WPW is commonly diagnosed on the basis of the electrocardiogram in an asymptomatic individual. In this case, it is manifested as a delta wave, which is a slurred upstroke in the QRS complex that is associated with a sho PR interval. The sho PR interval and slurring of the QRS complex are reflective of the impulse making it to the ventricles early ( the accessory pathway) without the usual delay experienced in the AV node. If a person with WPW experiences episodes of atrial fibrillation, the ECG shows a rapid polymorphic wide-complex tachycardia (without torsades de pointes). This combination of atrial fibrillation and WPW is considered dangerous, and most antiarrhythmic drugs are contraindicated. Ref Harrison 20th edition pg 1546 When an individual is in normal sinus rhythm, the ECG characteristics of WPW are a sho PR interval (less than 120 milliseconds in duration), widened QRS complex (greater than 120 milliseconds in duration) with slurred upstroke of the QRS complex, and secondary repolarization changes (reflected in ST segment-T wave changes). In individuals with WPW, electrical activity that is initiated in the SA node travels through the accessory pathway, as well as through the AV node to activate the ventricles both pathways. Since the accessory pathway does not have the impulse slowing propeies of the AV node, the electrical impulse first activates the ventricles the accessory pathway, and immediately afterwards the AV node. This gives the sho PR interval and slurred upstroke of the QRS complex known as the delta wave. In case of type A pre-excitation (left atrioventricular connections), a positive R wave is seen in V1 ("positive delta") on the precordial leads of the electrocardiogram, while in type B pre-excitation (right atrioventricular connections), a predominantly negative delta wave is seen in lead V1 ("negative delta" Ref Davidson 23rd edition pg 468