Heart 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
Scarring (fibrosis) of the heart's electrical
system caused by aging. This is the most common cause of heart
Use of certain medicines,
especially beta-blockers, calcium channel blockers, and digoxin.
Heart 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
first-degree 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.
does not usually require treatment. But this type of heart block may raise your risk of heart rhythm problems, such as atrial fibrillation.
Second-degree AV block
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
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
Complete or third-degree block
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
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
Complete 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
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 the branch.
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.
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 (2012). Specific arrhythmias: Diagnosis and treatment. In RO Bonow et al., eds., Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 9th ed., vol. 1, pp. 771–824. Philadelphia: Saunders.
Primary Medical Reviewer
E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer
Rakesh K. Pai, MD, FACC - Cardiology, Electrophysiology
How this information was developed to help you make better health decisions.