Atrioventricular Block

The heart's electrical signal starts in the sinus node, spreads through the upper chambers (atria), then passes through the atrioventricular node (AV node) before entering the lower chambers (ventricles). The AV node works like a gate that adds a short pause so the atria can finish filling the ventricles with blood. In atrioventricular block, or AV block, the signal at this gate or in the pathway below it is slowed or stopped. The disturbance can be permanent or come and go, and its effect depends on the degree and location of the block.

Degrees of AV Block

First-degree block means every signal still reaches the ventricles, only more slowly. It shows up on an ECG and usually causes no symptoms. Second-degree block means some signals are not conducted. Mobitz type I, or Wenckebach, is often found in fit young people or during sleep and is usually harmless. Mobitz type II usually occurs lower in the pathway and is more likely to progress to complete block. Third-degree block, or complete block, means no signals from the atria are conducted. The ventricles then beat on their own at a slow rate, often 30 to 45 beats per minute.

Causes

In adults, common causes are aging and hardening of the tissue in the heart's electrical system. Other causes include heart attack, medicines that slow conduction, heart valve surgery or procedures such as catheter-based aortic valve replacement (TAVI), inflammation of the heart muscle, sarcoidosis, and high potassium levels. There is also congenital AV block, for example in babies born to mothers with lupus.

Why the Degree Matters

The degree and location of the block determine treatment. First-degree block and Mobitz I without symptoms usually only need monitoring. Mobitz II block, high-grade block, and complete block usually require a permanent pacemaker even before symptoms appear, unless the cause is temporary and can be corrected. In younger people with no clear cause, the doctor also looks for an underlying heart muscle disease.

AV block is one source of a slow heartbeat. An overview of slow heartbeats from various causes is covered on the Bradycardia page.

AV block is often first seen on a routine ECG. The ECG shows the relationship between the electrical waves of the atria and the ventricles, so the doctor can determine the degree of block. The width of the QRS complex and changes when the heart rate increases give clues as to whether the block is in the AV node or lower in the pathway.

If the block comes and goes, a Holter monitor or long-term recorder is used to capture it. The doctor also looks for correctable causes: medicines that slow conduction, high potassium, heart attack, or inflammation of the heart muscle. Echocardiography assesses pumping function and the valves. In people under 60 with no clear cause, the doctor may recommend a cardiac MRI to look for sarcoidosis or other heart muscle diseases.

If your ECG shows AV block, discuss the results with a cardiologist even if you feel well, especially if it is described as second- or third-degree block. Also make an appointment if you tire easily, feel dizzy, get short of breath with light activity, or often feel as if your heart skips a beat.

Go to the emergency room right away if:

• You faint, especially suddenly and without warning
• Your pulse is very slow, with weakness, paleness, or cold sweats
• You develop chest pain, severe shortness of breath, or sudden confusion
• Your heart rate slows noticeably after valve surgery or TAVI