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Understanding Rhythms

Understanding Rhythms

Heart Block

Heart Block is a type of bradycardia (too-slow heartbeat) that also is called atrioventricular, or AV block. In this condition, the electrical signals that stimulate heart muscle contractions are partially or totally blocked between the upper chambers (atria) and the lower chambers (ventricles).

The atrioventricular (AV node) receives signals from the sinoatrial (SA) node, the heart's master pacemaker, and transmits them to the lower chambers (ventricles) of the heart.

The ventricles are the major pumping chambers of the heart. The electrical signal transmitted from the SA and AV nodes trigger the muscle contractions that pump blood out of the heart and into the lungs and body.

What are the types of heart block?

First-degree heart block. The electrical impulses are slowed as they pass through the conduction system, but all of them successfully reach the ventricles. First-degree heart block rarely causes any symptoms or problems, and well-trained athletes may have this. Medications can contribute to the condition. No treatment is generally necessary for first degree heart block.

Type I second-degree heart block (also known as Mobitz Type I second-degree AV block or Wenckebach AV block). In this condition, the electrical impulses are delayed further and further with each heartbeat until a beat is skipped entirely. The condition generally is not as serious as type II second-degree heart block, but it sometimes causes dizziness and/or other symptoms. Normal people may sometimes have this when they are sleeping.
Type II second-degree heart block (Mobitz Type II second-degree AV block) is also a condition in which some of the electrical impulses are unable to reach the ventricles. This condition is less common than Type I, but is generally more serious. In some cases, a pacemaker is implanted to treat the abnormally slow heartbeat that may result from this condition.

Third-degree heart block (also known as complete heart block or complete AV block) is when none of the electrical impulses can reach the ventricles. When the ventricles, (lower chambers), do not receive electrical impulses from the atria (upper chambers), they may generate some impulses on their own called functional or ventricular escape beats. Ventricular escape beats, natural backup signals, usually are very slow, however, and cannot generate the signals needed to maintain full functioning of the heart muscle.

Bundle Branch Block is when electrical impulses are slowed or blocked as they travel through specialized conducting tissue in the ventricles.


The Bundle Branches


A structure called the bundle of His emerges from the AV node and divides into thin, wire-like structures called bundle branches that extend into the right and the left ventricles. The electrical signal travels down the bundle branches to thin fibers that distribute the electrical impulse to the muscles of the ventricles, the major pumping chambers of the heart.

Bundle Branch Block often produces no symptoms, although some people may either faint (syncope) or feel as if they're going to faint (presyncope). If both bundles are diseased, heart block may result producing syncope (fainting) or preseyncope (feeling like fainting). When this happens, the heartbeat may be so slow that an artificial pacemaker is implanted, even if the heart is otherwise healthy. Treatment may also be necessary if bundle branch block is caused by an underlying heart condition, such as damage to the heart muscle from a heart attack (myocardial infarction). Depending on where the damage is located, a person may be diagnosed with one of several specific types of bundle branch block, including:

Left bundle branch block (LBBB) is when there is a defect in the left bundle branches that convey electrical signals to the ventricles; if the defect is in the right bundle branch, it is called right bundle branch block (RBBB). The left bundle is divided into two "paths" called the anterior and posterior fascicles. Bifascicular block is when defects occur in two of the divisions of the bundle branches (right bundle, left anterior fascicle and left posterior fascicle). For example, people with this condition may be diagnosed with "RBBB with left anterior hemiblock," and so on.


Trifascicular block occurs when an individual is diagnosed with a form of bifascicular block as well as a first-degree heart block.

What are the symptoms of heart block?

Symptoms of heart block may include the following. Sometimes, however, there are no symptoms at all.

  • Syncope (fainting)

  • Dizziness

  • Lightheadedness

  • Chest pain

  • Shortness of breath

What are some of the risk factors associated with heart block?

History of cardiac disease (for example, myocardial infarction, congestive heart failure, valvular failure).
Some medications or exposure to toxic substances can cause heart block or other types of bradycardia.
Other prior illnesses

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