How can I differentiate complete (third-degree) atrioventricular block from Mobitz I (Wenckebach) second-degree atrioventricular block on ECG and clinically?

Differentiating Complete Heart Block from Wenckebach (Mobitz I) on ECG

Complete (third-degree) heart block shows no relationship between P waves and QRS complexes with completely independent atrial and ventricular rhythms, whereas Wenckebach demonstrates progressive PR interval prolongation before a dropped QRS complex with maintained P-wave-to-QRS association in conducted beats. 1

Key ECG Distinguishing Features

Complete Heart Block (Third-Degree AV Block)

• No atrial impulses reach the ventricles—there is complete loss of AV conduction with P waves and QRS complexes occurring independently (AV dissociation). 1
• The atrial rate is faster than the ventricular rate, and P waves "march through" the QRS complexes with no consistent PR interval. 1
• The ventricular rate depends on the escape pacemaker location: junctional escape (40-60 bpm with narrow QRS) or ventricular escape (20-40 bpm with wide QRS). 2
• A narrow QRS escape rhythm suggests AV-nodal-level block with a more stable junctional escape, while a wide QRS indicates infranodal (His-Purkinje) block with an unreliable ventricular escape. 1

Wenckebach (Mobitz I Second-Degree AV Block)

• Progressive lengthening of the PR interval occurs before a P wave fails to conduct, creating a "dropped" QRS complex and group beating pattern. 1
• After the dropped beat, the PR interval resets to a shorter duration and the cycle repeats. 1
• The QRS complex is typically narrow because the block occurs at the AV node level. 1, 3
• The ratio of P waves to QRS complexes is greater than 1:1 (e.g., 3:2,4:3), but some atrial beats are conducted. 1

Clinical Differentiation

Hemodynamic Stability and Symptoms

• Complete heart block, especially with infranodal block and wide QRS escape, can progress rapidly to asystole and requires immediate intervention. 1, 2
• Patients with third-degree block may present with syncope, presyncope, hypotension, heart failure, or sudden death. 1, 2
• Wenckebach is often asymptomatic and commonly seen in healthy athletes, during sleep, or with increased vagal tone. 1, 3
• Wenckebach has a benign prognosis and rarely progresses to complete block. 1

Response to Interventions

• Atropine may improve AV-nodal-level blocks (including Wenckebach) but is completely ineffective for infranodal complete heart block. 1
• Complete heart block with infranodal escape will not respond to atropine but may sometimes improve with catecholamines. 1
• Wenckebach associated with AV-nodal disease responds to autonomic manipulation, whereas infranodal blocks do not. 1

Anatomic Location and Prognosis

Complete Heart Block

• Infranodal (His-Purkinje) complete block is associated with a slower, unpredictable ventricular escape mechanism and may progress rapidly and unexpectedly. 1
• AV-nodal complete block has a more reliable junctional escape rhythm but still requires monitoring on an individual basis. 1
• Complete heart block requires continuous arrhythmia monitoring until pacemaker implantation. 1

Wenckebach

• The block occurs at the AV node level and is associated with a faster, more reliable escape mechanism. 1
• Generally does not require in-hospital monitoring unless occurring frequently or during exercise. 1
• Monitoring may be considered on an individual basis but is generally not required for asymptomatic Wenckebach. 1

Critical Pitfalls to Avoid

• Do not mistake 2:1 AV block for either Wenckebach or Mobitz II—it cannot be classified without observing consecutive beats to assess PR interval behavior. 1
• Do not assume a narrow QRS escape rhythm in complete heart block is benign—it still represents complete AV dissociation requiring pacemaker evaluation. 1
• Do not delay transcutaneous pacing in hemodynamically unstable complete heart block to administer atropine, especially if the QRS is wide. 4
• In atrial fibrillation with a slow (<50 bpm) and regular ventricular response, complete AV block may be present even though P waves are not visible. 1
• Avoid misdiagnosing atrial bigeminy with blocked premature atrial contractions as true AV block. 1

Algorithmic Approach to Differentiation

1. Assess P-wave-to-QRS relationship:

   • No relationship (independent rhythms) → Complete heart block 1
   • Progressive PR prolongation before dropped beat → Wenckebach 1

2. Evaluate QRS width:

   • Narrow QRS in complete block → AV-nodal level (more stable) 1
   • Wide QRS in complete block → Infranodal (high risk, requires urgent pacing) 1, 2
   • Narrow QRS in Wenckebach → Typical AV-nodal block 1

3. Check hemodynamic status:

   • Unstable with complete block → Immediate transcutaneous pacing 4
   • Stable with Wenckebach → Observation, identify reversible causes 1, 3

4. Determine need for permanent pacing:

   • Complete heart block at any level → Class I indication for permanent pacemaker 4
   • Wenckebach → Rarely requires pacing unless symptomatic during exercise 1