leads simultaneously show a right bundle branch block pattern (wide QRS ≥120 ms, broad S waves in lead I, right axis deviation) 1
• This represents alternating bundle branch block or severe diffuse conduction system disease 1, 2, 3

This is distinct from typical RBBB, which shows the classic RSR' pattern in V1-V2, not in limb leads. 4, 5

Critical Prognostic Implications

This finding indicates extremely poor prognosis and high risk of complete heart block:

• Patients with masquerading bundle branch block have severe degeneration of the entire conduction system 2, 3
• The pattern is almost invariably associated with severe underlying heart disease, including triple vessel coronary disease 2
• Sudden cardiac death can occur within months of diagnosis, even in previously asymptomatic patients 3
• One case series documented death within 9 months despite close follow-up 3

Immediate Management Algorithm

Step 1: Confirm the Diagnosis

• Obtain a complete 12-lead ECG to document the pattern in both precordial and limb leads 1, 6
• Verify QRS duration ≥120 ms 4, 6
• Document the discordant bundle branch block patterns between lead groups 1, 2

Step 2: Exclude Reversible Causes (Class I Recommendation)

Before proceeding to permanent pacing, rule out: 1, 6

• Electrolyte abnormalities (particularly hyperkalemia) 6
• Perioperative hypothermia or inflammation near the AV junction from recent cardiac surgery 1, 6
• Acute Lyme disease (AV block may resolve with treatment) 1, 6
• Medication-induced conduction delay 1

Step 3: Cardiac Rhythm Monitoring

• Continuous cardiac monitoring is essential to detect progression to complete heart block 1
• Consider 24-hour Holter monitoring to assess for intermittent high-grade AV block 6
• Document any correlation between conduction abnormalities and symptoms 1

Step 4: Structural Heart Disease Evaluation

• Echocardiography to assess for:
  • Left ventricular dysfunction 1
  • Segmental wall motion abnormalities suggesting prior myocardial infarction 7
  • Infiltrative cardiomyopathies (sarcoidosis, amyloidosis) 1, 6
• Coronary angiography if ischemic heart disease is suspected 2

Step 5: Pacemaker Consideration (Class IIa-I Indication)

Permanent pacemaker implantation is strongly recommended because: 1, 6

• Alternating bundle branch block carries Class I indication for permanent pacing due to high risk of complete heart block 1
• Even asymptomatic patients require pacing when fascicular block is present 1
• The pattern indicates clear evidence of block in all three fascicles 1

Prophylactic pacing is particularly urgent if: 1, 6

• Sarcoidosis, amyloidosis, or neuromuscular diseases are present (even if block appears transient) 1, 6
• Patient develops any symptoms (syncope, presyncope, fatigue, dyspnea) 1
• Progression to Type II second-degree or third-degree AV block occurs 1

Special Diagnostic Considerations

Distinguish from Other RSR' Patterns

Normal variants that should NOT be confused with pathologic RSR' in limbs: 5, 8

• Incomplete RBBB in V1-V2 with normal QRS duration (<110 ms) is a benign finding in children and athletes 1, 5
• Lead misplacement (V1 recorded too high or too far right) can create false RSR' patterns 5
• The term "normal rsr'" should be avoided due to variable interpretation 5

Pathologic RSR' patterns requiring different management: 9, 8, 7

• RBBB with QR pattern in V1 suggests acute pulmonary embolism with high positive predictive value for cardiac arrest 9
• RSR' with wide QRS in precordial leads plus wall motion abnormalities indicates myocardial infarction scar 7
• RSR' with Brugada morphology (coved ST elevation ≥2 mm in V1-V3) requires ICD consideration 1, 8

Common Pitfalls to Avoid

• Do not dismiss as "incomplete RBBB" if the pattern appears in limb leads rather than V1-V2—this is masquerading BBB, not typical RBBB 1, 2, 3
• Do not delay pacemaker evaluation while awaiting other cardiac interventions (e.g., CABG)—patients can deteriorate rapidly 2
• Do not assume the patient is stable based on lack of symptoms—sudden death can occur in previously asymptomatic individuals 3
• Do not attribute the pattern to age alone—it represents severe pathology requiring intervention 2, 3

Follow-Up Strategy

Close monitoring is mandatory even after pacemaker implantation: 3

• Regular device interrogation to assess pacing dependency 1
• Ongoing evaluation for progression of underlying cardiac disease 2
• Aggressive management of coronary disease, heart failure, or infiltrative processes 1, 6, 2