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Wolff-Parkinson-White syndrome and atrioventricular reentrant tachycardias


Definitions

The anatomic basis of atrioventricular reentrant tachycardia (AVRT) is an abnormal connection ( accessory pathway [AP] ) between the atrial and ventricular myocardium. One limb of the reentrant circuit is the atrioventricular (AV) node and the other is the AP. On rare occasions the circuit includes two or more APs.

The term preexcitation refers to earlier activation of the ventricle by a wave front arising in the atrium than would be expected if conduction occurred via the normal atrioventricular conduction pathway.

Wolff-Parkinson-White (WPW) syndrome refers to the combination of preexcitation on an electrocardiogram and episodic tachycardias using the AP ( Fig. 15.1 ).

Fig. 15.1., Wolff-Parkinson-White syndrome caused by a left lateral accessory pathway.

The AP is located along the mitral or tricuspid annulus, and during sinus rhythm a typical pattern with the following characteristics is present on the electrocardiogram (ECG): (1) short PR interval (≤120 ms), (2) slurred upstroke or downstroke of the QRS complex (“delta wave”), and (3) a widened QRS complex during sinus rhythm. However, WPW patients with a fasciculoventricular pathway have a QRS width 120 ms or less. , Occasionally, preexcitation may not be fully apparent because of fusion of wave fronts progressing through the AP and the normal conduction system ( Figs. 15.2 and 15.3 ). In most cases accessory pathways giving rise to the WPW pattern are seen in structurally normal hearts.

Fig. 15.2., A 12-lead electrocardiogram (ECG) of a patient with frequent episodes of narrow-QRS tachycardia.

Fig. 15.3., Same patient as in Fig. 15.2 .

Electrophysiology and classification of accessory pathways

APs are single or multiple strands of myocardial cells that bypass the physiologic conduction system and directly connect atrial and ventricular myocardium. These AV connections are due to incomplete embryologic development of the AV annuli, without complete separation between the atria and ventricles. There are different types of APs. The most common ones connect the atrium and the ventricle along the mitral or tricuspid annulus. Approximately 60% are located along the mitral valve and are referred to as left free wall APs; 25% insert along the septal aspect of the mitral or tricuspid annulus; and approximately 15% insert along the right free wall. Because ventricular muscle is lacking in the proximity of the anterior leaflet of the mitral valve, left-sided APs are usually limited to the region of the mitral annulus at the attachment of the mural (posterior) leaflet. APs can be located in the superoparaseptal area in close proximity to the His bundle and AV node. Accessory pathways exhibit rapid conduction.

Concealed accessory pathways

Approximately 50% of APs conduct in both antegrade and retrograde directions, and the majority of the others conducted only in the retrograde direction are labeled as “concealed” because there is no evidence of preexcitation on the ECG. A small percentage conduct only in the anterograde direction. Concealed APs give rise only to orthodromic AVRT and occasionally have decremental properties. They are not associated with an increased risk of sudden cardiac death. No gender predilection is found, and these pathways tend to become clinically apparent at an earlier age than atrioventricular nodal reentrant tachycardia (AVNRT); however, significant overlap exists. Concealed APs are predominantly localized along the left free wall (65%) and less often at septal (30%) and right free wall locations. ,

Multiple accessory pathways

Multiple APs occur in up to 12% of patients with preexcitation and in up to 50% in patients with Ebstein anomaly. Characteristics suggestive of two or more APs are as follows:

  • 1.

    Delta wave pattern not typical of any single location.

  • 2.

    Changes in delta wave during atrial fibrillation or during right atrial versus coronary sinus (CS) pacing.

  • 3.

    Retrograde fusion during orthodromic AVRT.

  • 4.

    Changes in retrograde atrial activation sequence during orthodromic AVRT, either spontaneous or during radiofrequency (RF) ablation

Atypical accessory pathways

Atypical APs (previously called Mahaim fibers) are connections between the right atrium or the AV node (AVN) and the right ventricle into or close to the right bundle branch. Pathways with atypical characteristics can be atriofascicular, nodofascicular, or nodoventricular, depending on their proximal and distal insertions. , Left-sided atypical pathways have also been described but are extremely rare. They usually contain accessory nodal tissue, which results in decremental properties, and connect the atrium to the fascicles by crossing the lateral aspect of the tricuspid annulus, but posteroseptal locations can also be found in rare cases. Conduction is usually anterograde only, but concealed nodoventricular and nodofascicular fibers have also been described and may give rise to incessant tachycardias. , The baseline QRS of patients with atypical APs is normal or displays different degrees of manifest preexcitation with left bundle branch block morphology. Programmed or incremental atrial pacing results in manifest preexcitation when there has been sufficient delay in the AH interval. An increase in AV interval plus shortening of the HV interval and QRS widening is observed at shorter pacing cycle lengths. Atypical pathways can participate in antidromic AVRT ( Fig. 15.4 ).

Fig. 15.4., Atypical AP physiology.

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