Atrioventricular (AV) block is caused by one of the following mechanisms i.e. fibrosis or degeneration of the conduction system, ischemic heart disease, or medications.

• First-degree atrioventricular block
  • A delay in electrical heart conduction without significant interruption occurs from atria to ventricle.
  • This occurs at the level of atrioventricular node (AV node) or slightly below it.
  • In this situation, no hemodynamic instability happens.
• Second-degree atrioventricular block Mobitz type 1 (Wenckebach)
  • A block in the normal electrical conduction system occurs at the crest of AV node]as a result of failure of AV nodal cells to transmit the cardiac impulse to the ventricles.
  • This is related to a progressive fatigue of the cells and result is manifested as a dropped beat.
• Second-degree atrioventricular block Mobitz type 2
  • The electrical conduction system blockage occurs at the level below the AV node (His bundle, bundle branches, and heart fascicles.
• Third-degree atrioventricular block
  • There are two possible locations in this block with impulse conduction depending on the location of the block.
  • A narrow QRS complex with AV node intrinsic rate of 40 to 55 beats per minute is observed when the block is at the crest of AV node or above it.
  • A wide QRS complex with a ventricular pacemaker intrinsic rate of 20 to 40 beats per minute is observed when the block is below the AV node.^[1]

Cardiology