---
title: Atrioventricular block
description: What atrioventricular block is, why it develops, how it is detected, which treatment approaches clinical guidelines describe and when urgent help is needed.
type: disease
preset: av-block-third-degree
morePresets: [av-block-first-degree, av-block-second-degree-mobitz-1, av-block-second-degree-mobitz-2]
aliases:
  - AV block
  - heart block
  - Morgagni–Adams–Stokes attacks
sources:
  - tier: guideline
    org: minzdrav-rf
    title: Брадиаритмии и нарушения проводимости
    year: 2025
    doi: 10.15829/1560-4071-2025-6669
    url: https://cr.minzdrav.gov.ru/view-cr/160_2
  - tier: reference
    org: msd-manuals
    title: Atrioventricular Block
    year: 2024
    url: https://www.msdmanuals.com/professional/cardiovascular-disorders/specific-cardiac-arrhythmias/atrioventricular-block
  - tier: guideline
    org: nice
    title: Transient loss of consciousness ('blackouts') in over 16s (CG109)
    year: 2023
    url: https://www.nice.org.uk/guidance/cg109
sources_checked: 2026-10-12
review: null
---

Atrioventricular (AV) block is a partial or complete interruption of the conduction of the electrical impulse from the atria to the ventricles.<Cite n={[1, 2]} /> It belongs to the bradyarrhythmias: when impulse conduction is blocked, the ventricular rhythm can slow down.<Cite n={1} /> There are three degrees: first- and second-degree block are partial, third-degree block is complete.<Cite n={2} /> How the impulse normally travels through the heart is explained in the article on the [conduction system of the heart](../../anatomy/conduction-system/).

<Diagram name="av-block">
  <p slot="normal">
    Normally the impulse starts in the <Structure id="sa-node">sinus node</Structure> and spreads across the atria.<Cite n={1} /> It is
    then conducted from the atria to the ventricles through the <Structure id="av-node">atrioventricular node</Structure>, the <Structure id="bundle-of-his">bundle of
    His</Structure> and its branches; in the diagram every atrial impulse reaches the ventricles.<Cite n={1} />
  </p>
  <p slot="disease">
    The diagram shows an example of second-degree AV block: some atrial impulses are not conducted
    to the ventricles, and the ventricles skip a beat.<Cite n={[1, 2]} /> In the diagram the
    interval before the dropped beat does not lengthen, as in Mobitz type II.<Cite n={2} /> The
    block can be in the AV node or below it, in the bundle of His or its branches.<Cite n={1} /> In
    first-degree block all impulses get through, but more slowly; in third-degree block none
    do.<Cite n={1} />
  </p>
</Diagram>

## Why it happens

<Patient>

The most common cause is fibrosis and sclerosis (scarring and hardening) of the heart's conduction system, which develop with age.<Cite n={[1, 2]} /> The second most common is coronary heart disease: the conduction system is damaged both by a heart attack and by a long-standing shortage of blood supply.<Cite n={[1, 2]} />

Some medicines that slow impulse conduction can also cause the block.<Cite n={[1, 2]} /> The doctor then decides whether the person really needs to take that medicine.<Cite n={1} /> Other causes include valve disease, inflammation of the heart muscle (myocarditis), sarcoidosis, some infections and heart surgery.<Cite n={[1, 2]} /> AV block can also be congenital.<Cite n={1} />

</Patient>

<Student>

By aetiology, atrioventricular block is divided into acquired and congenital forms, and by course into transient (intermittent) and permanent (persistent) forms.<Cite n={1} /> The most common causes are idiopathic fibrosis and sclerosis of the conduction system (about 40% of patients) and ischaemic heart disease (20%).<Cite n={2} />

The Russian Ministry of Health guideline lists age-related idiopathic degenerative fibrosis (Lev–Lenègre disease) first among intrinsic causes; arteriolosclerosis associated with arterial hypertension also contributes to the degenerative process.<Cite n={1} /> In ischaemic heart disease the conduction system is damaged both by myocardial infarction and by chronic ischaemia.<Cite n={1} />

Causes can be reversible, for example drug effects or neurocardiac reflexes.<Cite n={1} /> Among extrinsic causes of bradyarrhythmias the guideline lists beta-blockers, calcium channel blockers, cardiac glycosides, class I and III antiarrhythmic drugs, hypokalaemia and hyperkalaemia, and hypothyroidism; among intrinsic causes, besides degeneration and ischaemic heart disease, it lists infiltrative processes (amyloidosis, sarcoidosis), infections, myocarditis and surgical trauma.<Cite n={1} />

