Wolff–Parkinson–White syndrome
Normally the only electrical road between the atria and ventricles runs through the AV node, which deliberately delays each signal. In Wolff–Parkinson–White syndrome a person is born with an extra wire — an accessory pathway — that bypasses this gatekeeper, letting part of the ventricle be activated early. This “pre-excitation” is the syndrome's defining feature.
On a resting ECG the head start shows up as a short PR interval and a slurred upstroke at the start of the QRS called a delta wave. The real trouble is that the extra pathway creates a ready-made loop: an impulse can run down one route and back up the other, setting off a fast re-entrant supraventricular tachycardia that causes sudden palpitations.
Most people with WPW do well, but a minority face a more serious risk: if atrial fibrillation develops, the accessory pathway can conduct extremely rapidly to the ventricles and, rarely, trigger ventricular fibrillation. For symptomatic patients, catheter ablation that destroys the accessory pathway is highly effective and often curative.
A young man with recurrent racing-heart spells has a resting ECG showing a delta wave and short PR interval; an electrophysiology study locates the accessory pathway, and catheter ablation cures him.
A delta wave with a short PR interval is the resting-ECG signature of pre-excitation.
In atrial fibrillation with WPW, common AV-node-blocking drugs can be hazardous because they push more impulses down the fast accessory pathway — a reason these cases need specialist handling.