There are three important QRS patterns that can be seen during sinus episodes.

In WPW syndrome, the ventricles are simultaneously activated by both the accessory pathway and the AV node. This dual activation presents as a fusion beat on the ECG. The QRS configuration in a fusion beat depends on which of the two sources of ventricular activation is most predominant. For example, when the ventricles are dominantly activated by impulses along the accessory pathway, the QRS complex bears more resemblance to that of the classic WPW preexcitation pattern—a short PR interval with a pronounced positive delta wave (see the ECG in figure 1). 

On the other hand, the more the ventricles are activated along the AV node, the more the QRS complex resembles that of a normal sinus rhythm. This makes detection of an accessory pathway more difficult. 

Sometimes patients present with insignificant shortening of the PR interval, an “abortive” delta wave, with minimally altered QRS complexes and repolarization. These patients may also present with varying levels of preexcitation and with relatively typical preexcitation patterns.

Rarely does a patient show full preexcitation of the ventricles entirely from accessory pathway impulses. Although when this does occur, the ECG pattern is very typical of WPW syndrome.

Some patients have a retrograde direction of impulses along the accessory pathway. These patients are diagnosed with a “concealed accessory pathway.” Electrophysiologic studies are required for the diagnosis because ECG morphologies resemble a regular sinus rhythm.

Anatomic Definition of Accessory Pathway Location ⁴³

Figure 3 provides an anatomic definition of the accessory pathway location. A left anterior oblique schematic of the heart shows the relationship of the tricuspid annulus (TA), mitral annulus (MA), His bundle (HIS), and coronary sinus (CS) with the accessory pathways’ anatomic locations. These locations are divided into five regions and defined by double lines: (1) left anterior (LA) and left lateral (LL); (2) left posterolateral (LPL) and left posterior (LP); (3) right and left midseptal (MS) and posteroseptal (PS); (4) right posterolateral (RPL) and right posterior (RP), and (5) right anteroseptal (RAS), right anterior (RA), and right lateral (RL).