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"If the AV junction is the pacemaker, the impulse will travel retrograde to the atrium. Remember that electricity goes every direction at once, so it's going to shoot down the bundle of his into the ventricles but it's also going to shoot to the atrium to depolarize them. If the P-wave is visible, it is ALWAY inverted in lead 2. Remember that the normal sinus rhythm the atria fired, QRS fired, then we a T-wave and it would go on from there. But now that it is going backwards, the P-wave will be upside down and everything else will stay normal. The PR interval will get shorter because there is no longer the nice long delay of the AV node because the delay is getting eaten up by the electricity going in other directions. Remember electricity is normally going towards the positive electrode in lead 2 so it will normally show a positive P-wave. So, when it is going backwards it makes sense why it's inverted.
The thing here is there is a lot of cells and the bundle too, so you tend to only see the inverted P-wave if it's one of the cells close to the Atria that takes over the pacemaker. This gives the Atria time to repolarize because most of the delay is still there. If it's one of the cells further down, the inverted P will still be there but it will more than likely be buried in the QRS. This is why we don't usually see a P-wave. "
*Conduction through the ventricles is normal because the right and left bundle branches are acting as normal. The problems with the heart are much higher up so the junction is taking control."
-When the AV junction takes over pacemaking responsibility, the atria and the ventricles will be depolarized at nearly the same time because the impulses spreads in two directions at one time. 