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Atrioventricular block

Last updated: February 26, 2021

Summarytoggle arrow icon

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. Second-degree AV blocks 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 third-degree AV block, 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 “Management approach to patients with AV block” for more information on investigations, monitoring, definitive treatment, and stabilization of unstable bradycardia.

Overview of atrioventricular blocks [1][2][3]
Type of AV block ECG findings Typical management

First-degree AV block

  • Low risk of progression to a higher degree heart block or sudden cardiac arrest
  • Asymptomatic patients require no treatment and can be followed-up as outpatients.
  • Consider an elective pacemaker for select patients.
Second-degree AV block Mobitz type I

2:1 AV block

  • Every second impulse from the atria is not conducted to the ventricles.
  • Regular rhythm
  • Narrow QRS complexes (< 0.12 s)
Mobitz type II

High-grade AV block

  • ≥ 2 consecutive impulses from the atria are not conducted to the ventricles.
  • Typically regular rhythm
  • Wide QRS complexes (> 0.12 s)
Third-degree AV block

See also “Causes of bradycardia.”

Etiology of atrioventricular blocks
Category

Examples [2]

Structural heart disease
Neurocardiogenic
Toxic/metabolic
Infectious
Endocrine
Neuromuscular

Mobitz type I (Wenckebach) [2]

First-degree and Mobitz type I second-degree AV blocks may be seen in healthy individuals, e.g., in athletes with increased vagal tone. Patients are often asymptomatic.

Mobitz type II [2]

  • Description
    • 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.: [2]
      • 3:2 block: regular AV block with 3 atrial depolarizations but only 2 atrial impulses that reach the ventricles (heart rate = ⅔ SA node rate)
      • 4:3 block: regular AV block with 4 atrial depolarizations but only 3 atrial impulses that reach the ventricles (heart rate = ¾ SA node rate)
    • 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”). [2]
  • Risk of progression to complete heart block: high (> 50%), as it is typically due to infranodal block (usually in the His-Purkinje system) [7]

Mobitz type II block can progress to third-degree heart block; therefore, all patients should be admitted for continuous cardiac monitoring and treatment.

Other variants

2:1 AV block [2]

High-grade AV block [2][7]

The differential diagnoses listed here are not exhaustive.

Approach

Management approach to patients with AV block [2]
Low-risk High-risk
Type of AV block
Unstable patients
Stable patients
  • Obtain cardiac imaging on all patients
    • Preferred initial test: TTE
    • Consider advanced cardiac imaging, e.g., MRI, CT, or TEE

Evaluation of underlying causes [2][13]

Management of low-risk AV block [2]

  • Patients on medication that can cause or exacerbate AV conduction
    • Monitor for progression with periodic ECGs.
    • Discontinue if patients have other preexisting conduction abnormalities. [13]

Indications for pacemaker placement [2]

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
    • Assess for correlation of symptoms using an ambulatory Holter monitor.
      • Permanent pacing is indicated if symptoms clearly correlate with AV block.
      • If symptoms do not correlate, continue monitoring as an outpatient

Management of high-risk AV block [2][13]

  • Potentially reversible cause
    • Consider temporary pacing.
    • Adapt management depending on the suspected underlying cause
    • Permanent pacemaker placement is indicated if:
      • AV block persists despite adequate treatment
      • Medication causing AV block is necessary and not replaceable
  • Irreversible cause
    • All patients with Mobitz Type II, high-grade, or third-degree AV block require the placement of a permanent pacemaker.
    • Consider a defibrillator if the AV block is caused by infiltrative processes or neuromuscular disease with conduction block.

All patients

Hemodynamically unstable patients (see “Unstable bradycardia”)

Hemodynamically stable patients

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