Image 1- Wandering Atrial Pacemaker
This rhythm is a common occurrence specially at night when the heart rate goes bradycardic. In strip A, there are distinct P waves in the earlier part with the atrial rate in the low 50's. In strip B, towards the latter part of the strip, distinct P waves can be seen and the atrial rate is about 60. In both strips, there is some irregularity of the R to R. We are deceived by the strip to think that it converted to junctional rhythm. However, if you look closely the morphology of the P wave just changed. If the QRS complexes were of junctional origin, then it should have been regular.
The change in P wave morphology can be explained by the shift of the pacemaker site.The pacemaker site in the sinoatrial node (SAN) shifts with respiration. The pacemaker site shifts higher the SAN, the heart rate and P wave amplitude or size in II, III and aVF increase. As the pacemaker shifts lower the SAN, the P wave amplitude and heart rate decrease.
You can also see the gradual change in P wave morphology from distinct upright, decreasing amplitude, to increasing amplitude as the rate increases. This gradual change in P wave morphology is the rhythm called wandering atrial pacemaker (or WAP).
WAP is a variant of sinus arrhythmia. There is passive transfer of dominant pacemaker focus from the sinus node to latent pacemakers. The change (in P wave shape) occurs gradually. There is only one pacemaker that is in control.
The change in P wave contour is gradual and after several cycles the pacemaker shifts back to the sinus node.
Do not confuse WAP with multifocal atrial rhythm (MAR).
For this rhythm, there is nothing to worry about.