Changing P-wave morphology: What is the mechanism?

A 76-year-old woman, affected by drugs resistant focal atrial tachycardia, underwent a catheter ablation procedure in our Hospital. During ablations we observed on the surface ECG a progressive modification of the second component of the P wave (delayed and then negative in inferior leads). These findings demonstrated, compared to the beginning of the procedure, a different propagation of the activation wave from the right to the left atrium, helped to identify the true origin of a focal atrial tachycardia. Moreover, this case underlined the importance of the Bachmann's bundle for the impulse propagation through the interatrial septum in normal hearts.