Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Atrioventricular Block


L. Brent Mitchell

, MD, Libin Cardiovascular Institute of Alberta, University of Calgary

Reviewed/Revised Jan 2023
Topic Resources

Atrioventricular block is a delay in the conduction of electrical current as it passes through the atrioventricular conduction system.

The electrical current that controls the contraction of heart muscle starts in the sinoatrial node, flows through the heart's upper chambers (atria) and then goes to the heart's lower chambers (ventricles) through a sort of electrical junction box called the atrioventricular node (AV node). The AV node is located in the lower part of the wall between the atria near the ventricles and provides the only electrical connection between the atria and ventricles. Otherwise, the atria are insulated from the ventricles by tissue that does not conduct electricity. The atrioventricular node delays transmission of the electrical current so that the atria can contract completely and the ventricles can fill with as much blood as possible before the ventricles are electrically signaled to contract.

The Conduction System

Causes of Atrioventricular Block

Most types of atrioventricular block are more common among older people. The most common causes are

Other causes include

Classification of Atrioventricular Block

Atrioventricular block is classified as

  • First-degree: Electrical conduction to the ventricles is delayed

  • Second-degree: Electrical conduction is intermittently blocked

  • Third-degree (complete): Electrical conduction is completely blocked

First-degree atrioventricular block

In first-degree atrioventricular block, every electrical impulse from the atria reaches the ventricles, but each is slowed for a fraction of a second as it moves through the atrioventricular node. First-degree atrioventricular block is common among well-trained athletes, teenagers, young adults, and people with a highly active vagus nerve. This disorder rarely causes symptoms and does not require treatment.

ECG: Reading the Waves

ECG: Reading the Waves

An electrocardiogram (ECG) represents the electrical current moving through the heart during a heartbeat. The current's movement is divided into parts, and each part is given an alphabetic designation in the ECG.

Each heartbeat begins with an impulse from the heart's pacemaker (sinus or sinoatrial node). This impulse activates the upper chambers of the heart (atria). The P wave represents activation of the atria.

Next, the electrical current flows down to the lower chambers of the heart (ventricles). The QRS complex represents activation of the ventricles.

The ventricles must then undergo an electrical change to get ready for the next heart beat. This electrical activity is called the recovery wave, which is represented by the T wave.

Many kinds of abnormalities can often be seen on an ECG. They include a previous heart attack (myocardial infarction), an abnormal heart rhythm (arrhythmia), an inadequate supply of blood and oxygen to the heart (ischemia), and excessive thickening (hypertrophy) of the heart's muscular walls.

Certain abnormalities seen on an ECG can also suggest bulges (aneurysms) that develop in weaker areas of the heart's walls. Aneurysms may result from a heart attack. If the rhythm is abnormal (too fast, too slow, or irregular), the ECG may also indicate where in the heart the abnormal rhythm starts. Such information helps doctors begin to determine the cause.

First-degree Atrioventricular Block

For first-degree block, conduction between the atrium (P) and ventricle (QRS) is slowed without skipped beats.

First-degree Atrioventricular Block

Second-degree atrioventricular block

In second-degree atrioventricular block, only some electrical impulses reach the ventricles. The heart may beat slowly, irregularly, or both.

There are two types of second-degree atrioventricular block, type 1 and type 2. Type 1 may be present in young, athletic people, but type 2 always indicates heart disease is present. Sometimes second-degree atrioventricular block progress to third-degree atrioventricular block.

Third-degree atrioventricular block

In third-degree atrioventricular block, no impulses from the atria reach the ventricles, and the ventricular rate and rhythm are controlled by the atrioventricular node, bundle of His, or the ventricles themselves. These substitute pacemakers are slower than the heart’s normal pacemaker (sinus or sinoatrial node) and are often irregular and unreliable. Thus, the ventricles beat very slowly—less than 50 beats per minute and sometimes as slowly as 30 beats per minute. Third-degree atrioventricular block is a serious abnormal heart rhythm (arrhythmia) that can affect the heart’s pumping ability. Fatigue, dizziness, and fainting are common. Symptoms are less severe when the ventricles beat faster than 40 beats per minute.

Third-degree Atrioventricular Block

In third-degree atrioventricular block, the atria (p) beat independently of the ventricles (QRS).

Third-degree Atrioventricular Block

Diagnosis of Atrioventricular Block

Treatment of Atrioventricular Block

  • Sometimes insertion of an artificial pacemaker

First-degree atrioventricular block generally requires no treatment.

A temporary pacemaker may be used in an emergency until a permanent one can be implanted. Most people need an artificial pacemaker (see figure ) for the rest of their lives, although heart rhythm may return to normal if the cause of the atrioventricular block resolves—for example, after the drug that caused the atrioventricular block is stopped or after recovery from a heart attack.

More Information

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Cardizem, Cardizem CD, Cardizem LA, Cardizem SR, Cartia XT , Dilacor XR, Dilt-CD , Diltia XT, Diltzac, Matzim LA, Taztia XT, TIADYLT ER, Tiamate, Tiazac
Calan, Calan SR, Covera-HS, Isoptin, Isoptin SR, Verelan, Verelan PM
Digitek , Lanoxicaps, Lanoxin, Lanoxin Pediatric
Cordarone, Nexterone, Pacerone
quiz link

Test your knowledge

Take a Quiz!