The preexcitation is the result of a congenital accessory pathway that conducts antegrade reaching the ventricles in advance of activation through the AVN-HPS (Figs. 4.1, 4.2, 4.3). The incidence of Wolff–Parkinson–White syndrome in children has been estimated around one in every 1,000 individuals. Ventricular preexcitation can be quite subtle, occasionally apparent only as a lack of a q-wave in the lateral precordial leads due to slow conduction in the accessory pathway; accelerated conduction through the AV node (as is common in children); or in later accessory pathway activation of a left-sided pathway (the most frequent site of a pathway).

While both manifest and concealed pathways can mediate reentrant tachycardia, patients with Wolff–Parkinson–White syndrome have a greater rate of recurrent SVT than those without preexcitation. The clinical course of AVRT is age related. If diagnosed as an infant, the preexcitation may resolve to complete disappearance as a child grows, usually by 18–24 months of age (30 % recurrence if diagnosed less than 1 year of age). However, if preexcitation or documented SVT is seen after this age, AVRT is unlikely to self-resolve (94 % recurrence rate).

Furthermore, patients with WPW are noted to have a very small, albeit increased, risk for a sudden cardiac arrest. During atrial fibrillation, a conduction of the fibrillatory impulses may preferentially cross a fast conducting manifest accessory pathway, bypassing the more slowly conducting AV node resulting in a rapid life-threatening ventricular response (>250–300 bpm) (Fig. 4.5b; Chap. 20, Fig. 20.​2). The incidence of sudden cardiac arrest in WPW patients has been estimated between one and 4.5 per 1,000 patient years, reflecting a higher incidence of atrial fibrillation in WPW patients. The highest prevalence of sudden cardiac arrest appears to be between the ages of 15 and 35 years. Patients with WPW syndrome who experience sudden cardiac death may have no history of a tachyarrhythmia. On the other hand, they may have had previous syncopal episodes. Potential risk factors for a sudden cardiac event with WPW include a younger age (<30 years), prior syncope or atrial fibrillation, male gender, familial WPW, or other heart disease.

The management of the asymptomatic patient with WPW has been addressed by the Pediatric and Congenital Electrophysiology Society in a recent consensus statement (2012). Patients who are 8 years of age or older and are found to have manifest preexcitation on an ECG without symptoms are recommended to undergo risk stratification, initially with noninvasive studies (i.e., exercise treadmill test; Holter monitor tracings). If inconclusive, invasive studies can be considered to determine those at higher risk for sudden cardiac arrest. In asymptomatic individuals under 8 years of age, the low risk of sudden death due to atrial fibrillation, allow conservative management and continued follow-up.

Mahaim fibers, as first described in 1938, are direct histologic links between either the AV node and the right ventricle (nodoventricular fibers) or the His bundle and the right ventricle (fasciculoventricular fibers). These nodo or fascicular ventricular fibers are anatomically common (though often inactive, i.e., not resulting in preexcitation or tachycardia) When they do produce ventricular preexcitation, it may be only in a “by-stander” roles and not participate in an arrhythmia reentrant circuit. In patients with a nodoventricular fiber, the PR interval varies relative to the fiber’s take-off from the node. In those with a fascicular-ventricular fiber arising from the His bundle and inserting into the ventricular myocardium the PR interval is normal. Rarely the nodoventricular fibers produce a wide QRS complex in the context of a different mechanism for SVT, such as AVNRT (Fig. 4.7), or even more rarely participate as a true antegrade limb of an atrioventricular circuit. Fasciculoventricular fibers have not been shown to participate in a reentrant circuit other than as a bystander. These bystander pathways are often best left alone and not ablated.

An uncommon right-sided atriofascicular fiber (somewhat confusingly also referred to as a Mahaim fiber) is located in the right posterior AV groove (Fig. 4.8). This accessory pathway has several features: (1) it typically conducts in only an antegrade direction producing preexcitation in a left bundle branch block configuration; (2) it inserts deep (not along the AV ring) in the right ventricle, often near the distal fascicle of the right bundle (moderator band); (3) the pathway demonstrates decremental conduction; (4) if present, the tachycardia is antidromic resulting in a wide QRS left bundle branch pattern SVT. These fibers were shown to be the cause of preexcitation characterized by left bundle branch block QRS morphology and wide QRS tachycardia. Intracardiac electrophysiologic studies demonstrate that the preexcitation and the wide QRS tachycardia are due to an atriofascicular accessory pathway coursing from the lateral right atrial tricuspid valve annular area to insert deeper in the right bundle rather than at the AV groove. As these fibers often have AV nodal properties, they may exhibit slow conduction velocity and “decremental conduction” with atrial extrastimulus and adenosine sensitive, resulting in AV block with adenosine administration. The hallmark of identifying the site for ablation is the identification of a fast action specialized conduction tissue electrogram distant in both location and timing from the His bundle electrogram, often near the moderator band. Ablation at that site terminates conduction within this pathway (eliminates the preexcitation), as well as the wide QRS tachycardia (Fig. 4.8). It can also be ablated on the right lateral annulus.

Abrupt disappearance of ventricular preexcitation on the electrocardiogram during exercise suggests an accessory pathway effective refractory period perhaps as long as 360–390 ms, placing the patient in a lower risk category (Fig. 4.9). Exercise testing or Holter monitoring during activity has been used as a noninvasive method to evaluate the conduction properties of a manifest accessory pathway. Abrupt loss of preexcitation during exercise treadmill testing occurred in 15 % of a predominately s pediatric group of patients. However, in practice, disappearance of ventricular preexcitation exercise testing can be difficult to interpret due to movement artifact on the ECG and enhanced AV node conduction from increased adrenergic tone during exercise resulting in gradual (and less) not abrupt diminution of the ventricular preexcitation.