ECG Topic - 3rd Degree Atrioventricular (AV) Block

Last Updated November 20, 2025

Introduction

Third-degree AV block, also called complete heart block, occurs when no atrial impulses conduct to the ventricles. The atria follow the sinus node rhythm, and the ventricles rely on an escape pacemaker below the site of block. Because the atria and ventricles are depolarizing independently, the hallmark ECG finding is AV dissociation with no conducted P waves and a ventricular escape rhythm that is slower than the atrial rate.

A patient with complete AV block may be awake and talking — or they may collapse without warning. In the ED, your job is to distinguish between:

• A junctional escape that might be temporarily stable
• A ventricular escape that signals imminent cardiovascular collapse.

Understanding the ECG patterns, escape foci, and mimics will help you rapidly diagnose and act before the blood pressure diagnoses it for you.

Electrophysiology

In complete heart block, the AV node or His–Purkinje system fails completely, preventing any atrial impulses from reaching the ventricles.

Two independent rhythms emerge:

• Atrial rhythm (P–P interval): Sinus node continues firing regularly.
• Ventricular rhythm (R–R interval): Escape pacemaker (junctional or ventricular) takes over

From Garcia Arrhythmia Recognition 2019

ECG: 3rd degree AV Block

Diagnostic Framework

1. Identify atrial rhythm
• Find P waves in multiple leads.
• Confirm regular P–P interval.
• Confirm atrial rate is faster than the ventricular rate.
• P waves may be found before QRS, inside QRS, riding on ST segment, or buried in T wave

2. Identify ventricular rhythm
• QRS complexes have their own regular rhythm.
• Ventricular rate is slower than atrial rate.
• QRS morphology reveals the escape site:
• Narrow = junctional escape.
• Wide = ventricular escape.
• If the ventricular rhythm is faster than the P–P rhythm → technically NOT complete heart block (this is competitive AV dissociation).

Junctional



From Garcia 2019

Ventricular


3. Look for any conducted P waves (critical step)
• In true 3rd degree AV Block
• No P wave reliably conducts.
• PR intervals are variable, constantly changing.
• No pattern repeats.
• No group-beating pattern
• No fixed PR interval on conducted beats
• If you find one P wave that truly conducts → consider alternative diagnoses (see following sections)
• High-grade AV block
• 2:1 AV Block
• Mobitz I or II

ECG Strips: AV node dissociation - Arrows signify hidden P waves



From Garcia 2019



Regular P–P + regular R–R at different rates + zero conducted P waves = COMPLETE AV BLOCK.

Escape Rhythms

Junctional Escape Rhythm (AV Nodal Block)

ECG findings

• Rate: 40–60 bpm
• QRS: Narrow or mildly wide (<120 ms)
• P waves: march independently, sometimes buried in QRS
• PR intervals: variable

Pearls

• More common with inferior MI (RCA), vagal tone, drug toxicity (BB, CCB, digoxin).
• Can be transient, sometimes resolves with reperfusion or withdrawal of AV nodal blockers.
• Still dangerous - can deteriorate suddenly.

ECG: 3rd degree AV Block with Junctional Escape

Ventricular Escape Rhythm (Infranodal Block)

ECG findings

• Rate: 20–40 bpm, often low 20s–30s
• QRS: Wide, bizarre, often >140–160 ms
• R–R regular but slow
• P waves: regular, unrelated

Pearls

• Seen with anterior MI, His–Purkinje degeneration, or massive structural disease.
• Extremely unstable - these patients can arrest.
• Requires immediate pacing and cardiology involvement.

ECG: 3rd degree AV Block with Ventricular Escape

Mimics and Differential Diagnosis

Competing Ventricular Rhythm

The ventricles outpace the sinus node with a faster escape or ectopic rhythm, creating AV dissociation without true conduction block.

• Accelerated junctional rhythm
• Idioventricular rhythm
• Ventricular tachycardia

Key: ventricular rate is faster than sinus rate.

ECG: Accelerated junction rhythm

ECG: Idioventricular rhythm

ECG: Ventricular Tachycardia

Isorhythmic AV Dissociation

Atrial and ventricular pacemakers fire at nearly identical rates, drifting in and out of alignment and mimicking block despite preserved conduction.

• Atrial and ventricular rates are similar.
• P waves drift in/out of QRS complexes.
• PR intervals may transiently look normal.

Key: ventricular rate is equal to sinus rate.

ECG: Isorhythmic AV dissociation

2nd Degree AV Block

• Mobitz 1: Progressive PR interval prolongation until a beat is dropped
• Mobitz 2: Fixed PR intervals on conducted beats with sudden dropped qrs complexes
• Both have grouped qrs beating

Key: If ANY predictable PR pattern exists, it’s not complete block.

ECG: 2nd degree AV Block Mobitz 1

ECG: 2nd degree AV Block Mobitz 2

High Grade AV Block

Multiple consecutive non-conducted P waves despite occasional preserved conduction, representing severe conduction system disease just short of complete heart block.

• Some P waves conduct.
• PR interval on conducted beats is fixed.
• Dropped beats between them.

Key: P wave Conduction ≠ complete heart block

ECG: High grave AV Block

Basic Algorithm

1. Are P waves present and regular?
2. Is the ventricular rate slower and independent?
3. Do ANY P waves conduct?
• If yes → NOT CHB.
4. Is the QRS narrow (junctional) or wide (ventricular)?
5. Is the patient stable enough to perfuse?
6. Prepare pacing

High Yield Pearls

• Localized inferior MI: Often causes AV nodal block → junctional escape → may respond to atropine → can resolve after reperfusion.
• Anterior MI: Destroys His–Purkinje system → CHB + wide-complex escape rhythm
• Hyperkalemia: Can produce pseudo-CHB; treat potassium immediately before diagnosing primary conduction disease.
• Drugs: CCB, BB, digoxin, amiodarone, clonidine can all cause reversible CHB.
• Don't rely on one lead: Use II, V1, V2, aVF to identify hidden P waves.
• Pads early: Place pacing pads during ECG acquisition in unstable bradycardic patients.

3rd Degree AV Block ECG Examples

ECG: 3rd degree AV Block

ECG: 3rd degree AV Block

ECG: 3rd degree AV Block

ECG: 3rd degree AV Block

ECG: Inferior/RV MI with 3rd degree AV Block

ECG Example: Biventricular VT evolving into 3rd degree AV Block (on Digoxin)

Related Posts

• ECG Exercise 14 - Identify the AV Block
• ECG Exercise 15 - Premature Contraction
• ECG Exercise 16 - Bradycardias
• ECG Topic - PR interval
• ECG Topic - 1st degree AV Block
• ECG Topic - 2nd degree AV Block

Sources

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This post is for education and not medical advice.