SA node is the ultimate power center of heart located in the junction of SVC and right atrium .In normal physiology it fires at a rate of 60 -90 /minute that dictates the ventricular rate .
SA node is a linear spindle shaped structure with a length of 1.5cm . The P cells with unique mitochodria are responsible for pace making activity . The ion responsible for pacemaker current is mainly calcium with the initial 25 % push given by sodium current as well . These cells are predominately under vagal control.Even though pace making activity is normally restricted to the SA node , the vagal innervation is such that the pacemaker has a potential to shifts it’s activity both functionally geographically.
In fact , there is constant flux of pacemaker activity with the entire length of SA node.The cranial aspect SA node has more fire power than its caudal tip . It is possible Sinus tachycardia and sinus bradycardia could represent minor changes in the firing focus in its cranio-caudal axis.Further the P cells of sinus node can spill all over the atria and even up to AV node.
What is wandering atrial pacemaker ?
This entity is poorly defined in literature. With pace making cells scattered all around there is no surprise to note dynamic pacemaker shifts even in healthy people. This is especially common in young athletes.
Wandering can occur
- Within SA node ( Shift of focus of p cell firing .No visible changes in ECG )
- Within SA node and atria
- Between SA node and AV node. (Sino-Junctional rhythm )
Effect on ECG
- Baseline bradycardia.
- Changing P wave morphology
- Change in PR intervals
- Intermittent absent (Rather concealed ) P wave if is also possible
- RR interval can also show minor variation.
Image Modifed from http://www.eheart.org
Clinical significance of Wandering pacemaker(WAP )
- A Benign condition generally has no clinical significance.
- It is often an expression of high vagal tone.
- Usually transient.
- Can be unmasked by beta or calcium blockers.
- Severe forms of wandering pace maker can be a marker of sinus node dysfunction and would need further evaluation
- In the coronary care units it is associated with infero-posterior MI when the vagal fibers are insulted.
Differential diagnosis .
- Some times it need to be differentiated form ectopic atrial rhythm /Low atrial/Coronary sinus rhythm etc .
- Sinus slowing followed by a functional escape and reemergence of sinus beat can be a termed as a form of wandering pacemaker
Final message
WAP : This attractive and descriptive ECG entity is largely insignificant in clinical cardiology .
It should not be confused with more dangerous cardiac arrhythmia like sinus pauses and arrest .
Dr.S.Venkatesan MD 