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Overview of Heart Block

By L. Brent Mitchell, MD, Professor of Medicine, Department of Cardiac Services, Libin Cardiovascular Institute of Alberta, University of Calgary

Heart block is a delay in the conduction of electrical current as it passes through 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.

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 (see Figure: Keeping the Beat: Artificial Pacemakers) 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.

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.

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