Heart block can have a number of different causes. It can occur if you have another heart condition or if you take certain medications.

A person can be born with heart block (congenital) or it can develop over time (acquired).

First-degree heart block

First-degree heart block is common among well-trained athletes. Vigorous and prolonged exercise can enlarge the heart muscles, which can cause mild disruption to the heart's electrical signals.

Other causes of first-degree heart block include:

• myocarditis – inflammation of the heart muscle
• low levels of potassium in the blood (hypokalaemia)
• low levels of magnesium in the blood (hypomagnesemia)

Certain medications can also cause first-degree heart block, including:

• medications for treating abnormal heart rhythms (antiarrhythmics), such as disopyramide
• medications used to treat high blood pressure (hypertension), such as calcium channel blockers
• digoxin – a medication used to treat heart failure

Second-degree heart block

Athletes can develop second-degree heart block for the reasons discussed above.

Some children born with congenital heart disease (heart defects present at birth) can also develop second-degree heart block.

Other causes of second-degree heart block include:

• damage that develops during a heart attack
• Lyme disease – a bacterial infection spread by tics
• certain medications, such as calcium-channel blockers (used to treat high blood pressure), amiodarone (used to treat abnormal heart rhythms) and pentamidine (used to treat some types of pneumonia)

Third-degree congenital heart block

Many cases of third-degree congenital heart block occur in babies whose mothers have an autoimmune condition, such as lupus (a long-term condition that causes inflammation in the body's tissues).

In autoimmune conditions, the immune system produces antibodies (proteins) that attack and damage tissue and cells.

The immune system is thought to mistake the unborn baby for a foreign object, as it would a virus or bacteria, and sends antibodies to attack it which damage the heart.

Some children with congenital heart disease are also born with third-degree heart block.

Third-degree acquired heart block

Many cases of third-degree heart block are caused by damage to the heart. The heart muscle can become damaged for a number of reasons, including:

• a complication of heart surgery – this is thought to be one of the most common causes
• coronary heart disease – where the heart doesn't receive enough blood due to acquired narrowing or closing of the coronary arteries
• a complication of radiotherapy – a treatment for conditions such as cancer, thyroid disorders and some blood disorders
• a serious infection, such as diphtheria (a bacterial infection that can cause heart inflammation) or rheumatic fever (a bacterial infection that damages the heart's valves)
• poorly controlled high blood pressure (hypertension)
• cancer that has spread from another part of the body to the heart
• a penetrating trauma to the chest, such as a stab wound or gunshot wound
• a complication of treatment for heart rhythm problems (arrhythmia), known as radiofrequency ablation

A number of medications can also cause third-degree heart block including:

• digoxin
• calcium-channel blockers
• beta blockers – used to treat high blood pressure
• tricyclic antidepressants – an older type of antidepressant
• clonidine – used to treat a sudden, sharp rise in blood pressure (hypertensive crisis)