Bundle branch blocks look scary on first pass, but they’re one of the most “pattern-recognition-friendly” ECG topics on Step 1/2. If you can (1) identify RBBB vs LBBB in V1/V6, (2) know the classic associations, and (3) remember the MI rule-outs, you’ll scoop up easy points.
The 10-second concept (what a BBB is)
A bundle branch block means ventricular depolarization is delayed through one bundle, so activation spreads cell-to-cell across myocardium instead of fast Purkinje conduction. That causes:
- Wide QRS ( ms)
- Secondary ST-T changes (often “discordant” with the QRS)
- A predictable shape in key leads (especially V1 and V6)
Visual mnemonic: “WiLLiaM” vs “MaRRoW”
This is the classic Step mnemonic for identifying BBB by V1 morphology:
-
LBBB = WiLLiaM
- In V1, QRS is W-shaped
- In V6, QRS is M-shaped
-
RBBB = MaRRoW
- In V1, QRS is M-shaped
- In V6, QRS is W-shaped
One-liner memory hook:
- LBBB: Left vent is late → left-sided leads (V5/V6) show a broad/notched R (“M”)
- RBBB: Right vent is late → right precordial lead (V1) shows rSR’ (“M”)
High-yield comparison table: LBBB vs RBBB
| Feature | LBBB (Left bundle branch block) | RBBB (Right bundle branch block) |
|---|---|---|
| QRS duration | Wide ms | Wide ms |
| Key lead to spot it | V1 + V6 | V1 + V6 |
| V1 appearance | Predominantly negative (deep S/QS), often “W” | rSR’ (terminal R’), “M” |
| V6 / I appearance | Broad, notched R (“M”); no Q in I, V5–V6 (classic teaching) | Wide terminal S in I and V6 (“W”) |
| “Delayed ventricle” | Left ventricle delayed | Right ventricle delayed |
| Typical axis | Often leftward, but variable | Often normal; can be rightward depending on cause |
| ST-T changes (secondary repolarization) | ST-T often discordant to QRS (e.g., ST depression/T inversion in lateral leads) | ST-T often discordant (e.g., T inversion/ST depression in V1–V3) |
| Big-time associations | Structural heart disease (HTN with LVH, CAD, cardiomyopathy), aortic stenosis; new LBBB can be ischemic | Pulmonary embolism, pulmonary HTN/cor pulmonale, ASD; can be benign (especially incomplete) |
| Step-style MI pearl | New LBBB + ischemic symptoms can be treated as STEMI-equivalent in classic teaching; look for Sgarbossa criteria | RBBB does not hide STEMI as much; still interpret ST elevation where possible |
| Incomplete BBB | QRS 110–119 ms with similar morphology | QRS 110–119 ms with similar morphology |
What each one “means” in one line (shareable)
- RBBB: “Right vent depolarizes late → rSR’ in V1 + wide S in I/V6.”
- LBBB: “Left vent depolarizes late → deep negative in V1 + broad/notched R in I/V6.”
How to identify in under 15 seconds (exam workflow)
- Confirm wide QRS: ms
- Look at V1:
- rSR’ (M) → RBBB
- QS or deep S (W/negative) → LBBB
- Confirm with V6:
- RBBB → broad S (“W”)
- LBBB → broad/notched R (“M”)
USMLE high-yield clinical correlations
RBBB: think “right heart strain”
Common testable associations:
- Pulmonary embolism (acute right heart strain can cause new RBBB)
- Pulmonary hypertension / cor pulmonale
- Atrial septal defect (esp. secundum ASD)
- Can also be an incidental finding (especially incomplete RBBB)
LBBB: think “structural left heart disease”
Common testable associations:
- Hypertension (LVH), CAD, dilated cardiomyopathy
- Aortic stenosis
- New LBBB in chest pain is a red flag because it can obscure ischemia and reflects significant conduction system disease.
MI + LBBB: the “don’t miss this” pearl
Because LBBB creates baseline ST-T abnormalities, diagnosing acute MI becomes tricky. On exams, when you’re given:
- ischemic symptoms + new (or presumably new) LBBB,
treat it as a STEMI-equivalent in many classic Step scenarios.
If they want specifics, they may hint at Sgarbossa criteria (not required to memorize in full for many Step questions, but the concept matters):
- Concordant ST elevation in leads with a positive QRS is concerning.
- Concordant ST depression in V1–V3 is concerning.
- “Excessively discordant” ST elevation can also be concerning.
Quick mini-table: “Which lead shows the late bump?”
| Block | Late depolarization shows up as… | Where you see it best |
|---|---|---|
| RBBB | Terminal R’ (extra upward deflection) | V1 (right-sided view) |
| LBBB | Broad/notched R (slurred upstroke) | I, V5–V6 (left lateral view) |
Rapid recap (what to memorize)
- Wide QRS + V1 pattern:
- RBBB: V1 rSR’ (“M”); V6 wide S
- LBBB: V1 QS/deep S (“W”); V6 broad notched R
- RBBB → often right heart strain causes (PE, pulm HTN, ASD)
- LBBB → structural left heart disease; can mask MI and may be treated as STEMI-equivalent with symptoms