Causes of Junctional Bradycardia
Junctional bradycardia results from either enhanced automaticity of an ectopic focus in the AV junction (causing junctional escape rhythm when the sinus rate falls below the junctional rate) or from underlying pathological conditions that suppress sinus node function, allowing the slower junctional pacemaker to emerge as the dominant rhythm. 1
Primary Mechanisms
Enhanced Automaticity
- The fundamental mechanism is enhanced abnormal automaticity from an ectopic focus in the AV node or His bundle, which produces a junctional rhythm when it exceeds the sinus rate or when sinus node function is impaired 1, 2
- The junctional escape mechanism originates specifically from the AV node or His bundle, producing narrow QRS complexes unless pre-existing bundle branch block is present 1
Sinus Node Suppression
- Junctional bradycardia emerges when the sinus node rate falls below the intrinsic junctional rate (typically 40-60 bpm), allowing the junctional pacemaker to become the dominant rhythm 1
Common Underlying Causes
Medication-Related Causes
- Digitalis toxicity is a major cause of nonparoxysmal junctional tachycardia and can manifest as junctional bradycardia 1, 2
- Beta-blockers are the most common drugs associated with drug-related bradycardia that can present with sinus bradycardia with junctional escape beats 3
- Non-dihydropyridine calcium channel blockers (diltiazem, verapamil) can cause profound bradycardia, especially when combined with beta-blockers 4
- The combination of beta-blockers and non-DHP CCBs may have more than additive effects in causing severe bradycardia 4
Cardiac Ischemia and Infarction
- Myocardial ischemia or infarction causes junctional rhythm due to altered automaticity in the conduction system 1, 2
- Acute myocardial injury can directly affect the AV junction, leading to enhanced automaticity 1
Electrolyte Abnormalities
- Hypokalemia is a significant contributor to junctional rhythm development 1, 2
- Hyperkalemia, particularly in combination with AV nodal blockers and renal failure (BRASH syndrome), can cause junctional bradycardia 5
Post-Cardiac Surgery
- Junctional rhythm is common after cardiac surgery, particularly in infants and children following congenital heart disease repair 1
- Post-operative inflammation and trauma to the conduction system can trigger junctional rhythms 1
Pulmonary Disease
- Chronic obstructive lung disease with hypoxia can trigger junctional rhythm 1, 2
- Hypoxia affects automaticity of the conduction system 1
Inflammatory Conditions
- Inflammatory myocarditis may lead to junctional rhythm due to inflammation near the conduction system 1, 2
Sinus Node Dysfunction
- When junctional rhythm results from sinus node dysfunction, patients may experience symptoms mimicking "pacemaker syndrome" due to loss of AV synchrony 1, 6
Clinical Context and Risk Factors
Patient-Specific Risk Factors
- Advanced age increases risk of drug-related bradycardia with junctional escape 4
- Renal or hepatic disease predicts marked bradycardia, particularly with medications 4
- Polypharmacy increases risk of drug-drug interactions causing bradycardia 4
Important Clinical Pitfall
- In approximately 25% of cases presenting with apparent drug-related bradycardia, the bradycardia persists despite drug withdrawal, indicating underlying intrinsic conduction system disease rather than purely drug-related etiology 3
- These patients require permanent pacemaker consideration rather than simple drug discontinuation 3
Diagnostic Approach
Initial Evaluation
- Determine if bradycardia is due to extrinsic (reversible) or intrinsic (permanent) causes 7
- Establish symptom-rhythm correlation through ECG documentation 7
- Evaluate for digitalis toxicity, electrolyte abnormalities (especially potassium), and myocardial ischemia 1, 2