Every time the heart squeezes it requires an electrical impulse. The impulse originates from an area in the top right corner of the heart known as the sinus node. Electricity then spreads across the upper two chambers of the heart. It travels to the lower two chambers through a specialized conducting pathway known as the AV node. Normally the only way for electricity to travel from the upper to the lower chambers is through the AV node.
What is Wolff-Parkinson-White syndrome?
A small percentage of people, about 3 in 1000, have extra conducting tissue from the top to the bottom chambers. This extra conducting tissue is termed an “accessory pathway.” The majority of the time the accessory pathway does not conduct electricity. However, if circumstances are just right, it is possible for electricity to travel down the normal pathway through the AV node and then immediately travel back up the accessory pathway. Subsequently, an electrical circuit can be established between the AV node and the accessory pathway. When electricity enters this circuit, supraventricular tachycardia(SVT) results.
Most accessory pathways can only conduct electricity backwards from the ventricles into the atrium ("concealed accessory pathways"). In the absence of SVT they do not conduct any electricity at all. Wolff-Parkinson-White syndrome refers to a special type of accessory pathway that can conduct electricity forward from the atria into the ventricles even in the absence of SVT. This means that it can be diagnosed on a routine ECG. The classic finding on the ECG in a patient with WPW is called a delta wave. Individuals with a concealed accessory pathway have a normal ECG at baseline.
Complications of Wolff-Parkinson-White syndrome
The most common complication of Wolff-Parkinson-White syndrome in children is supraventricular tachycardia (SVT). In SVT, electricity forms a circuit between the accessory pathway and the normal AV node. Rapid conduction through the circuit leads to a heart rate much faster than normal. More information on supraventricular tachycardia can be found in the article on SVT here.
The ability of the accessory pathway to conduct electricity forward from the atria to the ventricles presents an extra risk to patients with Wolff-Parkinson-White syndrome, unrelated to typical SVT. As these individuals enter their teenage and young adult years, there is a slightly increased incidence of atrial fibrillation. In atrial fibrillation the atria suddenly beat very rapidly, up to 300 to 400 times per minute. In a child with a normal heart, or even in one with “non-WPW” SVT, the AV node blocks a number of these impulses so that only a certain portion get through to the ventricles. However, in children with WPW the accessory pathway is often able to conduct all of these atrial impulses through to the ventricle. This results in an extremely fast heart rate in the ventricles which can be very dangerous.
Because of this risk, most children with Wolff-Parkinson-White syndrome need more specialized evaluation once they reach the mid teenage to early adult years. This can involve testing as simple as an exercise treadmill test, or as involved as a cardiac catheterization and electrophysiology study.
Treatment of Wolff-Parkinson-White syndrome
Infants and younger children with asymptomatic Wolff-Parkinson-White syndrome do not require any treatment at all. For those children that develop supraventricular tachycardia, there are a number of different treatment options. Some children have SVT so rarely (once or twice a year) that no treatment at all may be needed. If episodes are happening frequently, are extremely fast, or are worrisome to the patient, then treatment is usually indicated. A number of medicines have been devised to prevent SVT from starting in the first place. The most common class of medications are beta-blockers. Beta-blockers slow conduction through the normal pathways and prevent SVT from ever starting. They are about 75-90% effective in preventing SVT. Most beta-blockers have very few side effects. They are contraindicated in patients with asthma.
A potential cure for SVT and WPW can be achieved through a procedure called a radiofrequency ablation. This procedure involves a cardiac catheterization. A specialized catheter is passed through a vein in the leg into the heart. Using advanced electrical mapping techniques, the accessory pathway is located, and high-frequency energy is then applied to the pathway to destroy it. Occasionally the pathway may be frozen using a technique termed cryoablation. These procedures are approximately 95% effective. In most cases there are very few if any side effects or risks. A radiofrequency ablation can be an excellent option for patients with Wolff-Parkinson-White syndrome.