CHAPTER

5

Accessory Pathways

UNDERSTANDING AND EVALUATING ACCESSORY PATHWAYS (APs)

General Information

The ventricles are normally insulated from the atria by a fibrofatty connective tissue skeleton that also contains the valvular structures.

○In certain cases, an extranodal bypass tract (or accessory pathway, AP) connects the atrial and ventricular myocardium across the atrioventricular (AV) groove, bypassing the atrioventricular node (AVN).

○Such an AP allows for electrical activation of the ventricular myocardium before the impulse arrives via the AVN/His-Purkinje conduction system, causing pre-excitation.

Anatomy and Pathophysiology

Most APs have rapid, non-decremental conduction similar to myocardium or His-Purkinje tissue.

○ Bidirectional conduction is present in most bypass tracts.

▪ Bidirectional conduction could result in orthodromic or antidromic atrioventricular reciprocating tachycardia (AVRT) and pre-excited atrial arrhythmias/atrial fibrillation (AF).

○ Anterograde conduction could result in antidromic AVRT and pre-excited atrial arrhythmias/AF. It can be:

▪ Manifest: Anterograde conduction down AP results in pre-excitation on electrocardiogram (ECG).

▪ Latent: Anterograde activation is not manifest due to relatively greater degree of AVN conduction. The relative balance of AVN and AP conduction is dependent on:

• AVN conduction velocity (vagal maneuvers, premature atrial complexes [PAC], and adenosine increase pre-excitation by decreasing AV conduction)

• AP conduction velocity (increased pre-excitation with rapid conduction)

• The proximity of the bypass tract to the atrial impulse; for example, a sinus node impulse reaches the AVN or a right-sided AP earlier than it would a left-sided AP.

▪ Intermittent: Suggests a relatively long anterograde effective refractory period (ERP)

○ Retrograde: Exclusive retrogradely conducting pathways are called concealed APs.

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