AV Blocks: Types, Characteristics, and Management
Permanent pacing is the definitive treatment for symptomatic third-degree AV block and Mobitz type II second-degree AV block, while first-degree and Mobitz type I blocks often require only monitoring unless symptomatic. 1
Types of AV Blocks
First-Degree AV Block
- Defined as a prolonged PR interval >200 ms 1
- Conduction delay typically occurs at the AV node level, especially with narrow QRS complexes 1
- Generally benign and often requires no specific treatment 1
- May be caused by medications, electrolyte disturbances, or structural heart disease 1
- Only requires pacing if markedly prolonged (>300 ms) and causing symptoms due to improper timing of atrial systole 1
Second-Degree AV Block
Mobitz Type I (Wenckebach)
- Characterized by progressive PR interval prolongation until a P wave fails to conduct 1
- Usually associated with narrow QRS complexes 1
- Block typically occurs at the AV node level 1
- Often transient and asymptomatic 1
- Responds well to atropine in acute settings 1
- Permanent pacing generally not required unless symptomatic 1
Mobitz Type II
- Characterized by constant PR intervals before and after blocked P waves 1
- Usually associated with wide QRS complexes 1
- Block typically occurs below the AV node in the His-Purkinje system 1, 2
- Often symptomatic and may progress to complete heart block 1, 3
- Permanent pacing recommended even in asymptomatic patients 1
- Atropine often ineffective as treatment 1
2:1 AV Block
- Cannot be classified as Type I or Type II based on ECG appearance alone 1, 4
- Site of block may be nodal or infranodal 4, 3
- Management depends on QRS width, symptoms, and clinical context 1
Third-Degree (Complete) AV Block
- Complete absence of AV conduction with atrial and ventricular activity completely dissociated 1
- May occur at any anatomical level (AV node, His bundle, or bundle branches) 1
- Presents with slow ventricular escape rhythm 1
- Permanent pacing indicated for symptomatic patients (Class I) and most asymptomatic patients (Class IIa) 1
Management Strategies
Acute Management
- Assess hemodynamic stability and presence of symptoms 1
- For symptomatic bradycardia:
- Atropine 0.5 mg IV every 3-5 minutes (maximum 3 mg) as first-line for AV nodal blocks 1
- Avoid atropine in infranodal blocks (Mobitz II and third-degree with wide QRS) as it's likely ineffective 1
- Transcutaneous pacing for unstable patients not responding to medications 1
- Consider transvenous temporary pacing for persistent symptomatic bradycardia 1
Long-Term Management
First-degree AV block:
Mobitz Type I (Wenckebach):
Mobitz Type II:
Third-degree AV block:
Special Considerations
AV Block in Acute Myocardial Infarction
- Inferior MI: AV block often occurs at supra-Hisian level, is usually transient, and often responds to atropine 1, 5
- Anterior MI: AV block usually occurs at infra-Hisian level, carries worse prognosis, and often requires pacing 1
- Early AV block (within 6 hours) in inferior MI is often vagally mediated and resolves within 24 hours 5, 6
- Late AV block (>6 hours) in inferior MI is usually due to ischemic damage and has longer duration 5
Vagally Mediated AV Block
- Associated with slowing of sinus rate 6
- May mimic pathological AV block but is benign 6
- Managed as neurally mediated syncope if symptomatic 6
- No pacemaker indicated if asymptomatic 6
Neuromuscular Diseases with AV Block
- Permanent pacing recommended regardless of symptoms due to unpredictable progression of conduction disease 1
- Examples include myotonic muscular dystrophy and Kearns-Sayre syndrome 1