In third degree heart block there is complete failure of conduction through the AVN. Continuing ventricular activity depends upon the emergence of an escape rhythm.
If the block is in the AVN then the escape rhythm usually originates in the bundle of His and is fast enough to prevent symptoms. If however there is bundle damage the escape rhythm is generated lower down the conducting system resulting in a slow and unreliable heart beat and life- threatening asystoles.
Diagnostic criteria (1):
Atria and ventricles depolarize independently:
- P waves are regular in themselves and QRS waves are regular among themselves.
- Every P wave is not followed by a QRS complex.
- Some QRS complexes may coincidentally have preceeding P waves,
- Some QRS complexes may have P waves buried in them.
- Some P waves, buried in the QRS, will deform the QRS complex and may falsely give the impression of delta wave and therefore intermittent WPW syndrome, at first glance.
If the escape rhythm is junctional, then heart rate is 40-60/minute with narrow QRS complexes.
If the escape rhythm is idioventricular, then heart rate is 20-40/minute with wide QRS complexes.
Equalization of R-R intervals in a patient with atrial fibrillation suggests development of complete AV block.
In patients with atrial fibrillation who are already under Digoxin therapy, equalization of R-R intervals suggests development of complete AV block due to Digoxin intoxication.
Rarely, complete AV block may be congenital.
Reference:
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