What Is a Bundle Branch Block?
Electrical Conduction in the Heart
All of the cells of the heart have a negative charge and have a property called automaticity–the capability to spontaneously generate a rhythmic impulse of depolarization–an electrical change that can propagate to the next cell and begin depolarization there. In this way, an impulse can travel along a path.
When a cell depolarizes, it becomes refractory (silent), immune to a subsequent depolarization until it re-polarizes in preparation for the next depolarization. This allows a dominant (faster rhythmic) cell to lead the way in the depolarizations. In the heart, the dominant pace making cells are in the sinoatrial (SA) node.
From the SA node, the impulse causes the contraction of the atrium and travels to the junctional area between the atria and the ventricles. There, the atrioventricular (AV) node receives the impulse and sends it farther down a bundle of fibers–the Bundle of His–which separates into right and left main bundle branches. These themselves splinter into finer fibers that spread throughout the ventricles. Thus, a wave of depolarization travels from the SA node to the AV node, Bundle of His, right and left bundle branches, finally terminating in the Purkinje fibers.
The fact that the impulse has to travel, which takes time, means the wave of depolarizations traveling inferior and from right to left cause a sequential contraction of atria first and ventricles second; this generates a directional pumping action–out–for blood within the heart, one-way valves preventing back-flow.
A block at any point in the depolarization path can cause the directional pumping action of the heart to fail, the severity of which is based on where and how extensively the dissociation occurs. Such a failure results in an arrhythmia. Some failures are mild but others can significantly alter the cardiac output.
Blocks in the bundle branches are within the ventricles themselves, causing delays in the propagation of the wave of impulses. This is called “intraventricular conduction delay” (IVCD). The blocks can be total or partial (“fascicular”), accounting for a varied range of conduction and rhythm abnormalities associated with varying degrees of cardiac output impairment.
An ECG is a recording of the depolarizations and repolarizations as they travel through the heart. Electrodes placed on the skin around the heart allow different views of this wave from different directions. From this, a bundle branch block can be diagnosed, as well as can other electrical abnormalities in the heart.
Left Bundle Branch Block (LBBB)
LBBB occurs often indicates one of 4 conditions:
- Coronary artery disease
- Aortic valve disease
Right Bundle Branch Block (RBBB)
RBBB is typically due to:
- A congenital structural defect, such as an atrial septal defect
- Acquired heart disease, such as valvular disease or ischemia/myocardial infarction
How Is Bundle Branch Block Diagnosed?
In the depolarization (contraction) cascade–right to left and superior to inferior–that results in cardiac output, a disruption in the electrical conductive pathway that occurs below the atrioventricular (AV) node is called an intraventricular conduction delay (IVCD). The Bundle of His separates into right and left main bundle branch divisions. These divide further into “fascicles.” Disruptions in this circuit result in characteristic symptoms and electrocardiogram (ECG) changes.
Symptoms from Bundle Branch Block (BBB) and Fascicular Block
Delays in the depolarizing wavefront produces varying degrees of dyssynchrony of contractions, which affects ventricular pump efficiency and impairs cardiac output. Symptoms are caused by these inefficiencies in cardiac output:
- Blood pressure lability (instability)
Any such symptoms will require a neurological and cardiovascular investigation. The cardiovascular diagnostics are designed to pursue the presence of arrhythmias via electrocardiogram and functional assessment via cardiac ultrasonography (echocardiogram).
Since bundle branch blocks are electrical disturbances in the conductive pathway of the heart, the ECG stands as the gold standard of making the diagnosis. The familiar waveform of an ECG is altered, especially in that portion which represents the contraction (depolarization) of the ventricles–the QRS complex. Bundle branch blocks will delay conductivity selectively to one ventricle or the other, and one ventricle will lag behind in its contraction compared to the other:
The normal QRS complex, which is the combined ventricular contraction in normal circumstances, becomes wider (takes longer from beginning to end) when one ventricle takes longer than the other to contract. Since the many ECG views (directions of perspective, or “leads”) give a composite dimensional picture, the “axis” of the heart’s contraction can characterize which direction predominates in an imbalance of the usual axis.
