Bifascicular block, as seen on an ECG, refers to a conduction defect in the heart where two of the three main fascicles of the heart's electrical conduction system are blocked. This usually involves the right bundle branch and either the left anterior fascicle or the left posterior fascicle.
Understanding the Heart's Electrical System
The heart's electrical system ensures coordinated contraction of the heart muscle, allowing it to pump blood effectively. This system consists of:
- SA Node (Sinoatrial Node): The heart's natural pacemaker.
- AV Node (Atrioventricular Node): Delays the electrical signal briefly, allowing the atria to contract before the ventricles.
- Bundle of His: Conducts the electrical signal from the AV node to the ventricles.
- Right Bundle Branch (RBB): Carries the signal to the right ventricle.
- Left Bundle Branch (LBB): Divides into two fascicles:
- Left Anterior Fascicle (LAF): Carries the signal to the anterior and superior portions of the left ventricle.
- Left Posterior Fascicle (LPF): Carries the signal to the posterior and inferior portions of the left ventricle.
Bifascicular Block Explained
When a bifascicular block occurs, the electrical impulse has difficulty traveling through two of these pathways. This forces the impulse to take an alternate, slower route to activate the ventricles. The two most common types of bifascicular block are:
- Right Bundle Branch Block (RBBB) with Left Anterior Fascicular Block (LAFB): This combination is the more frequent of the two.
- Right Bundle Branch Block (RBBB) with Left Posterior Fascicular Block (LPFB): This combination is less common because the left posterior fascicle is thicker and has a dual blood supply, making it less susceptible to block.
ECG Findings
The ECG findings will vary depending on the specific combination of blocks present, but generally include:
- Prolonged QRS duration: Indicates delayed ventricular depolarization (widened QRS complex).
- RBBB pattern: Evident by specific morphology in leads V1 and V6.
- Left Axis Deviation (LAD): Often seen in LAFB.
- Right Axis Deviation (RAD): Often seen in LPFB.
Clinical Significance
Bifascicular block can be a benign finding in some individuals, but it can also indicate underlying heart disease. It is often associated with conditions such as:
- Ischemic heart disease (coronary artery disease)
- Hypertension
- Cardiomyopathy
- Degenerative disease of the conduction system
Because of the potential for progression to complete heart block (where all conduction to the ventricles is blocked), individuals with bifascicular block are carefully evaluated for symptoms like syncope (fainting) or pre-syncope (lightheadedness). In some cases, a pacemaker may be recommended to prevent complete heart block.
