1.Short PR interval: less than 0.12 seconds with a normal p-wave.

2.Abnormally wide QRS (equal to or greater then 0.11 seconds)

3.Presence of a delta wave (slurring of the QRS complex) When impulses travel down the accessory pathway in an anterograde manner, ventricular preexcitation results. This produces a short PR interval and a delta wave as seen in Wolff-Parkinson-White (WPW) syndrome (see Image 9) (Wolff, 1930).

4.Secondary ST segment and T-wave changes.

The delta wave: Slow conduction through the bypass tract and more rapid conduction through the Purkinje system. When the delta wave is negative, this simulates a Q-wave. This is known as a pseudo-infarction pattern, and is seen n up to 70% of patients with WPW.

ST and T-wave changes: abnormal depolarization results in abnormal repolarization patterns.

Significance: The WPW pattern is seen in approximately 0.20 percent of the general population. Paroxysmal tachycardias occur in approximately 13 percent of healthy individuals with WPW, however much higher incidences of tachycardia are seen in hospitalized or "cardiac" patients.

Atrial fibrillation and flutter is less common (20-35%) and commonly results in a pattern of wide QRS complexes. The ventricular rate may be as fast as 220-360 beats/minute. Due to the requisite short, fast refractory period these patients are a high risk for ventricular fibrillation.