Types of Heart Block

You can be born with heart block (congenital) or you can acquire it. Acquired heart block is more common.

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 electrical signal is slowed as it moves through the heart. When this occurs between the atria and the ventricles, it appears as a slightly longer, flatter line between the P and the R waves on the EKG.

First-degree heart block rarely causes any symptoms. Well-trained athletes and young people are at higher risk for first-degree heart block caused by an overly active vagus nerve. Activity in this nerve slows the heart rate. Some medicines, such as digitalis, also may trigger first-degree heart block.

First-degree heart block usually doesn't 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 can't reach the ventricles. On an EKG, the QRS wave doesn't follow each P wave as it normally would.

If the 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 different 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 wave doesn't follow the next P wave.

Sometimes people with 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 and 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, 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 and momentarily block the contraction of the ventricles.

Mobitz Type II

In this type, some of the electrical signals also 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 with 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 slow 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 than the QRS waves.

Complete heart block can be fatal. It can result in sudden cardiac arrest and death. This type of heart block needs emergency treatment. A temporary pacemaker may be used to keep the heart beating until you get a permanent pacemaker.