During the development the human embryonic heart, accessory pathways are necessary to sustain growth. By birth, each of these pathways should have undergone apoptosis, except for the AV node and His-Purkinje system. However, a small group of people (3 in 1,000) maintain at least one accessory pathway after birth.
The diagnosis of Wolff-Parkinson-White (WPW) syndrome is based on the presence of an accessory pathway that causes preexcitation and its characteristic ECG pattern described in this chapter.
An accessory conduction pathway allows impulses to conduct from the atria to the ventricles; this pathway triggers preexcitation of the heart. The accessory pathway may conduct impulses in a retrograde, antegrade, or bilateral direction. The pathway may be located anywhere along the atrioventricular ring or in the septum.
Electrophysiologic studies have mapped the location of these accessory pathways in patients; 50% are in the left lateral area, 30% are in the posteroseptal region, 10% are in the right anteroseptal area, and 10% are in the right lateral region.