• First-degree heart blocks are characterized by PR intervals longer than 0.20 second and all the P waves are followed by QRS complexes.
  
  
• Second-degree heart blocks are characterized by some P waves being blocked at the AV node. This results in some P waves occurring without following QRS complexes.
  
  
• Third-degree heart block is characterized by a complete dissociation between P waves and QRS complexes.

A hint for separating the heart blocks into degrees is that first- and third-degree blocks usually have regular QRS rates.

Atrioventricular block
Atrioventricular block is defined as a delay or interruption in conduction between the atria and ventricles. It may be due to (1) lesions along the conduction pathway (such as calcium, fibrosis, necrosis), (2) increases in the refractory period of a portion of the conduction pathway (such as may occur in the AV node when digitalis is administered), or (3) shortening of the supraventricular cycle length, i.e., rapid atrial rates, with encroachment on the normal refractory period (as with atrial flutter, in which 2:1 AV block at the level of the AV node occurs because the normal AV node refractory period will not allow conduction at a rate of 300 beats/min but will allow it at 150 beats/min).

AV block may be classified in two ways:
According to the degree of block:

• Partial blocks
  • First-degree AV block
  • Second-degree AV block (type I and type II), 2:1, and advanced (3:1 or greater)
• Third-degree or complete AV block

According to the site of block:

• AV node
• Infranodal
  • Bundle of His
  • Bundle branches

Each degree of block may occur either at the level of the AV node or below it. This distinction is not academic, since pathogenesis, treatment and prognosis differ.

This content is reviewed regularly. Last updated 12/5/08.