How do you differentiate and manage a dropped beat versus a second-degree atrioventricular (AV) block?

Differentiating and Managing Dropped Beat vs. Second-Degree AV Block

Mobitz type II second-degree AV block requires immediate pacemaker implantation regardless of symptoms due to high risk of progression to complete heart block, while a dropped beat due to concealed junctional extrasystoles (pseudo-AV block) requires no intervention. 1

Diagnostic Differentiation

Second-Degree AV Block

Second-degree AV block is characterized by intermittent failure to conduct atrial impulses to the ventricles and is classified into two main types:

1. Mobitz Type I (Wenckebach):

   • Progressive PR interval prolongation before the blocked P wave
   • PR interval shortens after the blocked P wave
   • Usually occurs in the AV node
   • Generally benign prognosis, especially with narrow QRS
   • Often associated with inferior MI or increased vagal tone 1

2. Mobitz Type II:

   • Constant PR intervals before and after the blocked P wave
   • No progressive PR prolongation
   • Usually occurs below the AV node (infranodal)
   • Associated with poor prognosis and high risk of progression to complete heart block
   • Often associated with wide QRS complexes 1, 2

Pseudo-AV Block (Dropped Beat)

• Caused by concealed junctional or His bundle extrasystoles
• Mimics second-degree AV block but is not true conduction block
• Extrasystoles are confined to specialized conduction system without ventricular depolarization
• Results in apparent "blocked" P waves 3, 4

Key Diagnostic Features to Differentiate

1. ECG Pattern Analysis:

   • Look for subtle extrasystoles that might be visible in some leads
   • Examine multiple ECG leads and rhythm strips
   • Check for consistent timing of the "dropped beats" - concealed extrasystoles often occur in bigeminal pattern 4

2. PR Interval Assessment:

   • In true Mobitz II, PR intervals remain constant before and after block
   • In pseudo-AV block, PR intervals may vary slightly 2, 5

3. QRS Morphology:

   • Wide QRS suggests infranodal block (favors Mobitz II)
   • Narrow QRS with apparent Mobitz II pattern is rare and should raise suspicion for pseudo-AV block 3

4. Response to Autonomic Maneuvers:

   • Vagal maneuvers may worsen AV nodal block (Mobitz I)
   • Minimal effect on infranodal block (Mobitz II)
   • May suppress junctional ectopy in pseudo-AV block 2

5. Holter Monitoring:

   • Look for coexistence of obvious Wenckebach patterns elsewhere in recording
   • True Mobitz II rarely coexists with Mobitz I in the same patient 3

Management Algorithm

For Suspected Second-Degree AV Block:

1. Confirm diagnosis:

   • 12-lead ECG to assess QRS morphology and duration
   • 24-48 hour Holter monitoring to determine frequency and pattern 6
   • Electrophysiological study if diagnosis remains uncertain 1

2. If Mobitz Type II confirmed:

   • Permanent pacemaker implantation is indicated regardless of symptoms (Class I recommendation) 1
   • Dual-chamber pacing (DDD) is preferred to maintain AV synchrony 6

3. If Mobitz Type I confirmed:

   • Asymptomatic with narrow QRS: Observation with regular follow-up
   • Symptomatic or wide QRS: Consider permanent pacing (Class IIa recommendation) 1
   • Evaluate for reversible causes (drug toxicity, ischemia) 1, 6

For Pseudo-AV Block (Dropped Beat):

1. No specific intervention required if asymptomatic
2. Treat underlying causes if identified:
   • Discontinue offending medications
   • Address electrolyte abnormalities
   • Manage ischemia if present 7
3. Follow-up with periodic ECG monitoring to ensure stability

Critical Pitfalls to Avoid

1. Misdiagnosing 2:1 AV Block: Cannot be classified as Mobitz I or II by definition - requires additional diagnostic evaluation 3, 2

2. Overlooking Vagal Influence: Vagal surge can cause simultaneous sinus slowing and AV nodal block that resembles Mobitz II 5

3. Failing to Recognize Pseudo-AV Block: Concealed junctional extrasystoles can perfectly mimic AV block and lead to unnecessary pacemaker implantation 4

4. Misinterpreting Atypical Wenckebach: Subtle PR prolongation may be missed, leading to incorrect diagnosis of Mobitz II 2

5. Ignoring QRS Width: Wide QRS with second-degree AV block strongly suggests infranodal disease requiring pacing 1

By carefully analyzing the ECG characteristics and clinical context, clinicians can accurately differentiate between true second-degree AV block and pseudo-AV block, ensuring appropriate management decisions that directly impact patient morbidity and mortality.