°II Degree

• Description

  • Progressive lengthening of the PR interval until a beat is dropped, which means a regular atrial impulse does not reach the ventricles (a normal P wave is not followed by a QRS complex)

  • Mostly regular rhythm separated by short pauses, which may lead to bradycardia (regularly irregular rhythm)

  • Rate of SA node > heart rate

• Risk of progression to complete heart block: typically low, as the block is most often at the level of the AV node

• 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.: [2]

    • 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”). [2]

• 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 [2]

• 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

  • 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

• Description

  • A block in which ≥ 2 consecutive P waves do not generate a ventricular response, e.g., 3:1 block

  • As some P waves do generate a ventricular response, high-grade AV block differs from third-degree heart block in which there is complete dissociation.

• Risk of progression to complete heart block: typically high, but depends on duration and reversibility of block