Atrioventricular (AV) block

Types of AV block:

There are 3 forms:

• 1^st degree block
• 2^nd degree (partial) block
• 3^rd degree (complete) block

First-degree block:

• Is simple prolongation of the PR interval to >0.22s
• Every atrial depolarisation is followed by conduction to the ventricles, but with delay

Second-degree block:

This occurs when some P waves conduct and some do not. There are 3 forms of 2^nd degree block:

• Mobitz type I block (Wenkebach block phenomenon):
• Progressive PR interval prolongation until a P wave fails to conduct
• The PR interval before the blocked P wave is much longer than the PR interval after the blocked P wave
• Mobitz type II block :
• Occurs when a dropped QRS complex is not preceded by progressive PR interval prolongation
• 2:1 or 3:1 (advanced) block:
• Occurs when every 2^nd or 3^rd P wave conducts to the ventricles

Third degree block:

This occurs when no P waves conduct to the ventricles. In this situation, life is maintained by a spontaneous escape rhythm that has either broad (>0.12s) or narrow (<0.12s) QRS complexes

Narrow complex escape rhythm:

• The escape rhythm occurs with an adequate rate (50-60bpm) and is relatively reliable
• May be caused by:
• Congenital heart disease
• MI
• Diphtheria
• Rheumatic fever
• Drug toxicity (e.g. digitalis, verapamil, ß-blockers)
• Endocarditis

• Treatment is often unnecessary (except for eradication of toxic causes)

Broad complex escape rhythm:

• The resulting rhythm is slow (15-40bpm) and relatively unreliable
• Dizziness and blackouts (Stokes-Adams attacks) often occur
• A permanent pacemaker should always be inserted as the mortality from this condition, even when asymptomatic, is considerably reduced by pacing