block refers to an abnormality in the way electricity passes through the normal electrical
pathways of the heart. The abnormality "blocks" the electrical impulse from continuing
through the normal pathways and usually results in a slower heart rate.
What causes heart block?
Heart block can be caused by:
(fibrosis) of the heart's electrical system caused by aging. This is the most common
cause of heart block.
Use of certain medicines,
especially beta-blockers, calcium channel blockers, and digoxin.
block is more common in older people and may be the result of age and a combination
of factors listed above. Heart block can occur in people who have had a heart attack.
When heart attacks cause heart block, it often goes away on its own. But if the heart
attack is extensive, the heart block may be permanent and require a pacemaker.
Where does the block occur?
The electrical activity
of the heart starts in the sinoatrial (SA) node in the upper chamber (atrium)
and travels through the atrioventricular (AV) node to reach the lower
chamber (ventricle). Heart block may occur at any point along this electrical pathway.
Heart block of the AV node can be of several types, and a doctor generally can diagnose
these by looking at the person's electrocardiogram(EKG, ECG).
What is atrioventricular (AV) block?
First-degree AV block
block, the electrical impulses take longer to travel between the upper chamber (atrium)
and lower chamber (ventricle) of the heart. This type of heart rhythm may or may not
be associated with a slow heart rate.
It does not usually require treatment.
But this type of heart block may raise your risk of heart rhythm problems, such as
Second-degree AV block
In second-degree heart block,
some of the electrical impulses are blocked between the upper and lower chamber of
the heart. These electrical impulses may or may not have a clear pattern. The blocking
of the impulse can come and go, resulting in "dropped heartbeats." A second-degree
type II block may progress to complete or third-degree heart block.
heart block can be categorized into two types:
Mobitz type I block (also
called Wenckebach) usually occurs in the AV node. It is common in young, healthy people
(especially during sleep). It usually does not cause symptoms and rarely requires
Mobitz type II block usually occurs below the AV node in other
conduction tissue. It may be part of aging. It is also seen in people with significant
heart disease or during a large heart attack. It may cause lightheadedness or fainting
(syncope). And it may progress to complete heart
block. This type frequently requires a pacemaker.
Complete or third-degree
In third-degree heart block, all of the electrical impulses are completely
blocked between the upper and lower chambers of the heart. When this occurs, the atria
and ventricles beat at completely different rates.
Complete heart block is caused
by the aging process, medicines, heart attacks, infiltrative heart diseases (amyloidosis,
sarcoidosis), and infectious diseases (endocarditis, Chagas disease). It may also
occur after heart surgery and can be present from birth (congenital).
heart block frequently causes symptoms of lightheadedness or fainting and usually
requires the placement of a permanent pacemaker. People who are born with complete
heart block (an uncommon congenital condition) often have no symptoms and may not
need treatment initially. But eventually they almost always require pacemaker placement.
What is bundle branch block?
Bundle branch block
can affect the heart's rhythm. The heart has structures, like wires, that are called
bundle branches. They are part of the heart's electrical pathway. When a branch is
diseased, it is called "blocked," because the electrical signals can't travel down
Some people with bundle branch block don't have any symptoms and
don't need treatment. But when a block causes the heart to beat too slowly, it can
cause symptoms such as tiredness and fainting spells. A pacemaker may be used to get
the heartbeat back to normal.
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Cheng S, et al. (2009).
Long-term outcomes in individuals with prolonged PR interval or first-degree atrioventricular
block. JAMA, 301(24): 2571-2577.
Olgin JE, Zipes DP (2015). Specific arrhythmias:
Diagnosis and treatment. In DL Mann et al., eds., Braunwald's Heart Disease: A Textbook
of Cardiovascular Medicine, 10th ed., vol. 1, pp. 748-797. Philadelphia: Saunders.
ByHealthwise Staff Primary
Medical ReviewerE. Gregory Thompson, MD - Internal
Medicine Martin J. Gabica, MD - Family Medicine Adam Husney, MD - Family Medicine Specialist
Medical ReviewerRakesh K. Pai, MD - Cardiology, Electrophysiology
This information does not replace the advice of a doctor.
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