Wandering atrial pacemaker
Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Associate Editor-In-Chief: Cafer Zorkun, M.D., Ph.D. 
This atrial arrhythmia occurs when the natural cardiac pacemaker site shifts between the SA node, the atria, and/or the AV node. This shifting of the pacemaker from the SA node to adjacent tissues is manifested electrocardiographically by transient changes in the size shape and direction of the P waves. A wandering pacemaker is usually caused by varying vagal tone. With increased vagal tone the SA node slows,allowing a pacemaker in the atria or AV Nodal area,which may become slightly faster briefly. After vagal tone decreases the SA node becomes the pacemaker again.Note that if the heart rate increases to above 100bpm, it is called multifocal atrial tachycardia. Possible causes are hypoxia, COPD and medication such as digoxin.
Shown below is an EKG image of a wandering atrial pacemaker. When several pacemakers are competing, p-waves with different origins and thus configurations occur. The rhythm is slightly different from beat to beat.
Copyleft image obtained courtesy of ECGpedia, http://en.ecgpedia.org/wiki/Main_Page