A bundle branch block is a delay or disruption in the electrical signals that tell your heart’s lower chambers (ventricles) to pump. Your heart has a built-in wiring system that coordinates each beat, and when part of that wiring is damaged, one ventricle contracts slightly out of sync with the other. Many people with a bundle branch block have no symptoms at all and discover it incidentally on an EKG. In other cases, it signals an underlying heart condition that needs attention.
How Your Heart’s Electrical Wiring Works
Every heartbeat starts with an electrical impulse in the upper right chamber, which travels down to a relay station between the upper and lower chambers called the AV node. From there, the signal enters a single cable of specialized fibers called the bundle of His, which runs down the wall separating the two ventricles. This cable then splits into two branches: a left bundle branch that sends signals to the left ventricle, and a right bundle branch that sends signals to the right ventricle. Those branches fan out into a fine network of fibers that trigger the muscle cells in each ventricle to contract at nearly the same instant, pushing blood out to your lungs and body.
When both branches work normally, the two ventricles squeeze together in tight coordination. A bundle branch block means one of those branches is damaged or blocked. The affected ventricle still contracts, but it has to rely on the slower spread of the electrical signal through surrounding muscle tissue rather than the fast-track wiring. The result is a slightly delayed, less efficient squeeze on one side.
Right vs. Left Bundle Branch Block
The two types are named for which branch is affected, and they carry different clinical weight.
A right bundle branch block (RBBB) means the signal to the right ventricle is delayed. The right ventricle handles a lighter job, pumping blood only to the lungs, so a delay here is often well tolerated. In a large study of over 9,000 heart failure patients, RBBB did not independently predict a higher risk of death or hospitalization. It can show up in otherwise healthy people and may not require any treatment.
A left bundle branch block (LBBB) tends to be more clinically significant. The left ventricle is the heart’s main pump, responsible for sending blood to the entire body. When its electrical activation is delayed, pumping efficiency drops more noticeably. In that same study, patients with LBBB had a 32% higher risk of heart failure hospitalization and a 10% higher risk of death compared to patients without a block, even after accounting for other health factors. LBBB is more likely to reflect underlying heart disease and typically prompts a more thorough workup.
The Left Bundle’s Extra Complexity
The left bundle branch itself splits into two smaller pathways called fascicles, one running toward the front of the heart and one toward the back. A block in just one fascicle is called a fascicular block (or hemiblock) and is generally milder. When the right bundle branch plus one left fascicle are both blocked, that’s a bifascicular block. If all three pathways are impaired, it’s called a trifascicular block, which poses a serious risk of complete heart block, dangerous rhythm problems, heart failure, and even sudden cardiac arrest.
Common Causes
The wiring damage behind a bundle branch block can come from many sources. Heart attacks are a major one, especially for LBBB, because reduced blood flow can injure the conduction fibers. Long-standing high blood pressure thickens the heart muscle over time, which can disrupt the electrical pathways. Cardiomyopathy, where the heart muscle itself becomes weakened or enlarged, is another frequent cause.
Age plays a role too. The conduction fibers can develop scar tissue (fibrosis) over decades, which is why bundle branch blocks become more common in older adults. Other causes include heart valve disease, congenital heart defects, infections that inflame the heart muscle (myocarditis), and chronic lung conditions that strain the right side of the heart. Occasionally, a bundle branch block shows up in younger, healthy individuals with no identifiable cause, particularly RBBB.
Symptoms You Might Notice
Most people with a bundle branch block feel nothing. The block itself doesn’t usually produce symptoms. When symptoms do occur, they’re typically driven by the underlying condition that caused the block, not the block alone. These can include lightheadedness or dizziness, fainting or near-fainting episodes, feeling unusually tired, or a sense that your heart is beating irregularly.
If you have LBBB and your left ventricle is already weakened, the added loss of coordination between the two ventricles can worsen shortness of breath and exercise intolerance. In cases where the block progresses to involve multiple pathways (bifascicular or trifascicular block), fainting episodes become more concerning because they may signal that the heart’s electrical system is close to failing entirely.
How It’s Diagnosed
Bundle branch blocks are diagnosed with an electrocardiogram (EKG), a quick, painless test that records your heart’s electrical activity through stickers on your chest. The key measurement is the width of the QRS complex, the portion of the EKG tracing that represents the ventricles contracting. Normally, this takes less than 120 milliseconds. A complete bundle branch block widens it beyond 120 milliseconds because one ventricle is activating late.
The shape of the QRS complex also tells your doctor which side is blocked. For a stricter definition of LBBB, some criteria require the QRS to be at least 130 milliseconds in women and 140 milliseconds in men. When the QRS is wider than normal but doesn’t quite reach 120 milliseconds, it may be called an incomplete bundle branch block, which is generally less concerning.
Once a block is identified, further testing depends on the type and your overall health. LBBB in particular often leads to additional evaluation with an echocardiogram (an ultrasound of the heart) to check how well the ventricles are pumping, and sometimes stress testing or other imaging to look for coronary artery disease.
Treatment and When It’s Needed
An isolated RBBB in someone with no other heart problems usually requires no treatment at all, just periodic monitoring. LBBB without symptoms or underlying disease may also be watched rather than treated, though it warrants closer follow-up.
Treatment becomes necessary when a bundle branch block coexists with other heart problems. The most important intervention is cardiac resynchronization therapy (CRT), a specialized type of pacemaker. CRT uses leads placed in both ventricles to coordinate their contractions, essentially compensating for the blocked pathway. It’s most beneficial for people with LBBB who also have a weakened heart pump (reduced ejection fraction) and symptoms of heart failure. Studies consistently show CRT improves symptoms, exercise capacity, and survival in this group.
For patients who need pacing for other reasons, such as a slow heart rate, and who are expected to require the pacemaker to drive the ventricles more than 40% of the time, current guidelines recommend pacing strategies that mimic the heart’s natural activation pattern. These include CRT or His bundle pacing, where the lead is placed directly on the natural conduction highway, rather than standard right ventricular pacing, which can actually create a pattern similar to LBBB over time.
In trifascicular block, the risk of progressing to complete heart block makes pacemaker implantation more urgent, even before severe symptoms develop. Complete heart block means no electrical signals reach the ventricles through normal pathways, and the heart relies on a slow backup rhythm that can be unreliable and life-threatening.
Living With a Bundle Branch Block
For the majority of people, especially those with an isolated RBBB, a bundle branch block is a finding on paper that doesn’t change daily life. You can exercise, work, and travel normally. The main practical consideration is that your EKG will always look abnormal going forward, which is worth mentioning to any new doctor or if you ever go to the emergency room for chest pain. LBBB in particular can mask the EKG signs of a heart attack, making diagnosis trickier and sometimes requiring additional tests like blood markers or imaging.
If you have LBBB with a known heart condition, regular follow-up with a cardiologist is important. Heart function can change over time, and a drop in pumping strength might make you a candidate for CRT even if you weren’t initially. Keeping blood pressure controlled, managing cholesterol, staying physically active, and addressing any underlying conditions are the most effective ways to protect the conduction system from further damage.