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Types of Heart Block

Some people are born with heart block (congenital), while others develop it during their lifetimes (acquired). Acquired heart block is more common than congenital heart block.

The three types of heart block are first degree, second degree, and third degree. First degree is the least severe, and third degree is the most severe. This is true for both congenital and acquired heart block.

First-Degree Heart Block

In first-degree heart block, the heart's electrical signals are slowed as they move from the atria to the ventricles (the heart's upper and lower chambers, respectively). This results in a longer, flatter line between the P and the R waves on the EKG (electrocardiogram).

First-degree heart block may not cause any symptoms or require treatment.

Second-Degree Heart Block

In this type of heart block, electrical signals between the atria and ventricles are slowed to a large degree. Some signals don't reach the ventricles. On an EKG, the pattern of QRS waves doesn't follow each P wave as it normally would.

If an electrical signal is blocked before it reaches the ventricles, they won't contract and pump blood to the lungs and the rest of the body.

Second-degree heart block is divided into two types: Mobitz type I and Mobitz type II.

Mobitz Type I

In this type (also known as Wenckebach's block), the electrical signals are delayed more and more with each heartbeat, until the heart skips a beat. On the EKG, the delay is shown as a line (called the PR interval) between the P and QRS waves. The line gets longer and longer until the QRS waves don't follow the next P wave.

Sometimes people who have Mobitz type I feel dizzy or have other symptoms. This type of second-degree heart block is less serious than Mobitz type II.

The animation below shows how your heart's electrical system works. It also shows what happens during second-degree Mobitz type I heart block. 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 first part of this animation is the same as the animation in the section on understanding the heart's electrical system and EKG results. If you want to skip directly to the frames on Mobitz type I heart block, click the "skip intro" link above the start, pause, and replay buttons.

The first part of the animation shows how an electrical signal moves through your heart and how an EKG records your heart's electrical activity. The second part of the animation shows how a pause in the heart's electrical signal can delay or block the contraction of the ventricles.

Mobitz Type II

In second-degree Mobitz type II heart block, some of the electrical signals don't reach the ventricles. However, the pattern is less regular than it is in Mobitz type I. Some signals move between the atria and ventricles normally, while others are blocked.

On an EKG, the QRS wave follows the P wave at a normal speed. Sometimes, though, the QRS wave is missing (when a signal is blocked).

Mobitz type II is less common than type I, but it's usually more severe. Some people who have type II need medical devices called pacemakers to maintain their heart rates.

Third-Degree Heart Block

In this type of heart block, none of the electrical signals reach the ventricles. This type also is called complete heart block or complete AV block.

When complete heart block occurs, special areas in the ventricles may create electrical signals to cause the ventricles to contract. This natural backup system is slower than the normal heart rate and isn't coordinated with the contraction of the atria. On an EKG, the normal pattern is disrupted. The P waves occur at a faster rate, and it isn't coordinated with the QRS waves.

Complete heart block can result in sudden cardiac arrest and death. This type of heart block often requires emergency treatment. A temporary pacemaker might be used to keep the heart beating until you get a long-term pacemaker.

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Heart Block Clinical Trials

Clinical trials are research studies that explore whether a medical strategy, treatment, or device is safe and effective for humans. To find clinical trials that are currently underway for Heart Block, visit

Know the Facts and Act Fast!

Collage image of new heart attack materials

When a heart attack happens, any delays in treatment can be deadly. 

Knowing the warning symptoms of a heart attack and how to take action can save your life or someone else’s.

The NHLBI has created a new series of informative, easy-to-read heart attack materials to help the public better understand the facts about heart attacks and how to act fast to save a life.

Click the links to download or order the NHLBI's new heart attack materials:

“Don’t Take a Chance With a Heart Attack: Know the Facts and Act Fast” (also available in Spanish)

“Heart Attack: Know the Symptoms. Take Action.”

“Learn What a Heart Attack Feels Like—It Could Save Your Life”

July 09, 2012 Last Updated Icon

The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.