Atrial extra systole is when an “extra” beat occurs. In other words, an extra beat is “inserted” between the regular beats.
Because the origin of this extra-systole could occur anywhere in the atria, right or left, the P wave and the PQ-interval may be different from the normal P wave.
In this example, the focus was located in the left atrium. Therefore the major direction of propagation is now opposite from the normal direction; hence the P-wave becomes opposite to the normal polarity (i.e. negative).
Note that the QRS and T waves are normal. This is because the extra impulse, after entering the AV-node, will propagate through the major Purkinje pathways in the same manner as the normal impulses.
In this type of atrial tachycardia, a re-entrant circuit is located in and around the AV-node!
These circuits can be quite small, mainly due to the very slow propagation in the AV-node (which produced the delay between the atria and the ventricles;
As in other re-entries, impulses will propagate from the circuit to surrounding areas, in this case both the atria and the ventricles.
If the circuit is located high in the AV-node, then the PQ-time will be very short or the P-wave may even be hidden in the QRS complex (top ECG in the figure).
If, however, the circuit is located low in the AV-node and close to the bundle of His, then propagation back to the atria will take a long time and the P-wave will occur after the QRS complex (bottom ECG in the figure).