Electrocardiogram showing Wolff-Parkinson-White syndrome

ECG showing Wolff-Parkinson-White syndrome. Note the characteristic short PR interval ("preexcitation syndrome") and the "delta wave." The short PR interval and the preexcitation syndromes is due to the accessory pathway permitting the impulse from the SAN to bypass the slow conducting AVN, allowing early depolarization of the ventricular myocardium. The delta wave and the slurring of the upstroke of the QRS is created when the preexcitation impulse in the myocardium initially activates ventricular myocardium that conducts at a slower rate as compared to the His-Purkinje system. The remainder of the QRS complex appears relatively normal as simultaneous activation of the ventricles through the AVN and His-Purkinje system provides normal conduction.