Among patients who experienced SVT while driving, 77% felt fatigue, 50% had symptoms of near-syncope, and 14% experienced syncope. Women had more symptoms in each category. See Online Data Supplement 1 for additional data on clinical presentation and differential diagnosis on the basis of symptoms. Concealed accessory pathway A pathway that conducts only retrogradely and does not affect the ECG pattern during sinus rhythm. Pre-excitation pattern An ECG pattern reflecting the presence of a manifest accessory pathway connecting the atrium to the ventricle. Pre-excited ventricular activation over the accessory pathway competes with the anterograde conduction over the AV node and spreads from the accessory pathway insertion point in the ventricular myocardium. Depending on the relative contribution from ventricular activation by the normal AV nodal/His Purkinje system versus the manifest accessory pathway, a variable degree of pre-excitation, with its characteristic pattern of a short P-R interval with slurring of the initial upstroke of the QRS complex (delta wave), is observed. Pre-excitation can be intermittent or not easily appreciated for some pathways capable of anterograde conduction; this is usually associated with a low-risk pathway, but exceptions occur. Asymptomatic pre-excitation (isolated pre-excitation) The abnormal pre-excitation ECG pattern in the absence of documented SVT or symptoms consistent with SVT. Wolff-Parkinson-White syndrome Syndrome characterized by documented SVT or symptoms consistent with SVT in a patient with ventricular pre-excitation during sinus rhythm. Atrioventricular reentrant tachycardia (AVRT) A reentrant tachycardia, the electrical pathway of which requires an accessory pathway, the atrium, atrioventricular node (or second accessory pathway), and ventricle. Orthodromic AVRT An AVRT in which the reentrant impulse uses the accessory pathway in the retrograde direction from the ventricle to the atrium, and the AV node in the anterograde direction. 