Understanding the Heart's Electrical System and EKG Results

Doctors use a test called an EKG (electrocardiogram) to help diagnose heart block. This test detects and records the heart's electrical activity. An EKG records the strength and timing of electrical signals as they pass through each part of the heart.

The data is recorded on a graph so your doctor can study your heart's electrical activity. Different parts of the graph show each step of an electrical signal's journey through the heart.

A Healthy Heart Cross-Section

The illustration shows a cross-section of a healthy heart and its inside structures. The blue arrow shows the direction in which oxygen-poor blood flows from the body to the lungs. The red arrow shows the direction in which oxygen-rich blood flows from the lungs to the rest of the body.

Each electrical signal begins in a group of cells called the sinus node or sinoatrial (SA) node. The SA node is located in the right atrium (AY-tree-um), which is the upper right chamber of the heart. In a healthy adult heart at rest, the SA node fires off an electrical signal to begin a new heartbeat 60 to 100 times a minute.

From the SA node, the signal travels to the right and left atria. This causes the atria to contract and pump blood into the heart's two lower chambers, the ventricles (VEN-trih-kuls). This is recorded as the P wave on the EKG.

The signal passes between the atria and ventricles through a group of cells called the atrioventricular (AV) node. The signal slows down as it passes through the AV node. This slowing allows the ventricles time to finish filling with blood. On the EKG, this is the flat line between the end of the P wave and beginning of the Q wave.

The electrical signal then leaves the AV node and travels along a pathway called the bundle of His. From there the signal travels into the right and left bundle branches. On the EKG, this is the Q wave.

As the signal spreads across the right and left ventricles, they contract and pump blood out to the lungs and the rest of the body. On the EKG, R marks the contraction of the left ventricle and S marks the contraction of the right ventricle.

The ventricles then relax (shown as the T wave on the EKG). This entire process continues over and over with each new heartbeat.

The animation below shows how your heart's electrical system works. Click the "start" button to play the animation. Written and spoken explanations are provided with each frame. Use the buttons in the lower right corner to pause, restart, or replay the animation, or use the scroll bar below the buttons to move through the frames.

The animation shows how an electrical signal moves through your heart and how an EKG records your heart's electrical activity.

For more information on the heart's electrical system, go to the Diseases and Conditions Index How the Heart Works article.