23 - Atypical bypass tracts

Abstract

A working definition of an atypical bypass tract (BT) is a conduction pathway that bypasses all or part of the normal conduction system but is not a rapidly conducting pathway connecting atrium and ventricle near the mitral or tricuspid annulus. Thus, pathways that connect the atrium to the His bundle (HB, atrio-Hisian BT), the atrioventricular node to the His-Purkinje system (HPS; nodofascicular BT) or the ventricle (nodoventricular BT), or the HPS to the ventricle (fasciculoventricular BT) fit into this designation. Additionally, a unique type of concealed atrioventricular (AV) BTs exhibits slow and decremental retrograde conduction properties and mediates a rare form of nearly incessant orthodromic AV reentrant tachycardia, known as permanent junctional reciprocating tachycardia, and will be discussed in this chapter. Atypical BTs comprise 3% to 5% of all BTs. Atriofascicular and long atrioventricular BTs comprise the majority (80%) of atypical BTs and usually exhibit the following characteristics: (1) unidirectional (anterograde-only) conduction (with rare exceptions); (2) long conduction times; and (3) decremental conduction. The hallmark of nodofascicular and nodoventricular BTs is their infra-atrial nature. Unlike other atypical BTs, nodofascicular and nodoventricular BTs can exhibit anterograde-only, bidirectional, or retrograde-only (concealed) conduction. Fasciculoventricular BTs do not give rise to any reentrant tachycardia. The goals of programmed electrical stimulation are the evaluation of the relationship among the His potential, the QRS, and the ventricular- His interval during atrial pacing and during tachycardia and the differentiation between the different types of atypical BTs. In addition, exclusion of a separate BT is necessary, especially if a rapid and fixed ventriculoatrial interval exists during incremental rate ventricular pacing.