Describe the regions of the left ventricle supplied by the left anterior and posterior hemifascicles, respectively
- Anterior: Anterior, superior, and lateral walls
- Posterior: Posterior, inferior, and medial walls
Under normal conditions, septal depolarization is initiated by the _______ bundle branch
Left
Compare QRS duration in left fascicular block vs. left or right BBB
In complete BBB the QRS is prolonged. In fascicular block the QRS is of normal duration
Left posterior fascicular block is __________ (more/less) common than left anterior fascicular block
less
List typical criteria for identifying RBBB
- QRS duration >= 120ms
- “M”-shaped QRS in V1 (rsR’ pattern)
- Prominent S in I and aVL
- Secondary repolarization abnormalities may be seen in V1-2
List typical criteria for identifying LBBB
- QRS duration >= 120ms
- Broad R-wave in I, aVL, and V6 (often notched)
- Lack of physiologic q-waves in I and V6
- Secondary repolarization abnormalities often seen in multiple leads:
- Downsloping ST-depression with TWI in I, aVL, V6
- Opposite pattern (deep S-wave with upsloping STE and upright T) in V1-3
Describe pathophysiology and ECG characteristics of incomplete BBB
- Intraventricualr conduction abnormality in which conduction is slowed, but not entirely stopped through one of the bundle branches
- Generally of similar etiology to complete block. Will often proceed to complete block with time
- ECG morphology is similar to complete block but with preserved QRS duration
List typical criteria for identifying isolated LAFB (left anterior fascicular block / left anterior hemiblock)
note that hemiblock is best assessed in the limb leads
- preserved QRS duration (100-120ms)
- left axis deviation (usually beyond -45°)
- small septal q in lateral limb leads (I, aVL)
- small R in inferior leads (II, III, aVF)
- intrinsicoid deflection >45ms in aVL
List typical criteria for identifying isolated LPFB (left posterior fascicular block / left posterior hemiblock)
note that hemiblock is best assessed in the limb leads
- preserved QRS duration (100-120ms)
- right axis deviation
- small q in inferior leads (II, III, aVF)
- small R in lateral limb leads (I, aVL)
- intrinsicoid deflection >45ms in aVF
- No additional evidence of RVH, as all these signs could be produced by RVH
QRS duration >120ms without RBBB or LBBB morphology is referred to as:
Non-specific intraventricular conduction delay
What is Bifascicular block and what is the most common cause?
RBBB + LAFB/LPFB
Most common cause is CAD
Assess the following ECG for signs of intraventricular conduction delay
- QRS >120ms
- Broad R-wave in lateral leads (I, aVL, V6)
- Deep S-waves with STE in V1-V3
- LAD
- Secondary repolarization abnormalities in lateral leads
Complete LBBB
Assess the following ECG for signs of intraventricular conduction delay
- Normal QRS duration
- Normal QRS morphology in V1 and V6
- RAD
- Small q-waves in inferior limb leads
- Small r-waves in lateral limb leads
LPFB
Assess the following ECG for signs of intraventricular conduction delay
- Prolonged QRS (~160ms)
- Left axis deviation (~-40°)
- rsR’ pattern in V1
- Wide terminal S in I, aVL, and V6
- TWI in V1-2
Bifascicular block: RBBB + LAFB
Assess the following ECG for signs of intraventricular conduction delay
- Wide QRS (160-200ms)
- Borderline LAD (QRS axis ~-30°)
- Wide terminal R-wave in V1-4
- Terminal s in V6, I, aVL
- TWI in V1-4
- Small r-waves in inferior limb leads
RBBB with atympical morphology (no “slurred s”) and potential LAFB
