The document defines left bundle branch block (LBBB) according to guidelines from the AHA/ACCF/HRS and Strauss et al. The AHA/ACCF/HRS defines LBBB as a QRS duration of 120 ms or greater with additional features including a broad notched R wave in certain leads and absence of q waves in others. Strauss et al defines LBBB as a QRS duration of 140 ms or greater in men and 130 ms or greater in women with additional features such as QS or rS patterns in leads V1 and V2. Guidelines from AHA/ACCF/HRS and ESC recommend cardiac resynchronization therapy for LBBB patients with a QRS duration of 150

1. ECG definitions of LBBB

2. AHA/ACCF/HRS
• QRS≥120 ms
• Additional Features
• Broad notched or slurred R wave in leads I,
aVL, V 5 and V 6, and an occasional RS pattern
in V 5 and V 6 attributed to displaced transition
of QRS complex
• Absence of q waves in leads I, V 5 and V 6
• R peak time > 60 ms in leads V 5 and V 6 but
normal in leads V 1 , V 2 and V 3
• Discordant ST segment and T waves

3. Strauss et al
• ≥140 ms in men
• ≥130 ms in women
• Additional features
• QS or rS in V1 and V2
• mid-QRS notching or slurring in two or
more contiguous leads of V 1 , V 2 , V 5 ,
V 6 , I and aVL

4. AHA/ACCF/HRS Class 1
Recommendation for CRT
• QRS≥150 ms
• LBBB morphology” as per AHA/ACCF/HRS
recommendations above

5. ESC Class 1
Recommendation for CRT
• QRS≥120 ms
• Additional features
• QS or rS in V1
• Broad (frequently notched or slurred) R
wave in leads I, aVL, V 5 or V 6,
• Absence of q waves in leads V 5 and V 6

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