The sinoatrial node (SA) is the default pacemaker and is located subepicardially and is crescent in shape. The atrioventricular node (AV) is a subendocardial structure situated in the inferior-posterior right atrium. It sits within an anatomic region bordered posteriorly by the coronary sinus ostium, superiorly by the tendon of Todaro and anteriorly by septal tricuspid valve annulus. This anatomic region is also commonly referred to as the triangle of Koch. The sinoatrial nodal artery supplies blood to the sinoatrial node, it branches off the right coronary artery in 60% of cases, whereas in 40% of cases, it comes from the left circumflex coronary artery. The blood supply to the AV node is from the AV nodal branch of the right coronary artery (90%) or the left circumflex artery (10%) depending on right or left dominant blood supply to the heart. The first septal perforator of the left anterior descending artery also supplies blood to the AV node. A junctional rhythm is where the heartbeat originates from the AV node or His Bundle which lies within the tissue at the junction of the atria and the ventricle. Generally, in sinus rhythm, a heartbeat is originated at the SA node. This electrical activity than travels through the atria to the AV node from where it reaches the Bundle of His from where the electrical signals travel to the ventricles through the Purkinje fibers.
The terminology used to identify the type junctional rhythm depends on its rate and is as follows:
When, the electrical activity of the SA node, is blocked or is less than the automaticity of the AV node/His Bundle a junctional rhythm originates. Numerous conditions and medications can lead to a diseased SA node and lead to the AV node/His Bundle to take over due to higher automaticity of the ectopic pacemaker.
Some of these conditions and medications are listed below.
Other than the above-listed causes of severe SA node disease which can result in a junctional rhythm, a high-grade second-degree heart block and a third-degree heart block can also result in a junctional rhythm. Digoxin toxicity can also lead to an accelerated junctional rhythm.
Junctional rhythm is typical among individuals who have a sinus node dysfunction (SND), and 1 in every 600 cardiac patients above the age of 65 within the United States has SND. Patient with sick sinus syndrome, young children, and athletes who have increased vagal tone may also intermittently exhibit junctional rhythm, especially during sleep. Junctional rhythm is reported equally among males and females.
A heart has numerous pacemaker sites within its conduction system which are independently able to keep the heart beating. And the rate of a heartbeat depends upon the pacemaker site, and as we go down its conduction system, the rate of spontaneous depolarization at pacemaker sites decrease. 
Pacemaker sites and their depolarization rates are listed below:
Histopathology of a disease SA node that can contribute to the AV node/His Bundle taking over as a pacemaker secondary to their intrinsic automaticity includes SA node fibrosis, atrophy, hypoplasia or amyloidosis. A normal SA node histopathology does not rule out reduced SA node automaticity contributing to a junctional rhythm.
Patients with junctional rhythm may present with a varied array of symptoms or may be asymptomatic. Symptoms mostly depend on the underlying cause of the junctional rhythm, for instance, a patient presenting with heart failure exacerbation may present with shortness of breath, wheezing, and lower extremity edema. Patients with rheumatic fever may present with a heart murmur secondary to the damage to the heart valves, fever, joint pain, and a rash with an electrocardiogram showing a junctional rhythm. Some patients might present with generalized complaints such as dizziness, fatigue, syncope/presyncope, and intermittent palpitations. Non-specific physical exam findings include pulsating veins and regular heart rate with the heart rate varying from 20 to over 100 beats per minute.
A significant component of an assessment for a patient who presents with signs and symptoms of a junctional rhythm is history and physical exam. These should include vital signs (respiratory rate, blood pressure, temperature, and heart rate), review of the patient's medication list, and an electrocardiogram. During their evaluation, it should be established if the patient is hemodynamically stable. The patient should be assessed for any underlying ischemic heart disease or heart failure. A healthcare provider should consider thyroid function testing, pulmonary function testing, including routine blood work, and echocardiography as part of their initial evaluation.
Treatment of a junctional rhythm primarily depends on the underlying cause of the rhythm. In circumstances where the junctional rhythm is a result of underlying sinus node dysfunction that is leading to asystole or bradycardia, it should not be terminated, for it is maintaining the heart rate. Therefore, before establishing a management plan for patients presenting with a junctional rhythm, an underlying etiology must be determined first. Otherwise healthy individuals who have junctional rhythm and are asymptomatic need no medical management as the rhythm is usually a result of their increased vagal tone suppressing the SA node intrinsic automaticity. In the setting of digoxin toxicity, a patient must be treated with atropine and digoxin-specific antibody. If a patient is refractory to these pharmacologic treatments and goes into junctional tachycardia intravenous phenytoin can be administered in a monitored setting as these patients can develop hypotension. In the pediatric population, persistent symptomatic junctional tachycardia is an indication for percutaneous radiofrequency ablation. In patients with sick sinus syndrome, or complete or high-grade AV block, a permanent pacemaker is indicated.
Prognosis is good when the rhythm is promptly identified by a healthcare provider.
If not identified promptly symptomatic complications such as syncope, fatigue or dizziness can occur.
If available patient education should be provided using resources familiar to the patient including online resources and pamphlets.
Always check serum digoxin level in patients who are on it for comorbidities. Also, check Lyme titers and empirically treat those patients who present with junctional rhythm and have possible exposure to ticks with antibiotics.
Educating patients at risk for this rhythm and making a closed loop communication between them and their providers can help further improve the management of these rhythms.
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