Accessory Atrioventricular Pathway Electrophysiology

Accessory Atrioventricular Pathway Electrophysiology involves the study and characterization of abnormal electrical conduction pathways that connect the atria and ventricles outside the normal atrioventricular (AV) nodal-His-Purkinje conduction system. These accessory pathways (APs) provide an alternative route for electrical impulses, potentially bypassing the AV node and leading to pre-excitation of the ventricles, arrhythmogenesis, and supraventricular tachycardias.

Anatomy and Physiology of Accessory Atrioventricular Pathways

Definition and Location

Accessory atrioventricular pathways are strands of myocardial fibers that directly connect atrial myocardium to ventricular myocardium, circumventing the specialized AV nodal conduction system. These fibers can be located anywhere along the AV ring but most commonly are found along the left free wall, right free wall, or posteroseptal region.

Normal AV Conduction vs. Accessory Pathways

In normal physiology, the AV node imposes a delay and acts as a gatekeeper, regulating the conduction of impulses from atria to ventricles. Accessory pathways lack this delay and can conduct impulses rapidly, resulting in early ventricular depolarization (pre-excitation). Unlike the AV node, many accessory pathways do not have decremental conduction properties.

Types of Accessory Pathways

The electrophysiological properties of APs vary and include:

• Atrio-ventricular bypass tracts: typical accessory pathways allowing bidirectional conduction.
• Mahaim fibers: specialized decrementally conducting pathways, often with nodoventricular or fasciculoventricular connections.
• Epicardial connections: some APs may have an epicardial location, affecting ablation strategies.

Electrophysiological Properties of Accessory Pathways

Conduction Characteristics

Accessory pathways can conduct electrical impulses:

• Antegrade conduction: from atria to ventricles, resulting in pre-excitation observed on ECG as a delta wave.
• Retrograde conduction: from ventricles to atria, allowing participation in reentrant tachycardias.
• Bidirectional conduction: capable of conducting impulses in both directions, facilitating arrhythmias such as atrioventricular reentrant tachycardia (AVRT).

Conduction Velocity and Refractoriness

Accessory pathways typically conduct impulses faster than the AV node but have variable refractory periods. Some pathways have short refractory periods, increasing the risk of rapid ventricular rates during atrial fibrillation, potentially leading to ventricular fibrillation.

Decremental Conduction

Most accessory pathways lack decremental conduction, meaning conduction velocity does not slow down with increased heart rates, unlike the AV node. However, some specialized pathways (e.g., Mahaim fibers) exhibit decremental conduction.

Electrophysiological Study and Mapping of Accessory Pathways

Intracardiac Electrogram Characteristics

During electrophysiological studies (EPS), accessory pathways are identified by:

• Early ventricular activation preceding the His bundle electrogram during antegrade conduction.
• Earliest atrial activation during retrograde conduction, indicating the atrial insertion site of the pathway.

Programmed Electrical Stimulation

Programmed stimulation protocols assess:

• Conduction properties (antegrade and retrograde).
• Effective refractory periods of the pathway.
• Induction of reentrant tachycardias involving the accessory pathway.

Mapping Techniques

Mapping includes:

• Activation mapping: locating the earliest site of ventricular or atrial activation.
• Pacing maneuvers: differentiating pathway conduction from AV nodal conduction.
• 3D electroanatomical mapping: aids in precise localization of pathway insertion sites, facilitating ablation.

Clinical Implications and Arrhythmogenesis

Pre-excitation Syndromes

Accessory pathways underlie pre-excitation syndromes such as Wolff-Parkinson-White (WPW) syndrome, characterized by a short PR interval and delta wave on ECG due to early ventricular depolarization.

Supraventricular Tachycardias

Accessory pathways participate in reentrant circuits causing AVRT, the most common tachycardia in patients with APs. The tachycardia can be orthodromic (antegrade AV nodal conduction, retrograde pathway conduction) or antidromic (antegrade pathway conduction, retrograde AV nodal conduction).

Risk of Sudden Cardiac Death

Accessory pathways with short refractory periods can conduct rapid atrial arrhythmias to the ventricles, risking ventricular fibrillation and sudden cardiac death, especially in patients with atrial fibrillation and pre-excitation.

Therapeutic Considerations and Ablation

Pharmacologic Management

Drugs affecting AV nodal conduction are used cautiously, as they may paradoxically enhance conduction via the accessory pathway. Antiarrhythmic agents that prolong refractoriness of the pathway or suppress conduction are preferred.

Catheter Ablation

Radiofrequency or cryoablation targeting the accessory pathway is the definitive treatment. Ablation success depends on accurate electrophysiological localization of the pathway, considering:

• Precise mapping of atrial and ventricular insertion sites.
• Identification of earliest activation signals.
• Avoidance of damage to normal conduction tissue.

Procedural Risks and Outcomes

Ablation carries risks such as AV block if the pathway is near the AV node or His bundle. Success rates exceed 90% in experienced centers, significantly improving morbidity by preventing tachyarrhythmias.

Summary of Electrophysiological Features

Accessory Atrioventricular Pathway Electrophysiology combines detailed understanding of the anatomical presence, conduction properties, and the role of these pathways in arrhythmogenesis. This knowledge is essential for diagnosis, risk stratification, and therapeutic intervention in patients with pre-excitation and reentrant supraventricular tachycardias.