Merck Manual

Please confirm that you are not located inside the Russian Federation


Overview of Heart Block


L. Brent Mitchell

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

Last full review/revision Jul 2019| Content last modified Jul 2019
Click here for the Professional Version
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Topic Resources

Heart block is a delay in the conduction of electrical current as it passes through the conduction system of the heart, including the atrioventricular node, bundle of His, or both bundle branches, all of which are located between the atria and the ventricles.

  • Some types of heart block cause no symptoms, but others cause fatigue, dizziness, light-headedness, and/or fainting.

  • Electrocardiography is used to detect heart block.

  • Some people require an artificial pacemaker.

Normal electrical pathway

Contraction of the muscle fibers in the heart is controlled by electricity that flows through the heart in a precise manner along distinct pathways at a controlled speed. The electrical current that begins each heartbeat originates in the heart’s pacemaker (called the sinus node or sinoatrial node), located in the top of the upper right heart chamber (right atrium). The rate at which the pacemaker discharges the electrical current determines the heart rate. This rate is influenced by nerve impulses and by levels of certain hormones in the bloodstream.

The electrical current from the sinoatrial node flows first through the right atrium and then through the left atrium, causing the muscles of these chambers to contract and blood to be pumped from the atria into the lower heart chambers (ventricles). The electrical current then reaches the atrioventricular node, located in the lower part of the wall between the atria near the ventricles. The atrioventricular node 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.

Tracing the Heart’s Electrical Pathway

The sinoatrial (sinus) node (1) initiates an electrical impulse that flows through the right and left atria (2), making them contract. When the electrical impulse reaches the atrioventricular node (3), it is delayed slightly. The impulse then travels down the bundle of His (4), which divides into the right bundle branch for the right ventricle (5) and the left bundle branch for the left ventricle (5). The impulse then spreads through the ventricles, making them contract.

Tracing the Heart’s Electrical Pathway

After passing through the atrioventricular node, the electrical current travels down the bundle of His, a group of fibers that divide into a left bundle branch for the left ventricle and a right bundle branch for the right ventricle. The electrical current then spreads in a regulated manner over the inner surface of the ventricles, from the bottom up, initiating contraction of the ventricles, which eject blood from the heart.

Heart blocks

Heart blocks can be divided into

Usually, no treatment is needed for either type when the block is incomplete (such as in first-degree atrioventricular block). However, an artificial pacemaker may be implanted in people who are at high risk of complete atrioventricular block (such as people with certain types of second-degree atrioventricular block or in people with third-degree atrioventricular block) to maintain the heart rate if complete heart block occurs.

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Others also read

Also of Interest


View All
High Blood Pressure
High Blood Pressure
The heart is a beating muscle that pumps blood to the body through a network of arteries....
3D Models
View All
Lymphatic System
3D Model
Lymphatic System