What is a Notched QRS in Lead aVL?
A notched QRS in lead aVL is a characteristic feature of left bundle branch block (LBBB) and left anterior fascicular block (LAFB), representing delayed or abnormal left ventricular activation that produces a broad, slurred, or notched R wave morphology in this lateral lead.
Diagnostic Significance by Condition
Left Bundle Branch Block (LBBB)
The presence of a broad notched or slurred R wave in lead aVL is one of the defining criteria for complete LBBB. 1
The specific LBBB criteria include:
- QRS duration ≥120 ms in adults 1
- Broad notched or slurred R wave in leads I, aVL, V5, and V6 1
- Absent Q waves in leads I, V5, and V6 (though a narrow Q wave may be present in aVL without myocardial pathology) 1
- R peak time >60 ms in leads V5 and V6 1
Recent high-quality evidence demonstrates that notched or slurred QRS complexes are identified in at least one of the lateral leads (I, aVL, V5, V6) in 100% of patients with true LBBB, including those with post-TAVR LBBB and conduction system pacing-correctable LBBB. 2
Left Anterior Fascicular Block (LAFB)
A qR pattern (small q wave followed by tall R wave) in lead aVL is the hallmark morphology for LAFB. 1
The diagnostic criteria for LAFB include:
- Frontal plane axis between −45° and −90° 1
- qR pattern in lead aVL 1
- R-peak time in lead aVL of ≥45 ms 1
- QRS duration <120 ms 1
- rS pattern (small r, deep S) in leads II, III, and aVF 1
Clinical Context and Interpretation
When Notching Indicates Conduction Disease
The notched appearance in aVL reflects abnormal ventricular depolarization sequence, where the electrical impulse travels through alternative pathways rather than the normal His-Purkinje system. 1
Key distinguishing features:
- In LBBB, the notching occurs in a broad R wave (QRS ≥120 ms) with associated findings in other lateral leads 1
- In LAFB, the notching appears as part of a qR complex with left axis deviation but normal QRS duration (<120 ms) 1
- The R-peak time measurement in aVL helps differentiate these conditions (≥45 ms for LAFB, >60 ms in V5/V6 for LBBB) 1
When Notching May Indicate Myocardial Scarring
Fragmented QRS patterns (RSR', rSR', or notched R waves) in left-sided leads including aVL, when QRS duration is ≤120 ms and bundle branch block is absent, may indicate significant myocardial scarring or left ventricular aneurysm. 3
This fragmented pattern has:
- 50% sensitivity for left ventricular aneurysm 3
- 94.6% specificity for left ventricular aneurysm 3
- High negative predictive value (95-98%) in post-myocardial infarction populations 3
Common Pitfalls and Caveats
Do not confuse the qR pattern of LAFB with the broad notched R wave of LBBB—the key differentiator is QRS duration and axis deviation. 1
In patients with atypical LBBB patterns, notching in aVL may still indicate true conduction system disease even when other criteria are not fully met, as 9.4% of patients with true LBBB (confirmed by TAVR-induced block or CSP-correction) do not completely meet Strauss criteria. 2
Electrode placement errors can alter QRS morphology in lateral leads, potentially creating or obscuring notching patterns, particularly when electrodes are placed in incorrect intercostal spaces. 4
Age-dependent normal values must be considered, as QRS duration criteria differ significantly between adults and children when evaluating conduction abnormalities. 5