ECG Exercise 14 - Identify The AV Block

Last updated 1/9/25

Zoom-In Instructions

Desktop: click ECG for full image Mobile: click 📷 for full image

Assume every patient is presenting with chest pain, weakness and dizziness.

ECG 1 (📷)

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What is the rhythm?

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Are there p waves?

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Do P waves have similar morphology within each lead?

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Is the PR interval fixed or changing?

Changing, progressively prolonging until QRS is dropped

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What is the P: QRS ratio? Are there grouped QRS complexes?

4:3
Yes there are groups of 3

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Is the QRS narrow or wide?

Narrow

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Based on these questions…What type of av block is this?

Sinus rhythm with 4:3 2nd degree av block type 1 (Mobitz 1/ Wenckebach)

Grouped qrs complexes suggest dropped beats indicating 2nd degree avb
A gradually prolonging PR narrows it down to type 1 (Mobitz 1/ Wenckebach)

More reading

AV Block: 2nd degree, Mobitz I (Wenckebach Phenomenon)

A review of the basic ECG features, causes and pathophysiology of Mobitz I AV Block (Wenckebach Phenomenon) with some example ECGs.

litfl.com

AV Block: 2nd degree, Mobitz I (Wenckebach Phenomenon)

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Source: ECG 1000

ECG 2 (📷)

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What is the rhythm?

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Are there p waves?

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Do P waves have similar morphology within each lead?

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Is the PR interval fixed or changing?

PR is fixed

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What is the P: QRS ratio? Are there grouped QRS complexes?

Initially 3:2 then 1:1
Groups of 2 qrs complexes are present in the first half of the ECG

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Is the QRS narrow or wide?

Narrow

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Based on these questions…What type of av block is this?

Sinus tachycardia with 2nd degree av block type 2 followed by sinus tachycardia
More reading

AV Block: 2nd degree, Mobitz II (Hay block)

A review of the ECG features of Mobitz II 2nd degree AV block - Originally termed Hay block (1906) by Mobitz in 1924. ECG Library LITFL

litfl.com

AV Block: 2nd degree, Mobitz II (Hay block)

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Source: ECG 827

ECG 3 (📷)

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What is the rhythm?

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Are there p waves?

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Do P waves have similar morphology within each lead?

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Is the PR interval fixed or changing?

Fixed but prolonged (≥200ms)

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What is the P: QRS ratio? Are there grouped QRS complexes?

1:1
There kind of looks like an inverted p waves at the end of some qrs complexes but I think this is part of the QRS complexes which are fragmented
No grouped qrs complexes

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Is the QRS narrow or wide?

Wide

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Based on these questions…What type of av block is this?

Sinus rhythm with 1st degree avb
More reading

First Degree Heart Block

Review of first degree AV block with some ECG examples - PR interval > 200ms (five small squares) LITFL ECG Library

litfl.com

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Source: ECG 439

ECG 4 (📷)

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What is the rhythm?

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Are there p waves?

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Do P waves have similar morphology within each lead?

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Is the PR interval fixed or changing?

Changing with no pattern

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What is the P: QRS ratio? Are there grouped QRS complexes?

2:1 but no grouped qrs complexes

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Is the QRS narrow or wide?

Narrow

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Based on these questions…What type of av block is this?

This is 3rd degree av block with a junctional escape rhythm

P waves are dissociated from qrs complexes
The QRS complexes are narrow suggesting a junctional rhythm

More reading

AV block: 3rd degree (complete heart block)

3rd degree AV block is characterised by complete AV dissociation, with no supraventricular impulses conducted to the ventricles

litfl.com

AV block: 3rd degree (complete heart block)

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Source: ECG 48

ECG 5 (📷)

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What is the rhythm?

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Are there p waves?

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Do P waves have similar morphology within each lead?

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Is the PR interval fixed or changing?

Fixed

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What is the P: QRS ratio? Are there grouped qrs complexes?

4:1 (see only 3?…look closely in the T wave)
No grouped qrs complexes

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Is the QRS narrow or wide?

Narrow

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Based on these questions…What type of av block is this?

This is considered a high grade block or 2nd degree av block P:QRS ≥ 3:1
This is not 3rd degree avb as the p waves are still conducting the qrs complexes (fixed PR) but the ventricular rate is low
Source of ECG

AV block: 2nd degree, “high-grade” AV block

Second degree heart block (2nd degree AV block) with a P:QRS ratio of 3:1 or higher, producing an extremely slow ventricular rate.

litfl.com

AV block: 2nd degree, “high-grade” AV block

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Source: LITFL

ECG 6 (📷)

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What is the rhythm?

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Are there p waves?

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Do P waves have similar morphology within each lead?

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Is the PR interval constant or changing?

Fixed but quite prolonged

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What is the P: QRS ratio?

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Is the QRS narrow or wide?

Narrow

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Based on these questions…What type of av block is this?

Sinus rhythm with 1st degree av block (PR ~400ms)

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Source: ECG 493

ECG 7 (📷)

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What is the rhythm?

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Are there p waves?

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Do P waves have similar morphology within each lead?

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Is the PR interval fixed or changing?

Constantly changing without pattern

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What is the P: QRS ratio? Are there grouped qrs complexes?

About 2:1
No grouped qrs complexes

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Is the QRS narrow or wide?

Wide with RBBB-like morphology
There is is also a LAFB (L axis + rS in II)

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Based on these questions…What type of av block is this?

This is 3rd degree av block with RBBB+LAFB (trifascicular block)
More reading

Trifascicular Block

Trifascicular block (TFB) refers to the presence of conducting disease in all three fascicles: RBBB, LAFB, LPFB. LITFL EKG Library

litfl.com

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Source: ECG 53

ECG 8 (📷)

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What is the rhythm?

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Are there p waves?

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Do P waves have similar morphology within each lead?

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Is the PR interval fixed or changing?

Changing, progressively prolonging

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What is the P:QRS ratio? Are there grouped qrs complexes?

3:1
Groups of 2 qrs complexes

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Is the QRS narrow or wide?

Narrow

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Based on these questions…What type of av block is this?

Sinus rhythm with 2nd degree av block type 1 (Mobitz 1/ Wenckebach)

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Source: ECG 1054

ECG 9 (📷)

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What is the rhythm?

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Are there p waves?

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Do P waves have similar morphology within each lead?

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Is the PR interval fixed or changing?

PR seems fixed

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What is the P: QRS ratio? Are there grouped qrs complexes?

4:1
No grouped qrs complexes

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Is the QRS narrow or wide?

Wide - LBBB pattern

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Based on these questions…What type of av block is this?

Sinus tachycardia (atrial rate) with high grade av block or complete heart block

PR is fixed so this could be 4:1 2nd degree av block / high grade av block
QRS is wide and ventricular rate is quite slow , so this could so be 3rd degree av block with slow ventricular escape

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Source: ECG 770

ECG 10 (📷)

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What is the rhythm?

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Are there p waves?

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Do P waves have similar morphology within each lead?

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Is the PR interval fixed or changing?

Fixed

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What is the P: QRS ratio? Are there grouped qrs complexes?

2:1
No grouped qrs complexes

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Is the QRS narrow or wide?

Wide - RBBB morphology with LAFB (L axis + rS in II)

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Based on these questions…What type of av block is this?

Sinus rhythm with 2nd degree 2:1 av block with bifascicular block (RBBB+LAFB)

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Source: ECG

This post is for education and not medical advice.