Heart block

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview[edit | edit source]

Heart block is defined as impaired or abnormal conduction of electrical impulses in the heart. With each heartbeat, an electrical signal spreads across the heart from the upper (atria) to the lower chambers (ventricles). As it travels, the signal causes the heart to contract and pump blood. This process repeats with each new heartbeat. Heart block occurs if the electrical signal is slowed or disrupted as it moves from the upper to the lower chambers of the heart. This abnormality can occur in the heart muscle (myocardium) or in the specialized electrical conduction system of the heart.

Classification[edit | edit source]

A heart block can be a blockage at any level of the electrical conduction system of the heart:

• SA Nodal Block: Blocks that occur within the sinoatrial node (SA node) are described as SA nodal blocks.
• AV Nodal Block: Blocks that occur within the atrioventricular node (AV node) are described as AV nodal blocks.
• Infra-Hisian Block: Blocks that occur below the AV node are known as Infra-Hisian blocks (named after the bundle of His).
• Bundle Branch Block: Blocks that occur within the left or right bundle branches are known as bundle branch blocks.
• Hemiblock: Blocks that occur within the fascicles of the left bundle branch are known as hemiblocks.

Clinically speaking, most of the important heart blocks are AV nodal blocks and Infra-Hisian blocks.

Types of SA nodal blocks[edit | edit source]

The SA nodal blocks rarely give symptoms. This is because if an individual had complete block at this level of the conduction system (which is uncommon), the secondary pacemaker of the heart would be at the AV node, which would fire at 40 to 60 beats a minute, which is enough to retain consciousness in the resting state.

Types of SA nodal blocks include:

• SA node Wenckebach (Mobitz type I)^[1]
• SA node Mobitz type II
• SA node exit block

In addition to the above blocks, the SA node can be suppressed by any other arrhythmia that reaches it. This includes retrograde conduction from the ventricles, ectopic atrial beats, atrial fibrillation, and atrial flutter.

The difference between SA node block and SA node suppression is that in SA node block an electrical impulse is generated by the SA node that doesn't make the atria contract. In SA node suppression, on the other hand, the SA node doesn't generate an electrical impulse because it is reset by the electrical impulse that enters the SA node.

Types of AV nodal blocks[edit | edit source]

There are four basic types of AV nodal block:

• First degree AV block
• Second degree AV block
  • Type 1 second degree AV block (Mobitz I) (also known as Wenckebach phenomenon)
• Third degree AV block (Complete heart block)

Types of infrahisian block[edit | edit source]

Infrahisian block describes block of the distal conduction system. Types of infrahisian block include:

• Type 2 second degree heart block (Mobitz II)
• Left bundle branch block
  • Left anterior fascicular block
  • Left posterior fascicular block
• Right bundle branch block

Of these types of infrahisian block, Mobitz II heart block is considered most important because of the possible progression to complete heart block.

Pathophysiology[edit | edit source]

Damage to the heart muscle and its electrical system by diseases, surgery, or medicines can cause acquired heart block. Heart block can be either congenital or acquired. Acquired heart block is more common than congenital heart block.

Risk Factors[edit | edit source]

The risk factors for congenital and acquired heart block are different.

Congenital Heart Block[edit | edit source]

• If a pregnant woman has an autoimmune disease, such as lupus, her fetus is at risk for heart block.

• Autoimmune diseases can cause the body to make proteins called antibodies that can cross the placenta. (The placenta is the organ that attaches the umbilical cord to the mother's womb.) These antibodies may damage the baby's heart and lead to congenital heart block.

• Congenital heart defects also may result in congenital heart block. These defects are problems with the heart's structure that are present at birth. Most of the time, doctors don't know what causes these defects.

• Heredity may play a role in certain heart defects. For example, a parent who has a congenital heart defect may be more likely than other people to have a child with the condition.

Acquired Heart Block[edit | edit source]

• Acquired heart block can occur in people of any age. However, most types of the disorder are more common in older people. This is because many of the risk factors are more common in older people.

• People who have a history of heart disease or heart attacks are more likely to have heart block. Examples of heart disease that can lead to heart block include heart failure, coronary heart disease, and cardiomyopathy (heart muscle diseases).

• Other diseases also may raise the risk of heart block. These include sarcoidosis and the degenerative muscle disorders Lev's disease and Lenegre's disease.

• Exposure to toxic substances or taking certain medicines, such as digitalis, also can raise your risk of heart block.

• Well-trained athletes and young people are at higher risk for first-degree heart block caused by an overly active vagus nerve. You have one vagus nerve on each side of your body. These nerves run from your brain stem all the way to your abdomen. Activity in the vagus nerve slows the heart rate.

Causes[edit | edit source]

Life Threatening Causes[edit | edit source]

Common Causes[edit | edit source]

Causes by Organ System[edit | edit source]

Causes in Alphabetical Order[edit | edit source]

Diagnosis[edit | edit source]

Symptoms[edit | edit source]

Symptoms depend on the level of the heart block:

• First-degree heart block rarely causes symptoms.
• Symptoms of second- and third-degree heart block include:

• Chest pain
• Dizziness
• Fainting
• Fatigue
• Heart failure symptoms
• Light-headedness
• Pre-syncope
• Shortness of breath
• Syncope

Contraindicated medications[edit | edit source]

Heart block is considered an absolute contraindication to the use of the following medications:

• Cyclobenzaprine

See also[edit | edit source]

• Electrical conduction system of the heart
  • Sinoatrial node (SA node)
  • Atrioventricular node (AV node)

• Trifascicular heart block

• Lev's disease

References[edit | edit source]

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