Sick sinus syndrome (SSS) is a group of signs and symptoms that, when they occur together, are an indication that an important part of the heart’s electrical system, called the sinus node, is not working well. The sinus node is a bundle of tissue, located in the upper right chamber of the heart (right atrium), that sets the pace at which the heart beats. The primary symptom of SSS is sinus bradycardia, a heart rhythm that is slower that 60 beats per minute. However, about half of patients with sick sinus syndrome may also experience rapid heartbeats or heart rates that alternate between abnormally fast and abnormally slow.
Many patients with sick sinus syndrome will not feel any symptoms until their heart rate drops below 50 beats per minute. Therefore, the condition is often not diagnosed until its advanced stages. The most common treatment is the implantation of an artificial pacemaker followed by the prescription of antiarrhythmic medications.
About sick sinus syndrome
Sick sinus syndrome (SSS), also known as sinus node dysfunction, is a relatively rare type of arrhythmia (abnormal heart rhythm). It is a condition in which the sinus node, which is located in the right atrium and serves as the heart’s natural pacemaker, is unable to set the pace of the heart as well as it should. Sick sinus is a syndrome, or a cluster of signs and symptoms that occur together, whether or not the cause of the condition is known.
Men and women of all ages can develop SSS, but it is rarely diagnosed in people under age 50. While it is rare in children, the condition may develop in pediatric patients who have had open-heart surgery, particularly after surgery to the atria (upper chambers of the heart). SSS develops over a period of years, and it can often be difficult to recognize until it is in its advanced stages.
Advanced SSS is often marked by alternating rapid and slow heart rhythms, neither of which produces a good cardiac output. Patients can often feel this yo–yo effect, which results in not enough blood circulating through the body. If left untreated, sick sinus syndrome can lead to heart–related complications such as heart failure. In addition, patients with SSS are at increased risk for heart attack, even after treatment. However, treatment for the disease dramatically lowers risk of complications.
Potential causes of sick sinus syndrome
There are many factors that may cause or contribute to the development of sick sinus syndrome (SSS). They include:
Cardiac ischemia. A lack of oxygen in heart muscles and tissue due to a restriction of blood flow to the heart, usually caused by coronary artery disease.
Heart attack. Scarring or death of heart muscle due to lack of oxygen. Oxygen-rich blood is blocked by a blood clot in a coronary artery, usually due to plaque-related narrowing of the artery (coronary artery disease).
Hyperkalemia. Too much potassium circulating in the blood. Potassium enters the body through food and is removed from the body as urine produced by the kidneys. Hyperkalemia is usually the result of kidney failure.
Antiarrhythmics. Medications used to treat arrhythmias (abnormal heart rhythms) have been connected to SSS. These medications may be withheld or reduced in the event of SSS. However, if a pacemaker is implanted to regulate the heart rhythm, antiarrhythmics may be prescribed to help prevent abnormally fast heart beats (tachycardia).
Beta blockers. Medications that reduce the workload of the heart and lower blood pressure. They are commonly prescribed for conditions such as angina (a certain type of chest pain, pressure or discomfort) or heart failure. They are also prescribed for people who have high blood pressure (hypertension).
Hypothyroidism. Abnormally low levels of thyroid hormones in the blood.
Sleep apnea. A sleep disorder in which a person’s breathing stops and starts many times during sleep.
Electrolyte disorders. Electrolytes are biochemical compounds that play an important role in controlling fluid levels, maintaining the body’s acid/base balance, nerve conduction, blood clotting and muscle contraction (including that of the heart). They include calcium, potassium, chloride and sodium.
Cardiomyopathy. A disease of the heart in which the muscles' ability to pump blood is inhibited or weakened, usually due to the enlargement, thickening and/or stiffening of the heart muscle.
Heart surgery. Operations on the heart, particularly the atria, are a common cause of SSS in children. The syndrome can be caused by thickened scarlike tissue forming on the atria, blocking the natural electrical patterns of the heart. In adults, there is a risk of developing SSS following catheter ablation (elimination) of part or all of the sinus node to treat tachycardia.
Diphtheria. An infectious disease that was formerly common in the United States, especially in children. Today diphtheria is rare in America due to use of a vaccine. However, it is epidemic in certain parts of the world, especially in Russia and other parts of the former USSR. The poison produced by a diphtheria infection can cause severe and permanent damage to the heart muscle, peripheral nerves and other tissues.
Inflammatory diseases. Various inflammatory diseases, such as rheumatic fever, pericarditis, Lyme disease and others, may cause SSS in rare cases.
Hemochromatosis. A condition that causes the body to absorb too much iron, leading to liver disease, diabetes and eventually to heart failure.
Muscular dystrophy. A group of conditions that causes weakness and difficulty with movement due to damage to muscles and nerves. Muscular dystrophy is caused by a genetic defect and currently has no cure.
