(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 Tracing the Heart's Electrical Pathway Tracing the Heart’s Electrical Pathway 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 ). 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.
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. Other causes include drugs (for example, beta-blockers and other drugs used for abnormal heart rhythms), excessive impulses from the vagus nerve, which inhibit the heartbeat (as may occur in well-trained athletes), 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 ).
Many types of sinus node dysfunction cause no symptoms. A persistent slow heart rate commonly causes weakness and tiredness. Fainting may occur if the rate becomes very slow.
A fast heart rate is often perceived by the person as palpitations Palpitations Palpitations are the awareness of heartbeats. The sensation may feel like pounding, fluttering, racing, or skipping beats. Other symptoms—for example, chest discomfort or shortness of breath—may... read more . When the fast heart rate stops, fainting may occur if the sinus node is slow in restarting normal heart rhythm.
A slow pulse (especially an irregular one), a pulse that varies greatly without any change in the person’s activity, or a pulse that does not increase during exercise suggests sinus node dysfunction. Doctors can usually diagnose sinus node dysfunction based on symptoms and the results of electrocardiography Electrocardiography Electrocardiography (ECG) is a quick, simple, painless procedure in which the heart’s electrical impulses are amplified and recorded. This record, the electrocardiogram (also known as an ECG)... read more (ECG), particularly when heart rhythm is recorded over a 24-hour period with a Holter monitor Holter Monitor: Continuous ECG Readings A standard electrocardiograph (ECG) records the heart's electrical activity for only a few seconds. This can detect abnormalities that are constant; however, sometimes abnormal heart rhythms... read more .
People with symptoms are usually given a permanent artificial pacemaker Artificial pacemakers 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 to accelerate the heart rate. If they also sometimes have a fast heart rate, they may also need drugs to slow the heart rate (such as a beta-blocker or a calcium channel blocker—see table Some Drugs Used to Treat Arrhythmias Some Drugs Used to Treat Arrhythmias 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 ).
The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
American Heart Association: Arrhythmia: Information to help people understand their risks of arrhythmias as well as information on diagnosis and treatment