What are

Rabbit Ears in QRS Complex

"Rabbit ears" refers to an rSR' pattern seen in the QRS complex, most commonly in leads V1-V2, which is the characteristic morphology of right bundle branch block (RBBB).

ECG Morphology and Definition

The term "rabbit ears" describes the distinctive double-peaked appearance of the QRS complex that resembles rabbit ears standing upright 1, 2. This pattern consists of:

• An initial small r wave
• A deep S wave
• A second, taller R wave (R'), creating the rSR' configuration 1, 2
• Most prominently visible in right precordial leads V1 and V2 1

Clinical Significance Based on QRS Duration

The interpretation of this rSR' pattern depends critically on the QRS duration:

Complete RBBB

• QRS duration ≥120 ms with rSR' pattern in V1-V2 2
• Associated with S waves of greater duration than R waves in leads I and V6 2
• Detected in approximately 1-2.5% of the general population 2

Incomplete RBBB

• Same rSR' morphology but QRS duration 110-119 ms 3
• Considered a normal variant in athletes when isolated and asymptomatic 1, 3
• Occurs in less than 2% of athletes' ECGs 3

Differential Diagnosis

When evaluating rabbit ears between QRS complexes, consider these important distinctions:

Normal variants that may show rSR' pattern:

• Incomplete RBBB in athletes (normal finding) 1
• Juvenile ECG pattern in adolescents <16 years (T-wave inversion may accompany the pattern in V1-V3) 1

Pathologic conditions to exclude:

• Brugada syndrome (downward coved or saddleback ST-segment elevations in V1-V3, distinct from simple rSR') 1
• Arrhythmogenic right ventricular cardiomyopathy (epsilon waves are low-amplitude terminal QRS notches, different from typical rabbit ears) 1
• Atrial septal defect (commonly presents with incomplete RBBB and fixed splitting of S2) 3

Clinical Evaluation Algorithm

For asymptomatic patients with isolated rSR' pattern:

• Document QRS duration to distinguish complete from incomplete RBBB 2, 3
• Assess for bifascicular block (RBBB plus left anterior or posterior fascicular block), which carries higher risk 2, 1
• No specific treatment required if isolated and asymptomatic 3

For symptomatic patients (syncope, presyncope, dizziness):

• Transthoracic echocardiography to exclude structural heart disease 2, 3
• Ambulatory ECG monitoring if atrioventricular block suspected 2
• Consider electrophysiology study if syncope present with HV interval measurement 2, 4

High-Risk Features Requiring Intervention

Permanent pacing is indicated (Class I recommendation) for: 2, 4

• Alternating bundle branch block (QRS complexes alternating between LBBB and RBBB morphologies)
• Syncope with RBBB and HV interval ≥70 ms or evidence of infranodal block on electrophysiology study

Permanent pacing is NOT indicated (Class III: Harm) for: 2

• Asymptomatic patients with isolated RBBB and normal 1:1 atrioventricular conduction

Special Populations

Athletes: Can participate in all competitive athletics if the rSR' pattern represents incomplete RBBB with no symptoms, no structural heart disease, and no family history of premature cardiac disease or sudden death 3

Congenital heart disease: In patients with repaired tetralogy of Fallot, RBBB is present in the vast majority, and an RBBB configuration during tachycardia does not reliably distinguish ventricular tachycardia from supraventricular tachycardia 1