Atrioventricular block (AV block) is characterized by an interrupted or delayed conduction between the atria and the ventricles. There are three degrees of AV block, categorized according to the extent of the delay or interruption. First-degree blocks are identifiable on ECG by a prolonged PR interval. are further divided into four subtypes: Mobitz type I (also called Wenckebach), Mobitz type II, 2:1 AV block, and high-grade AV block. In Mobitz type I blocks, a progressive prolongation of the PR interval culminates in a nonconducted P wave (“dropped beat”). Mobitz type II blocks generate dropped QRS complexes at regular intervals (e.g., 3:2, 4:3, or 5:4), often leading to bradycardia. A 2:1 AV block has a regular pattern in which every second atrial impulse is not conducted to the ventricles. In second-degree high-grade AV block, two or more consecutive P waves do not generate a ventricular response. A , also known as complete heart block, involves the total interruption of the electrical impulse between the atria and ventricles. The complete absence of conduction results in a ventricular escape rhythm, whose rate depends on the level at which the escape rhythm is generated. AV blocks may be asymptomatic or cause symptoms of bradycardia. Depending on the heart rate, symptoms can be severe and include heart failure or syncope. Asymptomatic patients with first-degree and Mobitz type I blocks usually only require observation, whereas higher-degree blocks necessitate permanent pacemaker insertion.
See “” for more information on investigations, monitoring, definitive treatment, and stabilization of
|Overview of atrioventricular blocks |
|Type of AV block||ECG findings||Typical management|
|Second-degree AV block||Mobitz type I|
|Mobitz type II|
|Third-degree AV block|
See also “.”
|Etiology of atrioventricular blocks|
|Structural heart disease|| |
- AV blocks result from the interruption of the electrical impulse anywhere within the atrioventricular conduction system, including the:
- Blocks at the level of the AV node: impulse passes through → normal propagation through conduction system →
- Infranodal block
- More distal AV blocks: typically result from more extensive damage to the conducting system → ↑ risk of progression to . 
- Asymptomatic (common): especially with first-degree and Mobitz type I blocks 
- Clinical features of end-organ hypoperfusion (due to bradycardia) may be present, including: 
- Irregular rhythms (e.g., Mobitz I): palpitations
Loss of atrioventricular synchrony (e.g., extreme 1° AV block or 3° AV block) : 
- Feeling of pulsations in the neck or chest
- Cannon A waves
Significant pauses of asystole
- Stokes-Adams attacks 
- Cardiac arrest
- Description 
- Risk of progression to complete heart block: low, as the block results from slowed conduction through the AV node 
Mobitz type I (Wenckebach) 
- Progressive lengthening of the PR interval until a beat is dropped, which means a regular atrial impulse does not reach the ventricles (a normal P wave is not followed by a QRS complex)
- Mostly regular rhythm separated by short pauses, which may lead to bradycardia (regularly irregular rhythm)
- Rate of SA node > heart rate
- Risk of progression to complete heart block: typically low, as the block is most often at the level of the AV node
Mobitz type II 
- Single or intermittent nonconducted P waves without QRS complexes
- The PR interval remains constant.
- The conduction of atrial impulses to the ventricles typically follows a regular pattern, e.g.: 
- While 2:1 block follows a regular pattern, it cannot be classified as Mobitz type I or II and is classified separately (see “2:1 AV block”). 
- Risk of progression to complete heart block: high (> 50%), as it is typically due to infranodal block (usually in the His-Purkinje system) 
2:1 AV block 
- Inhibited conduction of every second atrial depolarization (P wave) to the ventricles (heart rate = ½ SA node rate)
- Cannot be classified as Mobitz I or Mobitz II as only one PR interval is observed before the subsequent dropped complex
- Often a transient rhythm occurring on a baseline Mobitz I or Mobitz II rhythm
- Risk of progression to complete heart block: depends on level of block
High-grade AV block 
- Risk of progression to complete heart block: typically high, but depends on duration and reversibility of block
- Risk of progression to cardiogenic shock or cardiac arrest: : High; Sudden onset 3° AV block can result in ventricular asystole, which lasts until an takes over.
- : can be generated by sites that are usually located near the AV node or near the bundle of His.
- Idiopathic paroxysmal AV block 
- Left bundle branch block
- Right bundle branch block
- Left anterior fascicular block
- Left posterior fascicular block
- Sinus node dysfunction
- See also “ .”
The differential diagnoses listed here are not exhaustive.
Management of AV blocks
- All patients
|Management approach to patients with AV block |
|Type of AV block|
Evaluation of underlying causes 
- Order laboratory studies based on suspected etiology, e.g.:
- Identify medications that can impair AV conduction.
- Consider measuring drug levels: e.g. digoxin level
Management of low-risk AV block 
Patients on medication that can cause or exacerbate AV conduction
- Monitor for progression with periodic ECGs.
- Discontinue if patients have other preexisting conduction abnormalities. 
Indications for pacemaker placement 
Patients with an irreversible AV block and the following:
- Infranodal block
- Neuromuscular disease associated with AV block (known or suspected): refer to a specialist for possible pacemaker.
- Certain symptomatic patients
Management of high-risk AV block 
Potentially reversible cause
- Consider temporary pacing.
- Adapt management depending on the suspected underlying cause
- Discontinue or reduce medications causing AV block or follow detoxification strategies in case of:
- Perform PCI and avoid early pacemaker implantation < 72 h in patients with AV block caused by acute MI.
- See “Special considerations” in “Symptomatic, stable bradycardia” for further detail.
- Permanent pacemaker placement is indicated if:
- AV block persists despite adequate treatment
- Medication causing AV block is necessary and not replaceable
- Irreversible cause
Acute management checklist
- Review ECG.
- Evaluate for signs of end-organ hypoperfusion and apply transcutaneous pacing pads as needed.
- Admit with continuous cardiac monitoring: complete heart block, Mobitz II, or high-grade AV block.
- Can investigate as an outpatient unless hemodynamically unstable: 1° AV block, 2:1 heart block, or Mobitz I
- Identify and treat underlying causes.
Hemodynamically unstable patients (see “Unstable bradycardia”)
- Consider atropine with or without beta agonists. 
- Begin temporary cardiac pacing.
- Plan permanent pacemaker placement (with or without defibrillator) if cause is nonreversible.
Hemodynamically stable patients
- Consider assessment for pacemaker indications in patients with low-risk AV blocks.
- Plan permanent pacemaker placement (with or without defibrillator) in patients with: