Causes of Heart Block

Sinoatrial nodal blocks can be caused by a number of reasons including:

• Damage to the sinoatrial node itself – which is located in the right atrium of the heart;
• Damage to the myocardium – due to myocardial infarction;
• Hyperactivity of vagus nerve - a tenth cranial nerve responsible for parasympatic innervation of various organs.

Sinoatrial nodal blocks often occur in patients with:

• Congenital heart disease – a defect in the development of the heart and great vessels that is present at birth.
• Acquired valvular heart disease – a disease process that involves one or more heart valves, including aortic, mitral, pulmonary, and tricuspid, which leads to their stenosis or insufficiency.
• Myocarditis – inflammation of the heart muscle which is usually caused viruses, bacteria, or parasites.
• Coronary artery disease –a disease caused by the buildup of plaque on the interior lining of the coronary arteries, narrowing them and reducing the blood flow within the heart. This usually leads to a specific pain called angina pectoris, and is the leading cause of heart attack.
• Myocardial infarction – a condition during which the blood flow within a part of myocardium is impaired, causing hypoxia and eventually necrosis. Myocardial infarction usually occurs when atheroma (a characteristic buildup of cholesterol and calcium on the inner lining of the artery) gets damaged and attracts platelets. These platelets quickly obstruct the blood flow within the affected artery, which leads to hypoxia (lack of oxygen) in the tissues that this artery was supplying.

Sinoatrial nodal blocks can also occur as a result of taking certain medications in supratherapeutic doses:

• Beta-blockers – a class of drugs that targets beta receptors, which are found in myocardium, smooth muscles, arteries, airways, kidneys, and other tissues innervated by sympathetic nervous system. Beta-blockers are usually used to treat arrhythmias, hypertension, and secondary prevention of myocardial infarction.
• Calcium channel blockers – these drugs are mainly used as antihypertensive drugs (medications that lower blood pressure). Calcium channel blockers are also used to reduce chest pain caused by coronary artery disease, and to prevent cerebral vasospasm in people suffering from migraine.
• Cardiac glycosides – organic compounds that affect the force with which the cardiac muscle contracts. They are generally used in treatment of arrhythmias and congestive heart failure.
• Amidarone – a class III antiarrhythmic agent that blocks potassium channels prolonging repolarization of the cells.
• Class I antiarrhythmic agents – exhibit membrane stabilizing effect and are grouped together as they all interfere with sodium channels.

Other drugs and substances which can trigger SA nodal blocks include paclitaxel, lithium, dimethiyl sulfoxide, toluene, topical ophthalmic acetylcholine, alfentanil, fentanyl, reserpine, sufentanil, and clonidine.

In rare cases SA node can be affected by:

• Rheumatic fever – a disease caused by antibody cross-reactivity (antibodies that are formed to destroy proteins of streptococcus also affect proteins in certain tissues due to their similarity with microbial proteins) which appears a couple of weeks after streptococcal infection. This disease targets the heart, skin, joints, and the brain.
• Diphtheria – an illness of respiratory tract caused by Corynebacterium diphtheria. The most common symptoms are low fever, sore throat, and an adherent membrane on tonsils, pharynx and within the nasal cavity. In 20% of the cases this disease also affects the heart causing myocarditis.
• Electrical cardioversion (defibrillation) can also sometimes result in SA block.