Atrioventricular Block

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Introduction

Atrioventricular (AV) block (often referred to as “heart block”) involves the partial or complete interruption of impulse transmission from the atria to the ventricles. This interruption of impulse transmission results in characteristic ECG findings that different depending on the sub-type of AV block. The commonest cause of AV block overall is idiopathic fibrosis and sclerosis of the conduction system.

Any patient presenting with possible AV block would require a number of investigations to identify possible underlying causes including:

  • ECG – to help determine the subtype of AV block
  • Blood tests (e.g. FBC, U&Es, TSH, Troponin) – to rule out underlying causes
  • Echocardiogram – to rule out structural heart disease

Some forms of AV block can be managed conservatively, whereas other sub-types require intervention.

This article will explore each of the sub-types of AV block as follows:

  • First-degree AV block
  • Second-degree AV block (type 1)
  • Second-degree AV block (type 2)
  • Third-degree (complete) AV block

First-degree AV block

  • First-degree AV block involves the consistent prolongation of the PR interval (defined as >0.20 seconds) due to delayed conduction via the atrioventricular node.
  • Every P wave is followed by a QRS complex (i.e. there are no dropped QRS complexes, unlike some other forms of AV block discussed later).
  • First-degree AV block is relatively common and can often be an incidental finding.1

Aetiology

  • Enhanced vagal tone – often seen in athletes (non-pathological)
  • Post myocardial infarction
  • Lyme disease
  • Systemic lupus erythematosus
  • Congenital
  • Myocarditis
  • Electrolyte derangements
  • Drugs – particularly AV blocking drugs such as beta-blockers, rate-limiting calcium-channel blockers, digoxin and magnesium1
  • Thyroid dysfunction

ECG findings

  • Rhythm: regular
  • P wave: every P wave is present and followed by a QRS complex
  • PR interval: prolonged >0.2 seconds (5 small squares)
  • QRS complex: normal morphology and duration (<0.12 seconds)
First Degree AV block
First Degree AV block

Clinical features

History

  • Patients are typically asymptomatic

Clinical examination

  • Clinical examination is normally unremarkable

Management

  • AV blocking drugs should be stopped
  • No intervention is usually required if the patient is asymptomatic
  • If the patient is symptomatic, a pacemaker may be considered

Complications

  • First-degree AV block does not usually progress to higher grade AV blocks
  • Those with first-degree AV block may be at an increased risk of atrial fibrillation1

Second-degree AV block (type 1)

  • Second-degree AV block (type 1) is also known as Mobitz type 1 AV block or Wenckebach phenomenon.
  • Typical ECG findings in Mobitz type 1 AV block include progressive prolongation of the PR interval until eventually the atrial impulse is not conducted and the QRS complex is dropped.
  • AV nodal conduction resumes with the next beat and the sequence of progressive PR interval prolongation and the eventual dropping of a QRS complex repeats itself.

Aetiology2

  • Increased vagal tone – often seen in athletes (non-pathological)
  • Drugs – beta-blockers, calcium channel blockers, digoxin, amiodarone
  • Inferior myocardial infarction
  • Myocarditis
  • Cardiac surgery (mitral valve repair, Tetralogy of Fallot repair)

ECG findings

  • Rhythm: irregular
  • P wave: every P wave is present, but not all are followed by a QRS complex
  • PR interval: progressively lengthens before a QRS complex is dropped
  • QRS complex: normal morphology and duration (<0.12 seconds), but are occasionally dropped
Mobitz Type 1
Mobitz type 1 AV block

Clinical features

History

  • Patients are usually asymptomatic, but some can develop symptomatic bradycardia and present with symptoms such as pre-syncope and syncope.

Clinical examination

  • Irregular pulse
  • Bradycardia

Management

  • AV blocking drugs should be stopped
  • Second-degree AV block (type 1) is usually benign and rarely causing haemodynamic compromise2
  • Usually, no intervention is required if the patient is asymptomatic
  • If the patient is symptomatic a pacemaker may be considered

Complications

  • The patient may become haemodynamically compromised, although this is rare2

Second-degree AV block (type 2)

  • Second-degree AV block (type 2) is also known as Mobitz type 2 AV block.
  • Typical ECG findings in Mobitz type 2 AV block include a consistent PR interval duration with intermittently dropped QRS complexes due to a failure of conduction.
  • The intermittent dropping of the QRS complexes typically follows a repeating cycle of every 3rd (3:1 block) or 4th (4:1 block) P wave.

Aetiology

Mobitz type 2 AV block is always pathological, with the block typically occurring at either the bundle of His (20%) or the bundle branches (80%).

