What are the types of heart block?

Types of Heart Block

Heart block is classified into three main types: first-degree, second-degree, and third-degree (complete) heart block, with further subtypes based on anatomical location and conduction patterns. 1

First-Degree AV Block

• Definition: PR interval >200 ms with all P waves conducting to ventricles (1:1 conduction) 1
• Characteristics:
  • More accurately described as "AV delay" rather than block since all impulses are conducted 1
  • Usually benign, but extreme forms (PR >300 ms) can cause symptoms similar to pacemaker syndrome 1
  • Anatomically can be supra-His, intra-His, or infra-His 1

Second-Degree AV Block

Mobitz Type I (Wenckebach)

• Definition: Progressive prolongation of PR interval before a blocked beat, with shortening of PR interval after the blocked beat 1
• Characteristics:
  • Usually occurs at supra-His (AV node) level 1, 2
  • Generally benign prognosis when isolated 1
  • Narrow QRS complex is almost always AV nodal in origin 2
  • May be exacerbated by increased vagal tone 3

Mobitz Type II

• Definition: Sudden dropped beats with constant PR intervals before and after blocked beats 1
• Characteristics:
  • Almost always occurs at infra-His level 2
  • Usually associated with wide QRS complex 1
  • Higher risk of progression to complete heart block 4
  • Requires permanent pacing regardless of symptoms 2

2:1 AV Block

• Definition: Every other P wave conducts to ventricles 1
• Characteristics:
  • Cannot be classified as Type I or II based on ECG pattern alone 1, 2
  • Can be either nodal or infranodal in origin 2
  • QRS width helps determine likely site (narrow = likely nodal, wide = likely infranodal) 2

Advanced/High-Grade AV Block

• Definition: ≥2 consecutive P waves at normal rate not conducted to ventricles, with some AV conduction still present 5
• Characteristics:
  • Generally considered intra- or infra-Hisian in location 5
  • High risk of progression to complete heart block 5
  • Typically doesn't respond to atropine 5
  • Usually requires permanent pacing 5

Third-Degree (Complete) AV Block

• Definition: Complete absence of AV conduction with atrial and ventricular activity completely dissociated 1
• Characteristics:
  • Ventricular rhythm maintained by junctional or ventricular escape mechanism 5
  • Anatomically can be supra-His, intra-His, or infra-His 1
  • Associated with increased mortality, especially when symptomatic 1
  • Requires permanent pacing, particularly when symptomatic or associated with structural heart disease 1

Anatomical Classification

Heart blocks can also be classified based on anatomical location:

• Supra-His (AV nodal): Usually more benign, may respond to atropine
• Intra-His: Occurs within the His bundle
• Infra-His: Occurs below the His bundle, higher risk of progression, typically requires pacing 1

Clinical Pitfalls and Important Considerations

• A 2:1 AV block cannot be definitively classified as Mobitz I or II without additional testing 2
• Vagally-mediated AV block can mimic pathological AV block but is benign 5
• Isorhythmic dissociation can be confused with AV block when atrial and ventricular rates are similar 5
• Concealed His bundle or ventricular extrasystoles may mimic AV block (pseudo-AV block) 2
• In acute MI, the prognosis of AV block depends more on the extent of myocardial injury than on the block itself 1
• Infranodal block presenting with either Type I or II manifestations requires pacing regardless of QRS duration or symptoms 3

Understanding these classifications is crucial for appropriate management decisions, particularly regarding the need for temporary or permanent pacing to reduce morbidity and mortality in patients with heart block.