Bundle branch blocks (BBBs) are conduction delays within the ventricles of the heart. You can see the effects of these delays on an electrocardiogram (ECG), which shows up as changes in the QRS complex.
A right bundle branch block (RBBB) means there’s a delay in the right ventricle, while a left bundle branch block (LBBB) means there’s a delay in the left ventricle.
Here, we’ll explain the ECG criteria, underlying electrophysiology, possible causes, and clinical implications of both RBBB and LBBB. We’ll also look at how our understanding of BBBs is changing, especially when it comes to acute myocardial infarction (AMI).
Knowing what’s going on in the heart is more useful than just memorizing patterns on an ECG.
Basic Electrophysiology and Normal Conduction
In order to understand what happens in a bundle branch block, it’s helpful to know how the heart normally conducts electrical signals.
Typically, the heart’s electrical signal starts in the sinoatrial (SA) node, then travels through the atria to the atrioventricular (AV) node, and finally down the Bundle of His.
The Bundle of His then divides into right and left bundle branches, which carry the electrical signal to the right and left ventricles respectively. This synchronized activation of both ventricles is what allows the heart to beat efficiently.
On an ECG, this normal ventricular depolarization shows up as a narrow QRS complex (less than 120 ms). The precordial leads (V1-V6) are particularly important for spotting RBBB and LBBB patterns, because they let us see the heart’s electrical activity from different angles.
Specifically, V1 gives us a good view of the right side of the heart, while V6 (and other lateral leads) show us what’s happening on the left.
Right Bundle Branch Block (RBBB)
A right bundle branch block (RBBB) happens when electrical impulses have trouble traveling down the heart’s right bundle branch.
ECG Criteria for RBBB
If your doctor suspects you have a right bundle branch block, they will likely order an electrocardiogram (ECG) or EKG. On the EKG, a RBBB typically looks like this:
- QRS duration > 120 ms
- RSR’ pattern in leads V1-V3 (often described as “rabbit ears”)
- Wide S wave in lateral leads (I, aVL, V5, V6)
- ST depression and T-wave inversion in right precordial leads
Electrophysiology of RBBB
In RBBB, the right ventricle activates later than it should. The left ventricle depolarizes as expected, but the right ventricle is delayed.
Causes of RBBB
RBBB can be caused by a number of cardiac conditions, including:
- Right ventricular hypertrophy
- Cor pulmonale
- Pulmonary embolus
- Ischemic heart disease
- Rheumatic heart disease
- Congenital heart disease
- Myocarditis
- Cardiomyopathy
- Lenègre-Lev disease
A new RBBB in someone who is experiencing chest pain is cause for concern, as it could indicate acute myocardial infarction (OMI). The right bundle branch gets its blood supply from LAD perforators, so a blockage of that branch could show up as a new RBBB with or without LAFB.
Left Bundle Branch Block (LBBB)
A left bundle branch block (LBBB) happens when electrical impulses can’t travel normally through the left bundle branch. This delays activation of the left ventricle.
ECG Criteria for LBBB
To diagnose LBBB using an electrocardiogram (ECG), doctors look for these characteristics:
- QRS duration ≥ 120 ms (This shows how long it takes for the ventricles to depolarize.)
- Broad, notched (M-shaped) R waves in lateral leads (I, aVL, V5, V6).
- Absence of Q waves in lateral leads (V5-V6).
- QS or rS complex in V1-V3.
- ST-segment depression and T-wave inversion in lateral leads with tall R waves (This is called “appropriate discordance.”)
Electrophysiology of LBBB
In LBBB, the electrical impulse can’t travel through the left bundle branch in a timely fashion. This results in reversed septal activation.
Causes of LBBB
LBBB is usually a sign of some other problem in the heart. It’s rare to see LBBB without some kind of underlying organic disease.
Some things that can cause LBBB are:
- Infarction (tissue death due to lack of blood supply)
- Ischemia (reduced blood flow)
- Hypertension (high blood pressure)
- Cardiomyopathies (diseases of the heart muscle)
If your doctor diagnoses you with LBBB, they will likely want to investigate further to determine the underlying cause and recommend the appropriate treatment.
How to tell RBBB from LBBB
When trying to tell the difference between a right bundle branch block (RBBB) and a left bundle branch block (LBBB), you’ll want to focus on the precordial leads (V1-V6) on an EKG.
- RBBB: Lead V1 will usually show an RSR’ pattern, while V6 will show a wide S wave.
- LBBB: Lead V1 will usually show a QS or rS complex, while V6 will show a broad, notched R wave.
Instead of just memorizing these patterns, try to understand the electrical conduction pathways in the heart. This will help you better understand what you’re seeing on the EKG.
Also, keep an eye out for axis deviation, which is more common in LBBB (often a left axis deviation).
Clinical Significance and Management
When deciding on treatment, it’s important to know whether a bundle branch block (BBB) is new or old and how it’s affecting the patient.
Here’s what healthcare professionals will likely consider:
- They’ll compare your ECG to any previous ECGs to see if the block is new or if you’ve had it for a while.
- They’ll ask you about your medical history and any symptoms you’re experiencing.
A new left bundle branch block (LBBB) used to be considered the same as a STEMI (a type of heart attack) if the patient was experiencing chest pain. Now, a new LBBB is no longer considered a “STEMI-equivalent.” However, doctors will be on the lookout for ST-segment changes, which could indicate ischemia (reduced blood flow to the heart).
A new right bundle branch block (RBBB) in someone with chest pain could mean they’re having a heart attack, so doctors will consider that possibility.
People with complete or incomplete trifascicular block may need a pacemaker. They’ll also likely need to see a cardiologist.
Frequently Asked Questions
Is RBBB an emergency?
An isolated Right Bundle Branch Block (RBBB) is usually not an emergency, especially if you have no other symptoms. However, a new RBBB in the setting of chest pain or other concerning symptoms requires immediate medical evaluation, as it could indicate a serious underlying heart condition.
What is the Strauss criteria for LBBB?
The Strauss criteria are not the established criteria for diagnosing LBBB. The Strauss criteria were used to determine bi-fascicular block. The current criteria for LBBB diagnosis on an ECG include a QRS duration ≥ 120 ms, a broad notched R wave in leads I, aVL, V5, and V6, absent Q waves in those same leads, and delayed intrinsicoid deflection in V5 and V6.
How can you tell the difference between RBBB and LBBB?
The primary way to differentiate RBBB and LBBB is by examining the QRS complex on an ECG. In RBBB, you’ll typically see a wide QRS complex and an “rSR'” pattern (rabbit ears) in leads V1 and V2. LBBB is characterized by a wide QRS complex and broad, notched R waves in leads I, aVL, V5, and V6, often with inverted T waves.
What is the difference between right bundle and left bundle?
The right and left bundles are branches of the heart’s electrical conduction system. The right bundle carries electrical impulses to the right ventricle, while the left bundle branches into the left anterior and left posterior fascicles, delivering impulses to the left ventricle. They are responsible for the synchronized contraction of the ventricles.
Wrapping Up
Right bundle branch block (RBBB) and left bundle branch block (LBBB) are different conduction problems, and they look different on an EKG.
To diagnose and treat them correctly, it’s important to understand how the heart’s electrical system works and the overall clinical picture.
Interpreting bundle branch blocks, particularly when someone is having a heart attack, calls for a careful look at the latest guidelines and a healthy dose of clinical judgment.