QuestionSBA
QuestionChoice
This ECG does not show second degree heart block, mobitz type 1. An ECG of second degree heart block, mobitz type 1 typically shows progressive elongation of the PR interval followed by non-conduction of the P wave and a skipped QRS. This is not shown in this ECG, which shows complete non-conduction of the P waves
33525
c
Second degree heart block, mobitz type 1
QuestionChoice
This ECG does not show atrial fibrillation. An ECG in atrial fibrillation would be irregularly irregular with absent P waves. This ECG is regular and although P waves can be difficult to observe, they are visible on more detailed inspection of the ECG
33527
e
Atrial fibrillation
QuestionChoice
This ECG does not show second degree heart block, mobitz type 2. An ECG of second degree heart block, mobitz type 2 typically shows occasionally dropped QRS complexes (due to non-conduction of P waves) in the absence of progressive prolongation of the P-R interval. This can either be somewhat random or in a fixed ratio e.g. 2:1. This is in contrast to this ECG where no P waves are conducted, making it more in keeping with complete heart block
33524
b
Second degree heart block, mobitz type 2
QuestionChoice
Complete heart block is caused by failure of the atrioventricular node, preventing the electrical impulses generated by the sinoatrial node reaching the ventricles. This leads to a complete dissociation of the ventricles from the atria. In this case, this was caused by the combination of a beta blocker with a non-dihydropyridine calcium channel blocker, which should never be given in combination due to the risk of bradyarrhythmia and complete heart block. This is a typical ECG of complete heart block, with complete dissociation of the P waves from the QRS complexes. P waves are around the normal atrial rate of around 100-180/min and the QRS complexes are at ventricular escape rhythm which is around 20-40/min. If this patient remains untreated they are at significant risk of asystole and death
33523
a
Complete heart block
QuestionChoice
This is not a normal ECG. In a normal ECG, every P wave is followed by a QRS and every QRS is preceded by a P wave
33526
d
Normal ECG
Concept
Chapter
# Summary

Heart block refers to an obstruction in the electrical conduction system of the heart. This obstruction can occur at various points in the conduction system, including the sinoatrial node, atrioventricular node, Bundle of His, or bundle branches. Atrioventricular heart block specifically affects the conduction between the atria and ventricles. The severity of heart block can range from first degree, which is generally benign, to second degree (Mobitz Type I and II), to complete (third degree) heart block, which requires immediate management with a permanent pacemaker due to the risk of asystole. The underlying causes and management strategies differ for each type of heart block.

# Definition 

Heart block occurs due to an obstruction in the electrical conduction system of the heart. It can occur anywhere along the conduction system of the heart from the sinoatrial node to the atrioventricular node to the Bundle of His or within the bundle branches themselves. 

Sinoatrial node block rarely leads to symptoms as the atrioventricular node acts as a secondary pacemaker. Bundle branch blocks are a form of heart block but they are discussed in a separate section. The rest of this section will discuss atrioventricular block in more detail. 

Patients with atrioventricular heart block may be asymptomatic or may present with fatigue, lightheadeness, syncope, shortness of breath and most seriously in cardiac arrest or with sudden death. 

# First Degree Heart Block

## Definition

This is caused by prolonged conduction of electrical activity through the AV node. It can be identified on ECG by finding a PR interval >200ms.

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## Causes

* High vagal tone: e.g. athletes
* Acute inferior MI
* Electrolyte abnormalities: e.g. hyperkalaemia
* Drugs: e.g. NHP-CCBs, beta-blockers, digoxin, cholinesterase inhibitors

## Management

First degree heart block itself is benign and does not need treating. However, any pathological underlying cause should be reversed.

# Second Degree Heart Block 

Second degree heart block is split into Mobitz Type I and Mobitz Type II heart block. 

# Mobitz Type I

## Definition

Wenckebach phenomenon or Mobitz type I is a type of second degree heart block that is usually due to reversible conduction block at the AV node. It is characterised by progressive lengthening of the PR interval which results in a P wave that fails to conduct a QRS.

