Causes of Junctional Bradycardia
Junctional bradycardia (nonparoxysmal junctional rhythm with rates 70-120 bpm) is most commonly caused by digitalis toxicity, myocardial ischemia/infarction, post-cardiac surgery, hypokalemia, chronic obstructive lung disease with hypoxia, and inflammatory myocarditis. 1, 2
Primary Etiologies
Medication-Related Causes
- Digitalis toxicity is the most important medication-related cause of nonparoxysmal junctional tachycardia and should be suspected first in any patient on digoxin presenting with junctional rhythm 1, 2, 3
- Digoxin can produce accelerated junctional (nodal) rhythm as a manifestation of toxicity, particularly in the presence of hypokalemia 3
- Calcium channel blockers (particularly verapamil) combined with other medications can precipitate junctional bradycardia, especially when combined with hypermagnesemia 4, 5
- Beta-blockers are a common extrinsic cause of bradyarrhythmias including junctional rhythm 5
Cardiac Conditions
- Myocardial ischemia or infarction causes junctional rhythm through altered automaticity of the AV junction 1, 2
- Post-cardiac surgery is a major cause, particularly within 72 hours after surgical repair of congenital heart defects, due to direct trauma, ischemic injury, or stretch injury to the AV conduction tissues 2, 6
- Inflammatory myocarditis can trigger junctional rhythm due to inflammation near the conduction system 1, 2
Electrolyte and Metabolic Abnormalities
- Hypokalemia is a critical electrolyte disturbance that contributes to junctional rhythm and potentiates digitalis toxicity 1, 2, 3
- Hypermagnesemia, particularly when combined with calcium channel blockers, can precipitate junctional bradycardia 4
Pulmonary Disease
- Chronic obstructive lung disease with hypoxia can trigger junctional rhythm through autonomic and metabolic effects on the AV junction 1, 2
Conduction System Disorders
- Sinus node dysfunction can result in junctional escape rhythm when the sinus rate falls below the intrinsic junctional rate 2, 7
- Genetic channelopathies (such as KCNQ1 mutations in Short QT syndrome) can manifest as junctional bradycardia, particularly in pediatric patients 7
Mechanism
The underlying mechanism is enhanced automaticity arising from an ectopic focus in the AV junction, or it may represent an escape rhythm when higher pacemakers fail 1, 2. The rhythm originates from the AV node or His bundle, with each ventricular depolarization preceded by a His bundle deflection 1, 2.
Critical Clinical Pitfall
The most important feature about nonparoxysmal junctional tachycardia is that it may be a marker for a serious underlying condition requiring immediate attention 1. The rhythm itself is typically benign, showing characteristic "warm-up" and "cool-down" patterns that cannot be terminated by pacing maneuvers 1, 2. However, failure to identify and correct the underlying cause—particularly digitalis toxicity, acute myocardial ischemia, or severe electrolyte abnormalities—can lead to life-threatening complications 1, 2, 3.