A left bundle branch block (LBBB) is a delay or disruption in the electrical pathway that tells the left side of your heart when to squeeze. Normally, an electrical signal travels down two branches, one to each lower chamber (ventricle), so both sides contract in sync. When the left branch is blocked, the right ventricle fires first and the left ventricle activates late, throwing off the heart’s coordination. LBBB is usually discovered on a routine electrocardiogram (ECG) and can range from a harmless electrical quirk to a sign of significant heart disease.
How the Heart’s Wiring Works
Your heart has a built-in electrical system. A signal starts at the top of the heart, passes through a relay station between the upper and lower chambers, then enters a narrow cable of tissue called the His bundle. That cable splits into two branches: one heading to the right ventricle and one heading to the left. The left bundle branch emerges near the base of the aortic valve and then fans out into two smaller pathways that run toward the front and back walls of the left ventricle.
When this left branch is damaged or blocked, the electrical signal can’t travel its normal route. Instead, the right ventricle activates on schedule and the signal then has to spread across the muscular wall separating the two ventricles to reach the left side. That detour takes an extra 30 to 40 milliseconds on average. It may not sound like much, but the delay means different parts of the left ventricle contract out of sequence. The septum (the wall between the ventricles) contracts first during what should be a pressure-building phase, stretching the side and back walls. Those walls then contract late, stretching the septum in return. Over time, this tug-of-war pattern can reduce pumping efficiency.
What Causes It
LBBB is not a disease on its own. It’s a sign that something has changed the tissue of the left bundle branch. The most common culprits are conditions that put stress on or damage the heart muscle:
- High blood pressure, which thickens and stiffens the heart walls over years
- Coronary artery disease, which reduces blood flow to the heart tissue
- Heart attack, which can scar the conduction pathways
- Cardiomyopathy, a weakening or stiffening of the heart muscle
- Heart valve disease, particularly problems with the aortic valve
- Myocarditis, an infection or inflammation of the heart
- Heart failure
- Congenital heart defects
In some people, especially younger adults without other heart problems, LBBB appears with no identifiable cause. This “isolated” LBBB is generally considered lower risk, but it still warrants evaluation because an underlying condition may not yet be obvious.
Symptoms You Might Notice
Most people with LBBB feel nothing at all. The block itself doesn’t typically cause pain or shortness of breath. It’s frequently discovered by accident during a routine ECG or a pre-surgical evaluation. When symptoms do occur, they usually come from the underlying heart condition driving the block rather than the electrical delay itself. You might experience fatigue, lightheadedness, fainting, or a sense that your heart is beating irregularly. If LBBB develops suddenly during a heart attack, chest pain and other acute symptoms will be front and center.
How It’s Diagnosed on an ECG
LBBB shows up as a distinctive pattern on a standard 12-lead ECG. The hallmark is a widened QRS complex, the spike on the tracing that represents ventricular contraction. In a healthy heart, that spike lasts under 120 milliseconds. With LBBB, it stretches to 120 ms or longer, often beyond 150 ms. The shape of the spike changes too: in the lead watching the right side of the heart (V1), there’s a deep downward dip, while the lead watching the left side (V6) shows a broad, tall upward wave, sometimes with a notch or “M” shape at the top. Your doctor can spot this pattern in seconds, and no special equipment beyond a standard ECG machine is needed.
Why It Complicates Other Heart Tests
One frustrating reality of LBBB is that it interferes with the most common test for blocked coronary arteries: the exercise stress ECG. Doctors normally look for telltale changes in the electrical tracing while you walk on a treadmill. But LBBB distorts those same segments of the tracing at baseline, making it impossible to tell whether any changes during exercise are from poor blood flow or simply from the block itself. A standard treadmill stress test can still measure your exercise capacity, but it can’t reliably diagnose coronary artery disease when LBBB is present.
Instead, doctors typically use imaging-based stress tests. Stress echocardiography, where ultrasound images of the heart are taken during exercise or while a medication simulates exercise, performs well in this population. One approach uses a medication called dobutamine to increase the heart’s workload while an ultrasound watches for wall-motion abnormalities. Studies show this method reaches about 83 to 88 percent sensitivity and 92 to 96 percent specificity for detecting blocked arteries in different regions of the heart, with overall diagnostic accuracy around 87 to 93 percent. Nuclear imaging stress tests are another reliable alternative.
Long-Term Health Risks
LBBB is not just an electrical curiosity. In a large study of over 7,000 patients, those with LBBB had a 32 percent higher risk of being hospitalized for heart failure compared to patients without the block, after accounting for other health factors. Cardiovascular hospitalization risk was 13 percent higher, and overall mortality was 10 percent higher. These numbers reflect the reality that LBBB often travels alongside conditions that strain the heart, and the dyssynchrony it creates can gradually worsen pumping function even when the original cause is mild.
A new-onset LBBB deserves particular attention. If you’ve had normal ECGs in the past and a new one shows LBBB, your doctor will likely investigate with an echocardiogram to measure how well your heart is pumping and to look for structural problems. The context matters enormously: LBBB appearing during a heart attack is an emergency, while LBBB found incidentally in an otherwise healthy person calls for monitoring rather than immediate intervention.
Treatment Options
There’s no medication that “fixes” the blocked electrical pathway. Treatment focuses on managing whatever condition caused the block and, when necessary, restoring the heart’s coordinated squeeze with a device.
For people whose LBBB contributes to heart failure, cardiac resynchronization therapy (CRT) can be a significant help. CRT uses a specialized pacemaker with leads placed in both ventricles. By pacing both sides simultaneously, it bypasses the blocked pathway and restores synchronized contraction. International guidelines strongly recommend CRT for heart failure patients with LBBB and a QRS duration greater than 150 ms whose ejection fraction (a measure of pumping strength) is 35 percent or lower and who remain symptomatic despite medications. Some guidelines extend the recommendation to QRS durations as low as 120 ms in LBBB patients. The benefits are well established: CRT can improve symptoms, reduce hospitalizations, and in some patients reverse some of the heart’s structural decline.
If LBBB is isolated, meaning your heart structure and function are normal on imaging and you have no symptoms, treatment usually isn’t necessary. Regular follow-up with periodic echocardiograms lets your doctor catch any changes early.
Living With LBBB
For many people, LBBB has no impact on daily life. You can exercise, work, and travel normally if your heart function is preserved. The key is knowing what caused the block and staying on top of that underlying condition. If you have high blood pressure, keeping it controlled protects the conduction system from further damage. If coronary artery disease is the culprit, the usual strategies of medication, lifestyle changes, and sometimes procedures apply.
If you’ve been told you have LBBB, it’s worth keeping a copy of your ECG. Because the block changes the baseline electrical pattern of your heart, having a reference tracing on file helps doctors interpret future ECGs more accurately, especially in an emergency when they need to distinguish your “normal” LBBB pattern from new, urgent changes.