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QuestionChoice
First degree atrioventricular heart block involves a prolongation of the PR interval of more than 200ms. In this case, the ECG shows a PR interval of greater than 300ms. The ECG also shows sinus bradycardia
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First degree atrioventricular heart block
QuestionChoice
Second degree atrioventricular heart block (type one) is also known as Mobitz type one or Wenckebach phenomenon. It leads to progressive elongation of the PR interval, which will lead to an eventual missed QRS complex. In this case, the ECG shows a constant PR interval of greater than 300ms, which indicates first degree atrioventricular heart block
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Second degree atrioventricular heart block (type one)
QuestionChoice
Sick sinus syndrome, also known as sinus node dysfunction, can lead to sinus bradycardia, seen in the ECG. However, there is also prolongation of the PR interval, which is less likely to be seen in sick sinus syndrome
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Sick sinus syndrome
QuestionChoice
Third degree atrioventricular heart block occurs when there is no direct communication between the atria and the ventricles. Therefore, the P waves and QRS complexes are found to be independent of one another on ECG. In this case, the ECG shows a constant PR interval of greater than 300ms, which indicates first degree atrioventricular heart block
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Third degree atrioventricular heart block
QuestionChoice
Second degree atrioventricular heart block (type two), also known as Mobitz type two, leads to intermittent dropping of QRS complexes. This usually follows a pattern of 3:1 or 4:1. In this case, the ECG shows a constant PR interval of greater than 300ms, which indicates first degree atrioventricular heart block
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Second degree atrioventricular heart block (type two)
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thats such a horrible ecg picture and i cant even enlarge it 
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Click on it
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# 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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Topic
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Cardiology
A 60-year-old man attends his GP for a routine check-up; he is noted to have a low pulse rate. His ECG is as follows:

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What incidental finding is seen?
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