Sick sinus syndrome (SSS) is a type of bradycardia in which the sinoatrial (SA), or sinus node is not working as it should. The sinus node is a small cluster of cells in the upper right chamber, or atrium, of the heart. It contains special "pacemaker" cells that generate the electrical signals that regulate the pace and rhythm of the heartbeat. These signals travel from the sinus node to the AV node. From the AV node, the signal is conducted along pathways that spread into the ventricles, causing them to contract and pump blood into the lungs and throughout the body.
Sick sinus syndrome (SSS) is not a specific disease, but a group of signs or symptoms that indicate the SA node is not functioning properly.
Some patients with sick sinus syndrome also have rapid heartbeats. Or the heartbeat alternates between too fast and too slow. This is called bradycardia-tachycardia syndrome. Often there is a long pause (asystole) between heartbeats, especially after an episode of tachycardia.
SSS affects about 3 out of every 10,000 people. It becomes more common as we age.
Types of Sick Sinus Syndrome Sick sinus syndrome may be due to defects in the heart itself, or it can be related to factors outside the heart.
Some medications prescribed to treat high blood pressure, coronary artery disease and other conditions, for example, are known to cause bradycardia. This generally does not cause problems for most people, and drugs that slow the heart rate are useful treatments for some types of arrhythmias or other heart disorders. But they may cause low heart rates and bring out other symptoms of SSS in some individuals, such as elderly people with age-related degeneration of the SA node.
- Sinoatrial block is a type of SSS in which the electrical signals move too slowly through the sinus node.
- Sinus arrest is when the signal from the SA node has a pause in it's normal pacemaker activity.
Bradycardia also can result from abnormalities of the atrioventricular (AV) node, the part of the heart's conduction system that conducts electrical signals from the atria to the ventricles. These problems are commonly called heart block.
Symptoms Many people with sick sinus syndrome have no symptoms, or the symptoms may not seem serious enough for concern. The condition may not be diagnosed until it is in an advanced stage. Typically, the first sign an individual with SSS notices is a consistently slow heart rate or bradycardia. Often, the rate falls to 40 to 50 beats per minute or less before it is noticed.
For more information about the electrical conduction system of the heart and what can go wrong, go to The Normal Heart.
If you experience any sudden, unexplained episodes of fainting, consult your doctor.
Other symptoms may include:
- Fainting (syncope) can occur when the abnormal heart rhythm affects the heart's ability to pump a sufficient volume of blood to the brain. If you experience any sudden, unexplained episodes of fainting, consult your doctor.
- Fatigue and weakness may be signs that the heart is not pumping as well as it should. When the blood supply is inadequate, the body diverts blood to the most vital organs. This reduces blood flow to the muscles of the arms, legs or other parts of the body and the individual may lack energy or be unable to do normal tasks.
- Shortness of breath, or dyspnea. People with this symptom often describe the sensation of not being able to take in enough air, or say they are "out of breath."
- Chest pains, or angina, that indicate the heart is not getting enough blood.
- Disturbed sleep. The abnormal rhythm of the heart may interfere with sleep and wake people up repeatedly during the night. Some people have a condition called sleep apnea in which they stop breathing off and on during the night. This condition may contribute to SSS and other heart disorders by reducing oxygen supply to the heart.
- Confusion. When the blood supply to the brain is diminished, people may have trouble understanding what is happening around them or make errors in judgment. They also may seem drowsy, have problems with balance or movement, or appear anxious or agitated.
- Palpitations describe an unusual awareness of the beating of the heart. The heartbeat may seem too fast or slow, or to beat in a chaotic, irregular pattern. People may describe a "pounding" in the chest, as if the heart is beating with unusual force. People with SSS who have episodes of tachycardia (an abnormally rapid heartbeat) often describe them as "palpitations."
People with unexplained symptoms that may be early signs of sick sinus syndrome should consult their doctor as soon as possible.
Seeing the Doctor There are some signs that a doctor may discover during a physical examination that suggest a diagnosis of SSS, such as:
- A pulse that is slower than normal.
- An irregular heart rhythmthat can be heard with a stethoscope.
- Sinus "pauses," or gaps between signals from the SA node, that indicate the heart is not being paced at the normal, steady rate.
- "Escape" rhythms are electrical signals that originate from a part of the heart muscle other than the sinus node. If the SA node stops working or doesn't work properly, another pacemaker at the center of the heart called theAVnode may begin signaling the heart to beat. Less often, the lower chambers of the heart also can generate escape rhythms that take over for the function of the SA node.
- A heart rhythm that alternates between slow and fast.
- Low or low-normal blood pressure.
Sick Sinus Syndrome Associated Conditions There is no specific, known cause of sick sinus syndrome. Some factors, however, often are associated with the condition, such as: Age-related damage to the heart muscle. Wear-and-tear on the heart over time may weaken the SA node or damage the heart muscle that conducts the electrical signal. As we grow older, we also have fewer "pacing" cells in the heart.
- Heart attack (myocardial infarction). An infarct is an area of dead heart muscle caused by a lack of oxygen. This damage can affect the SA node, or give rise to abnormal electrical signals in other parts of the heart.
- Medications prescribed for conditions such as high blood pressure and other heart diseases may contribute to sick sinus syndrome. Patients with SA node problems who have a too-fast heart rate, for example, may develop symptoms of SSS when they take medications that slow this fast heartbeat.
- Hyperkalemia means there is too much potassium circulating in the blood. Normally, potassium is removed by urine produced in the kidneys. Hyperkalemia usually is a sign of kidney disease. It is on etype of "electrolyte disorder" and can cause abnormal heart rhythms.
- Other electrolyte disorders. Electrolytes are biochemical compounds that play a role in controlling fluid and chemical balances in the body, and in nerve conduction, and muscle contraction (including the heart muscle). Electrolytes include calcium, potassium, chloride and sodium.
- Thyroid disease. A low level of thyroid hormone in the blood (hypothyroidism) is associated with a too-slow heart rate.
- Sleep apnea, a condition in which a person's breathing stops and starts repeatedly during the night.
- Heart surgery. A common cause of SSS in children is heart surgery, especially in the upper chambers, or atria. Scarred tissue that forms in the atria can block the normal pattern of electrical signals from the SA node.
In rare cases, SSS may be associated with conditions such as:
- Diphtheria: The toxin produced by this infection can cause severe damage to the heart muscle. Diphtheria is uncommon in the U.S. because most American children are vaccinated against the disease, but it is common in some other parts of the world.
- Hemochromatosis: A disease in which the body absorbs too much iron.
- Muscular dystrophy: An inherited condition in which the muscles of the body are damaged and weak and do not function normally.
- Amyloidosis: A condition in which a body protein called amyloid is deposited in tissues or organs, including the heart.
Sick Sinus Syndrome Risk Factors Underlying diseases and other factors that are risk factors for heart disease in general also can contribute to a slow heart rhythm (bradycardia), or to other arrhythmias or heart disease. Many types of heart problems are associated with coronary artery disease (CAD), high blood pressure (hypertension), heart valve disease and high cholesterol. Lifestyle factors such as smoking, obesity, stress, lack of exercise, excessive use of caffeine or alcohol or a diet high in animal fats and cholesterol also are associated with heart disease.
There is no evidence that modifying these factors will prevent SSS or improve its symptoms, but they play important roles in the overall health of the heart.
