A left bundle branch block (LBBB) is a delay or disruption in the electrical pathway that triggers the left side of your heart to contract. Normally, an electrical signal travels down two branches (left and right) to make both sides of the heart squeeze simultaneously. When the left branch is blocked, the left ventricle still contracts, but it does so a fraction of a second later than the right, making the heart pump less efficiently.
LBBB shows up on an electrocardiogram (ECG) as a widened electrical signal, and it affects roughly 0.06% to 0.1% of the general population. It becomes more common with age and is frequently linked to underlying heart disease, though it occasionally appears in people with no other cardiac problems.
How the Heart’s Electrical System Works
Your heart has a built-in pacemaker called the sinus node, located in the upper right chamber. It fires an electrical impulse that travels to a relay station between the upper and lower chambers, then splits into two pathways: the right bundle branch and the left bundle branch. These branches carry the signal down into the thick muscular walls of the lower chambers (ventricles), causing them to contract in a coordinated squeeze that pushes blood out to the lungs and the rest of the body.
The left bundle branch itself divides into two smaller fascicles, one running toward the front of the heart and one toward the back. This dual design reflects how much work the left ventricle does. It’s the chamber responsible for pumping oxygenated blood to your entire body, so it’s thicker and more muscular than the right side. When the left branch is blocked, the electrical signal has to take a detour through the muscle tissue itself, which conducts electricity much more slowly than the specialized pathway. The result is a delayed, less synchronized contraction of the left ventricle.
Common Causes
In most cases, LBBB signals some form of structural heart disease. The most common underlying conditions include:
- High blood pressure: Chronic hypertension thickens the walls of the left ventricle over time, which can damage the conduction fibers.
- Coronary artery disease: Reduced blood flow to the heart muscle can injure or destroy the tissue that makes up the left bundle branch.
- Cardiomyopathy: Diseases that enlarge or stiffen the heart muscle frequently disrupt normal conduction.
- Heart valve disease: Particularly aortic valve problems, which place extra strain on the left ventricle.
- Heart attack: A new LBBB that appears suddenly can indicate an acute heart attack or be a consequence of a previous one.
In a smaller subset of people, LBBB appears without any detectable heart disease. This is sometimes called “isolated” LBBB and is more often seen in younger adults. Even in these cases, long-term follow-up studies have shown a higher risk of eventually developing heart failure or other cardiac problems compared to people with normal conduction, so it’s not something to ignore entirely.
Symptoms You Might Notice
LBBB by itself often causes no symptoms at all. Many people learn they have it only after getting an ECG for an unrelated reason. When symptoms do occur, they usually come from the underlying condition causing the block rather than from the block itself.
That said, the loss of synchronized pumping can reduce the heart’s efficiency by 5% to 10% in some estimates. Over time, this may contribute to fatigue, shortness of breath during exertion, dizziness, or episodes of fainting. People with pre-existing heart failure tend to notice the effects more, because their hearts are already working at reduced capacity and can’t absorb the additional loss of coordination.
How It’s Diagnosed
An ECG is the only way to diagnose LBBB. The hallmark finding is a QRS complex (the part of the tracing that represents ventricular contraction) wider than 120 milliseconds, along with a characteristic shape in specific leads of the ECG. A normal QRS complex is 80 to 100 milliseconds, so the widening reflects that slower, roundabout path the electrical signal takes through the left ventricle.
Once LBBB is identified, doctors typically order additional testing to look for an underlying cause. An echocardiogram (an ultrasound of the heart) checks the size and pumping strength of the chambers, valve function, and wall thickness. Stress testing, cardiac MRI, or coronary angiography may follow depending on symptoms and risk factors. A new LBBB in someone with chest pain is treated as a medical emergency because it can indicate an ongoing heart attack.
LBBB and Heart Attack Diagnosis
One important complication of having LBBB is that it makes reading future ECGs much harder. The altered electrical pattern masks the classic signs of a heart attack on a standard tracing. For years, emergency guidelines treated any new LBBB with chest pain symptoms as equivalent to an ST-elevation heart attack, warranting immediate intervention. Current guidelines have become more nuanced, using additional criteria to distinguish a true heart attack from the baseline LBBB pattern, but the diagnosis remains challenging. If you know you have LBBB, keeping a copy of your baseline ECG accessible can help emergency physicians compare and spot new changes quickly.
Treatment Options
There is no medication that “unblocks” the left bundle branch. Treatment focuses on managing whatever underlying condition caused the block. If high blood pressure is the culprit, controlling it can prevent further damage. If coronary artery disease is involved, restoring blood flow through procedures or medication addresses the root problem.
For people who develop heart failure alongside LBBB, a treatment called cardiac resynchronization therapy (CRT) can make a significant difference. CRT uses a specialized pacemaker with leads placed in both ventricles, timing the electrical signals so the two sides contract together again. Clinical trials have shown that CRT improves symptoms, exercise capacity, and survival in heart failure patients with LBBB and a QRS duration of 150 milliseconds or more. In fact, patients with LBBB tend to respond better to CRT than those with other types of conduction delays, likely because the device directly corrects the specific timing problem LBBB creates.
Not everyone with LBBB needs CRT. It’s generally reserved for people whose heart’s pumping efficiency (ejection fraction) has dropped below 35% and who have persistent symptoms despite standard heart failure treatment. For people with LBBB but normal heart function, the typical approach is periodic monitoring with echocardiograms every one to two years to catch any decline early.
Long-Term Outlook
Prognosis depends almost entirely on what’s happening beneath the LBBB. In people with no underlying heart disease, the condition can remain stable for decades. A study following patients with isolated LBBB over more than 10 years found that while their risk of cardiovascular events was higher than the general population, many remained asymptomatic and never required intervention.
When LBBB accompanies heart failure, the outlook is more serious but has improved considerably with modern treatments. CRT has reduced hospitalization rates and improved survival in this group. The key variable is how early the condition is caught and how well the underlying disease is managed. Regular follow-up matters, even when you feel fine, because the transition from stable LBBB to declining heart function can happen gradually enough that symptoms sneak up on you.
Age also plays a role in interpretation. LBBB in someone under 50 with no other cardiac findings carries a different weight than LBBB appearing for the first time in a 70-year-old with high blood pressure and diabetes. In either case, the finding warrants a thorough cardiac workup rather than a wait-and-see approach.