Junctional Rhythm

Introduction
Background

Cardiac rhythms arising from the atrioventricular (AV) junction occur as an automatic tachycardia or as an escape mechanism during periods of significant bradycardia with rates slower than the intrinsic junctional pacemaker.

The AV node (AVN) has intrinsic automaticity that allows it to initiate and depolarize the myocardium during periods of significant sinus bradycardia or complete heart block. This escape mechanism, with a rate of 40-60 beats per minute, produces a narrow QRS complex because the ventricle is depolarized using the normal conduction pathway. An accelerated junctional rhythm (rate >60) is a narrow complex rhythm that often supersedes a clinically bradycardic sinus node rate (see Media files 1-2). The QRS complexes are uniform in shape, and evidence of retrograde P wave activation may or may not be present.


Junctional bradycardia due to profound sinus node dysfunction. No atrial activity is apparent.



Note the retrograde P waves that precede each QRS complex.

Less commonly, the AV junction develops abnormal automaticity and exceeds the sinus node rate at a time when the sinus rate would be normal (see Media file 3). These junctional tachycardias are most often observed in the setting of digitalis toxicity, recent cardiac surgery, acute myocardial infarction, or isoproterenol infusion.


Accelerated junctional rhythm is present in this patient. Note the inverted P waves that precede each QRS complex, with a rate of 115 bpm.

Pathophysiology

The junctional rhythm initiates within the AV nodal tissue. Accelerated junctional rhythm is a result of enhanced automaticity of the AVN that supersedes the sinus node rate. During this rhythm, the AVN is firing faster than the sinus node, resulting in a regular narrow complex rhythm. These rhythms may demonstrate retrograde P waves on ECG findings, and the rates can vary from 40-60 beats per minute.

Changes in autonomic tone or the presence of sinus node disease that is causing an inappropriate slowing of the sinus node may exacerbate this rhythm. Young healthy individuals, especially those with increased vagal tone during sleep, are often noted to have periods of junctional rhythm that is completely benign, not requiring any intervention.

Rarely, the AVN develops enhanced automaticity and overtakes a "normal" sinus node. This occasionally is observed in digitalis toxicity, following cardiac surgery (typically valve replacement), during acute myocardial infarction, or during isoproterenol infusion.
Frequency
United States

Junctional rhythms are common in patients with sick sinus syndrome or in patients who have significant bradycardia that allows the AV nodal region to determine the heart rate.
Mortality/Morbidity
The heart rate during a junctional rhythm often determines whether the patient has symptoms.
Presence of AV disassociation can lead to symptoms in patients because of atrial conduction and subsequent contraction when the tricuspid valve is closed (ie, canon a waves).
Periods of junctional rhythm are not necessarily associated with an increase in mortality. If an obvious cause is present, such as complete heart block or sick sinus syndrome, then the morbidity or mortality is directly related to that and not to the junctional rhythm mechanism, which is serving as a "backup rhythm" during the periods of bradycardia. Accelerated junctional rhythms may be a sign of digitalis toxicity.
Sex

Junctional escape rhythms, which are common in younger and/or athletic individuals during periods of increased vagal tone (eg, sleep), occur equally in males and females.
Age
This rhythm may occur in persons of any age.
Junctional rhythms during sleep are common in children and in athletic adults.
Clinical
History

Junctional rhythms may be accompanied by symptoms or may be entirely asymptomatic.

Palpitations, fatigue, or poor exercise tolerance: These may occur during a period of junctional rhythm in patients who are abnormally bradycardic for their level of activity.
Dyspnea: Sudden onset of symptoms and sudden termination of symptoms may occur, especially in the setting of complete heart block.
Presyncope (near syncope): The underlying cause of the junctional rhythm is the most significant predictor of symptoms. For instance, AV dissociation with complete heart block, defined as an atrial rate that is faster than the junctional escape rate, is more likely to cause symptoms than AV dissociation with a sinus rate slower than the competing junctional pacemaker. Additionally, syncope or presyncope may occur from an acute decrease in heart rate.

Physical
A predominant junctional rhythm may be associated with structural heart disease, sick sinus syndrome, or both, during which the junctional escape rhythm supersedes the sinus rate and provides a safety mechanism.
During a predominant junctional rhythm, the pulse usually is regular and the heart rate may be within reference range. Frequently, the junctional rhythm is 40-60 beats per minute.
Prominent jugular venous pulsations (ie, cannon a waves) may be present due to the right atrium contracting with a closed tricuspid valve.
Causes
Sick sinus syndrome (including drug-induced)
Digoxin toxicity
Ischemia of the AVN, especially with acute inferior infarction involving the posterior descending artery, the origin of the AV nodal artery branch.
Acutely after cardiac surgery, especially in children within 4 days after surgery for congenital cardiac defects
Acute inflammatory processes (eg, acute rheumatic fever, lyme disease), which may involve the conduction system
Diphtheria
Other drugs (eg, beta-blockers, calcium blockers, most antiarrhythmic agents) that cause sinus bradycardia
Metabolic states with increased adrenergic tone
Isoproterenol infusion

http://emedicine.medscape.com/article/155146-overview