Preexcitation

Pre-excitation syndromes

• Wolff-Parkinson-White (WPW) and Lown-Ganong-Levine (LGL) syndromes
• These involve the presence of an accessory pathway connecting the atria and ventricles
• The accessory pathway conducts impulses faster than normal, producing a short PR interval (<120 msec)
  
• The accessory pathway also acts as an anatomical re-entry circuit, making patients susceptible to re-entry tachyarrhythmias
• Patients present with episodes of paroxsymal supraventricular tachycardia (SVT), specifically atrioventricular re-entry tachycardia (AVRT), and characteristic features on the resting 12-lead ECG.

Warning!

Inhibtion of AV node conduction (e.g. vagal manoeuvres or drugs such as adenosine, beta-blockers, calcim channel blockers and digoxin) can increase pre-excitation leading to ventricular fibrillation!

Wolff-Parkinson-White syndrome

The characteristic features of WPW are (each is only found in about 75% of WPW cases):

• Short PR interval  (<120 msec in adults, <90 msec in children)
• Broad QRS (>120msec in adults, >90 msec in children)
• Delta wave — a slurred upstroke to the QRS complex seen in most leads, perhaps best in V4.

There may also be secondary St and T wave changes, typically in the opposite direction to the delta wave.

For a great clinical case on WPW, click here.

Lown-Ganong-Levine syndrome

The features of LGL syndrome are:

• very short PR interval
• normal P waves and QRS complexes
• absent delta waves

A short PR interval is also seen with AV nodal (junctional) rhythm (see PR interval and PR  segment)