What is a junctional rhythm in a patient with underlying heart disease, such as coronary artery disease or cardiomyopathy?

What is a Junctional Rhythm?

A junctional rhythm is an 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 70-130 bpm for nonparoxysmal forms, and often showing AV dissociation on ECG. 1, 2

Anatomical Origin and Mechanism

• The unifying feature of all junctional rhythms is their origin from either the AV node or the His bundle, distinguishing them from atrial or ventricular arrhythmias 2, 3
• The mechanism involves enhanced (abnormal) automaticity from an ectopic focus in the AV junction, rather than reentry 1, 4
• Each ventricular depolarization is preceded by a His bundle deflection, confirming the junctional origin 2

Clinical Types and Heart Rate Ranges

Junctional rhythms exist on a spectrum based on rate:

• Junctional escape rhythm: 40-60 bpm, serving as a backup pacemaker when higher pacemakers fail 2, 5
• Nonparoxysmal junctional tachycardia (accelerated junctional rhythm): 70-130 bpm, the most common form in adults 1, 2
• Focal (paroxysmal) junctional tachycardia: 110-250 bpm, rare in adults but can be serious 1, 2, 6

ECG Characteristics

Key diagnostic features include:

• Narrow QRS complexes (typically <120 ms) unless pre-existing bundle branch block is present 2, 5
• AV dissociation is often present, which when identified excludes AVRT and makes AVNRT highly unlikely 1, 2
• P waves may be absent, inverted (retrograde), or dissociated from the QRS complex 5
• When P waves are visible, they may occur before, during, or after the QRS complex 5
• Nonparoxysmal forms show characteristic "warm-up" and "cool-down" patterns that cannot be terminated by pacing maneuvers 2

Common Underlying Causes in Patients with Heart Disease

In patients with coronary artery disease or cardiomyopathy, junctional rhythm typically signals serious underlying pathology:

• Digoxin toxicity is the most common cause of nonparoxysmal junctional tachycardia 1, 2
• Acute myocardial infarction or ischemia frequently triggers junctional rhythms due to altered automaticity 1, 2, 7
• Electrolyte abnormalities, particularly hypokalemia, commonly precipitate junctional rhythms 2
• Post-cardiac surgery complications, especially in the first 72 hours 2, 6
• Inflammatory myocarditis affecting the conduction system 2
• Chronic obstructive lung disease with hypoxia 2

Clinical Significance in Structural Heart Disease

• Nonparoxysmal junctional rhythm may be a marker for serious underlying conditions requiring immediate investigation 2
• In the context of hypertrophic cardiomyopathy, junctional rhythm can induce left ventricular outflow tract gradients due to loss of effective atrial systole, potentially worsening hemodynamics 7
• In adults, junctional tachycardia typically has a relatively benign course if the underlying cause is addressed, unlike in infants where it can lead to heart failure 2, 6

Critical Diagnostic Pitfalls

• Do not confuse irregular junctional tachycardia with atrial fibrillation or multifocal atrial tachycardia, as the presence of AV dissociation distinguishes junctional rhythm 1, 2
• The presence of AV dissociation excludes AVRT and makes AVNRT highly unlikely, preventing misdiagnosis of reentrant tachycardias 1
• Always investigate for digoxin toxicity, acute MI, and electrolyte abnormalities before attributing the rhythm to a benign cause 2

Management Principles

Treatment centers on addressing the underlying condition rather than the rhythm itself:

• Withhold digitalis immediately if toxicity is suspected, using digitalis-binding agents only if ventricular arrhythmias or high-grade AV block develop 2
• Correct electrolyte abnormalities urgently, particularly hypokalemia 2
• Treat myocardial ischemia if present with standard acute coronary syndrome protocols 2
• For symptomatic focal junctional tachycardia, intravenous beta blockers are reasonable first-line therapy (Class IIa) 1, 2
• Catheter ablation may be reasonable when medical therapy fails, though it carries a 5-10% risk of AV block requiring permanent pacemaker 1, 2