Definition

- WPW syndrome is a combination of a congenital accessory pathway (Bundle of Kent) and episodes of tachyarrhythmia

- This pathway may communicate between the left atrium and the left ventricle, in which case it is termed a "type A pre-excitation", or between the right atrium and the right ventricle, in which case it is termed a "type B pre-excitation"

- Problems arise when this pathway creates an electrical circuit that bypasses the AV node. The AV node is capable of slowing the rate of conduction of electrical impulses to the ventricles, whereas the bundle of Kent lacks this capability.

- This can cause the patient to transition into tachydysrhythmias (e.g. VF)

- Incidence of 0.1 – 3.0 per 1000

- Associated with a small risk of sudden cardiac death

ECG features of WPW

- PR interval <120ms
- QRS prolongation >110ms
- ST Segment and T wave discordant changes (in the opposite direction to the major component of the QRS complex)
- Delta wave (slurring slow rise of initial portion of the QRS)

Pseudo-infarction pattern can be seen in up to 70% of patients – due to negatively deflected delta waves in the inferior / anterior leads (“pseudo-Q waves”), or as a prominent R wave in V1-3 (mimicking posterior infarction)

Type A vs Type B

Type A (left-sided pathway): dominant R wave in V1 and right axis deviation
Type B (right-sided pathway): no dominant R wave in V1 and left axis deviation

Howevr it should be noted that in the majority of cases WPW is associated with left axis deviation

See the below ECG; there is no dominant R wave in V1 and left axis deviation, implying a right sided pathway, hence Type B WPW.

Pathophysiology

Pre-excitation refers to early activation of the ventricles due to impulses by
passing the AV node via an accessory pathway

Accessory pathways, also known as bypass tracts, are abnormal conduction 
pathways formed during cardiac development and can exist in a variety of 
anatomical locations and in some patients there may be multiple pathways

In WPW the accessory pathway is often referred to as the Bundle of Kent,
or atrioventricular bypass tract

An accessory pathway can conduct impulses either anterograde,
towards the ventricle, retrograde, away from the ventricle, or in both directions

The majority of pathways allow conduction in both directions, with retrograde only conduction occurring in 15% of cases, and antegrade only conduction rarely seen

The direction of conduction affects the appearance of the ECG in sinus rhythm and during tachyarrhythmias

Causes of Tachyarrythmia:

Tachyarrythmia can be facilitated by the formation of a reentry circuit
involving the accessory pathway, termed atrioventricular reentry tachycardias (AVRT)

Tachyarrythmia may also be facilitated by direct conduction from the atria to the ventricles via the accessory pathway, bypassing the AV node, seen with atrial fibrillation or atrial flutter in conjunction with WPW

Other Associated Cardiac Conditions

- HOCM (Hypertrophic Obstructive Cardiomyopathy)
- Mitral valve prolapse
- Ebstein's anomaly
- Secundum ASD

Note: additionally thyrotoxicosis

Managment

Definitive treatment: radiofrequency ablation of the accessory pathway

Medical therapy: sotalol*, amiodarone, flecainide

*sotalol should be avoided if there is coexistent atrial fibrillation
as prolonging the refractory period at the AV node may increase the rate
of transmission through the accessory pathway, increasing the
ventricular rate and potentially deteriorating into ventricular
fibrillation

WPW Quiz

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