Wolff-Parkinson-White (WPW) Syndrome was first reported in 1930.
It is a congenital abnormality seen in up to 1 in 500 otherwise normal EKGs. Electrically active muscle fibers (Kent Bundles) bridge the atria and ventricles and cause pre-excitation of the ventricles (via the HIS-Purkinje system) by bypassing the normal electrical circuitry of the heart.
This is seen as a characteristic “delta wave” on EKG, only with the orthodromic version (Type A) of WPW. This accessory pathway is able to conduct faster than the AV node. WPW is a reentry mechanism with an accessory pathway.
In WPW, a critically timed premature atrial beat that occurs during the refractory period of the accessory pathway typically initiates CMT (circus movement tachycardia).
The impulse, therefore, travels solely down the AV node but returns retrograde through the accessory pathway, resulting in CMT (orthodromic conduction). It results in a narrow complex heart rhythm limited by the refractory period of the AV node. The QRS interval is narrow because the impulses travel antegrade (orthodromically) through the AV node and regular because circus movement occurs at a regular rate. If one’s rate is above 220 bpm, but otherwise looks on EKG to be SVT, one must think about orthodromic WPW.
Orthodromic CMTs are 10-15 times more likely than antidromic CMTs. (4)
If the accessory pathway goes in the opposite direction, that is, through the accessory pathway, to the ventricle, and then retrograde through the AV node, it is said to be antidromic (Type B WPW). With this type of WPW, one may not see the characteristic “delta waves” seen with orthodromic WPW, but instead, they may see what appears to be ventricular tachycardia (described below).
Antidromic CMTs are wide and potentially faster because of the relatively short refractory period of most accessory pathways. They are termed antidromic because anterograde transmission occurs down the accessory pathway, creating pre-excitation of the ventricle adjacent to it. These dysrhythmias are regular due to the nature of the circus movement. (5)

Differential diagnoses include ventricular tachycardia (V Tach), which also is regular (unless it is Torsade de Pointes) or PSVT with aberrancy. Consider any regular wide-complex tachycardia to be V Tach until proven otherwise; however, as in regular narrow-complex tachycardias.

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