In the lower region of the AV junction, where the AV node meets the bundle of His, there is a group of pacemaker cells responsible for the generation of junctional rhythms, or atrioventricular junctional rhythms. The impulse generated from the AV junction produces an unusual pattern of conduction. Since the impulse is generated in the middle of the heart and the impulse travels in two directions, the atria and ventricles depolarize at the same time.

In a normal ECG, lead II shows a positive deflection of the P wave and the QRS complex. In junctional rhythms, the impulse also travels a retrograde conduction path to the atria toward the negative electrode, so the P wave has a negative deflection (see Figure 5.1).

Figure 5.1. Atrioventricular Junctional Rhythm

Retrograde conduction causes the P waves to show a negative deflection in lead II.