Diagnostic Considerations
Wolff-Parkinson-White (WPW) syndrome can result in supraventricular tachycardia (SVT) that uses an arteriovenous accessory pathway (AP). The AP may also act as an innocent bystander and allow conduction during other supraventricular dysrhythmias, such as atrial fibrillation (AF) or atrial flutter. Paradoxically, the use of digoxin and perhaps other atrioventricular (AV) nodal blocking agents may favor conduction through the AP, causing potentially lethal ventricular dysrhythmias (ventricular fibrillation [VF]) or hemodynamic instability during AF.
The most common mechanism in pediatric patients is AP-mediated SVT, which is a reentrant tachycardia. The main differential during SVT is whether the AP is concealed (ie, conducts only from ventricle to atrium) and thus is an SVT with narrow QRS.
An automatic mechanism may be differentiated from reentry by the following:
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Presence of a warmup or cool-down period (which is due to its relation with catecholamine levels)
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Inability to initiate or terminate SVT with programmed atrial stimulation
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Usual unresponsiveness to electrical cardioversion
A regular tachycardia of sudden onset and termination that allows for some cycle-length oscillation (which can be usually initiated and terminated with programmed atrial stimulation) and that is usually responsive to electrical cardioversion favors a reentrant mechanism.
Differential diagnosis of accessory pathway syndromes
Few entities that involve paroxysms of SVT with a WPW pattern electrocardiogram (ECG) in sinus rhythm may be differentiated from AP-mediated SVT. Examples include the following:
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Low atrial focus, which occasionally produces the appearance of a short PR interval
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Atriofascicular APs (so-called Mahaim fibers)
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Lown-Ganong-Levine (LGL) syndrome
The term “Mahaim fibers” refers to atriofascicular bypass tracts that connect the right atrium to the distal right bundle (see the image below). These pathways usually represent a duplication of the AV node and the distal conducting system. They typically occupy the right ventricular free wall. Their proximal end resides adjacent to the lateral tricuspid annulus and exhibits slow conduction, with AV node–like characteristics. The distal end, which conducts rapidly, inserts into the distal right bundle branch or the apical region of the right ventricle.
If Mahaim fibers are present, the ECG findings are a normal or long PR interval and an abnormally wide QRS complex with a left-bundle appearance. Preexcitation may not be apparent during sinus rhythm but can be demonstrated with premature right atrial stimulation. Because retrograde conduction is absent, only an antidromic AV reentry tachycardia (ie, preexcited tachycardia) can develop.
In the presence of atriofascicular tracts, preexcited tachycardia has a left bundle-branch block (LBBB) pattern, a long AV interval (due to the long conduction time over the accessory pathway), and short ventriculoatrial (VA) intervals. If right bundle-branch block (RBBB) develops, it may prolong the tachycardia cycle length (slow the AV reentrant tachycardia [AVRT]). Any right free wall bypass tract-mediated SVT will be prolonged with RBBB but should be unaffected by LBBB aberrancy.
Conversely, left free wall bypass tract AVRT will slow with LBBB (ipsilateral to the accessory pathway). Septal pathways may be slightly affected by either RBBB (anteroseptal) or LBBB (posteroseptal), but this would be detected only on electrophysiologic study (EPS). The eponym Coumel is also applied to this rule.
Given the pathway length and decremental conduction properties (similar to AV nodal conduction), a preexcited QRS complex with a short PR interval essentially rules out Mahaim fibers.
In LGL syndrome, patients have a short PR interval and SVT but no delta wave. This is typically caused by an atrio-Hisian pathway leading to accelerated AV conduction, but not causing paroxysmal SVT. In patients with LGL syndrome who have an atrio-Hisian tract, the QRS complex remains normal and the short atrio-Hisian interval remains fixed during atrial pacing at rapid rates.
