Left Bundle Branch Block
QRS 120 ms or more
Left Bundle Branch Block
cautionThe left ventricle is activated slowly from the right, so the whole QRS is broad and leftward: a deep QS in V1, a broad notched R in I and V6, and ST and T turned the other way.
On the trace
Read the tracing above in this order; it shows the complete block (the Variant control beside the title switches to incomplete). Tap any lead to see its own measurements. In the box that opens, the QRS view colours each part of the complex by the part of the heart that drew it, and the Normal button draws the same lead without the block in grey behind it. The Normal switch under the tracing does the same for the whole ECG.
- Measure the QRS.
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143 ms across the twelve leads. It is wide, so the next steps decide which block. - Look at V1.
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One deep, entirely negative deflection: a QS, 15.7 mm deep. There is no small r at the start, because the septum no longer depolarises towards V1. - Look at I, V5 and V6.
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A broad R with a small notch near its top, and no q in front of it. The whole QRS points at these leads, for longer than normal. - Check where the delay falls.
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The R-peak time in V5 and V6 is 90 ms, well over the 60 ms limit. The delay is on the left. - Look at the ST and T.
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They point the other way from the QRS. Under the deep QS of V1 to V3 the ST is raised, 1.4 mm at J in V1, and climbs into a tall upright T. Under the broad R of I, V5 and V6 the ST is depressed and the T inverted. - Check the axis.
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+30°, normal. LBBB can come with any axis; a left axis is common (see below).
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| QRS duration | 120 ms or more | 143 ms here. Strauss's stricter definition asks for 140 ms in men and 130 ms in women. |
| V1 (and V2) | QS, or rS with a tiny r | QS here, 15.7 mm deep in V1 and 17.7 mm in V2. |
| I, aVL, V5, V6 | Broad, notched or slurred R | Notched in I, V5 and V6 here. An RS in V5 or V6 can occur when the transition moves. |
| Septal q | Absent in I, V5 and V6 | A small q may remain in aVL. |
| R-peak time | Over 60 ms in V5 and V6 | 90 ms here. |
| ST and T | Opposite to the main QRS deflection | ST +1.4 mm in V1, −0.9 mm in V6; T upright in V1 to V3, inverted in I, V5 and V6. |
| Axis | Any | +30° here. |
These are the criteria of the 2009 AHA/ACCF/HRS recommendations on intraventricular conduction, with the mid-QRS notching that Strauss added in 2011.
Mechanism
The left bundle normally starts ventricular activation twice over. One branch reaches the left side of the septum first, and the septum depolarises from left to right: the small r in V1 and the small q in I, V5 and V6. The main bundle carries the impulse into the left ventricle, which depolarises quickly and, being larger, dominates the QRS.
In left bundle branch block both jobs fail. The septum is now activated from the right bundle's side, so it depolarises from right to left: the septal r in V1 and the septal q on the left are lost. The left ventricle has lost its fast route and is reached through the septum, cell to cell, which conducts far more slowly. Its large force, pointing left and backwards, now lasts the whole of a long QRS.
V1 sits in front and to the right, so it sees that force going away from it the whole time: one deep QS. I, V5 and V6 sit on the left and see it coming towards them: one broad R. The slow front crosses the septum and then the free wall, and the notch on the R, or the W at the bottom of the QS, marks the moment between them.
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Why the ST and T turn the other way
The left ventricle depolarises late, so it also repolarises late, and repolarisation runs against the direction of the slow depolarisation. The ST and T therefore point away from the QRS: raised where the QRS is deeply negative, depressed where it is broadly positive. This is appropriate discordance. It is part of the block, not a sign of ischaemia, and a raised ST of 1 to 3 mm in V1 to V3 is expected.
What is not explained by the block
ST and T that point the same way as the QRS are not part of LBBB: ST elevation in a lead with a positive QRS, ST depression in V1 to V3, or T inversion in V1 to V3. Each points to a primary problem, most importantly acute ischaemia, and is what the Sgarbossa criteria look for.
