Than when AV nodes which can be blocked=atrio-ventricular block (AV heart block)=HB is a type of heart block in which the conduction between the atria and ventricles of the heart is impaired. Under normal conditions, the sinoatrial node (SA node) in the atria sets the intial pace for the heart, and these impulses travel down to the atriums than passing the AV valves into the ventricles and up the purkinje fibers . In an AV block, this message does not reach the ventricles or is impaired along the way. The ventricles of the heart have their own pacing mechanisms, which can maintain a lowered heart rate in the absence of SA stimulation.

The causes of pathological AV block are varied and include ischemia, infarction, fibrosis or drugs, and the blocks may be complete or may only impair the signaling between the SA and AV nodes.

Types of AV blocks: 1^st degree HB which is NSR with a prolonged PR interval and can live a totally normal life with it. PR interval is the time from beginning of the p wave to the beginning of the qrs wave.  In all degrees of HB the PRI interval is prolonged. 

2^nd degree HB-Mobitz I called Wenkebach is a NSR with a progressing increasing PR interval longer and longer till it actually drops a QRS and you see just the p wave without a QRS once and the rhythm restarts the pattern all over again.

Than 2^nd degree HB-Mobitz II and this is a NSR with a prolonged PRI that measures the same amount each time but even though its not progressively getting longer its worse than Mobitz I always.  This is because with the same measurement prolonged PRI interval you only see 2 or 3 or more  p waves without the QRS and these need immediate external subcutaneous pacing or just as effective if its not available (external subcutaneous pacing) than a dopamine IV drip 2-10 mcg/kg/min or epinephrine IV drip 2 to 10 mcg/min.

Than there is complete HB 3^rd degree heart block where no conduction from atriums to ventricles is going throught so the atriums are contracting the way they want and the ventricles contracting the way they want and total disassociation between the upper and lower chambers. Treatment is a most patients with acquired complete heart block will require a permanent pacemaker or an implantable cardioverter defibrillator (ICD).  Temporarily till OR try a subcutaneous pacing but may not work at all.