Sinus Node Dysfunction

(See also Overview of Abnormal Heart Rhythms Overview of Abnormal Heart Rhythms Abnormal heart rhythms (arrhythmias) are sequences of heartbeats that are irregular, too fast, too slow, or conducted via an abnormal electrical pathway through the heart. Heart disorders are... read more .)

Dysfunction of the heart’s natural pacemaker (the sinus or sinoatrial node) may result in a persistently slow heartbeat (sinus bradycardia) or complete cessation of normal pacemaker activity (sinus arrest—see figure ). When activity ceases, another area of the heart usually takes over the function of the sinus node. This area, called an escape pacemaker, may be located lower in the atrium, in the atrioventricular node, in the conduction system, or even in the ventricle.

All types of sinus node dysfunction are more common among older people. Some drugs and an underactive thyroid gland Hypothyroidism Hypothyroidism is underactivity of the thyroid gland that leads to inadequate production of thyroid hormones and a slowing of vital body functions. Facial expressions become dull, the voice... read more (hypothyroidism) can cause sinus node dysfunction. However, the cause is usually unknown. When the cause is unknown, the disorder is called sick sinus syndrome.

An important subtype of the sick sinus syndrome is the bradycardia-tachycardia syndrome, in which periods of slow heart rhythms (bradycardia) alternate with periods of fast atrial arrhythmias (tachycardia), such as atrial fibrillation and atrial flutter Atrial Fibrillation and Atrial Flutter Atrial fibrillation and atrial flutter are very fast electrical discharge patterns that make the atria (upper chambers of the heart) contract very rapidly, with some of the electrical impulses... read more .

Sinus node dysfunction affects mainly older people, especially those with another heart disorder or with diabetes Diabetes Mellitus (DM) Diabetes mellitus is a disorder in which the body does not produce enough or respond normally to insulin, causing blood sugar (glucose) levels to be abnormally high. Urination and thirst are... read more . The most common cause is formation of scar tissue (fibrosis) in the sinus node. The cause of fibrosis is usually unknown, but known causes of sinus node dysfunction include drugs (for example, beta-blockers and other drugs used for abnormal heart rhythms), excessive impulses from the vagus nerve, which inhibit the heartbeat, and many disorders that limit blood flow (for example, coronary artery disease Overview of Coronary Artery Disease (CAD) Coronary artery disease is a condition in which the blood supply to the heart muscle is partially or completely blocked. The heart muscle needs a constant supply of oxygen-rich blood. The coronary... read more ) or cause inflammation (for example, rheumatic fever Rheumatic Fever Rheumatic fever is inflammation of the joints, heart, skin, and nervous system, resulting from a complication of untreated streptococcal infection of the throat. This condition is a reaction... read more or inflammation of the heart muscle, called myocarditis Myocarditis Myocarditis is inflammation of the muscle tissue of the heart (myocardium) that causes tissue death. Myocarditis may be caused by many disorders, including infection, toxins and drugs that affect... read more ).