Amyloidosis. A condition in which a body protein called amyloid is deposited in tissues or organs. This can occur in the lungs, kidneys, bowels or the brain where it is associated with Alzheimer’s disease. Accumulation of amyloid in the heart may cause arrhythmias or result in heart failure.
In addition, there are several conditions that are commonly associated with SSS. However, it is unknown whether these conditions are the cause of SSS. They include:
Valvular heart disease. Any dysfunction or abnormality of one or more of the heart’s valves. Problems with the aortic valve or the mitral valve are most commonly associated with SSS.
Congenital heart disease such as corrected transposition of the heart vessels.
Signs and symptoms of sick sinus syndrome
Symptoms of sick sinus syndrome (SSS) may develop over a lengthy span of time, even over a period of years. Alternatively, many patients experience no symptoms. Because the symptoms may not seem significant when they occur individually, people may not seek treatment until SSS has reached an advanced stage. Symptoms include:
Syncope (fainting) spells.
Bradycardia (a heart rate that is consistently less than 60 beats per minute).
Palpitations (a feeling in the chest of a fast or pounding heartbeat).
Angina(chest pain) caused by a lack of blood flow in the heart.
Heart failure (new or worsening).
Dizziness or lightheadedness.
Shortness of breath.
Muscle aches.
Confusion.
Fatigue or unusual tiredness.
Disturbed sleep.
In addition to these symptoms, the physician may also be able to detect signs of SSS that may not be noticeable to the patient. Signs of SSS can include various types of arrhythmias (abnormal heart rhythms). These rhythms may be too fast (tachycardia), too slow (bradycardia) or alternating between fast and slow (brady-tachy syndrome). More than half of patients with SSS experienced brady-tachy syndrome. Specific arrhythmias associated with SSS include:
Atrial fibrillation. A condition in which the upper chambers of the heart (atria) quiver instead of beat. This is caused by faulty electrical signals in the heart that signal the heart to beat. Atrial fibrillation is a significant risk factor for stroke.
Drug-resistant sinus bradycardia. A condition in which the heart beats abnormally slow (less than 60 beats per minute). The sinus bradycardia that is associated with SSS cannot usually controlled with medications.
Sinus pauses. A pause in the signals originating from the sinus node that normally triggers the heartbeat. Sinus pauses are caused by a number of things and are not usually a concern. The pause usually lasts less than three seconds and produces no symptoms. However, when sinus pauses are detected along with other signs, SSS is suspected.
Escape rhythms. Any heart rhythm that is produced by signals that do not originate from the sinus node. The sinus node is the heart’s natural pacemaker. If it stops working, the center of the heart (the AV junction) will begin signaling the heart to beat at 40 to 60 beats per minute. This slower pace is called an “escape rhythm.” To a lesser extent, the upper and lower chambers of the heart (atria and ventricles) can also pace the heart.
Chronotropic incompetence. Inadequate increase of heart rate during activity which may result in fatigue and exhaustion with exertion. This can be diagnosed by exercise stress test and by observing variation of heart rate during Holter monitoring.
Diagnosis methods for sick sinus syndrome
Diagnosis of sick sinus syndrome (SSS) is usually made after a medical history and physical examination by a cardiologist. The syndrome is difficult to diagnose because patients often have few symptoms, and many patients are elderly and have other cardiac conditions. In general, a physician looks for the following combinations of signs, in addition to arrhythmias, to make a diagnosis of SSS.
A heart rate that is consistently slower than normal (60 beats per minute), which may or may not be accompanied by episodes of a rapid heart rate (more than 140 beats per minute).
Normal or low blood pressure.
Symptoms that appear only during episodes of arrhythmia.
An electrocardiogram (EKG) result that shows one or more of the various arrhythmias associated with sick sinus syndrome.
One or more of the following tests may be used in diagnosing sick sinus syndrome:
EKG. A recording of the heart’s electrical activity as a graph on a moving strip of paper or video monitor. The highly sensitive electrocardiograph machine helps detect heart irregularities, disease and damage by measuring the heart’s rhythms and electrical impulses.
Holter monitor. A continuous EKG that is temporarily attached to an ambulatory (freely moving) patient and run for a 24-hour period (though it can be used for up to five days).
Cardiac catheterization. A group of diagnostic tests that are performed with the use of a catheter, or a thin, hollow tube that is inserted into a large blood vessel and fed to a target area (e.g., the heart). Through the catheter, the physician can measure pressure, obtain a tissue sample or perform a coronary angiogram.
Tilt table testing. A test in which the patient is strapped to a table that is then tilted. It is used to help determine the cause of unexplained fainting (syncope).
