EKG Discussions

The Hemiblocks

The hemiblocks (left anterior and left posterior) have little interest or utility in the interpretation of an acute 12-lead EKG. When coupled with a RBBB they are more significant because of the resulting bifascicular block. However, in the interest of complete coverage of EKG interpretation…

Left anterior hemiblock
• Small Q in leads I and aVL and small R in leads III and aVF
• Prominent R in I and aVL and deep S in III and aVF
• Left axis deviation (between -30° and -90°)
• QRS slightly prolonged at <110ms

Left anterior hemiblock (LAH)

Left posterior hemiblock
• Small Q in leads III and aVF, and small R in I and aVL
• Prominent R in III and aVF and deep S in I and aVL
• Right axis deviation
• QRS slightly prolonged

Left posterior hemiblock (LPH)

When a hemiblock occurs with a RBBB it is identified primarily by axis deviation, a finding not part of the EKG criteria for RBBB. RBBB with a hemiblock becomes a bifascicular block.
• RBBB with left axis deviation adds left anterior hemiblock
• RBBB with right axis deviation adds left posterior hemiblock

RBBB with left anterior hemiblock due to left axis deviation (-60°)

RBBB with left posterior hemiblock due to right axis deviation

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