Causes include:3

  • Myocardial infarction
  • Idiopathic fibrosis of the conducting system (Lenegre’s or Lev’s disease)
  • Cardiac surgery (especially surgery occurring close to the septum – e.g. mitral valve repair)
  • Inflammatory conditions (rheumatic fever, myocarditis, Lyme disease)
  • Autoimmune (SLE, systemic sclerosis)
  • Infiltrative myocardial disease (amyloidosis, haemochromatosis, sarcoidosis)
  • Hyperkalaemia
  • Drugs (e.g. beta-blockers, calcium channel blockers, digoxin, amiodarone)
  • Thyroid dysfunction

ECG findings

  • Rhythm: irregular (may be regularly irregular in 3:1 or 4:1 block)
  • P wave: present but there are more P waves than QRS complexes
  • PR interval: consistent normal PR interval duration with intermittently dropped QRS complexes
  • QRS complex: normal (<0.12 seconds) or broad (>0.12 seconds)
    • The QRS complex will be broad if the conduction failure is located distal to the bundle of His 3
Mobitz Type 2
Mobitz type 2 AV block

Clinical features

History

  • Some patients are asymptomatic
  • Other patients may experience:
    • Palpitations
    • Pre-syncope
    • Syncope

Clinical examination

  • You may detect a ‘regularly irregular’ pulse, where there is a pattern of how many atrial depolarisations (P waves) lead to ventricular depolarisation (QRS waves) such as 3:1 block.1

Management

  • Because of the risk of progression to complete AV block, patients should be placed on a cardiac monitor as soon as possible.
  • The underlying cause of the AV block should be investigated.
  • Temporary pacing or isoprenaline may be required if the patient is haemodynamically compromised due to bradycardia.
  • A permanent pacemaker is usually inserted if there are no readily reversibly causes identified.

Complications

  • Patients are at risk of progressing to symptomatic complete AV block, in which the escape rhythm is likely to be ventricular and thus too slow to maintain adequate systemic perfusion.
  • Patients are also at risk of developing asystole. 3

Third-degree (complete) AV block

  • Third-degree (complete) AV block occurs when there is no electrical communication between the atria and ventricles due to a complete failure of conduction.4
  • Typical ECG findings include the presence of P waves and QRS complexes that have no association with each other, due to the atria and ventricles functioning independently.
  • Cardiac function is maintained by a junctional or ventricular pacemaker.4
  • Narrow-complex escape rhythms (QRS complexes of <0.12 seconds duration) originate above the bifurcation of the bundle of His. A typical heart rate would be >40bpm.
  • Broad-complex escape rhythms (QRS complexes >0.12 seconds duration) originate from below the bifurcation of the bundle of His. These escape rhythms produce slower, less reliable heart rates and more significant clinical features (e.g. heart failure, syncope).

Aetiology

Causes include the following:4

  • Congenital:
    • Structural heart disease (e.g transposition of the great vessels)
    • Autoimmune (e.g maternal SLE)
  • Idiopathic fibrosis:
    • Lev’s disease (fibrosis of the distal His-Purkinje system in the elderly)
    • Lenegre’s disease (fibrosis of the proximal His-Purkinje system in younger individuals)
  • Ischaemic heart disease:
    • Myocardial infarction
    • Ischaemic cardiomyopathy
  • Non-ischaemic heart disease:
    • Calcific aortic stenosis
    • Idiopathic dilated cardiomyopathy
    • Infiltrative disease (e.g sarcoidosis, amyloidosis)
  • Iatrogenic:
    • Post ablative therapies and pacemaker implantation
    • Post cardiac surgery
  • Drug-related:
    • Digoxin
    • Beta-blockers
    • Calcium channel blockers
    • Amiodarone
  • Infections:
    • Endocarditis
    • Lyme disease
    • Chagas’ disease
  • Autoimmune conditions:
    • SLE
    • Rheumatoid arthritis
  • Thyroid dysfunction

ECG findings

  • Rhythm: variable
  • P wave: present but not associated with QRS complexes
  • PR interval: absent (as there is atrioventricular dissociation)
  • QRS complex: narrow (<0.12 seconds) or broad (>0.12 seconds)
    • Depending on the site of the escape rhythm (see introduction)
Third Degree Heart Block
Third-degree AV block 5

Clinical features

History

  • Palpitations
  • Pre-syncope/syncope
  • Confusion
  • Shortness of breath (due to heart failure)
  • Chest pain
  • Sudden cardiac death

Clinical examination

  • Irregular pulse
  • Profound bradycardia
  • Haemodynamic compromise (e.g. prolonged capillary refill time and hypotension)

Management

  • Patients should be placed on a cardiac monitor
  • Transcutaneous pacing/temporary pacing wire or isoprenaline infusion may be required
  • Some rhythms (particularly narrow-complex escape rhythms) may respond to atropine
  • A permanent pacemaker is generally required

Complications

  • Ventricular arrhythmias leading to sudden cardiac death.4

Reviewer

Professor Faizel Osman

Cardiology Consultant

UHCW NHS Trust


References

  1. Patient. ECG Identification of Conduction Disorders. Published in 2016. [https://patient.info/doctor/ecg-identification-of-conduction-disorders].
  2. Life In The Fast Lane. AV Block: 2nd degree, Mobitz I (Wenckebach Phenomenon). Published in 2019. [https://litfl.com/av-block-2nd-degree-mobitz-i-wenckebach-phenomenon/].
  3. Life In The Fast Lane. AV Block: 2nd degree, Mobitz II (Hay block). Published in 2019. [https://litfl.com/av-block-2nd-degree-mobitz-ii-hay-block/].
  4. Parveen Kumar and Michael Clarke. Kumar & Clark’s Clinical Medicine. Published in 2017.
  5. Author: James Heilman, MD (Own work) – Delta wave – via Wikimedia Commons – Licence: CC BY-SA 3.0

 

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