What is an atrioventricular block?

An atrioventricular (AV) block is a conduction problem between atria and ventricles. The sinoatrial node may fire normally. The atria may depolarise normally. But somewhere between atrium and ventricle, the impulse is delayed or fails to conduct.

Unlike atrial fibrillation or atrial flutter, the issue is not chaos. It is interruption.

Normal conduction (the baseline story)

Normal ECG logic is simple:

• Sinoatrial node fires
• P wave appears (atrial depolarisation)
• AV node delays conduction slightly (this is normal physiology)
• QRS appears (ventricular depolarisation)

The PR interval normally ranges from 120 to 200 milliseconds. When the PR interval lengthens beyond that, or when impulses fail to conduct, AV block begins.

First-degree AV block

Delayed — but delivered.

ECG criteria

• PR interval greater than 200 milliseconds
• Every P wave conducts to a QRS
• No dropped beats
• Rhythm typically regular

Clinical context

Often seen with increased vagal tone and with atrioventricular nodal blocking drugs (beta blockers, non-dihydropyridine calcium channel blockers, digoxin). It may also appear in inferior myocardial infarction. Most patients are asymptomatic.

P wave present before every QRS?
      ↓ Yes
PR interval greater than 200 ms?
      ↓ Yes
No dropped beats?
      ↓ Yes
→ First-degree AV block

Mobitz Type One (Wenckebach)

Progressive fatigue.

ECG criteria

• Progressive PR prolongation
• Followed by a dropped QRS
• Then the cycle repeats

Common settings

Young individuals with high vagal tone, sleep, inferior myocardial infarction, or medication effects. Many patients remain stable and asymptomatic.

PR interval changes beat to beat?
      ↓ Yes
Does PR progressively lengthen?
      ↓ Yes
Is there a dropped QRS after the progressive lengthening?
      ↓ Yes
→ Mobitz Type One (Wenckebach)

Mobitz Type Two

Sudden failure.

ECG criteria

• PR interval is constant (for the conducted beats)
• Dropped QRS complexes occur unexpectedly
• No progressive PR prolongation

Common causes

Anterior myocardial infarction, degenerative conduction disease, post-surgical injury, infiltrative disease. Symptoms may range from none to syncope depending on ventricular response and escape rhythm reliability.

PR interval fixed?
      ↓ Yes
Sudden dropped QRS?
      ↓ Yes
No progressive lengthening beforehand?
      ↓ Yes
→ Mobitz Type Two

Complete heart block (Third-degree AV block)

Electrical divorce.

ECG criteria

• P waves and QRS complexes are both present
• No consistent relationship between P waves and QRS complexes
• Atrioventricular dissociation with an escape rhythm

Are P waves present?
      ↓ Yes
Are QRS complexes present?
      ↓ Yes
Is there any consistent relationship between P and QRS?
      ↓ No
→ Complete (Third-degree) AV block

Clinical approach to atrioventricular block

Step 1 — assess stability first

Prioritise physiology. Unstable features include hypotension, altered mental status, ischaemic chest pain, and acute heart failure. If unstable, proceed to urgent bradycardia management and pacing strategies as appropriate.

Step 2 — classify the pattern

Use one practical rule: what is the PR doing and are beats dropped?

• First-degree: PR prolonged, no dropped beats
• Mobitz Type One: PR progressively prolongs, then drops
• Mobitz Type Two: PR fixed, sudden drops
• Complete block: P waves and QRS are independent

Management overview (first-line concepts)

• First-degree: usually observation; review drugs and reversible causes
• Mobitz Type One: observation unless symptomatic; treat reversible triggers
• Mobitz Type Two: treat as high risk; pacing is often required
• Complete heart block: pacing is typically required urgently

Perioperative considerations 🏥

In theatre and critical care, atrioventricular block may appear due to increased vagal tone, myocardial ischaemia, electrolyte disturbance, drug effects, hypoxia, and procedural stimulation. In this setting, a “new block” is often a physiologic alarm — the conduction system is revealing stress.

Summary table