What is heart block?

Heart block is a problem with the heart's electrical system that causes your heart to beat too slowly (bradycardia). The condition is also called atrioventricular (AV) block. The AV node is a cluster of cells that connects the electrical activity in the top chambers of the heart (atria) to the bottom chambers of the heart (ventricles). If you have heart block, the electrical signal from the AV node to the ventricles that controls your heartbeat is partly or completely blocked from reaching the ventricles.

What are the types of heart block?

Heart block can be first, second or third degree, depending on the extent of electrical signal impairment.

First-degree heart block: The electrical impulse still reaches the ventricles, but moves more slowly than normal through the AV node.

Second-degree heart block is classified into two categories: Type I and Type II.

Type I, also called Mobitz Type I or Wenckebach’s AV block): The less serious form of second-degree heart block. The electrical signal gets slower and slower until your heart actually skips a beat.

Type II, also called Mobitz Type II: Some of the electrical signals do not reach the ventricles, and the pattern is irregular. Your heartbeat may be slower than normal.

Third-degree heart block: The electrical signal from the atria to the ventricles is completely blocked. To make up for this, a section of the ventricles acts like a pacemaker to create electrical signals. The signals keep the heart pumping blood, but at a much slower rate than normal.

Who is at risk of having heart block?

Heart block can be present at birth (congenital), but that is not usually the case. Your risk of having heart block gets greater as you age and is higher if you have heart disease.

First-degree heart block is common among well-trained athletes, teenagers, young adults, and people with a highly active vagus nerve.

What causes heart block?

The most common cause of heart block is heart attack. Other causes include heart disease, problems with the heart’s structure and rheumatic fever. Heart block can also be caused by damage to the heart during open heart surgery, as a side effect of some medications or exposure to toxins.

What are the symptoms of heart block?

First-degree heart block often does not cause symptoms. It may be found during a routine electrocardiogram (ECG). The heart rate and rhythm are usually normal.

Symptoms of second- and third-degree heart block include fainting, chest pain and feeling dizzy, tired or short of breath. Symptoms of third-degree heart block, the are more intense due to the slow heart rate. If you have severe symptoms get medical attention right away.

How is heart block diagnosed?

Electrophysiologists are doctors who specialize in the electrical activity of the heart. Your primary doctor or cardiologist may refer you to an electrophysiologist to find out if you have heart block.

An evaluation includes an exam, review of your health history and medications, and tests.

An ECG records your heart’s electrical activity and shows problems with your heart. This is a useful test to check for heart block. You may need to wear an ambulatory monitor for 24 to 48 hours so the doctor can collect more information about your heart’s electrical activity. Common devices are a Holter monitor and event recorder. If you need to use a monitor, you will get detailed information about how to use it.

An electrophysiology study can also help diagnose heart block. This involves inserting a long, thin tube called a catheter through a blood vessel and guiding it to the outside of your heart. The test allows your doctor to map your heart’s electrical activity and find any problems. If you need this test, you will get more detailed information.

Last reviewed by a Cleveland Clinic medical professional on 07/18/2019.

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy