ECG Diagnosis of Bundle Branch Block

QRS > 0.12sec

Look at V1:

  • Terminal R = RBBB as excitation spreading from left to right
  • Terminal S = LBBB as excitation spreading away from right

Confirm I:(& aVL V5&6)

  • Terminal S = RBBB as excitation going away from left side
  • Terminal R = LBBB as excitation heading towards left

The above equates to pattern recognition of MaRrow/ WiLliam in V1-6.

With LBBB associated ST/T opposite to QRS, poor R progression in V1-6, RS in V5,6 left axis deviation.

Diagnosing AMI in LBBB

The Sgarbossa criteria only apply in LBBB (see rules above)

In true LBBB, there must not be any Q wave in the lateral leads

True LBBB. Note NO Q wave in lateral leads

Sgarbossa criteria

Of acute MI with LBBB (any of following)

  • ST elevation ≥ 1mm concordant with QRS
  • ST depression ≥ 1mm in V1-3
  • ST elevation ≥ 5mm discordant with QRS

Hemi-blocks – QRS ≤ 0.12sec

Left Anterior Superior Fascicular Block:

Left ventricle stimulated from LPIF so direction of excitation is Inferior to Superior and Right to Left so

  • left axis deviation < -45°
  • R in I > R in II or III
  • qR in aVL
  • deep S in II III aVF

Left Posterior Inferior Fascicular Block: (Rare in isolation as anatomically less discrete than LASF – often accompanies RBB as bifascicular block, or in association with a significant degree of ischaemic damage.)

Left ventricle stimulated from Superior to Inferior and Left to Right:

  • Right axis deviation > 110°
  • Small r & deep S in I
  • R in III > II
  • qR in III

Trifasicular Block

Indicating advanced disease:

  • RBB and LASF with 1st degree AV block
  • RBB and LPIF with 1st degree AV block
  • LBB with 1st degree AV block
  • Alternating RBB and LBB

Risk of sudden progression to complete heart block or sudden death low unless associated with acute MI or symptomatic bradycardic episodes - prophylactic pacing for these patients.

Content by Dr Íomhar O' Sullivan (30/10/2007) from College of Emergency Medicine Learning Resources. Last review Dr Íomhar O' Sullivan 10/06/21.