Sick Sinus Syndrome

sinus rhythm
SCIENCE PHOTO LIBRARY/Getty Images

There are two general categories of cardiac arrhythmias that can cause the heart to beat too slowly, and that often require insertion of a permanent pacemaker. One of these is heart block, in which electrical impulses from the atria (upper chambers of the heart) are blocked from reaching the ventricles (the lower chambers of the heart). But the more common indication for a pacemaker is a condition called sick sinus syndrome.

With sick sinus syndrome, sinus bradycardia (slow heart rate) is present because of disease of the sinus node(the tiny structure in the right atrium that generates the heart’s electrical impulse), and the bradycardia is severe enough to cause symptoms. In addition, sick sinus syndrome is often accompanied by episodes of atrial fibrillation. Sick sinus syndrome is a disorder of older people, and is most commonly seen in people over 70 years of age.

What Causes Sick Sinus Syndrome?

Sick sinus syndrome usually occurs when the sinus node is affected by a form of fibrosis associated with aging. “Fibrosis” means that normal tissue is replaced by a form of scar tissue. When fibrosis affects the sinus node, bradycardia can result. When bradycardia is caused by a problem with the sinus node, it is termed sinus bradycardia.

 

The same age-related fibrosis that affects the sinus node can also affect the atrial muscle, leading to atrial fibrillation. 

And sometimes, this fibrosis can also affect the AV node. If it does, the sinus bradycardia may be accompanied by episodes of heart block — so there may be two causes for bradycardia.

While other medical conditions — such as amyloidosissarcoidosisChagas diseasehypothyroidism, or cardiac trauma — can also cause sinus bradycardia, the fibrosis of aging is by far the most common cause of sick sinus syndrome.

What Symptoms Are Associated With Sick Sinus Syndrome?

The most prominent symptoms are usually those due to a slow heart rate — easy fatiguability, lightheadedness, or (in severe cases) syncope

In some people, symptoms will only occur during exercise. In these cases the chief problem is an inability to increase the heart rate appropriately during activity — which can cause extreme fatigue during exertion. This condition is called "chronotropic incompetence."

Sick Sinus Syndrome and Atrial Fibrillation

People with sinus node disease who also have episodes of atrial fibrillation will frequently experience symptoms of bradycardia, which is caused by their sinus node disease, as well as symptoms of tachycardia (fast heart rate), especially palpitations. In these cases the tachycardia is almost always produced by episodes of atrial fibrillation.

People who have episodes of both slow and fast heart rates are said to have bradycardia-tachycardia syndrome, or "brady-tachy syndrome."

The most troublesome symptom associated with brady-tachy syndrome is syncope. The loss of consciousness usually occurs immediately after an episode of atrial fibrillation terminates, and is produced by a prolonged pause in the heart rate.

This prolonged pause occurs because, when the sinus node is already "sick," an episode of atrial fibrillation tends to even further suppress its function. So, when the atrial fibrillation suddenly stops, the sinus node may require several seconds to "wake up" and begin generating electrical impulses again. During this interval, there may be no heart beat at all for 10 or more seconds — leading to extreme lightheadedness, or syncope.

How Is Sick Sinus Syndrome Diagnosed?

Diagnosing sick sinus syndrome is typically not difficult. The correct diagnosis is usually pretty apparent when a person who complains of typical symptoms is found to have significant sinus bradycardia on their electrocardiogram (ECG). The "brady-tachy" variety of sick sinus syndrome is diagnosed when a patient with sinus node disease is also discovered to have episodes of atrial fibrillation.

Because the fibrosis that causes sinus node disease sometimes affects the AV node, people with brady-tachy syndrome may also have a partial heart block, and therefore, a relatively slow heart rate when they are in atrial fibrillation. So, whenever a person with atrial fibrillation is discovered to have a relatively slow heart rate (in the absence of medication aimed at slowing the heart rate), that should give the doctor a strong clue that sick sinus syndrome is likely also present.

Doctors can make the diagnosis of chronotropic incompetence simply by observing the patient's heart rate during exercise — for instance, during a stress test. Because chronotropic incompetence is a fairly common condition in the elderly and is readily treatable (with a rate-responsive pacemaker), it is important for older people who are experiencing fatigue with mild or moderate exertion to make sure their doctors do the proper evaluation.

How Is Sick Sinus Syndrome Treated?

Virtually all people with sick sinus syndrome should be treated with a permanent pacemaker.

A pacemaker is especially important for people who have the brady-tachy form of sick sinus syndrome, for two reasons. First, these people have a relatively high risk of experiencing syncope (from those prolonged pauses when the atrial fibrillation terminates). And second, many of the drugs that are often used to treat atrial fibrillation — beta blockerscalcium channel blockers, and antiarrhythmic drugs — may make sinus node disease much worse. Implanting a pacemaker will prevent syncope, and will allow the doctor to treat atrial fibrillation much more safely.

Sources:

Epstein AE, DiMarco JP, Ellenbogen KA, et al. ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities: a Report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to Revise the ACC/AHA/NASPE 2002 Guideline Update for Implantation of Cardiac Pacemakers and Antiarrhythmia Devices): developed in collaboration with the American Association for Thoracic Surgery and Society of Thoracic Surgeons. Circulation 2008; 117:e350.

Josephson, ME. Sinus Node Function. In: Clinical Cardiac Electrophysiology: Techniques and Interpretations, 4th, Lippincott, Williams, & Wilkins, Philadelphia 2008. p.69-92.

Continue Reading