What Causes Junctional Rhythm
Junctional rhythm is primarily caused by enhanced automaticity from an ectopic focus in the AV junction, most commonly triggered by serious underlying conditions including digitalis toxicity, myocardial ischemia/infarction, hypokalemia, post-cardiac surgery complications, chronic obstructive lung disease with hypoxia, and inflammatory myocarditis. 1
Fundamental Mechanism
The unifying mechanism across all junctional rhythms is enhanced abnormal automaticity originating from the AV node or His bundle, which takes over as the dominant pacemaker when it fires faster than the sinus node or when sinus node function is impaired. 1, 2 Research confirms that each ventricular depolarization in junctional rhythm is preceded by a His bundle deflection, localizing the origin to the proximal His bundle at its junction with the AV node rather than the AV node itself. 3
Major Underlying Causes
Most Common Etiologies in Adults
Digitalis toxicity is the most common cause of nonparoxysmal junctional tachycardia and should be suspected immediately in any patient on digoxin presenting with junctional rhythm. 1, 2 The FDA label confirms that digoxin overdosage commonly manifests as cardiac arrhythmias including junctional rhythms, particularly when combined with hypokalemia. 4
Myocardial ischemia or infarction causes altered automaticity in the AV junction, making this a critical diagnosis to exclude. 1, 2
Hypokalemia and other electrolyte abnormalities are major precipitants, particularly when combined with digitalis therapy. 1, 2, 4
Post-cardiac surgery complications represent a distinct entity where direct trauma, ischemic injury, or stretch injury to the AV conduction tissues during surgical repair triggers junctional rhythm, typically within 72 hours postoperatively. 1, 5
Less Common but Important Causes
Chronic obstructive lung disease with hypoxia can trigger junctional automaticity through metabolic stress on the conduction system. 1, 2
Inflammatory myocarditis may lead to junctional rhythm when inflammation affects tissues near the conduction system. 1
Sinus node dysfunction can allow the junctional escape mechanism to emerge as the dominant pacemaker, producing rates of 40-60 bpm. 1
Clinical Context: Two Distinct Presentations
Nonparoxysmal Junctional Tachycardia (70-120 bpm)
This is the more common form in adults and typically represents a marker for serious underlying pathology rather than a primary arrhythmia. 1 It shows characteristic "warm-up" and "cool-down" patterns and cannot be terminated by pacing maneuvers, distinguishing it from reentrant mechanisms. 1 The American College of Cardiology emphasizes that the mainstay of management is correcting the underlying abnormality rather than treating the rhythm itself. 1
Focal Junctional Tachycardia (110-250 bpm)
This is a very uncommon arrhythmia that is rare even in pediatric populations and even less common in adults. 1 It results from enhanced automaticity and may be exercise or stress-related, potentially leading to heart failure if incessant and untreated. 1, 2
Important Clinical Pitfall
Do not confuse the mechanism of junctional rhythm during radiofrequency ablation procedures with pathologic junctional rhythms. Research demonstrates that junctional rhythm observed during slow pathway ablation for AVNRT results from enhanced automaticity at multiple sites in the AV nodal transitional zone due to thermal injury, not from the underlying disease processes listed above. 6, 7 This procedural junctional rhythm is a transient phenomenon and should not be conflated with the pathologic junctional rhythms caused by the conditions described in this answer.