A bundle branch block is a delay or obstruction in the electrical pathway that triggers your heart’s lower chambers to pump. It shows up on an electrocardiogram (ECG) as a widened electrical signal, and in many cases, especially with a right bundle branch block, it causes no symptoms and requires no treatment. Left bundle branch block, however, is more closely tied to underlying heart disease and carries a higher long-term risk.
How Your Heart’s Electrical System Works
Every heartbeat starts with a small burst of electricity from a cluster of cells called the SA node, located at the top of your heart. That signal travels downward, first telling the upper chambers (atria) to squeeze blood into the lower chambers (ventricles). The signal then pauses briefly at the AV node, a checkpoint near the center of your heart that holds the impulse just long enough for the upper chambers to finish emptying.
From there, the signal enters a highway of specialized nerve fibers called the bundle of His, which runs down the wall separating your two ventricles. This highway splits into two lanes: the right bundle branch, serving the right ventricle, and the left bundle branch, serving the left ventricle. These branches fan out into a network of tiny fibers that deliver the signal to the muscle cells, causing both ventricles to contract at almost exactly the same moment. That simultaneous squeeze is what pumps blood efficiently to your lungs and body.
What Happens When a Branch Is Blocked
When one of these branches is damaged or scarred, the electrical signal can’t travel down it normally. The affected ventricle still contracts, but it does so a fraction of a second late, triggered by a slower, roundabout spread of electricity from the other side. On an ECG, this shows up as a wider-than-normal QRS complex, which is the spike that represents ventricular contraction. A normal QRS complex lasts less than 100 milliseconds. In a complete bundle branch block, it stretches to 120 milliseconds or longer. When it falls between 100 and 119 milliseconds, it’s considered an incomplete block.
That slight delay in one ventricle’s squeeze doesn’t always cause problems. For many people, the heart compensates well enough that they never feel a thing. But in others, particularly when the left branch is affected or when heart function is already weakened, the lack of synchrony between the two ventricles can reduce pumping efficiency over time.
Right vs. Left Bundle Branch Block
The two types differ significantly in what they suggest about your heart health.
A right bundle branch block (RBBB) is relatively common and often benign. It can appear in people with no heart disease at all, sometimes as an incidental finding during a routine physical or pre-surgical ECG. When it does have an underlying cause, the most common culprits include a blood clot in the lungs (pulmonary embolism), high blood pressure in the lung arteries, inflammation of the heart muscle, congenital heart defects like a hole between the upper chambers, or a heart attack.
A left bundle branch block (LBBB) is a different story. It almost always signals some form of structural heart disease. The most frequent causes include coronary artery disease, long-standing high blood pressure, aortic valve disease, and cardiomyopathy, a condition where the heart muscle becomes enlarged or stiff. Among people with dilated cardiomyopathy specifically, LBBB shows up in 10% to 26% of cases. Finding a new LBBB on an ECG typically prompts further testing to identify the underlying problem.
Symptoms and How It’s Found
Most people with a bundle branch block don’t feel anything. The condition is usually discovered on an ECG done for another reason, such as a pre-operative screening, a routine check-up, or an evaluation for chest pain or shortness of breath. When symptoms do occur, they’re generally caused by the underlying condition rather than the block itself. These can include fatigue, lightheadedness, or fainting episodes.
Your doctor can spot a bundle branch block on a standard 12-lead ECG by looking at the shape and width of the QRS complex. Right and left blocks produce distinct patterns in different leads of the ECG, making it straightforward to tell them apart. The characteristic pattern of RBBB, sometimes called the “bunny ear” pattern because of its double-peaked appearance, is visible in leads that look at the right side of the heart.
Long-Term Risks
For an isolated right bundle branch block in someone with no other heart disease, the prognosis is generally excellent. Many people live their entire lives with one and never experience complications.
Left bundle branch block carries more concern. A study published in JAMA Network Open found that people with LBBB had roughly five times the risk of developing heart failure compared to those without it, even after adjusting for other risk factors. They also had nearly five times the odds of experiencing a measurable decline in their heart’s pumping strength over a five-year period. The increased risk of death in the same study was modest and not statistically definitive, but the strong link to heart failure means LBBB warrants ongoing monitoring.
In rare cases, a bundle branch block can progress to a complete heart block, where electrical signals fail to reach the ventricles at all. This is more likely when both branches are partially damaged, or when the block occurs alongside other conduction abnormalities.
Treatment Depends on the Underlying Cause
A bundle branch block itself isn’t treated directly. If there’s no underlying heart disease, no treatment is needed. If the block is caused by high blood pressure, coronary artery disease, or another condition, managing that condition is the priority.
For people who have both LBBB and heart failure with reduced pumping function, a specialized device called a cardiac resynchronization therapy (CRT) device can make a significant difference. This is essentially a pacemaker with an extra lead that stimulates both ventricles simultaneously, restoring the coordinated squeeze that the blocked branch can no longer deliver. The greatest benefit is seen in patients whose QRS complex is wider than 150 milliseconds and who have a left bundle branch block pattern. For those with non-LBBB patterns, the evidence of benefit is weaker and generally limited to people with more severe heart failure symptoms and wider QRS measurements.
A standard pacemaker may be recommended if the bundle branch block progresses to a more advanced conduction problem, such as complete heart block, where the heart rate drops dangerously low.
Living With a Bundle Branch Block
If you’ve been told you have a bundle branch block, what it means for your daily life depends almost entirely on whether you have underlying heart disease. Most people with an isolated block, particularly RBBB, face no activity restrictions. They exercise, work, and travel without limitations.
The most practical thing your diagnosis provides is a baseline ECG for comparison. If you ever develop chest pain or other cardiac symptoms in the future, doctors can compare a new ECG to your known pattern and more accurately determine whether something has changed. Current guidelines from the American Heart Association and American College of Cardiology emphasize that clinical risk assessment tools are more useful for predicting outcomes than a single resting ECG finding alone. In other words, the block on your ECG is one piece of the puzzle, not the whole picture.