Bundle Branch Block

Acute (infarct, inflammation) or chronic (remodeling from CAD, HTN, fibrosis)
- ➔ damage to conduction system (bundle branches)
- ➔ delayed conduction ➔ widened QRS (> 0.12s or 3 small boxes)

Normal conduction

Down left and right bundle ➔ typical QRS

Signal progression:

SA node ➔ AV node ➔ Bundle of His
Right bundle ➔ RV depolarization
- ➔ RV contraction
Left bundle blocked
- Slower LV Depolarization comes from septum
- Delayed conduction ➔ widened QRS

The M, W may not be present or obvious

V1 - BROAD S wave ➔ W
V6 - BROAD & NOTCHED R wave ➔ M
Lateral leads (I, aVL) - broad & notched
- since they "look" in the same direction as V6

Generally, chest pain + new LBB: think STEMI
- Difficult to dx MI in presence of LBB
  - Need to understand concordance, appropriate vs excessive discordance, Sgarbossa criteria

Appropriate discordance: ST segments/T waves go in opposite direction to the main vector of the QRS complex
If you're feeling really extra
- Sgarbossa Criteria
- New Electrocardiographic Algorithm for the Diagnosis of Acute Myocardial Infarction in Patients With Left Bundle Branch Block

Signal progression:

SA node ➔ AV node ➔ Bundle of His
Left bundle ➔ LV depolarization
- ➔ LV contraction
Right bundle blocked
- RV Depolarization comes from LV & septum
- Delayed conduction ➔ widened QRS

Normally activated left ventricle ➔ early part of the QRS appears normal (not all wide like LBB)

Initial part of QRS is unchanged

V1 - RSR' and inverted T

(An extra R wave & appropriate discordance)

V6 - Wide & slurred S (& lateral leads)

The LV contributes the most electrically to how the QRS looks
In RBB the signal makes it down left bundle just fine
- ➔ the initial part of the QRS appearing normal-ish

Next: Brady Tachy TX