How do you differentiate between idioventricular and junctional rhythms on an electrocardiogram (ECG)?

Differentiating Idioventricular and Junctional Rhythms on ECG

The key differences between idioventricular and junctional rhythms on ECG are QRS morphology, heart rate, and origin of the rhythm - with idioventricular rhythms showing wide QRS complexes (>120ms) with left bundle branch block pattern and slower rates (20-40 bpm for escape rhythm, 40-120 bpm for accelerated), while junctional rhythms display narrow QRS complexes with rates of 40-60 bpm for escape rhythm or 70-120 bpm for nonparoxysmal junctional tachycardia. 1, 2

Key Distinguishing Features

QRS Morphology

• Junctional rhythm: Narrow QRS complex (unless there's a pre-existing bundle branch block) 2
• Idioventricular rhythm: Wide QRS complex (>120ms) with typical left bundle branch block morphology 3

Heart Rate

• Junctional rhythm:
  • Junctional escape rhythm: 40-60 bpm 1
  • Nonparoxysmal junctional tachycardia: 70-120 bpm 2
  • Focal junctional tachycardia: 110-250 bpm 2
• Idioventricular rhythm:
  • Ventricular escape rhythm: 20-40 bpm 1
  • Accelerated idioventricular rhythm (AIVR): 40-120 bpm 3

Origin of Rhythm

• Junctional rhythm: Originates from the AV node or His bundle 2
• Idioventricular rhythm: Originates from ventricular tissue (Purkinje fibers or ventricular myocardium) or right bundle branch 3

Additional Diagnostic Features

P Wave Relationship

• Junctional rhythm:
  • P waves may be absent, inverted (retrograde), or dissociated from QRS 2
  • When present, P waves may occur before, during, or after the QRS complex 1
• Idioventricular rhythm:
  • Complete AV dissociation with independent P waves 3
  • Often shows isorhythmic AV dissociation (similar atrial and ventricular rates) 3

Response to Interventions

• Junctional rhythm:
  • May respond to atropine or sympathomimetics 2
  • Often related to digoxin toxicity, which should be considered 4
• Idioventricular rhythm:
  • Often responds to beta-blockers (particularly metoprolol) 3
  • May be accelerated by exercise, stress, or isoproterenol 3

Clinical Context Considerations

Common Causes

• Junctional rhythm:
  • Digoxin toxicity 2, 4
  • Myocardial ischemia/infarction 2
  • Electrolyte abnormalities (especially hypokalemia) 2
  • Medication effects (verapamil, diltiazem) 5
  • Post-cardiac surgery 2
• Idioventricular rhythm:
  • Reperfusion after myocardial infarction 1
  • Idiopathic (particularly in young patients) 3
  • Cardiomyopathy 3

Diagnostic Pitfalls

• Atrial premature contractions can mimic junctional rhythm if blocked or conducted with aberrancy 6
• Concealed junctional rhythms may be present but electrocardiographically silent, requiring His bundle recordings for diagnosis 4
• Idioventricular rhythm can be confused with ventricular tachycardia, especially when rates are higher 3

Management Implications

• Correct identification is crucial as management differs:
  • For junctional rhythm: Address underlying cause (e.g., stop digoxin if toxicity suspected, correct electrolyte abnormalities) 2
  • For idioventricular rhythm: Beta-blockers may be effective; catheter ablation can be considered for symptomatic cases 3

Algorithmic Approach to Differentiation

1. Assess QRS width:
   • Narrow (<120ms): Likely junctional 2
   • Wide (>120ms): Likely idioventricular 3
2. Determine heart rate:
   • Very slow (20-40 bpm): Likely ventricular escape 1
   • 40-60 bpm: Could be junctional escape or slow AIVR 1, 3
   • 60-120 bpm: Could be nonparoxysmal junctional tachycardia or AIVR 2, 3

   • 120 bpm: Likely focal junctional tachycardia or ventricular tachycardia 2

3. Analyze P-QRS relationship:
   • Retrograde P waves: Suggests junctional origin 2
   • Complete AV dissociation: Favors idioventricular rhythm 3
4. Consider clinical context:
   • Recent MI with reperfusion: Suggests AIVR 1
   • Digoxin use or electrolyte abnormalities: Suggests junctional rhythm 2, 4