What is happening in the heart
A normal heart has exactly one electrical connection between the upper chambers (atria) and the lower chambers (ventricles): the AV node. The AV node is deliberately slow — it gives the atria a moment to empty into the ventricles before the next squeeze, and it acts as a gatekeeper that limits how fast signals from above can reach the ventricles below.
In Wolff-Parkinson-White, a person is born with an additional strand of heart muscle — an accessory pathway — that crosses the electrical insulation between the atria and ventricles. The result is a second connection between the two floors of the heart. Unlike the AV node, this extra pathway conducts quickly and without the safety-valve slowdown.
The classic ECG
When the accessory pathway is active, a piece of the ventricle gets “pre-excited” — it depolarizes a beat early, before the rest of the ventricle catches up through the normal wiring. On the ECG that produces three classic findings:
- A short PR interval — the signal arrives in the ventricle sooner than usual.
- A delta wave — a slurred, slow upstroke at the start of the QRS complex (the early-firing piece of ventricle).
- A wide QRS — because the pre-excited beat blends with the normal one.
WPW pattern vs WPW syndrome
This distinction matters more than any other.
- WPW pattern means the ECG looks like WPW but the patient has never had a symptomatic arrhythmia. Often found incidentally on a pre-employment, sports, or pre-op ECG.
- WPW syndrome means the pattern is there and the patient has had a documented fast rhythm related to it.
Most people with the pattern never develop the syndrome.
The two arrhythmias to know
Orthodromic AVRT — the common one
Most WPW arrhythmias are a form of SVT. The signal travels down the normal AV node, into the ventricles, and then back up the accessory pathway to the atria — looping around and around at 150-220 bpm. The QRS is narrow (because the ventricles are being activated through the normal wiring), and the rhythm is regular. It feels just like other forms of SVT: sudden onset, rapid palpitations, often with chest discomfort or lightheadedness, then a sudden stop.
Pre-excited atrial fibrillation — the dangerous one
Rare, but the reason WPW deserves real respect. If a WPW patient develops atrial fibrillation — the chaotic 400-plus-beats-per-minute storm of the upper chambers — the AV node normally blocks most of those signals and protects the ventricles. But an accessory pathway has no such safety valve. It can conduct nearly every one of those impulses straight to the ventricles, driving the ventricular rate up to 250-300 bpm. On the ECG it looks irregularly irregular with wide, bizarre QRS complexes. At those rates the ventricles can deteriorate into ventricular fibrillation, which is sudden cardiac arrest.
This is the rare but real cause of sudden death in WPW, and it’s why we don’t shrug off asymptomatic WPW in every patient.
How we risk-stratify
The key question is: how fast can this accessory pathway conduct? A pathway that fatigues quickly is low-risk. One that can conduct at very high rates is dangerous.
- Low-risk features include WPW that disappears during exercise stress testing (the pathway can’t keep up), and intermittent pre-excitation on monitoring.
- High-risk features include young age, persistent pre-excitation at high heart rates, and certain pathway locations.
The most precise tool is an electrophysiology (EP) study. During the study, we deliberately induce atrial fibrillation and measure the shortest pre-excited RR interval — the fastest beat-to-beat conduction the pathway will allow. A short interval (typically under 250 milliseconds) flags a higher-risk pathway.
How we treat it
For symptomatic patients, the conversation is short: catheter ablation cures WPW in well over 95% of cases. We position thin catheters in the heart through the leg veins, map exactly where the pathway crosses, and deliver a small targeted burn (or freeze) at that spot. The pathway is gone, the ECG normalizes, and the arrhythmia risk goes with it. Most patients go home the same day.
Medications (beta-blockers, calcium channel blockers) can reduce episodes but are rarely used long-term given how effective ablation is. Two medications — digoxin and verapamil — are specifically avoided in pre-excited atrial fibrillation, because they can paradoxically speed conduction down the accessory pathway.
For asymptomatic patients, the decision depends on age, occupation, and lifestyle. We typically recommend a risk assessment — at minimum a stress test, and often an EP study — for children, competitive athletes, pilots, commercial drivers, and others where a sudden event would be catastrophic.
What to expect at your visit
We’ll review the ECG that raised the question, talk through any symptoms, and walk through the risk-stratification options. If you’ve already had an SVT episode, we’ll usually recommend an EP study with ablation — most patients are surprised at how quick and definitive the procedure is. If you’re asymptomatic, we’ll spend more time on the tradeoffs and let you make an informed decision.