Heart block

Definition

Heart block refers to a delay in the normal flow of electrical impulses that cause the heart to beat. They are further classified as first-, second-, or third-degree block.

Description

The muscles of the heart contract in a rhythmic order for each heart beat, because electrical impulses travel along a specific route called the conduction system. The main junction of this system is called the atrioventricular node (AV node). Just as on a highway, there are occasionally some delays getting the impulse from one point to another. These delays are classified according to their severity.

In first-degree heart block, the signal is just slowed down a little as it travels along the defective part of the conduction system so that it arrives late traveling from the atrium to the ventricle.

In second-degree heart block, not every impulse reaches its destination. The block may affect every other beat, every second or third beat, or be very rare. If the blockage is frequent, it results in an overall slowing of the heart called bradycardia.

Third-degree block, also called complete heart block, is the most serious. When no signals can travel through the AV node, the heart uses its backup impulse generator in the lower portion of the heart. Though this impulse usually keeps the heart from stopping entirely, it is too slow to be an effective pump.

Causes & symptoms

First-degree heart block is fairly common. It is seen in teenagers, young adults and in well-trained athletes. The condition may be caused by rheumatic fever, some types of heart disease and by some drugs. First-degree heart block produces no symptoms.

Some cases of second-degree heart block may benefit from an artificial pace-maker. Second-degree block can occasionally progress to third-degree.

Third-degree heart block is a serious condition that affects the heart's ability to pump blood effectively. Symptoms include fainting, dizziness and sudden heart failure. If the ventricles beat more than 40 times per minute, symptoms are not as severe, but include tiredness, low blood pressure on standing, and shortness of breath.

Young children who have received a forceful blunt chest injury, can experience first-, or second-degree heart block.

Diagnosis

Diagnosis of first-, and second-degree heart block is made by observing it on an electrocardiograph (ECG).

Third-degree heart block usually results in symptoms such as fainting, dizziness and sudden heart failure, which require immediate medical care. A physical exam and ECG confirm the presence of heart block.

Treatment

Some second- and almost all third-degree heart blocks require an artificial pacemaker. In an emergency, a temporary pacemaker can be used until an implanted device is advisable. Most people need the pacemaker for the rest of their lives.

Prognosis

Most people with first- and second-degree heart block don't even know they have it. For people with third-degree block, once the heart has been restored to its normal, dependable rhythm, most people, most people live full and comfortable lives.

Key Terms

Atrioventricular node (AV node)

Highly specialized area of the heart muscle which transmits electrical impulses.

Bradycardia

A slow heart rate, usually under 60 beats per minute.

Further Reading

For Your Information

Books

• McGoon, Michael D., Editor-in-Chief. Mayo Clinic Heart Book: The Ultimate Guide to Heart Health. New York: William Morrow and Company, Inc., 1993

  Periodicals

• Grady, Thomas A. et al. "Prognostic Significance of Exercise-induced Left Bundle-branch Block." Journal of the American Medical Association 279(2)(January 14, 1998): 153(4).

  Organizations

• American Heart Association. 7320 Greenville Avenue, Dallas, TX 75231. 1-800-889-7943.

Gale Encyclopedia of Medicine. Gale Research, 1999.