What’s happening in the heart
Every heartbeat is an electrical event before it becomes a mechanical one. The signal starts in a small patch of cells at the top of the heart called the sinus node, sweeps across both upper chambers (the atria), and then reaches a single gateway called the AV node. From there it travels down a thicker piece of wiring called the His bundle, which immediately splits into two main branches:
- The right bundle runs down the right side of the wall between the two lower chambers and lights up the right ventricle.
- The left bundle is broader and quickly fans out into smaller branches called fascicles that light up the much larger left ventricle.
When everything works, both ventricles squeeze almost at the same instant — a tightly coordinated beat. A bundle branch block means one of those branches is slow or not conducting at all. The signal still gets to both ventricles, but the side with the blocked wire has to wait for the impulse to travel the long way around through the heart muscle itself. That side ends up beating slightly later, and the overall beat becomes less synchronized.
Right bundle branch block (RBBB)
When the right bundle is the problem, the right ventricle gets its signal late. The right side of the heart is smaller and less critical for pumping, so the cost of this delay is usually small.
Left bundle branch block (LBBB)
When the left bundle is blocked, the left ventricle — the heart’s main pumping chamber — receives its signal late, and the squeeze becomes uncoordinated. Different parts of the left ventricle contract at different times, which can reduce pumping efficiency, especially in a heart that’s already weakened.
Fascicular blocks
The left bundle isn’t a single wire — it splits into two main fascicles (and sometimes a third). If just one of those branches is blocked, we call it a fascicular block (left anterior or left posterior). On its own, this is usually a mild finding.
Why it matters
Bundle branch blocks matter for three reasons, each of which we weigh case by case.
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What they tell us about the rest of the heart. RBBB is often a normal variant or simply a sign of aging in the wiring. LBBB, on the other hand, is rarely innocent in adults — it usually points to an underlying condition such as longstanding high blood pressure, narrowed heart valves, prior heart attacks, or weakening of the heart muscle. When we see a new LBBB, we go looking for the cause.
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Whether they cause symptoms. A bundle branch block on its own usually doesn’t cause symptoms. But when LBBB is combined with a weak heart muscle, the loss of coordinated pumping can worsen heart failure — fatigue, breathlessness, and exercise limitation.
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What they predict. Some patterns of conduction disease are stable and will likely never progress. Others — particularly bifascicular block (both the right bundle and one of the left fascicles affected) or trifascicular block (bifascicular block plus a slow AV node) in a patient who has fainted — raise concern that the wiring could fail more completely and require a pacemaker.
How we diagnose it
The diagnosis comes from a standard 12-lead ECG — the same simple test you may have had in any clinic or emergency room. Each type of block has a recognizable signature: the QRS complex (the part of the ECG that represents the ventricles firing) becomes wider than normal and takes on a characteristic shape.
From there, we usually want to know two more things:
- Is the block new, or has it been there for years? Comparing with any previous ECG is one of the most useful things we can do. A new LBBB, especially in someone with chest pain, is treated as a possible heart attack until proven otherwise.
- What’s the rest of the heart doing? An echocardiogram (an ultrasound of the heart) tells us about the muscle strength, valves, and chamber sizes. In some cases we add stress testing, cardiac MRI, or a coronary evaluation.
If you have unexplained fainting along with bundle branch block, we sometimes also do an electrophysiology study to measure how well the wiring below the AV node is actually conducting.
How we treat it — or just observe
When we observe
- Isolated RBBB in someone with a structurally normal heart needs no treatment. We note it and move on.
- Isolated LAFB or LPFB (a single fascicular block) is usually just observed.
- Stable bifascicular block without symptoms is also typically followed rather than treated.
When we pace
We recommend a pacemaker if there’s evidence that the wiring is failing more broadly — for example, episodes of fainting tied to documented pauses, second-degree or third-degree AV block, or an EP study showing severely delayed conduction below the AV node.
When we consider CRT
In patients with LBBB plus a weak heart muscle (a low ejection fraction) and heart failure symptoms despite good medical therapy, a special device called cardiac resynchronization therapy (CRT) uses an extra pacing wire to restore coordinated pumping. This can meaningfully improve symptoms and survival in well-selected patients. RBBB does not usually benefit from CRT in the same way.
What to expect at your visit
We’ll review your ECG, ask about symptoms — especially fainting, dizziness, breathlessness, or fatigue — and look at any prior tracings to see whether the block is new. Depending on the pattern, we may order an echocardiogram or longer-term rhythm monitoring. For most patients, the conversation ends with reassurance and routine follow-up. For a smaller group, we’ll talk about whether a pacemaker or CRT device is the right next step, and we’ll walk you through that decision carefully.