Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Sinus Node Dysfunction


L. Brent Mitchell

, MD, Libin Cardiovascular Institute of Alberta, University of Calgary

Last full review/revision Feb 2021| Content last modified Feb 2021
Click here for the Professional Version
Topic Resources

Sinus node dysfunction is an abnormality in the natural pacemaker of the heart that causes a slow heart rate.

  • People may have no symptoms, or they may feel weak or tired or have palpitations.

  • Electrocardiography is used to make the diagnosis.

  • A permanent artificial pacemaker is usually needed.

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).

Symptoms of Sinus Node Dysfunction

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.

Diagnosis of Sinus Node Dysfunction

  • Electrocardiography

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.

Treatment of Sinus Node Dysfunction

  • Inserting an artificial pacemaker

People with symptoms are usually given a permanent artificial pacemaker 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).

More Information

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.

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Others also read

Test your knowledge

Overview of Aneurysms and Aortic Dissection
In aortic aneurysms, the pressure of blood inside the artery forces a weak area in the artery’s wall to balloon outward. In which of the following areas do three-quarters of aortic aneurysms occur? 
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID

Also of Interest

Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID