Ventricular pre-excitation is clinically important because of its frequent association with tachyarrhythmis, some of which may be life threatening. This association (ventricular pre-excitation and tachyarrhythmias) constitutes the Wolff-Parkinson-White Syndrome, also referred to as the WPW syndrome. It is so named in recognition of the 3 Boston physicians who, in 1930, described a series of young patients having a short PR interval and bundle branch block pattern on the electrocardiogram, and paroxysms of tachycardia. This ECG is from a 28 yr old patient with an episode of sudden, unexplained syncope. After looking at the tracing, try to answer these questions: 1) What is the PR interval? 2) What is the QRS duration? 3) In what leads is the delta wave negative? 4) In what leads is the delta wave positive? 5) Where do you think the bypass tract is located?