Updated June 14, 2015.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.
"Heart block" -- also called atrioventricular block, or AV block -- is a relatively common problem affecting the heart's electrical system. Heart block is one of the two major causes of bradycardia (slow heart rate).

• Read about sinus bradycardia, the other major cause of bradycardia.

In heart block, the heart's electrical impulses are partially or completely blocked as they attempt to travel from the atrial chambers of the heart to the ventricular chambers.

If the heart block is severe enough, it may slow the heart rate to dangerously low levels.

• Read about the heart's electrical system.

Depending on its severity, heart block may produce a range of symptoms, from none at all to dizziness, syncope (loss of consciousness) or even death. Heart block that is severe enough to produce symptoms, or that threatens to become that severe, can be successfully treated with a pacemaker.
So if you have heart block, the main issue for you and your doctor will be to determine whether your heart block is serious enough to require a pacemaker.

When Does Heart Block Require A Pacemaker?

Deciding whether a person with heart block needs a pacemaker depends on several factors, including: the cause of the heart block, the "degree" of the heart block, the location of the heart block, and the symptoms it is causing.

What Is Causing Heart Block?

Brief episodes of heart block are not always dangerous, or even abnormal. Transient heart block is often seen in young, healthy people who experience a sudden increase in the tone of their vagus nerves.

This increased vagal tone often happens with nausea or vomiting, or in response to pain, fright or sudden stress. This form of heart block is not life-threatening and does not indicate any underlying problem with the heart's electrical system. It disappears immediately once the triggering event has subsided, and almost never requires insertion of a pacemaker.
On the other hand, heart block can also occur with various cardiac diseases, especially coronary artery disease, heart failure, or myocarditis. There are also familial forms of heart block. In general, when heart block is produced by heart disease, it means there is a permanent disorder of the cardiac electrical system. This kind of heart block often gets worse over time, so pacemakers are often required.

What Is The Degree Of The Heart Block?

Heart block is categorized by doctors into one of three "degrees." Your doctor can determine the degree of your heart block with an electrocardiogram (ECG).
First degree block means that each cardiac impulse eventually makes it from the atria to the ventricles, but conduction of the impulse is slowed.

Second degree block means that some of the impulses are successfully conducted to the ventricles, but some are not.

Third degree block means that all of the impulses are blocked. Third degree heart block is also referred to as "complete heart block."

With third degree heart block, the patient's survival depends on the existence of subsidiary pacemaker cells below the site of the block, that is, cells that generate their own electrical impulses, which allow the heart to keep on beating. The heart rhythm produced by these subsidiary pacemaker cells is called an "escape rhythm." Often, these escape rhythms are notoriously unreliable and fragile. Sometimes the presence of an escape rhythm is considered a medical emergency.

In general, the higher the degree of heart block, the more likely the need for a pacemaker. Pacemakers are almost always required with third degree block, often with second degree block, but only rarely with first degree block.

What Is The Location Of The Heart Block?

During the normal heart rhythm, the heart's electrical impulse must pass across the junction between the atria and the ventricles. This AV junction consists of two structures: the AV node, and the His bundle.
In determining the severity of heart block, it is important to know where in the AV junction the block is occurring. That is, is the block within the AV node, or is it in the His bundle (or the bundle branches, which are given off by the His bundle)? In most cases, the doctor can determine the location of the heart block simply by examining the ECG. Sometimes, however, an electrophysiology study is needed.

• Read more about the AV junction.

Heart block that occurs within the AV node (which is called "proximal" heart block) is usually relatively benign, and often does not require a permanent pacemaker. This is because when the block is occurring within the AV node, subsidiary pacemaker cells in the AV node just beyond the site of the block often take over the rhythm of the heart. This so-called "junctional escape rhythm" tends to be relatively stable, and does not pose a threat to life.
On the other hand, when third block occurs in the His bundle or below the His bundle (which is called "distal" heart block), the patient is left with a much more dangerous "ventricular escape rhythm," in which the subsidiary pacemaker cells are located within the ventricles. These pacemaker cells are unreliable, and are prone to suddenly stopping. Because it tends to worsen over time, even in cases where it is currently causing only first or second degree block, distal heart block is considered dangerous.

What Are The Symptoms Associated With Heart Block?

Heart block that is not producing any symptoms whatsoever usually does not require a pacemaker, unless it is either third degree heart block, or distal heart block.
If heart block is producing symptoms- - especially dizziness or syncope -- then treating it with a pacemaker is usually required. The exception is when the heart block is known to be transient -- for instance, when it is due to a temporary increase in vagal tone.

Pacemakers For Heart Block -- Summary

If you have heart block, here are the main factors that will determine whether you need a pacemaker:

• If your heart block was due to a transient increase in vagal tone, a pacemaker is almost never needed.
• If you have first degree heart block, a pacemaker is unnecessary unless the block is distal.
• If you have second degree heart block that is either producing symptoms or is distal, a pacemaker is needed. Second degree block that is not causing symptoms and is proximal usually does not require a pacemaker.
• If you have third degree heart block, then unless it is due to a transient increase in vagal tone, a pacemaker is almost always required.

So: If you have been told you have heart block or AV block, you and your doctor will need to consider all these factors in order to make the right decision regarding whether you need a pacemaker. Fortunately, gathering the necessary information is usually a relatively quick and straightforward process, and only occasionally requires an invasive electrophysiology study.
<sub>Sources:

Levine SA, Miller H, Penton GB. Some clinical features of complete heart block. Circulation 1956; 13:801.

Epstein AE, DiMarco JP, Ellenbogen KA, et al. ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to Revise the ACC/AHA/NASPE 2002 Guideline Update for Implantation of Cardiac Pacemakers and Antiarrhythmia Devices): developed in collaboration with the American Association for Thoracic Surgery and Society of Thoracic Surgeons. Circulation 2008; 117:e350.</sub>