Atrioventricular block

Second-Degree AV Block

Mobitz Type I (Wenckebach)

• Pathophysiology: Progressive fatigue of the AV node.
• ECG Findings:
  • Progressive lengthening of the PR interval until a beat (QRS) is dropped.
  • “Going, going, gone.”
  • The PR interval after the dropped beat is shorter than the one before the drop.
  • R-R interval shortens as the PR interval lengthens.
• Location: Usually intranodal (AV node).
• Management:
  • Asymptomatic: Observation.
  • Symptomatic: Atropine, Isoproterenol.

Mobitz Type II

• Pathophysiology: Intermittent block, usually below the AV node (His-Purkinje system). Structural damage.
• ECG Findings:
  • Constant PR interval in conducted beats.
  • Intermittent dropped QRS complexes (e.g., 2:1 or 3:1 block).
  • QRS complexes are often wide (bundle branch block).
• Significance: High risk of progression to 3rd-degree block.
• Management:
  • Pacemaker is usually indicated.
  • Contraindicated: Atropine (can worsen block/conduction ratio in distal blocks).

Description

Single or intermittent nonconducted P waves without QRS complexes The PR interval remains constant. The conduction of atrial impulses to the ventricles typically follows a regular pattern, e.g.:

• 3:2 block: regular AV block with 3 atrial depolarizations but only 2 atrial impulses that reach the ventricles (heart rate = ⅔ SA node rate)
• 4:3 block: regular AV block with 4 atrial depolarizations but only 3 atrial impulses that reach the ventricles (heart rate = ¾ SA node rate) While 2:1 block follows a regular pattern, it cannot be classified as Mobitz type I or II and is classified separately (see “2:1 AV block”). Risk of progression to complete heart block: high (> 50%), as it is typically due to infranodal block (usually in the His-Purkinje system)

2:1 AV block 

• Description
  • Inhibited conduction of every second atrial depolarization (P wave) to the ventricles (heart rate = ½ SA node rate)
  • Cannot be classified as Mobitz I or Mobitz II as only one PR interval is observed before the subsequent dropped complex (can fit into both types)
  • Often a transient rhythm occurring on a baseline Mobitz I or Mobitz II rhythm
• Risk of progression to complete heart block: depends on level of block 
  • Block at the level of the AV node (more common): low
  • Infranodal block (less common): high

Ventriculophasic sinus arrhythmia

Sinus rate variation of this type with complete heart block is called ventriculophasic sinus arrhythmia.