Why the usual signs of myocardial infarction cannot be read
The block moves the ST by itself, so the ordinary STEMI thresholds do not apply. Tap V1 and the lead panel says so. The Sgarbossa criteria read the ST against the QRS instead: ST elevation of 1 mm or more in a lead with a positive QRS, or ST depression of 1 mm or more in V1 to V3, both concordant. Smith's modification adds discordant ST elevation of at least a quarter of the depth of the preceding S. On this tracing V1's ST is 1.4 mm against a 15.7 mm QS, about a tenth, so none is met. LBBB also hides old Q waves, and makes the voltage criteria for left ventricular hypertrophy unreliable.
LBBB with a left axis
LBBB often comes with left axis deviation, when the disease reaching the left bundle also involves the left anterior fascicle, or when the left ventricle itself is diseased. Lead III and aVF then become predominantly negative. It usually goes with more advanced heart disease.
Incomplete and rate-related LBBB
The same shape with a QRS of 110 to 119 ms is incomplete LBBB: the left bundle conducts, only slowly. Choose Incomplete beside the title for its tracing and how to tell it apart.
Some people conduct normally at rest and develop LBBB only above a certain heart rate.
Clinical impact
Unlike right bundle branch block, LBBB is rarely found in a healthy heart. Its common causes are hypertension, coronary artery disease, cardiomyopathy, aortic stenosis and age-related fibrosis of the conduction system. It can follow aortic valve surgery or transcatheter valve implantation.
The slow activation makes the two sides of the left ventricle contract out of step, which lowers the pumping efficiency of a heart that is often already weak. In heart failure, LBBB is a marker of worse outcome, and it is also the pattern that responds best to resynchronisation pacing.
In a patient with chest pain, LBBB hides the usual signs of acute myocardial infarction. New LBBB on its own is no longer treated as equivalent to a STEMI, but LBBB with ongoing ischaemic symptoms is managed as a possible one.
Management
Find the cause
A new LBBB needs an echocardiogram to look at left ventricular function and the valves, and a search for coronary disease where the history suggests it.
Treat chest pain as a possible infarct
With ongoing ischaemic symptoms, apply the Sgarbossa and Smith criteria, compare with old tracings, and treat as a possible STEMI. European guidance advises the same strategy as for ST elevation when symptoms persist.
Resynchronise the failing heart
With symptomatic heart failure, an ejection fraction of 35% or less and a QRS of 150 ms or more despite medical treatment, cardiac resynchronisation therapy is recommended: pacing both ventricles together restores the lost timing.
Look for the rest of the conduction system
Syncope, a long PR or alternating bundle branch block raises the question of complete heart block, which may need a pacemaker.
Differential
Also wide, but the late forces point right and forwards: an R′ in V1 and a broad terminal S in I and V6, with the septal q kept.
Tall R on the left, deep S in V1 and discordant ST-T, but the QRS is under 120 ms and the septal q in V5 and V6 is usually kept.
Pacing from the right ventricle activates the left ventricle late, from the right, and gives the same LBBB-like shape. A pacing spike before each QRS gives it away.
A wide-complex tachycardia can have an LBBB-like shape. A broad initial r in V1 or V2, a slow descent to the nadir of the QS, or any q in V6 point to VT. In a rhythm, look for AV dissociation.
References
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, Part III: Intraventricular Conduction Disturbances — Circulation, 2009
- Defining left bundle branch block in the era of cardiac resynchronization therapy — American Journal of Cardiology, 2011
- Electrocardiographic diagnosis of evolving acute myocardial infarction in the presence of left bundle-branch block — New England Journal of Medicine, 1996
- Diagnosis of ST-elevation myocardial infarction in the presence of left bundle branch block with the ST-elevation to S-wave ratio in a modified Sgarbossa rule — Annals of Emergency Medicine, 2012
- 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy — European Heart Journal, 2021
- 2023 ESC Guidelines for the management of acute coronary syndromes — European Heart Journal, 2023