(See also Overview of Abnormal Heart Rhythms.)
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). 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) 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. 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) or cause inflammation (for example, rheumatic fever or inflammation of the heart muscle, called myocarditis).
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. 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 (ECG), particularly when heart rhythm is recorded over a 24-hour period with a Holter monitor.