Heart block

Summary

• Heart block (AV block) is a delay or interruption in the transmission of an impulse from the atria to the ventricles.
• There are different types of heart block: first-degree AV block, more correctly called prolonged AV conduction, second-degree heart block, and third-degree heart block.
• The abnormal heart rhythm most commonly causes dizziness and fainting spells. The severity of these symptoms will depend on the nature and extent of the heart block and any underlying diseases contributing to the block.
• Heart block is easily diagnosed on ECG. However, for patients whose symptoms are intermittent, a special Holter ECG may be needed.
• Because many cases of heart block are due to damage of the conducting system as a result of heart attacks, any step taken to prevent heart attacks will help prevent this complication.

What is heart block?

Heart block (AV block) is a delay or interruption in the transmission of an impulse from the atria to the ventricles. This is caused by an anatomical or functional impairment in the conduction system. The disturbance may be temporary or permanent. Conduction may be delayed, intermittent or even absent. Each type of problem has a specific descriptive name.

To understand heart block, you need to understand the mechanism of how the heart beats. There is a specialised system of electrical fibres in the heart with two main nodes, or hubs, from which the electrical impulses spread.

The electrical impulse originates at the sino-atrial (SA) node - the "sinus impulse" - and then spreads to the atrio-ventricular (AV) node. From the AV node, the impulse is then delivered to the two ventricles, causing them to contract normally. This normal state is called "sinus rhythm".

Any abnormality in the conduction of this sinus impulse to the ventricles may signal the development of heart block.

The conduction system of the heart thus includes the SA node, the AV node, and the fibres (called the "Bundles of His") conducting the impulses to the ventricles.

In heart block, it is mainly the AV node that is affected, although problems may arise at any point along the system.

What causes heart block?

The AV node is supplied by two different parts of the autonomic nervous system: the parasympathetic and sympathetic. This is the system that controls involuntary actions in the body.

The sympathetic nervous system has nerves distributed to the heart, blood vessels and lungs, intestines and other organs in the abdomen. It governs functions in these areas by reflex action in balance with the parasympathetic nervous system, which supplies the same areas, but often provides the opposite function to the sympathetic nervous system.

The AV node is sensitive to what is called autonomic tone - the action of the two branches of the autonomic nervous system together.

Heart block may be congenital.

Other than the congenital form, there are three broad categories accounting for the vast majority of causes for heart block:

• Underlying structural abnormalities of the node. This is the most common cause and includes degenerative changes (50%), which may be familial or associated with calcification of the heart valve rings. Ischaemic heart disease (40%), including damage caused by a heart attack, is another common cause.
• Drugs slowing conduction at the AV node include digitalis, beta blockers and calcium-channel blockers. These are often used for patients with underlying heart disease and for hypertension, thus compounding the problem.
• Change in autonomic tone.
• Other causes include congenital block; damage to the conduction system during catheter procedures (e.g. closure of a VSD); cardiomyopathies (HOCM, amyloid, sarcoid); myocarditis; malignancies; and electrolyte disturbances.

Different types of heart block

The different types include:

• First-degree AV block, more correctly called prolonged AV conduction
• Second-degree heart block
• Third-degree heart block

Each of these types of heart block is characterised by the different way in which the conducting system is disturbed.

Symptoms and signs

The abnormal heart rhythm most commonly causes dizziness and fainting spells. The severity of these symptoms will depend on the nature and extent of the heart block and any underlying diseases contributing to the block.

How is it investigated?

Heart block is easily diagnosed on ECG. However, for patients whose symptoms are intermittent, a special Holter ECG may be needed. Here, the patient wears a portable mini ECG for 24-48 hours: this will record any abnormalities, allowing the cardiologist to make an accurate diagnosis.

How is it treated?

Once the type of block has been identified, any associated causes must be sought. If any of these are reversible, e.g. electrolyte disturbances or inappropriate drugs, this is treated and the block is re-assessed. Some associated causes are reversible, e.g. damage to the conduction system from a heart attack.

If the block remains, the cardiologist may recommend the insertion of a pacemaker, which takes over the function of the heart's damaged system. The pacemaker does not cure the underlying condition, but can regulate and maintain a normal effective heart rate.

There are other specialised circumstances in which a pacemaker may not be required, but full investigation is needed by a specialist in this field before any decision can be made.

Persons who have permanent pacemakers may need to observe certain precautions, e.g. not storing cellphones in pockets directly over the unit, not lingering when passing through security systems, avoiding MRI scans unless the radiologist gives approval, and avoiding blood-thinning medications unless prescribed by a cardiologist who is aware of the presence of a pacemaker.

What is the outcome?

The outcome will depend largely on the cause of the block. If a reversible cause is identified, e.g. a drug, then removing that cause will cure the problem. If no reversible problems are found and a pacemaker is inserted, the outcome is highly successful.

Prevention

Congenital abnormalities of the conducting system can obviously not be prevented.

Judicious use of medication, such as beta-blockers or digitalis, may prevent heart attacks while still achieving the desired therapeutic effect.

Because many cases of heart block are due to damage of the conducting system as a result of heart attacks, any step taken to prevent heart attacks will help prevent this complication. These steps will mainly involve lifestyle factor modifications.

When to see your doctor

See your doctor urgently in any of the following situations:

• If you experience an episode of unexplained dizziness or fainting, especially if you know you have a heart problem or are on any of the medications mentioned (digitalis, beta-blockers or calcium-channel blockers).
• If you are being treated for high blood pressure and experience dizzy spells or fainting.

Reviewed by Dr A.G. Hall (B.Soc.Sc.(SW), MB,Ch.B),
December 2008.

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