What is a junctional rhythm?

What is a Junctional Rhythm?

A junctional rhythm is a cardiac arrhythmia originating from the AV node or His bundle, characterized by narrow QRS complexes (unless pre-existing bundle branch block exists), heart rates typically between 40-60 bpm for escape rhythms or 70-120 bpm for nonparoxysmal junctional tachycardia, and often demonstrating AV dissociation with absent, inverted, or dissociated P waves. 1

Anatomical Origin and Mechanism

• The unifying anatomical feature of all junctional rhythms is their origin from either the AV node or His bundle 1
• Each ventricular depolarization in junctional rhythm is preceded by a His bundle deflection, confirming the junctional origin 1
• The mechanism involves enhanced automaticity from an ectopic focus in the AV junction, rather than reentry 1, 2
• The junctional escape mechanism does not require participation of either atrium or ventricle for its propagation 1

Classification by Heart Rate

Junctional rhythms are classified into distinct types based on their rate:

• Junctional escape rhythm: 40-60 bpm, serving as a backup pacemaker when the sinus node fails 1
• Nonparoxysmal junctional tachycardia: 70-120 bpm (some sources report 70-130 bpm), showing typical "warm-up" and "cool-down" patterns that cannot be terminated by pacing maneuvers 1
• Focal junctional tachycardia: 110-250 bpm (some sources report 120-220 bpm), a rare and potentially serious arrhythmia that may lead to heart failure if incessant 1, 3

ECG Characteristics

• QRS complex: Narrow (<120 ms) unless pre-existing bundle branch block is present 1, 4
• P waves: May be absent, inverted (retrograde), or dissociated from the QRS complex 4
• P wave timing: When present, P waves may occur before, during, or after the QRS complex 4
• AV dissociation: Often present, though one-to-one retrograde conduction may be transiently observed 1
• The presence of AV dissociation excludes AVRT and makes AVNRT highly unlikely 1

Common Underlying Causes

Nonparoxysmal junctional tachycardia may be a marker for serious underlying conditions:

• Digitalis toxicity: The most common cause of nonparoxysmal junctional rhythm 1
• Myocardial ischemia or infarction: Due to altered automaticity 1
• Post-cardiac surgery complications: Particularly in children with congenital heart disease 1, 3
• Electrolyte abnormalities: Particularly hypokalemia 1
• Chronic obstructive lung disease with hypoxia 1
• Inflammatory myocarditis: Due to inflammation near the conduction system 1

Clinical Significance

• Junctional escape rhythms are more reliable and faster than ventricular escape rhythms (which originate below the His bundle at 20-40 bpm) 1
• The location of the junctional escape focus above the bundle branches explains why it is generally more responsive to autonomic manipulation compared to ventricular escape rhythms 1
• In adults, junctional tachycardia typically has a relatively benign course, whereas in infants and children it can lead to heart failure or uncontrollable tachyarrhythmia 1
• Focal junctional tachycardia is very uncommon, rare in the pediatric population and even less common in adults 1
• Congenital junctional ectopic tachycardia has a high rate of morbidity and mortality, with death occurring in 35% of cases 3

Differentiation from Other Arrhythmias

Critical distinction from AVNRT and AVRT:

• The presence of AV dissociation in junctional rhythm excludes AVRT and makes AVNRT highly unlikely 1
• In AVNRT, the P wave is typically inscribed at the end of the QRS complex (short RP tachycardia), whereas junctional rhythms show variable or absent P wave relationships 5

Differentiation from idioventricular rhythm:

• Idioventricular rhythms show wide QRS complexes (>120ms) with left bundle branch block pattern and slower rates (20-40 bpm for escape rhythm, 40-120 bpm for accelerated) 4
• Junctional rhythms display narrow QRS complexes unless pre-existing bundle branch block exists 4

Management Principles

• The mainstay of management is correcting the underlying abnormality rather than treating the rhythm itself 1
• For symptomatic cases, beta blockers are reasonable for both acute treatment and ongoing management 1
• Intravenous diltiazem, procainamide, or verapamil may be reasonable for acute treatment when beta blockers are ineffective 1
• Catheter ablation may be reasonable when medical therapy is ineffective or contraindicated for focal junctional tachycardia, but carries a 5-10% risk of AV block 1
• Junctional rhythm with rates >50 bpm may be physiologically normal and require no treatment if the patient is asymptomatic 1