The ECG can also discern areas of ischemia or prior myocardial infarction that could contribute to the BBB.
The cardiac ultrasound uses Doppler technology to ascertain flow through the chambers of the heart, which is affected in bundle branch blocks that cause alterations in cardiac output.
Its B (brightness) mode can evaluate structural integrity, diagnosing valve disease that may be associated with BBB, or even thrombus formation related to the atrial fibrillation that is more frequent in those suffering from BBB.
Congenital abnormalities, a major source of right BBB, can be identified, both as deviations in architecture and in disorders of flow dynamics.
Management of Bundle Branch Block
Right Bundle Branch Block (RBBB)
The structural heart diseases associated with RBBB are:
- Cor pulmonale
- Pulmonary embolism
- Myocardial ischemia/infarction
- Congenital heart disease
Outcomes depend on comorbidities (underlying heart disease), and those with RBBB without heart disease have excellent long-term outcomes.
If there are no symptoms and there is no underlying heart disease, treatment is limited to just serial observation (close surveillance for ECG follow-up and for symptoms that may occur).
If there are symptoms in the pre-syncope to syncope range, treatment is with an implanted cardiac pacemaker if no reversible causes are identified, e.g., electrolyte disturbances, medications, etc.
Left Bundle Branch Block (LBBB)
If a patient with LBBB is asymptomatic and has no underlying heart disease, no therapy is indicated other than close surveillance with serial ECGs and for the emergence of symptoms.
If a patient is symptomatic (pre-syncope, syncope, and other cognitive changes) and if there are no reversible causes (electrolyte disturbances, medications, etc.), a permanent pacemaker is indicated to treat any symptomatic conduction system disturbances.
Decreases in left ventricular fraction cardiac output may require a selective pace making protocol that involves using cardiac resynchronization therapy. Such pacemakers selectively pace the ventricles or only the left ventricle to reestablish a normal cardiac output.
How to Prevent Bundle Branch Block
Prevention of bundle branch block (BBB) is virtually impossible when it is due to congenital, genetic, or long-standing cardiac pathology that has damaged the bundle portions of the heart’s electrical conductive system.
Some cardiac conditions are amenable to improvement and such therapy-derived improvements serve as an aid in preventing a worsening severity of BBB and the symptoms that would ensue:
- Hypertension, using antihypertensives and weight management
- Coronary artery disease, by addressing hypertension, dyslipidemia (cholesterol and triglyceride elevations), diet to mitigate metabolic syndrome, glycemic control (in diabetics), and smoking cessation
- Iatrogenic causes (via medication substitution with equally efficacious drugs that impact the conduction system less)
- Surgical and ablation changes mitigated by post-operative surveillance and care
- Conduction disturbances due to reversible electrolyte imbalance conditions that are easily corrected in diabetics or renal disease patients
From the opposite point of view, when BBB is present, patients should be evaluated for hypertension, coronary disease, myocarditis, valve disease, and other cardiomyopathy. In most patients, this can be accomplished with a careful history and physical examination. When present, prevention within the scope of bundle branch block is essentially prevention of consequences of these other sometimes life-threatening conditions.
Symptoms of LBBB and RBBB center on the idea of effectiveness of cardiac output, which is related to syncope and pre-syncope. Such patients can prevent these cognitive disruptions with an implanted pacemaker.
BBB can be incomplete or complete, and if a patient with BBB has severe cardiac output alteration (low left ventricular ejection fraction or heart failure) prevention of further cardiac deterioration and/or cardiac-related death can be accomplished with cardiac resynchronization therapy. This is a process that paces both ventricles or only the left ventricle.
The key to prevention of BBB sequelae is close observation for symptoms, evaluation of dysrhythmia with ECG, assessing functional cardiac status with echocardiography, and stepping in to override the arrhythmia when symptoms emerge or there is a concerning alteration to cardiac output.