Wolff-Parkinson-White (WPW) Syndrome is a potentially lethal abnormality within the heart’s conduction system that affects about 0.2% of the general population. About 70% of those with WPW have no history of cardiac disease. WPW is more common in men, and is most often diagnosed in children and young adults after they present to the ER with an arrhythmic event.
WPW is associated with an accessory pathway, usually the bundle of Kent, that is an extra lane of conduction between the atria and ventricles of the heart. This extra pathway is located in the myocardial wall and is faster than the normal lane of atrioventricular conduction due to the lack of an AV nodal pause. Many people with WPW may be asymptomatic their entire life. Most patients who do have symptoms present with a tachycardic emergency.
The AV node pauses normal conduction to allow for proper filling of the ventricles after atrial depolarization. Since the accessory pathway allows a faster lane of conduction, this can lead to premature ventricular depolarization, also known as pre-excitation,1 which can lead to what is known as a re-entry tachycardia.
The pause caused by the AV node can be depicted on an ECG as the PR segment, which starts at the end of atrial depolarization--the end of the P wave. The PR segment lies on the isoelectric line and ends at the start of the QRS complex. This segment becomes nearly nonexistent when WPW is present. Since the heart’s conduction bypasses the AV node by way of the accessory pathway, ventricular depolarization occurs directly after atrial depolarization. This inhibits maximum filling and cardiac output, according to Frank Starling’s law.
Because of the abnormal conduction of a heart with WPW, certain ECG findings may indicate this condition. In addition to a shortened PR segment, a delta wave may be present. This is a slurring upslope from the end of the P wave to the peak of the R wave that occurs due to the immediate ventricular depolarization following atrial depolarization.
Let's look at some examples of the way conduction might be altered in the presence of WPW. In the first example (Figure 1), an impulse is initiated in the SA node, then travels simultaneously down both the accessory pathway and the normal AV tract to the bundle of His. This causes the classic WPW presentation of a short PR segment and a delta wave.
The second example (Figure 2) shows what is called antidromic conduction.2 In this cycle, the impulse originates somewhere in the right atrium but conducts in a retrograde fashion through the accessory pathway and re-enters the atrium via the AV junction. This is an example of a re-entry tachycardia. This particular rhythm is called AVRT (atrioventricular reciprocating tachycardia), and is commonly misinterpreted as ventricular tachycardia due to its wide and fast appearance. Since the AV node is bypassed, atrial rhythms may be much faster than normal. A 3:1 atrial flutter can easily become a 1:1 atrial flutter at rates in excess of 300 bpm. These patients may need immediate intervention.
Figure 3 Conduction may also be orthodromic.3 Impulses may enter the ventricles in a normal physiologic manner, but they re-enter the atrium through the accessory pathway. The antegrade conduction may keep the QRS complex narrow, but the heart rate may accelerate due to a re-entry circuit. A bundle branch-like pattern may indicate this form of conduction.
Another possible presentation of WPW is FBI (fast, broad and irregular), which is specific for atrial fibrillation with WPW. By definition, this is a wide complex tachycardia (WCT). The American Heart Association (AHA) designed the WCT algorithm specifically for aberrancy like this. In the presence of WPW, traditional treatments may be contraindicated.
Any AV nodal slowing agent, including adenosine, diltiazem and amiodarone, may cause an adverse reaction in the presence of WPW. Of those three medications, AHA recommends amiodarone. It is important to anticipate a lethal re-entry tachycardia with administration of any AV nodal slowing agent.4 Synchronized cardioversion is indicated for the unstable tachycardic patient.