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, 2

First-Degree AV Block

• Definition: PR interval >200 ms with all P waves conducting to ventricles (1:1 conduction) 1
• More accurately described as "AV delay" rather than block since all impulses are conducted 2
• Anatomical location:
  • If QRS complex is narrow, conduction delay is usually in the AV node
  • If QRS is wide, delay may be either in AV node or His-Purkinje system 1
• Clinical significance:
  • Usually benign, but extreme forms (PR >300 ms) can cause symptoms similar to pacemaker syndrome 2, 3
  • Pacing not recommended unless PR interval fails to adapt during exercise and is long enough (usually >300 ms) to cause symptoms 1

Second-Degree AV Block

Type I (Mobitz I or Wenckebach)

• Definition: Progressive prolongation of PR interval until an atrial stimulus fails to be conducted, with shortening of PR interval after the blocked beat 1
• ECG features:
  • Progressively increasing PR interval until a P wave is not conducted
  • Often subtle increase in PR interval in the last cycles before the blocked P wave 1
• Anatomical location: Usually in the AV node; deterioration to higher degree block uncommon 1
• Clinical significance: Generally benign when isolated and occurring at the AV node level 4, 5

Type II (Mobitz II)

• Definition: Constant PR interval before and after blocked P waves with normal sinus rhythm 1
• ECG features:
  • Constant PR interval before and after blocked P waves
  • No visible changes in AV conduction time before and after blocked impulse 4, 5
• Anatomical location: Usually in the His-Purkinje system, especially with wide QRS 1
• Clinical significance:
  • Higher risk of progression to complete heart block 1
  • All correctly defined type II blocks are infranodal 4, 5
  • Pacing recommended, especially with wide QRS 1

2:1 AV Block

• Definition: Every other P wave conducts to the ventricles 1
• Clinical note: Cannot be definitively classified as Mobitz I or II without additional testing 4, 5
• Can be nodal or infranodal in origin 5

Advanced/High-Grade AV Block

• Definition: ≥2 consecutive P waves at constant physiologic rate that do not conduct to ventricles, with some evidence of AV conduction 1

Third-Degree (Complete) AV Block

• Definition: No atrial impulses conducted to ventricles; complete dissociation between atrial and ventricular activity 1
• ECG features: Ventricles depolarized by an escape rhythm 1
• Clinical significance:
  • Associated with increased mortality, especially when symptomatic 2
  • Permanent pacing improves survival, especially in patients with syncope 1

Anatomical Classification

Heart blocks can also be classified based on anatomical location:

1. Supra-Hisian (AV nodal): Usually associated with narrow QRS and better prognosis 1
2. Intra-Hisian: Within the His bundle 1
3. Infra-Hisian: Below the His bundle; higher risk of progression, typically requiring pacing 1, 2

Important Clinical Considerations

• QRS width provides clues to block location: narrow QRS type I block is almost always AV nodal, whereas type I block with bundle branch block is infranodal in 60-70% of cases 4, 5
• Infranodal blocks require pacing regardless of form or symptoms 5
• Vagally mediated AV block can mimic type II block but is generally benign 4
• Concealed His bundle or ventricular extrasystoles may mimic both type I and type II block (pseudo-AV block) 4

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