Rabbit Ears on aVL: Left Anterior Fascicular Block
A "rabbit ear" pattern (qR pattern) in lead aVL is a diagnostic criterion for left anterior fascicular block (LAFB), representing a specific intraventricular conduction disturbance that requires additional ECG findings for definitive diagnosis. 1
Diagnostic Criteria for LAFB
According to the AHA/ACCF/HRS standardized ECG interpretation guidelines, the qR pattern in aVL is one component of a four-part diagnostic algorithm 1:
Required findings for LAFB diagnosis:
- Frontal plane axis between -45° and -90° (marked left axis deviation)
- qR pattern in lead aVL (the "rabbit ears" appearance)
- R-peak time in lead aVL ≥45 ms
- QRS duration <120 ms (distinguishes from complete bundle branch block)
Clinical Significance
The qR pattern alone is insufficient for diagnosis—you must verify all four criteria. The pattern reflects altered ventricular activation due to blockage in the anterior fascicle of the left bundle branch system, causing the electrical impulse to travel through the posterior fascicle first, then activate the anterior and lateral left ventricle in a delayed fashion.
Key Clinical Context
When LAFB matters most:
- Often benign in isolation without structural heart disease
- May indicate underlying cardiac pathology when accompanied by other conduction abnormalities
- Can mask or alter ECG signs of left ventricular hypertrophy
- May coexist with right bundle branch block (bifascicular block), which carries higher risk for progression to complete heart block
Common Pitfalls to Avoid
Do not confuse with:
- Mid-septal RV pacing: Research shows that a QR pattern in aVL can also indicate mid-septal right ventricular lead positioning in paced rhythms (sensitivity 53.3% for this location) 2. Always check if the patient has a pacemaker before attributing the pattern to LAFB.
- Isolated left axis deviation: The qR pattern must be present; left axis deviation alone is insufficient.
- Complete LBBB: In LBBB, the QRS duration is ≥120 ms, and a narrow q wave may be present in aVL even without myocardial pathology 1.
Practical Approach
Step 1: Confirm the qR morphology in aVL (small q wave followed by dominant R wave with notching)
Step 2: Measure the frontal plane axis—must be between -45° and -90°
Step 3: Measure QRS duration—must be <120 ms
Step 4: Measure R-peak time in aVL—must be ≥45 ms
Step 5: Rule out pacemaker rhythm or other conduction abnormalities
Only when all criteria are met should you diagnose LAFB. The isolated finding of a qR pattern in aVL warrants systematic evaluation but does not constitute a diagnosis by itself.