A junctional rhythm is a protective heart rhythm that occurs when the atrioventricular node (AV node) takes over as the heart’s pacemaker. To fully understand a junctional rhythm, let’s first take a look at the cardiac conduction system and see how it operates in a normal, healthy heart.
Cardiac Conduction System
The SA node is the heart’s natural pacemaker and is located in the right atrium. The SA node generates an electrical impulse at a rate of 60-100 bpm and sends it to both atria by way of the internodal tracts. This causes both atria to contract simultaneously and force blood into both ventricles. The electrical impulse then continues down to the AV node, which is located between the atria and the ventricles. The AV node slows down and regulates the impulse, then sends it through the bundle of His to the left and right ventricle. From the bundle of His, the impulse travels through the Purkinje fibers, which are inside the ventricular muscle, causing the ventricles to contract and pump blood throughout the body.
A junctional rhythm occurs when the AV node takes over as the primary pacemaker because either the SA node failed or the AV node blocked the atrial impulse. The AV node only generates an electrical impulse at a rate of 40-60 bpm, so you typically see a much slower heart rate. There are some circumstances, however, where the AV node develops an abnormal automaticity and exceeds the SA node rate. This is defined as either an accelerated junctional rhythm or junctional tachycardia, depending on the rate. A junctional rhythm is protective — the AV node serves as a backup for the SA node — so it should not be suppressed. The reason being is it would be more dangerous for a lower pacemaker site, such as the bundle of His, to assume the role of pacing the heart. Generally, the lower or more distal the pacemaker site, the less reliable and effective the pacing is.
How to Identify a Junctional Rhythm
A junctional rhythm can be identified by having a regular R-R interval and one of the following P wave characteristics:
- Absent P Waves: This occurs when the AV node sends an electrical impulse to the atria and ventricles at the same time.
- Inverted P Waves: This occurs when the AV node sends an electrical impulse to the atria first.
- Post QRS P Waves: This occurs when the AV node sends an electrical impulse to the ventricles first.
The absence or change of the P wave indicates the primary pacemaker has shifted from the SA node to the AV node.
Junctional Rhythm Types
- Junctional Rhythm: The AV node is the primary pacemaker with a rate of 40-60 bpm.
- Accelerated Junctional Rhythm: The AV node is the primary pacemaker with a rate of 60-100 bpm.
- Junctional Tachycardia: The AV node is the primary pacemaker with a rate > 100 bpm.
Visit the Junctional Rhythms page to learn more about the different types of junctional rhythms.
Junctional Rhythm Symptoms
Junctional rhythms can be completely asymptomatic or accompanied by any of the following:
Junctional Rhythm Causes
- Sick sinus syndrome
- Digoxin toxicity
- Ischemia of the AV node
- Acute inflammatory process that may involve the conduction system (e.g., acute rheumatic fever, lyme disease)
- Drugs that can cause bradycardia (e.g., beta-blockers, calcium blockers, antiarrhythmic agents)
- Metabolic states with increased adrenergic tone
- Isoproterenol infusion