Third Degree Heart Blocks (Complete AV Dissociation)
Third degree
blocks are characterized by a complete AV
nodal block resulting in no depolarization of the ventricles
(i.e. no ventricular contraction takes place). The electrical
signal from the SA node is blocked between the atria and
ventricles of the heart. This conduction dysfunction generally
occurs between the AV junction and the bundle of His. Therefore,
the ventricles must create their own impulse in order for
contraction to occur. Both the atria and ventricles function as
two separate units each with its own rate (atria, 60 bpm and
ventricles, 20-40 bpm).
This is a lethal dysrhythmia due to the fact that it can
evolve into ventricular standstill or 
asystole. Since the
independent firing rate of the ventricles is 20-40 bpm, perfusion
of the entire system will not be adequate enough to sustain life.
Causes of third degree heart block include Digitalis toxicity, MI
and massive heart disease. Patients with third degree heart
block usually need a pacemaker.
Third degree
blocks are characterized by a complete AV
nodal block resulting in no depolarization of the ventricles
(i.e. no ventricular contraction takes place). The electrical
signal from the SA node is blocked between the atria and
ventricles of the heart. This conduction dysfunction generally
occurs between the AV junction and the bundle of His. Therefore,
the ventricles must create their own impulse in order for
contraction to occur. Both the atria and ventricles function as
two separate units each with its own rate (atria, 60 bpm and
ventricles, 20-40 bpm). 
asystole