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........................................................................ retrograde limb of the re-entrant circuit. Orthodromic AVRT tends to be a rapid tachycardia, with frequencies ranging from 150 to, rarely, >220 b.p.m. During tachycardia (Figure 18), the following ECG features can be present: (i) RP interval constant and, usually but not invariably, up to one-half of the tachycardia CL; (ii) narrow QRS; (iii) functional BBB usually associated with an AP ipsilateral to the blocked bundle, especially in young patients (aged <40 years); and (iv) ST-segment depression.

11.3.4 Antidromic atrioventricular re-entrant tachycardia Antidromic AVRT occurs in 3 - 8% of patients with WPW syndrome.402404 The re-entrant impulse travels from the atrium to the ventricle through the AP with anterograde conduction; meanwhile, retrograde conduction occurs over the AVN or another AP, usually located in a contralateral position to ensure longer travel distances, thus allowing for sufficient recovery of refractoriness of the respective elements of the re-entrant circuit. In 30 - 60% of patients with spontaneous antidromic AVRT, multiple APs (manifest or concealed), which could act or not as the retrograde limb during the AVRT, may be detected. Antidromic AVRT has the following ECG features, illustrated in Figure 18: (i) a wide QRS complex (fully pre-excited) and (ii) an RP interval that is difficult to assess as the retrograde P wave is usually inscribed within the ST-T segment.

11.3.5 Accessory pathway as a bystander In the presence of focal AT, atrial flutter, AF, or AVNRT, the QRS complexes can be pre-excited when the AP acts as a bystander, and is not a critical part of the re-entry circuit.

11.3.6 Pre-excited atrial fibrillation Paroxysmal AF has been found in 50% of patients with WPW, and may be the presenting arrhythmia in affected patients.405,406 These patients are typically young and have no structural heart disease. High-rate AVRT may potentially initiate AF. AF with fast ventricular response over an overt AP with a short anterograde refractory period is a potentially life-threatening arrhythmia in patients with WPW syndrome, due to potential degeneration into VF.

11.3.7 Concealed accessory pathways Concealed APs give rise only to orthodromic AVRT. Their true prevalence is unknown because they are not detectable on the resting surface ECG, but only at occurrence of AVRT, or during electrophysiology testing.45 No sex predilection is found and these pathways

Figure 17 Localization of accessory pathways in the presence of maximum (spontaneous or evoked) pre-excitation.400 Accessory pathway locations are green when right-sided and red when left-sided. Left posterolateral accessory pathways can have 0, 1, or 2 inferior leads with positive polarity, whereas nodo-Hisian accessory pathways can have 1, 2, or 3 inferior leads with positive polarity. Right-sided accessory pathways are framed orange or yellow when the V3 lead is negative or positive, respectively. Left posterior accessory pathways are framed blue when the V1/I ratio is <1 or purple when V1/I ratio is >_1. AP = accessory pathway; DCS = deep coronary sinus; LL = left lateral; LPL = left posterolateral; LPS = left paraseptal; NH = nodo-Hisian; RA = right anterior; RL = right lateral; RP = right posterior; RPS = right paraseptal. Downloaded from academic.oup.com/eurheartj/article/41/5/655/5556821 by guest on 07 October 2026