Electrophysiology study (EPS). A test that uses an electrode catheter to locate a particular pathway through which electrical impulses travel in the heart and to determine the extent of any abnormal impulses traveling through it.
Stress test. An electrocardiogram performed while the patient exercises in a controlled manner on a treadmill or stationary bicycle at varied speeds and elevations.
Atropine Test. Atropine sulfate is a medication that, when given intravenously, usually increases the patient’s heart rate to over 100 beats per minute. The bradycardia (slow heart rate) associated with SSS, however, is drug resistant, meaning it does not respond to medications. When atropine sulfate is injected into a patient with SSS, their heart rates rarely rise above 90 beats per minute. Therefore raising the heart rate by atropine sulfate injection is usually attempted before the diagnosis of SSS is confirmed.
Adenosine Test. Recently, a different compound called adenosine has also been researched as a test for SSS. Adenosine is a natural metabolic substance that affects heart rate. When given intravenously, adenosine inhibits the sinus node, resulting in a decreased heart rate. Researchers have proposed that patients with SSS may be more sensitive to this reaction than healthy patients. Therefore, patients with SSS will have a marked slowing of the heart rate (a sinus pause) when given adenosine. While it is not a standard test at this time, researchers hope that the accuracy of the adenosine test will eventually eliminate the need for more invasive procedures, such as the electrophysiology study in the diagnosis of SSS.
Treatment and prevention of SSS
Because of the nature of the symptoms, sick sinus syndrome (SSS) often is not diagnosed until its advanced stages. If the syndrome is detected even though it is producing no symptoms, no treatment is necessary. However, if the patient is taking medications that may worsen the condition, those medications likely will be stopped and/or replaced with safer alternatives.
Treatment is necessary once SSS enters the advanced stages. The most common treatment is a combination of an artificial pacemaker and antiarrhythmic medications.
Pacemakers are devices implanted into the patient’s chest to replace the sinus node as the pace setter for the heart’s rhythm. They are implanted during a minor surgical procedure, and a short stay in the hospital may be required. When treating SSS, a dual-lead (or dual-chamber) pacemaker is the most common type used. Dual-lead pacemakers send electrical leads to both an atrium (one of two upper heart chambers) and a ventricle (one of two lower heart chambers), stimulating both chambers. This helps prevent the atrial fibrillation commonly associated with SSS. In addition, the dual–chamber pacemaker has the ability to synchronize the contraction of the atrium and ventricle to more closely resemble the natural action of the heart.
Patients may still experience certain arrhythmias following pacemaker insertion, including:
Paroxysms of atrial tachycardia. A sudden and rapid rhythm in the atria.
Atrial flutter. An abnormally fast signal coming from the atria at a rate of 160 to 240 times per minute. The atrioventricular node is only able to process 1 beat of the ventricles for every four signals coming from the atria.
Atrial fibrillation. A condition in which the upper chambers of the heart (atria) quiver instead of beat at a regular rate. In this case, the physician may also prescribe an anticoagulant to prevent blood clots from forming in the atria and causing a stroke.
Antiarrhythmic medications may be prescribed along with the pacemaker to control the abnormal heart rhythms. The most common antiarrhythmic medications prescribed for SSS are calcium channel blockers. Also called calcium antagonists, these are non-habit-forming medications that prevent the flow of calcium ions into the muscle cells of the heart and blood vessels, causing them to widen and relax. As a result, the heart and blood vessels relax, increasing the supply of oxygen-rich blood to the heart, lowering blood pressure and reducing the heart’s workload.
The pacemaker protects the patient from having their heart rate drop too low from these medications. That is why the same medications that are avoided by SSS patients without a pacemaker are then sometimes prescribed once the pacemaker is in place. The pacemaker is a safety mechanism to prevent the heart from slowing down, while the antiarrythmic drugs prevent the heart from going to fast. The pacemaker/antiarrhythmic drug combination helps keep the heart rate in a normal range depending on the body's needs.
Occasionally, SSS is accompanied by angina(chest pain caused by a restriction in blood flow to the heart). In these cases, one or more catheter-based procedures may be performed to remove, breakup or compress any plaque buildup that may be causing the restriction in blood flow.
Other than making healthy lifestyle changes to promote overall health (e.g., reducing use of alcohol and caffeine, losing weight), there is no known strategy to prevent SSS.
Questions for your doctor
Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about sick sinus syndrome (SSS):
Do I have any of the symptoms associated with SSS?
I haven't noticed any cardiovascular symptoms, could I still have SSS?
Do I have any underlying conditions that may contribute to the development of SSS?
My child has had open-heart surgery, is he/she at increased risk of developing SSS?
Are there any tests available that can help determine whether I have SSS?
How advanced is my condition?
What type of treatment would you recommend for the treatment of SSS?
Are there any lifestyle changes I can make to reduce the chances of developing SSS?
