Conduction Block

Conduction Block is a pathological interruption or delay in the normal propagation of electrical impulses through the cardiac conduction system. It results in impaired transmission of action potentials from the atria to the ventricles or within the specialized conduction pathways, leading to disturbances in heart rhythm and synchronization. The conduction block can be partial or complete and can occur at various levels of the cardiac conduction system, including the sinoatrial (SA) node, atrioventricular (AV) node, His bundle, bundle branches, or Purkinje fibers.

Types of Conduction Block

Sinoatrial (SA) Block

SA block refers to impaired conduction of impulses from the sinoatrial node to the atrial myocardium. This can cause pauses or dropped beats as the atria fail to depolarize properly. SA block is usually classified into first-degree (prolonged conduction), second-degree (intermittent failure), and third-degree (complete block).

Atrioventricular (AV) Block

AV block involves delayed or blocked conduction through the AV node or the His-Purkinje system, impairing impulse transmission from the atria to the ventricles. It is classified into three degrees:

• First-degree AV block: Prolonged PR interval on ECG without dropped beats, indicating slowed conduction.
• Second-degree AV block: Intermittent failure of conduction causing dropped ventricular beats; subdivided into:
  • Mobitz type I (Wenckebach): Progressive PR interval prolongation before a dropped beat.
  • Mobitz type II: Sudden dropped beats without prior PR prolongation.
• Third-degree AV block (complete heart block): No atrial impulses conduct to the ventricles; atria and ventricles beat independently.

Intraventricular Conduction Block

This type of block occurs within the ventricular conduction pathways, mainly affecting the His bundle, bundle branches, or Purkinje fibers.

• Bundle Branch Block (BBB): A block in either the right or left bundle branch causing delayed depolarization of the respective ventricle. It is detected on ECG by characteristic widening and morphology changes in QRS complexes.
  • Right Bundle Branch Block (RBBB)
  • Left Bundle Branch Block (LBBB)
• Hemiblocks (Fascicular blocks): Blocks in one of the left bundle branch fascicles (anterior or posterior), altering the axis of ventricular depolarization.

Pathophysiology of Conduction Block

Conduction block arises from structural or functional abnormalities in the cardiac conduction tissue. Common causes include ischemic injury (such as myocardial infarction), fibrosis, inflammation, degenerative changes, electrolyte imbalances, drug effects, or congenital defects. These factors impair ion channel function or cause physical disruption of conduction fibers, leading to slowed or halted electrical impulse propagation.

The conduction system relies on rapid depolarization and efficient cell-to-cell communication via gap junctions. Damage to these components alters the velocity or safety of conduction, producing blocks. Depending on the location and severity, conduction block results in asynchronous contraction, reduced cardiac output, and arrhythmias.

Electrocardiographic Manifestations

ECG is the primary diagnostic tool for conduction block and reveals characteristic alterations in wave intervals and morphology:

• PR interval prolongation indicates delayed AV nodal conduction.
• Dropped QRS complexes indicate intermittent failure of impulse conduction.
• Widened QRS complexes with specific morphologies indicate bundle branch blocks.
• AV dissociation with independent atrial and ventricular rhythms suggests complete AV block.

Recognition of these patterns enables localization of the block and guides treatment decisions.

Clinical Significance and Consequences

Conduction blocks can be asymptomatic or cause symptoms such as dizziness, syncope, palpitations, fatigue, or heart failure, depending on the extent of rhythm disturbance and hemodynamic compromise. Complete heart block and certain bundle branch blocks may cause severe bradycardia, ventricular arrhythmias, or sudden cardiac death.

Early identification and management of conduction block are crucial to prevent complications. Treatment options vary from observation in mild cases to pacemaker implantation in advanced blocks causing symptomatic bradycardia or hemodynamic instability.

Diagnostic Evaluation

Besides ECG, other diagnostic tools assist in evaluating conduction block:

• Holter monitoring: Detects intermittent or transient blocks over prolonged periods.
• Electrophysiological study (EPS): Invasive assessment of conduction pathways to precisely localize blocks and assess conduction properties.
• Imaging: Echocardiography or cardiac MRI may identify structural heart disease contributing to conduction abnormalities.

Management of Conduction Block

Management depends on the type and severity of the block:

• Reversible causes such as electrolyte imbalances or drug toxicity should be corrected.
• Pharmacological therapy may involve atropine or isoproterenol in acute settings to enhance conduction.
• Pacemaker implantation is the mainstay for symptomatic second- or third-degree AV blocks and some bundle branch blocks with associated symptoms or high risk.
• Treatment of underlying cardiac disease (e.g., revascularization in ischemia) may improve conduction.

Summary Table of Conduction Block Types and Characteristics

Summary of Electrophysiological Mechanisms

The normal cardiac conduction system transmits impulses rapidly via specialized fibers with high conduction velocity. Conduction block disrupts this process by:

• Reducing conduction velocity below a critical threshold.
• Creating areas of conduction delay or block due to fibrosis or ischemia.
• Altering refractoriness, leading to failure of impulse propagation.

These disruptions cause asynchronous electrical activation, leading to inefficient contraction and arrhythmogenesis.