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Shifting (wandering) pacemaker
|Classification and external resources|
A wandering atrial pacemaker, (WAP), is an atrial arrhythmia that occurs when the natural cardiac pacemaker site shifts between the sinoatrial node (SA node), the atria, and/or the atrioventricular node (AV node). This shifting of the pacemaker from the SA node to adjacent tissues is identifiable on ECG Lead II by morphological changes in the P-wave; sinus beats have smooth upright P waves, while atrial beats have flattened, notched, or diphasic P-waves. It is often seen in the very young, very old, in athletes and rarely causes symptoms or requires treatment.
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 briefly become slightly faster. After vagal tone decreases, the SA Node assumes its natural pace.
A wandering atrial pacemaker, also termed multifocal atrial rhythm, is present when there are three or more ectopic foci within the atrial myocardium that serve as the dominant pacemaker (waveform 5). Since they discharge in random fashion, the pacemaker location is continuously shifting and may be located anywhere in the atrial myocardium. As a result, there is a changing vector of atrial activation that causes a changing P wave morphology and PR interval duration. A dominant P wave (sinus or atrial) cannot be identified. The rate is less than 100 beats per minute.
The RR intervals have variable cycle lengths since the ectopic foci exhibit differences in automaticity and rates of impulse generation. The rhythm is therefore irregularly irregular, and it can be confused with atrial fibrillation. However, in contrast to atrial fibrillation, distinct P waves are present. Sinus arrhythmia may also be irregularly irregular; however, one P wave morphology and PR interval are seen in this situation. This arrhythmia may also be confused with sinus rhythm with multifocal premature atrial contractions, although in this situation a dominant sinus P wave can be identified and there are periods of RR interval regularity. Ventricular conduction is normal with wandering pacemaker, and thus the QRS complex is normal.