Entities that involve wide-QRS SVT must be differentiated from ventricular tachycardia (VT). Examples include the following:
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Aberrantly conducting orthodromic SVT, which is wide-QRS SVT with the AV node as the antegrade limb but with bundle branch block, must be differentiated from VT
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Antidromic tachycardia, which is wide-QRS SVT due to ventricular preexcitation through an AP, must also be differentiated from VT and from Mahaim fiber tachycardia
Sometimes, accessory AV fibers connect to the AV node itself or the His bundle or bundle branches and insert into the ventricular myocardium. These are called nodoventricular or fasciculoventricular tracts. Patients with fasciculoventricular connections show a short His-ventricle (HV) interval and no change in the QRS complex during rapid atrial pacing. Fasciculoventricular AV pathways do not participate in clinically meaningful arrhythmias.
Other forms of tachycardia in patients with WPW syndrome
Patients with WPW syndrome can have other tachycardias where the AP is just a bystander, such as atrioventricular nodal reentrant tachycardia (AVNRT) or atrial tachycardia that conducts to the ventricle over the bypass tract.
Atrial flutter or AF may also occur in the atrium, unrelated to the AP, and can conduct rapidly to the ventricle. Patients with WPW syndrome who have AF frequently have inducible reciprocating tachycardias. Interruption of the AP with ablation may or may not prevent recurrence of the AF.
AF presents a potentially serious risk. It typically has a cycle length of 120-200 ms or 300-500 bpm. Atrial flutter is typically at a cycle length of 200 ms or 300 bpm. At rapidly conducted rates (ie, a rate faster than 250 bpm or an AP refractory period shorter than 240 ms), the risk of sudden death occurs as a result of potential conversion from rapid AF to VF. However, such a phenomenon is uncommon, occurring at an estimated frequency of about less than 0.1%.
Patients who have intermittent preexcitation or those who lose ECG evidence of preexcitation abruptly with exercise or when injected intravenously with procainamide generally have a long AP refractory period. These patients are thought to have a low risk of developing a rapid ventricular rate should atrial flutter or AF develop (see the image below).
However, a more recent study suggested that children with intermittent preexcitation do not have a lower-risk APs as measured by EPS criteria or reduced symptoms. [24] The investigators evaluated data from 295 children (1996-2013), comparing those with persistent, intermittent, or loss of preexcitation Holter/Holter testing. Although there were no baseline differences in high-risk pathways among these groups, the inclusion of isoproterenol values demonstrated an increased frequency of high-risk pathways among children with loss of preexcitation on Holter/Holter testing (54% vs 16% persistent, 11% intermittent).
Differential Diagnoses
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Genetics of von Gierke Disease (Glycogen-Storage Disease Type 1)
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Genetics of Glycogen-Storage Disease Type II (Pompe Disease)
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Classic Wolff-Parkinson-White electrocardiogram with short PR, QRS >120 ms, and delta wave.
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Preexcited atrial fibrillation.
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Variants of Wolff-Parkinson-White syndrome (unusual accessory pathways).
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Accessory pathway potential and local AV fusion at successful RF ablation site with loss of preexcitation and return of normal HV interval.
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Electrocardiogram of asymptomatic 17-year-old male who was incidentally discovered to have Wolff-Parkinson-White pattern. It shows sinus rhythm with evident preexcitation. To locate accessory pathway (AP), initial 40 ms of QRS (delta wave) is evaluated. Note that delta wave is positive in I and aVL, negative in III and aVF, isoelectric in V1, and positive in rest of precordial leads. Therefore, this is likely posteroseptal AP.
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12-lead electrocardiogram from asymptomatic 7-year-old boy with Wolff-Parkinson-White pattern. Delta waves are positive in I and aVL; negative in II, III, and aVF; isoelectric in V1; and positive in rest of precordial leads. This predicts posteroseptal location for accessory pathway.
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12-lead electrocardiogram showing short PR interval and delta waves consistent with presence of accessory pathway.