Persistent first-degree AV block occurs in no more than 1% of adults over 20, rising to 5% or more after 50 and up to 30% in people over 65; the incidence of acquired advanced second-degree and complete AV block is estimated at 200 cases per million per year.<Cite n={1} />

</Student>

<HeartModel highlight={['av-node', 'bundle-of-his', 'right-bundle-branch', 'left-bundle-branch']} />

## Symptoms

<Patient>

AV block may cause no symptoms at all, and its signs are often non-specific.<Cite n={1} /> First-degree block rarely causes symptoms.<Cite n={2} />

A persistently slow rhythm can cause tiredness, tiring easily, sluggishness, apathy and reduced mental performance.<Cite n={1} /> Exercise is often harder to tolerate: tiredness and breathlessness come on quickly.<Cite n={1} /> Sudden conduction problems can cause dizziness, confusion, feeling faint and fainting.<Cite n={1} /> A slow rhythm can make angina, high blood pressure and chronic heart failure worse.<Cite n={1} />

</Patient>

<Student>

The clinical picture of bradyarrhythmias includes cerebral and cardiac symptoms.<Cite n={1} /> Acute disturbances cause sudden dizziness and confusion and, in more severe cases, presyncope and syncope up to full Morgagni–Adams–Stokes attacks.<Cite n={1} /> In Mobitz type II, patients may be asymptomatic or experience light-headedness, presyncope and syncope, depending on the ratio of conducted to blocked beats.<Cite n={2} />

In third-degree block, an escape rhythm originating above the bifurcation of the His bundle produces narrow QRS complexes, a relatively rapid and reliable heart rate, and mild symptoms (fatigue, postural light-headedness, effort intolerance).<Cite n={2} /> An escape rhythm originating below the bifurcation produces wider QRS complexes and a slower, unreliable heart rate, with more severe symptoms: presyncope, syncope and heart failure.<Cite n={2} /> With an escape rhythm from below the bifurcation of the His bundle, the risk of asystole-related syncope and sudden death is greater.<Cite n={2} />

</Student>

## How it is diagnosed

<Patient>

AV block is found on an [electrocardiogram](../../diagnostic/ecg/) (ECG).<Cite n={[1, 2]} /> If the problem comes and goes, the rhythm is recorded for a long time, with a [Holter monitor](../../diagnostic/holter-monitoring/) for a day or longer or with event recorders, including implantable ones, so that the recording can be matched with the symptoms.<Cite n={1} />

The doctor also looks for causes that can be removed, including those related to medicines the person takes.<Cite n={1} /> In some forms of block, a heart ultrasound (echocardiography) is done if heart disease is suspected.<Cite n={1} />

</Patient>

<Student>

The diagnosis is made according to the classification, based on documented confirmation by ECG and/or long-term ECG monitoring together with the clinical symptoms.<Cite n={1} /> A 12-lead ECG is recommended for all patients with suspected bradycardia or a conduction disorder.<Cite n={1} />

ECG features by degree:

- First degree: every normal P wave is followed by a QRS complex, but the PR interval is longer than normal (more than 0.20 s).<Cite n={2} />
- Second degree, Mobitz type I (Wenckebach phenomenon): the PR interval lengthens from beat to beat until the atrial impulse is not conducted and the QRS complex is dropped; the next conducted beat has the shortest PR interval, and the sequence repeats.<Cite n={[1, 2]} /> If the QRS complex is narrow, the block is nearly always in the AV node.<Cite n={2} />
- Second degree, Mobitz type II: the PR interval is constant, some beats are not conducted, and QRS complexes are dropped.<Cite n={[1, 2]} /> Mobitz type II is always pathological.<Cite n={2} /> The block is in the His bundle in 25% of patients and in the bundle branches in the rest.<Cite n={2} />
- Advanced (high-grade) second-degree block: every second P wave, or several P waves in a row, are blocked.<Cite n={[1, 2]} /> When every second P wave is blocked (2:1), Mobitz type I and type II are difficult to tell apart, because two P waves are never conducted in a row.<Cite n={2} />
- Third degree: no P waves are conducted and complete AV dissociation develops; cardiac function is maintained by a junctional or ventricular escape rhythm.<Cite n={[1, 2]} />

