Wandering pacemaker
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Wandering atrial pacemaker (WAP) is an
atrial The atrium ( la, ātrium, , entry hall) is one of two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular valves. There are two atr ...
rhythm where the pacemaking activity of the heart originates from different locations within the atria. This is different from normal pacemaking activity, where the
sinoatrial node The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells. The sinus node is approximat ...
(SA node) is responsible for each heartbeat and keeps a steady rate and rhythm. Causes of wandering atrial pacemaker are unclear, but there may be factors leading to its development. It is often seen in the young, the old, and in athletes, and rarely causes symptoms or requires treatment. Diagnosis of wandering atrial pacemaker is made by an
ECG Electrocardiography is the process of producing an electrocardiogram (ECG or EKG), a recording of the heart's electrical activity. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the hear ...
.


Pathophysiology

The SA node is considered the primary pacemaker of the heart. In wandering atrial pacemaker, there are other locations within the atria besides the SA node that are responsible for each heartbeat. This is unusual because the
SA node The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is an ellipse, oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of Cell (biology), cells known as pacemaker cells. The s ...
,
AV node The atrioventricular node or AV node electrically connects the heart's atria and ventricles to coordinate beating in the top of the heart; it is part of the electrical conduction system of the heart. The AV node lies at the lower back section of t ...
,
bundle of His The bundle of His (BH) or His bundle (HB) ( "hiss"Medical Terminology for Health Professions, Spiral bound Version'. Cengage Learning; 2016. . pp. 129–.) is a collection of heart muscle cells specialized for electrical conduction. As part of t ...
,
bundle branches The bundle branches, or Tawara branches, are offshoots of the bundle of His in the heart's ventricle. They play an integral role in the electrical conduction system of the heart by transmitting cardiac action potentials from the bundle of His to ...
, and
Purkinje fibers The Purkinje fibers (; often incorrectly ; Purkinje tissue or subendocardial branches) are located in the inner ventricular walls of the heart, just beneath the endocardium in a space called the subendocardium. The Purkinje fibers are specia ...
are the structures that have pacemaking capability. The atrial and ventricular muscle tissue do not have this capability. Originally, it was believed that the atria had different
ectopic foci An ectopic pacemaker, also known as ectopic focus or ectopic foci, is an excitable group of cells that causes a premature heart beat outside the normally functioning SA node of the heart. It is thus a cardiac pacemaker that is ectopia (medicine), ...
that were spontaneously
depolarizing In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is esse ...
, each foci acting as a pacemaker for the heart. However, the atrial muscle tissue isn't able to spontaneously depolarize. Therefore, the mechanism is by extension of the self-depolarizing tissue between the SA and AV node. Over time, the extension of tissue becomes a tract with spontaneous depolarization capability. This allows for any location along the tract to spontaneously depolarize, hence different locations that are responsible for each heartbeat.


Causes

Wandering atrial pacemaker may be seen in young, healthy individuals as well as in the elderly and those with
lung disease The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...
. The cause of wandering atrial pacemaker is unclear. Increased tone from the
vagus nerve The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and right ...
may factor into the rhythm appearing in young, healthy individuals who exercise. The vagus nerve is a part of the parasympathetic nervous system that helps control heart rate and contractility of the heart. Through exercise, there is increased input to the heart from the vagus nerve, which causes the heart to beat at a slower rate. This is manifested by a lower resting heart rate than may be seen in the average person. For elderly individuals, the rhythm may be caused by
sinus node dysfunction Sinus node dysfunction (SND), also known as sick sinus syndrome (SSS), is a group of abnormal heart rhythms (arrhythmias) usually caused by a malfunction of the sinus node, the heart's primary pacemaker. Tachycardia-bradycardia syndrome is a varian ...
. This is where the heart's pacemaker, the SA node, has become damaged. For individuals with lung disease, the rhythm could be related to the underlying lung pathology. In rare cases,
digoxin toxicity Digoxin toxicity, also known as digoxin poisoning, is a type of poisoning that occurs in people who take too much of the medication digoxin or eat plants such as foxglove that contain a similar substance. Symptoms are typically vague. They may inc ...
can cause wandering atrial pacemaker.


Symptoms

Wandering atrial pacemaker doesn't usually have symptoms because it is commonly a benign rhythm. It is usually found incidentally on an ECG for other medical indications that require a heart rhythm screening. If a patient does have symptoms, it may manifest as skipped heartbeats. Upon physical examination, it can be found by having an irregularly irregular rhythm, similar to how atrial fibrillation is described. An ECG would then be performed to find the underlying cause of the rhythm disturbance.


Diagnosis

The
heart rhythm The cardiac conduction system (CCS) (also called the electrical conduction system of the heart) transmits the signals generated by the sinoatrial node – the heart's pacemaker, to cause the heart muscle to contract, and pump blood through the ...
is seen through an electrocardiogram. To make the diagnosis, there must be at least 3 different
P-wave A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any ...
morphologies in a single ECG lead due to the shifting of the pacemaker in the atria. This is different from normal
sinus rhythm A sinus rhythm is any cardiac rhythm in which depolarisation of the cardiac muscle begins at the sinus node. It is characterised by the presence of correctly oriented P waves on the electrocardiogram (ECG). Sinus rhythm is necessary, but not su ...
where one will see the same P-wave morphology through the same lead because each beat is started from the SA node. The P-wave is normally upright or positive in leads I and II, and therefore may be the helpful when determining changing P-wave morphologies. Other common changes that are seen on ECG with wandering atrial pacemaker include differing PR intervals and PP intervals. Another heart rhythm similar to wandering atrial pacemaker is
multifocal atrial tachycardia Multifocal (or multiform) atrial tachycardia (MAT) is an cardiac arrhythmia, abnormal heart rhythm, specifically a type of supraventricular tachycardia, that is particularly common in older people and is associated with exacerbations of chronic o ...
. Both arrhythmias have at least 3 different P-wave morphologies in a single ECG lead, but the heart rate is different. When the heart rate is lower than 100 beats per minute, the heart rhythm is considered wandering atrial pacemaker. When the heart rate is greater than 100 beats per minute, the heart rhythm is considered multifocal atrial tachycardia.


Treatment

Treatment is rarely required because, in most cases, it is asymptomatic. If symptoms develop, medication can be pursued for symptomatic relief. In the setting of suspected sinus node dysfunction manifesting as wandering atrial pacemaker, evaluation for pacemaker placement may be done due to sinus node damage. In cases of digoxin toxicity, a physician may decrease the dose, change medications, or cease digoxin therapy.


References


External links

{{Circulatory system pathology Cardiac arrhythmia