(See also Overview of Abnormal Heart Rhythms and Overview of Heart Block.)
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.
Atrioventricular block is classified as
Most types of atrioventricular block are more common among older people. The most common causes are
Other causes include
A few cases of atrioventricular block are caused by drugs, particularly those that slow conduction of electrical impulses through the atrioventricular node (such as beta-blockers, diltiazem, verapamil, digoxin, and amiodarone), rheumatic heart disease, or sarcoidosis that affects the heart.
Tracing the Heart’s Electrical Pathway
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.
ECG: Reading the Waves
Second-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. When the ventricles beat faster than 40 beats per minute, symptoms are less severe.
Diagnosis
Electrocardiography (ECG) is used to detect atrioventricular block. Each degree of block produces a particular pattern. First-degree atrioventricular block can be detected only by (ECG), which shows the conduction delay.
Treatment
First-degree atrioventricular block generally requires no treatment.
Some people with second-degree atrioventricular block require an artificial pacemaker. Almost all people with third-degree atrioventricular block require an artificial pacemaker.
A temporary pacemaker may be used in an emergency until a permanent one can be implanted. Most people need an artificial pacemaker (see figure Keeping the Beat: Artificial Pacemakers) 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 is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
-
American Heart Association: Arrhythmia: Information to help people understand their risks of arrhythmias as well as information on diagnosis and treatment
