Wolff-Parkinson-White Syndrome

Quite recently I came across one of Ambulance Driver's posts A Little Cardiology Geekery. This was a great story about a patient that he ran on. The patient presented with an unstable tachycardic arrhythmia. Instead of turning to the drug box, AD diagnosed WPW and successfully treated with synchronized cardioversion.

This sparked some great discussion within the comments section. I had always been taught to stay away from Adenosine or Calcium Channel Blockers for these patients but only recently learned that Amiodarone isn't the best treatment either. I thought maybe it would be a good idea to further elaborate on WPW right here for those who may not know this stuff yet. Its also a good chance for me to learn something new.

What is WPW?

WPW or Wolff-Parkinson-White syndrome is a condition where an accessory pathway exists between the atrium and ventricle. This means that there is an extra path of conduction, and its a faster one. You know how the AV node pauses the impulse from the SA node to allow for ventricular filling? Well the extra pathway, also known as bundle of kent, has no pause, no yield sign so-to-speak. So when the patient with WPW has an impulse that starts at the SA node like a healthy heart, but travels quickly through the bundle of kent this causes an early depolarization of the ventricles. Pre-mature ventraculation ventricular depolarization, most commonly called pre-excitation. Check out the image below.

This is an image showing a possible location of the accessory pathway. Check out the next image to see how this can lead to conduction abnormalities.

With WPW, the conduction bypasses the AV node. Since the PR-interval is a representation of the pause at the atrioventricular junction, without the pause you get no PR-interval. One of the classic findings of WPW on an ECG is a shortened PR interval followed by a delta wave. A delta wave is a slurring upslope of the QRS, this causes the complex to be wider than normal. The next image shows the use of the bundle of kent for antidromic conduction.

Antidromic atrioventricular reciprocating tachycardia(AVRT). Commonly mistaken for ventricular tachycardia because of its morphology, antidromic conduction is still created in the atrium. In antidromic conduction the impulse travels down the bundle of kent and pre-excites the ventricles retrogradely. Since the AV node is bypassed atrial rhythms may be much faster than normal. Your 3:1 atrial flutters can now be 1:1! So if you have an abnormal pacemaker, or an atrium that wants to beat faster than normal, it can. This is pretty dangerous and may in fact lead to ventricular tachycardia or ventricular fibrillation. The images below show orthodromic conduction.

Orthodromic AVRT. This is a little bit more difficult to diagnose. Since the conduction works an an antegrade direction, just like normal conduction, it remains narrow. The rate is accelerated because of the bundle of kent being used as a reentry circuit. The electrical impulse can quickly re-enter the atrium via the extra pathway. This isn't good. Take a gander at the next image.

There are many different possible locations of extra pathways. In fact you can have more than one extra pathway. The picture above just shows a possible location in the left atrioventricular wall.

WPW Facts

• The kent bundle is the most common bypass tract in the heart
• WPW affects about 0.2% of the general population, and about 70% of them have no evidence of cardiac disease
• About 60-70% of all WPW cases are men
• WPW is most commonly recognized and diagnosed in children and young adults after they present to the ER with an arrhythmic event.

Recognition of WPW on the ECG

Not all patients with WPW will present with ECG changes, but the ones that do are at the highest risk for sudden cardiac arrest.

ECG characteristics:

1. Very fast rate, your SVT, or A-fib/flutter patients may have much faster rates than normal(well normal for an accelerated rhythm).
2. Delta wave
3. Shortened PR-Interval
4. Fast broad and irregular rhythm that isn't torsades

Take a look at these ECGs:

The above ECG shows classic characteristics of WPW.

Above is an image that shows a fast, broad, and irregular rhythm. Read below for more on this presentation from a Circulation article [1].

This grossly irregular, rapid right bundle-branch block tachycardia represented preexcited atrial fibrillation (AF) with anterograde conduction over a left posterolateral accessory pathway, leading to irregularly irregular preexcited ventricular complexes with varying degrees of preexcitation. The shortest RR interval during preexcited AF was 160 ms, indicating a short anterograde refractory period of the accessory pathway and an increased risk for the development of ventricular fibrillation and sudden death.

The differential diagnosis of preexcited AF includes ventricular tachycardia and atrial fibrillation with aberrancy. The presence of an apparent fusion and capture beat could lead one to the diagnosis of ventricular tachycardia, because these findings are considered diagnostic for ventricular tachycardia. Morphologically speaking, the tachycardia is ventricular in origin. Nevertheless, the marked cycle-length variation of more than 100% strongly argues against ventricular tachycardia and instead favors preexcited AF as the underlying arrhythmia mechanism. The right superior axis deviation and the fact that longer RR intervals lead to more fully preexcited QRS complexes compared with shorter RR cycles (the concertina phenomenon) virtually exclude AF with solely aberrant conduction.

Preexcited atrial fibrillation with rapid ventricular response reveals a typical electrocardiographic pattern that is often diagnostic at first glance. Because of its characteristic ECG features (fast, broad, and irregular), this tachyarrhythmia has been named FBI tachycardia. This potentially life-threatening clinical condition is obviously a case for the emergency rather than the secret service.

Above is an example of an accelerated junctional rhythm. What looks like an upslope is in fact a P wave. The PR-interval in this ECG is shortened due to the rhythm originating in the AV junction.

Treatment

Treatment of a tachycardic WPW is different than treating patients with normal conduction tachycardia. If you close down the normal pathway, or try to slow it down, you will be rerouting any normal conduction right to that extra pathway. You might as well administer 3mg of epinephrine IV push.

Avoid your calcium channel blockers, beta blockers, and adenosine. Amiodarone is still being accepted but Procainamide is superior. Below is the abstract from Medical Mythology disputing the use of Amiodarone in the presence of WPW.

ABSTRACT
Wolff–Parkinson–White (WPW) syndrome with atrial fibrillation (AF) is a potentially life-threaten- ing problem requiring rapid conversion to sinus rhythm. The most recent American Heart Associa- tion guidelines for the treatment of patients with WPW, published in conjunction with the 2000 Advanced Cardiac Life Support (ACLS) guidelines, suggests that intravenous amiodarone is a first- line therapy for AF–WPW; however the evidence suggests this is a potentially dangerous myth.

The safest and most effective treatment for any unstable tachycardia with a pulse is synchronized cardioversion. As always, follow your protocols no matter what you read here. Here are a couple more links from The Prehospital 12-Lead Blog. Link 1 & Link 2.

Here are two videos of physician interviews on the topic of WPW.

WPW RECOGNITION TEST

[1]Sergio Richter, MD; Pedro Brugada, MD, PhD "FBI (Fast Broad Irregular), A Case for the Secret Service?" Circulation, 2006

*All images, including ECGs, are from various web sites found with Google. They are only to be used for educational purposes.