An atrioventricular block (or AV block) involves the impairment of the conduction between the atria and ventricle

Ventricle (heart)

In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...

s of the heart

Heart

The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

.

The causes of pathological AV block are varied and include ischaemia, infarction, fibrosis or drugs. Certain AV blocks can also be found as normal variants, such as in athletes or children, and are benign. Strong vagal stimulation may also produce AV block. The cholinergic receptor types affected are the muscarinic receptors.

There are three types:

• First degree AV block - PR interval
  PR interval
  In electrocardiography, the PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. It is usually 120 to 200 ms long. On the usual 25 mm/s ECG tracing, this corresponds to 3 to 5 small boxes. The PR interval reflects the time the electrical impulse takes to...
  
   greater than 0.20sec.
• Second degree AV block - Type 1 (aka Mobitz 1, Wenckebach): Progressive prolongation of PR interval with dropped beats (the PR interval gets longer and longer; finally one beat drops) . Type 2 (aka Mobitz 2, Hay): PR interval remains unchanged prior to the P wave which suddenly fails to conduct to the ventricles.
• Third degree AV block - No association between P waves and QRS complexes.

External links

• http://www.umm.edu/imagepages/1429.htm
• http://emedicine.medscape.com/article/161919-overview