What is the definition of high-grade Atrioventricular (AV) block?

Definition of High-Grade AV Block

High-grade (also called high-degree or advanced) atrioventricular block refers to a situation where two or more consecutive P waves at a normal physiologic rate are not conducted to the ventricles, with evidence that some atrioventricular conduction is still present. 1

Distinguishing Features of High-Grade AV Block

High-grade AV block is distinct from other forms of AV block in several important ways:

• Consecutive non-conducted P waves: The hallmark feature is ≥2 consecutive P waves that fail to conduct to the ventricles
• Some conduction remains: Unlike complete (third-degree) AV block, there is still evidence of some intermittent AV conduction
• Normal atrial rate: The P waves occur at a constant physiologic rate (<100 bpm)
• Clinical significance: Generally considered to be intra- or infra-Hisian in location 1

Comparison with Other Types of AV Block

To understand high-grade AV block, it's important to compare it with other forms of AV block:

1. First-degree AV block:

   • PR interval >200 ms
   • All P waves conduct to ventricles (no dropped beats)
   • More accurately described as "AV delay" rather than block 1

2. Second-degree AV block:

   • Mobitz type I (Wenckebach): Progressive PR prolongation before a dropped beat, with inconstant PR intervals
   • Mobitz type II: Sudden dropped beats with constant PR intervals before and after
   • 2:1 AV block: Every other P wave conducts (cannot be classified as Mobitz I or II) 1

3. Third-degree (complete) AV block:

   • No AV conduction at all
   • Complete dissociation between atria and ventricles
   • Ventricular rhythm maintained by junctional or ventricular escape mechanism 1

Clinical Implications

High-grade AV block has significant clinical implications:

• Anatomical location: Generally considered to be intra- or infra-Hisian (below the AV node) 1
• Response to interventions: Unlike AV nodal block, high-grade AV block typically will not respond to atropine but may sometimes improve with catecholamines 1
• Escape mechanism: Associated with a slower and more unpredictable ventricular escape rhythm compared to AV nodal block 1
• Progression risk: May progress rapidly and unexpectedly to complete heart block 1
• Treatment: Generally treated with permanent pacing due to risk of progression and poor escape mechanism 1

Common Pitfalls in Diagnosis

When evaluating for high-grade AV block, be aware of these potential diagnostic pitfalls:

• Isorhythmic dissociation: May be confused with AV block when atrial and ventricular rates are similar 1
• Atrial bigeminy: Repetitive premature atrial contractions can mimic AV block 1
• Vagally-mediated AV block: In unusual circumstances (especially at night with sinus slowing), high-degree AV block with a narrow QRS may have a vagal etiology 1
• 2:1 AV block: Cannot be classified as Mobitz I or II, requires careful evaluation to determine the level of block 1

High-grade AV block is a significant conduction abnormality that typically requires permanent pacing, especially when symptomatic or associated with structural heart disease, due to its high risk of progression to complete heart block and associated morbidity and mortality 1.