Bundle Branch Blocks and Fascicular Blocks
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Direct answer
A bundle branch block is a conduction delay in the right or left bundle branch, widening the QRS to 120 milliseconds or more with a characteristic pattern. Left bundle branch block gives a broad, notched R wave in leads I, aVL, V5 and V6 with no septal q waves, while right bundle branch block gives an rSR prime pattern in V1 with a wide S wave in I and V6. Fascicular blocks are narrow-QRS axis shifts caused by a block in the left anterior or left posterior fascicle.
What you must remember
- LBBB criteria: QRS of 120 milliseconds or more, a broad notched R in I, aVL, V5 and V6, absent septal q waves, and a deep QS or W pattern in V1.
- RBBB criteria: QRS of 120 milliseconds or more, an rSR prime in V1, and a broad slurred S wave in I and V6.
- WiLLiaM MaRRoW: a mnemonic for lead patterns, with a W in V1 and an M in V6 for LBBB, and an M in V1 and a W in V6 for RBBB.
- Left anterior fascicular block: left axis more negative than about minus 45 degrees, qR in aVL, rS in II, III and aVF, and a QRS under 120 milliseconds.
- Left posterior fascicular block: rightward axis, usually beyond plus 90 degrees, with a narrow QRS, and it is diagnosed only after other causes of right axis deviation are excluded.
- Paradoxical splitting of S2: in LBBB the aortic valve closes late, so the split widens on expiration instead of inspiration.
- Bifascicular block: RBBB with LAFB or LPFB, which carries a risk of progression to complete heart block.
- LBBB and ischaemia: Sgarbossa criteria, such as concordant ST elevation, help detect acute MI in the presence of LBBB.
Reading the QRS pattern in leads V1 and V6
A 70-year-old man with hypertension has a QRS of 140 milliseconds on a routine ECG, and the examiner asks which block is present. The answer depends on the morphology in V1 and the lateral leads, not on the width alone.
| Feature | LBBB | RBBB | LAFB |
|---|---|---|---|
| QRS duration | 120 ms or more | 120 ms or more | Under 120 ms |
| Lead V1 | Deep QS or W shape | rSR prime (M shape) | Usually normal |
| Lead V6 | Broad notched R, no q wave | Wide slurred S wave | Normal |
| Frontal axis | Often normal or leftward | Usually normal | Left, below minus 45 degrees |
Secondary ST and T changes run opposite to the QRS in bundle branch block, so ST elevation in LBBB cannot be read in the usual way. In a patient with a wide QRS, the first step is to measure the width and then match the morphology, rather than naming the block from the limb leads alone.
Where the exam tries to catch you
Bundle branch and fascicular questions test criteria and mnemonics.
- Trap: Calling a QRS of 110 milliseconds complete bundle branch block. Correct: Complete bundle branch block needs a QRS of 120 milliseconds or more, and narrower widths describe incomplete block.
- Trap: Diagnosing RBBB from QRS width alone. Correct: Morphology matters most, and an rSR prime in V1 with a wide S in I and V6 defines RBBB.
- Trap: Assigning a left axis to LAFB without further thought. Correct: Left axis deviation also arises from inferior MI and LVH, so LAFB is a diagnosis of exclusion.
- Trap: Reading an expiratory-widening split S2 as normal physiology. Correct: It is paradoxical splitting, seen in LBBB, severe aortic stenosis and RV pacing.
Frequently asked questions
What QRS duration defines complete bundle branch block?
A QRS of 120 milliseconds or more defines complete bundle branch block, with characteristic morphology in the precordial and lateral leads. Narrower QRS complexes with a similar pattern are called incomplete bundle branch block.
What does the WiLLiaM MaRRoW mnemonic help identify?
WiLLiaM MaRRoW is a mnemonic for the lead V1 and V6 patterns that distinguish LBBB from RBBB. It helps confirm which bundle is blocked once the QRS has been measured as wide.
Why is left bundle branch block important in acute myocardial infarction?
Left bundle branch block hides the ST changes of ischaemia because repolarisation runs discordant to the QRS. Sgarbossa criteria, including concordant ST elevation, are used to detect acute MI in LBBB.
Which conditions cause paradoxical splitting of S2?
Paradoxical splitting of S2 occurs when aortic valve closure is delayed, as in LBBB, severe aortic stenosis and right ventricular pacing. The split widens on expiration instead of inspiration, which is a common point in heart sound questions.