Long-term rhythm monitoring is recommended to detect conduction disorders and to correlate the ECG with symptoms.<Cite n={1} /> Transthoracic echocardiography is recommended in Mobitz type II, high-grade or third-degree AV block if structural heart disease is suspected.<Cite n={1} />

</Student>

## How it is treated

<Patient>

Treatment depends on the degree of block, the symptoms and the cause; the doctor makes the decision.<Cite n={[1, 2]} /> First-degree block usually needs no treatment.<Cite n={2} />

First, causes that can be removed are looked for and dealt with, such as the effect of a medicine.<Cite n={1} /> If the block went away after such a medicine was stopped, it can come back even without the medicine, so follow-up continues.<Cite n={2} />

If a slow rhythm has caused a dangerous condition, doctors use medicines that speed up the rhythm for a short time, and temporary pacing.<Cite n={1} />

The main planned treatment is a permanent pacemaker.<Cite n={[1, 2]} /> This is a small device placed under the skin, usually below the collarbone; through a lead it sends an impulse to the heart when the heart's own rhythm slows down or pauses.<Cite n={1} /> Fitting it does not require open-heart surgery.<Cite n={1} /> In acquired Mobitz type II, advanced second-degree and third-degree block that is not caused by reversible or physiological factors, a pacemaker is recommended even without symptoms: in block below the AV node, symptoms can start suddenly when the block becomes complete and show as fainting.<Cite n={[1, 2]} />

</Patient>

<Student>

Treatment is divided into correction of acute, potentially reversible conditions and planned treatment: in acute conditions, drug therapy and temporary pacing; in chronic conduction disorders, permanent pacing with an implantable pacemaker.<Cite n={1} /> Acute treatment of bradycardia due to AV block begins with identifying and removing the causative factors and with drug therapy that can temporarily increase the heart rate.<Cite n={1} /> If the block is drug-induced, it is assessed whether the causative drug is essential for this patient.<Cite n={1} /> In symptomatic second- and third-degree AV block with haemodynamic compromise that does not respond to drug therapy, temporary pacing is recommended until a permanent pacemaker is implanted.<Cite n={1} />

In acquired Mobitz type II, high-grade or third-degree AV block not due to reversible or physiological causes, pacemaker implantation is recommended regardless of symptoms: in infranodal block, symptoms can start suddenly as the block becomes complete and present as syncope.<Cite n={1} /> In first-degree and Mobitz type I block with clear symptoms typical of AV block, permanent pacing is recommended.<Cite n={1} />

First-degree AV block may be physiological in younger people with high vagal tone and in well-trained athletes, and Mobitz type I in younger and more athletic people.<Cite n={2} /> In Mobitz type I at the level of the AV node, treatment is often unnecessary: it is given if the block causes symptomatic bradycardia and transient or reversible causes have been excluded.<Cite n={2} />

</Student>

## When to call an ambulance

AV block can show itself as sudden fainting: in block below the AV node, symptoms can start suddenly when the block becomes complete.<Cite n={1} /> If the escape rhythm comes from the lower parts of the conduction system, the risk of fainting due to cardiac arrest (asystole) and of sudden death is higher.<Cite n={2} /> Emergency care for bradyarrhythmias with fainting is started by the ambulance team.<Cite n={1} /> NICE treats fainting in a person with any degree of heart block on the ECG as a red flag: such a person is referred urgently for cardiovascular assessment, and a specialist reviews the referral within 24 hours (the NICE guideline covers people over 16).<Cite n={3} />

If someone has lost consciousness, call your local emergency number (112 in the EU, 911 in the US and Canada, 999 in the UK).<Cite n={1} /> The site's general safety rule, repeated at the bottom of every page: "If you have chest pain, severe shortness of breath or have fainted, call your local emergency number immediately (112 in the EU, 911 in the US and Canada, 999 in the UK)." Sudden dizziness, confusion or feeling faint can also be signs of an acute conduction problem.<Cite n={1} /> The Russian Ministry of Health guideline names feeling faint, weakness or dizziness that may be linked to a slow rhythm as a reason for hospital admission, so do not put off seeing a doctor.<Cite n={1} /> If dizziness or feeling faint comes back in a person with a pacemaker, this is a reason for an unscheduled visit to the doctor, and if they lose consciousness, call an ambulance.<Cite n={1} />

<Quiz />