Atrioventricular (AV) block, also known 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 differ depending on the subtype of AV block.
Idiopathic fibrosis and sclerosis of the conduction system is the most common cause of AV block.
Any patient presenting with AV block requires investigation to identify underlying causes, such as an ECG to help determine the subtype of AV block, laboratory investigations to rule out underlying causes, and an 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, including first-degree AV block, second-degree AV block (type 1), second-degree AV block (type 2), and third-degree (complete) 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 common and can often be an incidental finding.
Causes of first-degree AV block include: enhanced vagal tone (often seen in athletes and 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 magnesium), and thyroid dysfunction.
ECG findings in first-degree AV block include: regular rhythm, every P wave present and followed by a QRS complex, prolonged PR interval >0.2 seconds (5 small squares), and normal morphology and duration of the QRS complex (<0.12 seconds).
Patients are usually asymptomatic and clinical examination is normally unremarkable. Any atrioventricular blocking drugs should be stopped, and no intervention is usually required if the patient is asymptomatic. If the patient is symptomatic, a pacemaker may be considered.
First-degree atrioventricular block does not usually progress to higher grade AV blocks. Those with first-degree AV block may be at an increased risk of atrial fibrillation.
Second-degree atrioventricular 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.
Causes of second-degree AV block (type 1) include: increased vagal tone, drugs (beta-blockers, calcium channel blockers, digoxin, amiodarone), inferior myocardial infarction, myocarditis, and cardiac surgery (mitral valve repair, Tetralogy of Fallot repair).
ECG findings in second-degree AV block (type 1) include: irregular rhythm, every P wave is present but not all are followed by a QRS complex, PR interval progressively lengthens before a QRS complex is dropped, and normal QRS morphology and duration (<0.12 seconds), but are occasionally dropped.
Patients with second-degree AV block (type 1) are typically asymptomatic, but some may develop symptomatic bradycardia and present with symptoms such as pre-syncope and syncope.
Typical clinical findings may include:
AV blocking drugs should be stopped. Second-degree AV block (type 1) is usually benign and rarely causes haemodynamic compromise.
Usually, no intervention is required if the patient is asymptomatic. If the patient is symptomatic a pacemaker may be considered.
The patient may become haemodynamically compromised, although this is rare.
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.
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 of second-degree AV block (type 2) include:
Second degree AV (type 2) block, also known as Mobitz type 2 block, is a disorder of the heart's electrical conduction system where atrial impulses are regularly, or occasionally, not conducted to the ventricles. The causes of this type of block include:
ECG findings in second-degree AV (type 2) include:
Symptoms may include:
Clinical examination 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.
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 reversible causes identified.
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.
Third-degree (complete) AV block occurs when there is no electrical communication between the atria and ventricles due to a complete failure of conduction.
Typical ECG findings include the presence of P waves and QRS complexes that have no association with each other, from the atria and ventricles functioning independently.
Cardiac function is maintained by a junctional or ventricular pacemaker.
Narrow-complex escape rhythms (QRS complexes of <0.12 seconds duration) originate above the bifurcation of the bundle of His, typically with a heart rate of >40bpm. Broad-complex escape rhythms (QRS complexes >0.12 seconds duration) originate from below the bifurcation of the bundle of His, with slower, less reliable heart rates and more significant clinical features (e.g. heart failure, syncope).
Causes of third-degree (complete) AV block include:
ECG findings in third-degree (complete) heart block include:
Third-degree (or complete) AV block is a type of heart block where electrical signals between the atria and ventricles are completely blocked. This results in an abnormal rhythm of the heart, called a bradycardia (slower than normal heart rate).
Typical symptoms of third-degree heart block may include palpitations, pre-syncope/syncope, confusion, shortness of breath (due to heart failure), chest pain and sudden cardiac death.
Typical clinical findings in third-degree heart block may include an irregular pulse, profound bradycardia, haemodynamic compromise (e.g. prolonged capillary refill time and hypotension).
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 usually required.
The main complication is sudden cardiac death due to ventricular arrhythmias.