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## Causes

* MI (mainly inferior)
* Drugs such as beta/calcium channel blockers, digoxin
* Professional athletes due to high vagal tone
* Myocarditis
* Cardiac surgery

## Management

It is generally asymptomatic and does not require any specific management as the risk of high AV block/complete heart block is low. If symptoms do arise, ECG monitoring may be required, precipitating drugs must be stopped and if they are bradycardic with adverse features they should be treated with atropine.

# Mobitz Type II

## Definition

Mobitz type II block is a type of second degree AV block where there are intermittent non-conducted P waves. The _PR interval is constant_ (may be normal or prolonged) and there may no pattern or fixed ratios such as 2:1 or 3:1 block. It is usually caused by conduction system failure, especially at the His-Purkinje system.

In most cases there is a broad QRS indicating a distal block in the His-Purkinje system and many patients have pre-existing left bundle branch block/bifascicular block.

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## Causes

* Infarction: particularly anterior MI which damages the bundle branches
* Surgery: mitral valve repair or septal ablation
* Inflammatory/autoimmune: rheumatic heart disease, SLE, systemic sclerosis, myocarditis
* Fibrosis: Lenegre's disease
* Infiltration: sarcoidosis, haemochromatosis, amyloidosis
* Medication: beta-blockers, calcium channel blockers, Digoxin, amiodarone

## Management

Definitive management is with a permanent pacemaker as these <u>_patients are at risk of complete heart block_</u> and at risk of becoming haemodynamically unstable.

# Complete (Third degree) Heart Block

## Definition

Complete heart block occurs when atrial impulses fail to be conducted to the ventricles. Sufficient cardiac output may be secondary to a ventricular or junctional escape rhythm.

ECG shows severe bradycardia and complete dissociation between the P waves and the QRS complexes. These patients are at high risk of asystole, ventricular tachycardia and cardiac arrest. 

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## Causes

* Myocardial infarction: especially inferior
* Drugs acting at the AVN: beta blockers, dihydropyridine calcium channel blockers, or adenosine
* Idiopathic fibrosis

## Management

Management of complete (third degree) heart block is via the acute bradycardia guideline (see below). Permanent pacemaker insertion is eventually required due to the risk of sudden death.

Acute management:

For emergencies, always follow an A-E structure. Identify reversible causes (dyselectrolytaemias, drugs, cardiac causes etc.) 

*If there are adverse signs (e.g. shock, syncope, heart failure, myocardial ischaemia):* 

* **1st line** = **500 micrograms atropine IV**
  * Atropine blocks the vagal nerve which increases firing rate of the SAN. 
* **2nd line** = if the first dose of atropine is not working can consider giving additional doses of atropine 500mcg up to 3mg until response. Alternatively,  **transcutaenous pacing** or **isoprenaline** or **adrenaline** or **alternative drugs** including aminophylline, adrenaline, glucagon (in beta-blocker or calcium channel blocker overdose).  

*If there are no adverse signs but a risk of asystole, or a satisfactory response to 500mcg atropine:*

* Risk of asystole: recent asystole, mobitz type II block, complete heart block + broad QRS, ventricular pauses >3s. 
* **1st line** = administer **500 micrograms atropine IV**.  Alternatively,  **transcutaenous pacing** or **isoprenaline** or **adrenaline** or **alternative drugs** including aminophylline, adrenaline, glucagon (in beta-blocker or calcium channel blocker overdose).  

*If there are no adverse signs and there is no risk of asystole*

* Observe

# NICE Guidelines

[NICE Guidelines on Pacing](<https://www.nice.org.uk/guidance/ta88/chapter/2-clinical-need-and-practice>) 

# References

[Life in the Fast Lane Heart Block ECG Summary](https://litfl.com/heart-block-and-conduction-abnormalities/)

[Resuscitation Council Adult Bradycardia Algorithm](<https://www.resus.org.uk/sites/default/files/2020-05/G2015_Adult_bradycardia.pdf>)
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First degree heart block.
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Mobitz type I.
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Complete heart block.
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Mobitz type II.
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Heart Block
Topic
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ECG Interpretation
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Topic
35
Cardiology
A 27 year old patient attends the GP with physical symptoms of anxiety. They are subsequently started on Propanolol. They present to the emergency department the same day with severe breathlessness and lightheadedness. They have a past medical history of cluster headaches for which they take verapamil for prophylaxis. Their ECG is shown below:

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What is the most likely diagnosis?
a