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................................................................... whereas those that conduct in the retrograde direction only are more frequent (<_50%). When the AP conducts antegradely, ventricular pre-excitation is usually evident at rest during sinus rhythm and the AP is referred to as ‘manifest’. Conversely, APs are referred to as ‘concealed’ if they exclusively conduct retrogradely. Concealed APs may have decremental properties.395 The term ‘latent AP’ denotes an AP that is not, or is barely, visible due to location or faster conduction through the AVN.

Multiple APs occur in <_12% of patients with pre-excitation, and in <_50% in patients with Ebstein’s anomaly.396 AVRT is the most common tachycardia associated with APs. Two mechanisms of re-entry are possible according to the antegrade or retrograde conduction over the AVNHPS, and are classified as orthodromic and antidromic AVRT.

11.3.2 WolffParkinsonWhite syndrome WPW syndrome refers to the presence of an overt (manifest) AP, thus resulting in the so-called pre-excitation, in combination with usually recurrent tachyarrhythmias.397 During sinus rhythm, a typical pattern in the resting ECG with the following characteristics is present: (i) a short PR interval (<_120 ms); (ii) slurred upstroke (or downstroke) of the QRS complex (‘delta wave’); and (iii) a wide QRS complex (>120 ms). In most cases, APs giving rise to the WPW pattern are seen in structurally normal hearts. Rare, familial forms of pre-excitation associated with LV hypertrophy and multisystem disease [mutations in the protein kinase adenosine monophosphate-activated non-catalytic subunit gamma 2 (PRKAG2) gene, Danon and Fabry disease, and others] have also been described.398

Several surface ECG algorithms have been developed that can be applied for the localization of APs in the presence of overt preexcitation (Figures 16 and 17).399401 Pre-excitation on the surface ECG can be intermittent and can even disappear permanently (in <_35% of cases) over time. Furthermore, various degrees of preexcitation are possible depending on the location of the AP as well as on AVN conduction properties.

11.3.3 Orthodromic atrioventricular re-entrant tachycardia Orthodromic AVRT accounts for >90% of AVRTs and for 20 - 30% of all sustained SVTs. The re-entrant impulse conducts from the atrium to the ventricle through the AVNHPS, which is the anterograde limb of the re-entrant circuit, whereas the AP conducts from the ventricle to the atrium, and serves as the

Figure 16 The St George’s algorithm for the localization of accessory pathways.399 þve = QRS complex-positive; ve = QRS complex-negative; þ/- = QRS complex equiphasic; AP = accessory pathway; LAL = left anterolateral; LP = left posterior; LPL = left posterolateral; LPS = left posteroseptal; MS = mid-septal; RAS = right anteroseptal; RL = right lateral; RP = right posterior; RPS = right posteroseptal. Downloaded from academic.oup.com/eurheartj/article/41/5/655/5556821 by guest on 07 October 2026