3rd degree (complete heart block)
Severe bradycardia due to absence of AV conduction
The ECG shows complete AV dissociation, with independent atrial and ventricular rates
Pathophysiology
Complete heart block is essentially the end point of either Mobitz I or Mobitz II AV block
It may be due to progressive fatigue of AV nodal cells as per Mobitz I (e.g. secondary to increased vagal tone in the acute phase of an inferior MI)
Alternatively, it may be due to sudden onset of complete conduction failure throughout the His-Purkinje system, as per Mobitz II.
This can be secondary to septal infarction in acute anterior MI, or as a result of progression of conducting system disease causing true trifascicular block
The former is more likely to respond to atropine and has a better overall prognosis
Causes
The causes are the same as for Mobitz I and Mobitz II second degree heart block.
The most important aetiologies are:
Inferior myocardial infarction
AV-nodal blocking drugs (e.g. calcium-channel blockers, beta-blockers, digoxin)
Idiopathic degeneration of the conducting system (Lenegre’s or Lev’s disease), causing true trifascicular block
Clinical significance
Patients with third degree heart block are at high risk of ventricular standstill and sudden cardiac death
They require urgent admission for cardiac monitoring, backup temporary pacing and usually insertion of a permanent pacemaker
Differential diagnosis
Complete heart block should not be confused with:
High grade AV block: A type of severe second degree heart block with a very slow ventricular rate but still some evidence of occasional AV conduction
AV dissociation: This term indicates only the occurrence of independent atrial and ventricular contractions and may be caused by entities other than complete heart block (e.g. “interference-dissociation” due to the presence of a ventricular rhythm such as AIVR or VT)
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