Atrioventricular block

Atrioventricular block (AV block) is characterised by a partial or complete block of conduction from the atria to the ventricles, usually at the level of the AV node.

We distinguish multiple different types:

• 1st degree AV block – the conduction of atrial impulses to the ventricles is delayed
• 2nd degree AV block – some atrial impulses fail to reach the ventricles
  • Mobitz type I (Wenkebach)
  • Mobitz type II
• 3rd degree AV block – all atrial impulses fail to reach the ventricles

Etiology

There are multiple different types, but the major causes of all are similar:

• Idiopathic
• Myocarditis
• Hyperkalaemia
• Drugs slowing AV node conduction (beta blockers, digoxin, CCBs)
• Ischaemic heart disease
• Hypothyroidism
• Infiltrative heart diseases (amyloidosis, sarcoidosis)

1st degree AV block

1st degree AV block is characterised by a PQ/PR interval which is longer than normal (> 200 ms), but all atrial impulses reach the ventricles. It can occur in normal, healthy people.

1st degree AV block is almost always asymptomatic and harmless, not requiring treatment.

2nd degree AV block Mobitz type I

In Mobitz type I 2nd degree AV block the PQ interval is progressively longer with each beat, until a beat is dropped (a P wave is not followed by a QRS complex). It is also called Wenckebach block or Wenckebach pattern.

Like 1st degree AV block, this can occur in healthy people and is often asymptomatic.

2nd degree AV block Mobitz type II

In Mobitz type II 2nd degree AV block some atrial impulses (P waves) are suddenly not conducted to the ventricles (not followed by a QRS), often at a regular interval, like 3:2, 2:1, 4:3, etc. If more than one atrial impulse in a row is not conducted, it’s called “high grade AV block” and is more severe.

Most people with Mobitz type II are symptomatic. They experience typical symptoms of AV block, like fatigue, dyspnoea, presyncope, syncope, or even sudden cardiac arrest.

Mobitz type II can quickly progress to 3rd degree AV block, and so all patients must be continuously monitored. If the patient is haemodynamically unstable, they should be treated with 0,5 mg IV atropine and temporary cardiac pacing. Atropine increases AV conduction and may partially reverse the block.

While being monitored and once stabilised, reversible causes of AV block should be excluded and treated if present. If no reversible cause can be found and treated, the patient requires a permanent pacemaker as the risk of sudden cardiac death is high.

3rd degree AV block

In 3rd degree AV block (complete heart block), no atrial impulses are conducted to the ventricles. This causes an escape rhythm to kick in, originating somewhere more downstream from the site of the block. With the atria being paced by the SA node and the ventricles being paced by some other focus, the P waves and QRS waves will be independent of each other on the ECG.

3rd degree AV block is almost always symptomatic. The symptoms depend on the rate of the escape rhythm, and include fatigue, dyspnoea, presyncope, syncope, or even sudden cardiac arrest.

If the patient is haemodynamically unstable, they should be treated with atropine and temporary cardiac pacing. Once stabilised, reversible causes of AV block should be excluded and treated if present. If no reversible cause can be found and treated, the patient requires a permanent pacemaker.