How do you differentiate between complete heart block and Wenckebach's (Mobitz type I second-degree atrioventricular (AV) block) on an electrocardiogram (ECG)?

Differentiating Complete Heart Block from Wenckebach on ECG

The key ECG difference between complete heart block and Wenckebach (Mobitz type I second-degree AV block) is that in complete heart block there is no conduction between atria and ventricles with complete AV dissociation, while in Wenckebach there is progressive PR prolongation before a dropped beat followed by a reset shorter PR interval.

Key Diagnostic Features

Complete Heart Block (Third-Degree AV Block)

• Complete dissociation between P waves and QRS complexes
• Regular P waves at normal sinus rate
• Regular QRS complexes at a slower escape rhythm (junctional or ventricular)
• No relationship between P waves and QRS complexes
• QRS morphology:
  • Narrow QRS (≤120 ms): Block likely in AV node or within His bundle
  • Wide QRS (>120 ms): Block likely below the His bundle 1

Wenckebach (Mobitz Type I Second-Degree AV Block)

• Progressive lengthening of PR intervals before a dropped beat (non-conducted P wave)
• First PR interval after the dropped beat is shorter than the last conducted PR interval
• Group beating pattern due to periodically dropped QRS complexes
• Usually narrow QRS complexes (block typically at AV node level)
• Often associated with a slowing sinus rate 1

Clinical Significance and Prognosis

Complete Heart Block

• Usually associated with either junctional or ventricular escape mechanism
• May be paroxysmal or persistent
• Often symptomatic (syncope, presyncope)
• Poor prognosis if untreated, especially if block is below His bundle
• Usually requires permanent pacing 1

Wenckebach

• Often asymptomatic, especially when occurring at rest
• Usually represents block at the AV node level
• Better prognosis than complete heart block
• Often seen in healthy, active patients
• May cause symptoms if frequent or occurring during exercise
• Usually more responsive to autonomic manipulation (atropine, isoproterenol) 1

Common Pitfalls in Differentiation

1. 2:1 AV Block Confusion: Cannot be classified as Mobitz I or II based on ECG pattern alone, as progressive PR prolongation cannot be observed with only one conducted beat 2

2. Pseudo-AV Block: Concealed His bundle or ventricular extrasystoles may mimic both Wenckebach and complete heart block 2

3. Isorhythmic Dissociation: When atrial and ventricular rates are similar, it may mimic AV block 1

4. Atrial Bigeminy: Repetitive premature atrial contractions may be mistaken for AV block 1

5. Wide QRS in Wenckebach: While typically associated with narrow QRS, Wenckebach can occasionally present with bundle branch block, making it harder to differentiate from complete heart block 1

Special Considerations

• In complete heart block with atrial fibrillation, the ventricular response is slow (<50 bpm) and regular 1

• Careful assessment of the relationship between P waves and QRS complexes is essential, particularly when the rates are similar 1

• Mobitz type I (Wenckebach) block is almost always at the level of the AV node, while complete heart block can occur at various levels of the conduction system 1

• Progression from Wenckebach to complete heart block is possible, especially in patients with underlying heart disease 3

By systematically analyzing the PR intervals, QRS morphology, and the relationship between P waves and QRS complexes, clinicians can accurately differentiate between these two important forms of heart block, which is crucial for appropriate management and prognostication.