Sick sinus syndrome or sinus node dysfunction is a defined range of problems with an individual's heart rhythm where the sinus node does not function normally. The rhythm of the heart is controlled by an area of specialized cells in the right atrium of the heart called the sinus node. In healthy individuals, a steady tempo of regular electrical impulses is produced by the sinus node. However, these electrical signals are irregularly paced in individuals affected by sick sinus syndrome. These abnormal heart rhythms may be too slow, too fast, interrupted by long pauses, or any combination of these rhythm issues. Common symptoms indicative of sick sinus syndrome include fatigue, breathlessness, chest pain, palpitations, bradycardia, dizziness, fainting, and confusion. If sick sinus syndrome cannot be managed through the use of medication, a surgical procedure is usually needed.
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Bradycardia-tachycardia syndrome is a sick sinus syndrome complication characterized by the alternation of bradycardia or an abnormally slow heartbeat and tachycardia or an abnormally fast heartbeat. Both bradycardia and tachycardia occur as a result of a problem with the conduction or initiation of signals meant to trigger cardiac action. The reason an issue arises with the signals is due to a dysfunction of the ion channels, which play a crucial role in regulating heart rhythm. The specific heart rhythm pattern exhibited in individuals affected by bradycardia-tachycardia syndrome can also be a result of electrophysiological and structural remodeling of the heart muscle. This remodeling causes a modification of the atrial sinus node or the group of cells responsible for producing a steady pace of impulses that signal the heart muscle to contract and relax. It is not clear whether the tachycardia causes bradycardia or vice versa in bradycardia-tachycardia syndrome patients. Depending on the exact cause of this complication, treatment may include the use of a pacemaker, a surgical procedure, or medication.
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A sinoatrial block is a disruption in the normal heart rhythm that may occur in individuals with sick sinus syndrome. A healthy heart has an initial action impulse generated in the sinoatrial node and carried through the heart's upper chambers or atria. The impulse then moves down the internodal atrial pathways to the atrioventricular (AV) node, and it travels through the AV bundle triggering the contraction of the lower chambers of the heart or ventricles. In individuals affected by sinoatrial block, the impulse fired by the sinoatrial node is blocked or delayed on its way to atria or the upper chambers of the heart. This interruption of the electrical impulse causes a lag between the time the sinoatrial node fires off the impulse, to the time when the atria depolarize and contract. Depending on the degree of the sinoatrial block, the electrical impulses may or may not move from the atria and reach the ventricles. Individuals affected by a sinoatrial block are asymptomatic, or they will experience spells of dizziness from bradycardia precipitated by the blockage.
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Scar Tissue From Previous Heart Surgery
The presence of scar tissue from previous heart surgery can cause an individual to develop sick sinus syndrome. Scarring or fibrosis of the sinus node may occur as a result of sinoatrial artery stenosis, a clot in the sinoatrial artery, rheumatic fever, pericarditis, diphtheria, Lyme disease, Chagas disease, amyloidosis, hemochromatosis, scleroderma, or other inflammatory and infiltrative diseases. Because the body naturally repairs damage to its tissues by replacing it with scar tissue, the damage from a previous heart surgery will not heal back to its original state. Healthy cardiac muscle tissue is conductive and flexible, while scar tissue is dense, fibrous, and is not conductive. Scar tissue forms in the heart near the sinus node as the body repairs local surgical damage. This change of tissue characteristics may interfere with the healthy function of the sinus node. Scarring can cause the sinoatrial node to fire too early, too late, or fire less frequently than it should. Such abnormalities of the sinoatrial node are what characterizes sick sinus syndrome.
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Atrial fibrillation is an irregular heart rhythm where the atria beat abnormally, chaotically, and out of coordination with the ventricles of the heart. This lack of coordination is a result of fast and erratic electrical signals being fired from the sinoatrial node. These impulses do not all successfully reach the atrioventricular node that connects the electrical pathway between the atria and ventricles. The result is the atria contracts and relaxes faster than the ventricles because of the difference in the frequency of electrical impulses between the two. Atrial fibrillation can cause an individual to develop heart failure or have a stroke. It is a common complication seen in sick sinus syndrome patient. The reason for this is because the dysfunction that causes atrial fibrillation lies within the firing of electrical impulses by the sinoatrial node, which should fire impulses at a steady and even rate so the chambers of the heart will contract and relax at the proper times. The aberrant and rapid impulses that result in atrial fibrillation can occur due to sick sinus syndrome.
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Heart failure describes an individual with a heart that is not pumping as efficiently as it should be. When an individual is dealing with heart failure, the body's need for oxygen and blood are not being met by the heart's pumping. The heart will try to compensate for its decreased function in one or more ways that can cause changes in the structure and or anatomy of the muscle. At first, the heart's methods of compensation, which include a faster heart rate, larger muscle mass, and higher blood pressure, will keep the heart functioning. However, the heart eventually becomes unable to fill up with enough blood, or it fills up with too much blood for the muscle to pump out. These problems happen because of anatomical and or structural changes that develop in the heart. The sinus node may become damaged as a result of a lack of blood supply, changes in tissue arrangements, scarring, and several other factors precipitated from heart failure. When the sinus node is unable to function normally and produces specific symptoms, the patient is affected by sick sinus syndrome due to heart failure.