What are the ECG findings and causes of junctional rhythm?

Junctional Rhythm: ECG Findings and Causes

ECG Findings

Junctional rhythm on ECG is characterized by a QRS rate that is faster than the resting P-wave or sinus rate, typically <100 beats/min with a narrow QRS complex (unless baseline aberrancy exists), and may show AV dissociation or marked irregularity in the junctional rate. 1

Key Electrocardiographic Features

• P-wave characteristics: P waves may be absent, inverted (retrograde), or dissociated from the QRS complexes 1
• QRS morphology: Narrow QRS complexes (<120 ms) unless conducted with aberrancy 1
• Heart rate: Typically ranges from 40-60 beats/min for junctional escape rhythm, but can be faster (up to <100 beats/min) 1
• Rhythm regularity: Can be regular or irregular with marked variability in the junctional rate 1

P-Wave Relationships

When P waves are visible in junctional rhythms, they demonstrate specific patterns:

• Retrograde P waves: May appear as narrow negative deflections in inferior leads (II, III, aVF) 1, 2
• Timing: P waves may be buried within the QRS complex, appear immediately before, or immediately after the QRS 1, 2
• AV dissociation: The junctional pacemaker may fire independently of atrial activity, creating dissociation between P waves and QRS complexes 1

Distinguishing Features

Junctional rhythms can mimic the pattern of slow-fast AVNRT but are distinguished by:

• Variable conduction to the atria (unlike the fixed relationship in AVNRT) 1
• Marked irregularity in the junctional rate (less common in AVNRT) 1
• AV dissociation when present (not seen in AVNRT) 1

Causes of Junctional Rhythm

Physiological Causes

In highly trained athletes, junctional escape rhythm is considered a normal variant related to increased vagal tone, with heart rates ≥30 beats/min considered normal in the absence of symptoms. 1

• Athletic training: Enhanced parasympathetic tone suppresses the sinus node, allowing junctional escape 1
• Sinus node suppression: When the primary sinus pacemaker slows below the intrinsic junctional rate, escape beats occur 3

Medication-Induced Causes

• Calcium channel blockers: Verapamil can suppress sinus node function and induce junctional bradycardia, particularly when combined with other factors 4
• Beta-blockers: Excessive beta-blockade may slow the sinus node sufficiently to allow junctional escape 1
• Digoxin toxicity: Can enhance AV nodal automaticity and suppress sinus node function 4
• Electrolyte disturbances: Hypermagnesemia combined with AV nodal blocking agents (e.g., verapamil) can precipitate junctional rhythms 4

Pathological Causes

• Increased sympathetic activity: Catecholamine surge (from epinephrine administration or stress) can accelerate AV nodal pacemaker cells, creating an accelerated junctional rhythm 3
• Acute stress response: Panic attacks have been documented to transiently induce junctional escape rhythms through autonomic dysregulation 5
• Sinus node dysfunction: Intrinsic disease of the sinus node allows lower pacemakers to assume control 3
• Myocardial ischemia or infarction: Particularly involving the inferior wall, which affects sinus node blood supply 4

Clinical Context Considerations

The cause of junctional rhythm must be determined by clinical context—a junctional escape rhythm from sinus suppression requires different management than an accelerated junctional rhythm from enhanced automaticity. 3

• Escape rhythm: Results from failure of higher pacemakers (sinus node), typically with rates 40-60 bpm 3
• Accelerated junctional rhythm: Results from enhanced automaticity of the AV junction, typically with rates 60-100 bpm 3
• Transient vs. persistent: Transient junctional rhythms (as with panic attacks or anesthesia) may resolve spontaneously, while persistent rhythms require investigation for underlying pathology 5, 3

Common Pitfalls

• Misdiagnosis as AVNRT: Junctional tachycardia may mimic slow-fast AVNRT but shows AV dissociation and/or marked irregularity 1
• Overlooking medication interactions: Polypharmacy in elderly patients (e.g., calcium channel blockers plus magnesium supplements) can precipitate junctional rhythms 4
• Assuming pathology in athletes: Junctional escape rhythms are normal findings in highly trained athletes and should resolve with activity 1