Electrocardiogram
The classic ECG morphology of WPW is described as a shortened PR interval, a widened QRS complex, and a delta wave. In reality, however, morphology varies greatly.
Depending upon the location of the accessory pathway in relation to the sinus node and the relative transmission characteristics of the accessory pathway and the AV node, the morphology of the ECG may vary from a classic presentation to near normal.
In some cases, the electrical impulse's arrival into the ventricle occurs slightly earlier through the accessory pathway, creating preexcitation.
QRS is widened because the ventricles initially are activated via the accessory pathway outside the normal conducting system in the muscle tissues, producing slow initial forces and a delta wave. This is known as a revealed accessory pathway because it is easily identifiable on ECG.
In other cases, however, arrival of the electrical impulse to the ventricle occurs nearly simultaneously through both the accessory pathway and the AV node.
When this occurs, preexcitation is absent and ECG appears normal.
Thus, morphology of the ECG depends directly upon the degree of preexcitation.
An accessory pathway that does not manifest on ECG is revealed when the rate exceeds the refractory period of the AV node. This has been described as latent.
A latent accessory pathway can conduct both anterograde and retrograde transmissions.
An accessory pathway in which only retrograde transmission of impulses can occur is called concealed and is used only during circus movement tachycardias (CMTs).
