Pre-excitation occurs when there is an extra electrical pathway in the heart that allows electrical impulses to travel through and cause the ventricles to contract before they normally would. This extra pathway can cause an abnormal heartbeat known as Wolff-Parkinson-White syndrome. People with pre-excitation may experience episodes of a very fast heartbeat, known as tachycardia, which can cause dizziness, fainting, or in some cases even cardiac arrest if not treated promptly.