A bundle branch block (BBB) is a condition involving a delay or complete obstruction along the electrical pathways that carry impulses to the heart’s lower chambers (ventricles). These electrical signals prompt the ventricles to contract and pump blood, so a block means the left and right sides contract slightly out of sync. This disruption is a common finding, often detected incidentally during routine medical testing. The severity depends heavily on its specific location and whether underlying heart disease is present.
How the Heart’s Electrical System Works
The heart relies on an intrinsic electrical conduction system to maintain a steady rhythm and coordinate its pumping action. This process begins at the sinoatrial (SA) node, the heart’s natural pacemaker, which generates the initial electrical impulse. The signal spreads across the upper chambers (atria), causing them to contract and push blood into the ventricles.
The impulse then travels to the atrioventricular (AV) node, where it pauses briefly to allow the ventricles time to fully fill with blood. From there, the signal moves rapidly down a single tract called the Bundle of His. This tract quickly divides into the specialized right and left bundle branches, which deliver the signal to the muscular lower chambers.
These branches distribute the signal simultaneously to the corresponding right and left ventricles via a fine network of Purkinje fibers. This synchronization is necessary for the ventricles to contract forcefully and efficiently. A bundle branch block occurs when one of these two main electrical highways is damaged or obstructed.
When the signal is blocked in one branch, it must detour through the unblocked side and slowly spread across the muscle tissue to reach the affected side. This delayed path causes the two ventricles to contract sequentially instead of simultaneously. The resulting asynchronous contraction can sometimes reduce the heart’s overall pumping efficiency, particularly in individuals with pre-existing heart problems.
Different Types and Associated Conditions
Bundle branch blocks are categorized based on which side of the heart’s electrical system is affected by the delay. A Right Bundle Branch Block (RBBB) involves a delay in the electrical signal reaching the right ventricle. This type is relatively common and is frequently found in individuals who have no other apparent heart disease.
RBBB on its own generally does not cause symptoms or require specific treatment. The presence of RBBB warrants a thorough medical evaluation, but it is typically viewed as less clinically significant when the rest of the heart structure is normal. The block primarily affects the thinner-walled right ventricle, which has less impact on overall cardiac output.
Conversely, a Left Bundle Branch Block (LBBB) affects the signal traveling to the left ventricle, the heart’s main pumping chamber. LBBB is less common than RBBB and is considered more significant because it rarely occurs in the absence of underlying structural heart disease. Its presence indicates a need for detailed cardiac investigation.
The LBBB causes the left ventricle to contract later than the right, leading to a noticeable asynchronous beat that can impair the heart’s ability to pump blood efficiently. Conditions strongly associated with LBBB include severe high blood pressure (hypertension), coronary artery disease, and heart failure. Other causes involve cardiomyopathy, heart valve disease, or scar tissue from a prior heart attack.
Diagnosis and When Treatment is Necessary
A bundle branch block is diagnosed primarily through an Electrocardiogram (ECG or EKG), which records the heart’s electrical activity. The delay in the electrical signal traveling through the ventricles is identified by a widened QRS complex on the ECG tracing. A complete block is defined by a QRS duration of 120 milliseconds or longer, indicating slower ventricular activation.
Since the block itself usually does not cause specific symptoms, diagnosis leads to further investigation to identify any underlying heart problems. Doctors often use an echocardiogram, a specialized ultrasound of the heart, to visualize the muscle structure and pumping function. This test helps determine if the block is an isolated finding or a marker of disease like heart failure.
In many cases, particularly with an isolated RBBB in an otherwise healthy individual, no direct treatment for the block is necessary. Treatment is instead directed toward managing the underlying conditions, such as using medication to control high blood pressure or heart failure. Regular monitoring is maintained to check for any progression in the electrical disturbance or the underlying disease.
A pacemaker may be considered if the block causes symptoms like fainting (syncope) or if it progresses to a more severe form of heart block. For patients with heart failure and LBBB, a specialized device called a Cardiac Resynchronization Therapy (CRT) device may be recommended. The CRT device helps re-coordinate the contraction of the two ventricles to improve pumping efficiency and reduce symptoms.