Last updated on Jun 17, In normal adults, the heart beats regularly at a rate of 60 to times per minute. And the pulse felt at the wrist, neck or elsewhere matches the contractions of the heart's two powerful lower chambers, called the ventricles. The heart's two upper chambers, called the atria, also contract to help fill the ventricles.
But this milder contraction occurs just before the ventricles contract, and it is not felt in the pulse. Under normal circumstances, the signal for a heartbeat comes from the heart's sinus node. It's the natural pacemaker located in the upper portion of the right atrium.
From the sinus node, the heartbeat signal travels to the atrioventricular node or "A-V node," which is located between the atria. It's made up of a series of modified heart muscle fibers located between the ventricles. The signal enters the muscles of the ventricles. This causes the ventricles to contract and produces a heartbeat. Cardiac arrhythmias sometimes are classified according to their origin as either ventricular arrhythmias originating in the ventricles or supraventricular arrhythmias originating in heart areas above the ventricles, typically the atria.
They also can be classified according to their effect on the heart rate, with bradycardia indicating a heart rate of less than 60 beats per minute and tachycardia indicating a heart rate of more than beats per minute. Sinus node dysfunction - This usually causes a slow heart rate bradycardia , with a heart rate of 50 beats per minute or less. The most common cause is scar tissue that develops and eventually replaces the sinus node.
Why this happens is not known. Sinus node dysfunction also can be caused by coronary artery disease, hypothyroidism, severe liver disease, hypothermia, typhoid fever or other conditions. It also can be the result of vasovagal hypertonia, an unusually active vagus nerve. Supraventricular tachyarrhythmias - This diverse family of cardiac arrhythmias causes rapid heartbeats tachycardias that start in parts of the heart above the ventricles.
In most cases, the problem is either an abnormality in the A-V node or an abnormal pathway that bypasses the typical route for heartbeat signals. Atrial fibrillation - This is a supraventricular arrhythmia that causes a rapid and irregular heartbeat, during which the atria quiver or "fibrillate" instead of beating normally. During atrial fibrillation, heartbeat signals begin in many different locations in the atria rather than in the sinus node.
Although these abnormal signals manage to trigger to contractions per minute within the atria, the extraordinarily high number of heartbeat signals overwhelms the A-V node. As a result, the A-V node sends sporadic, irregular signals to the ventricles, causing an irregular and usually rapid heartbeat of to beats per minute. But the ventricular rate can be slower.
The disordered heartbeat of atrial fibrillation cannot pump blood out of the heart efficiently. This causes blood to pool in the heart chambers and increases the risk of a blood clot forming inside the heart. The major risk factors for atrial fibrillation are age, high blood pressure, heart valve abnormalities, diabetes, and heart failure. A-V block or heart block - In this family of arrhythmias, there is some problem conducting the heartbeat signal from the sinus node to the ventricles. There are three degrees of A-V block:. First-degree A-V block, where the signal gets through, but may take longer than normal to travel from the sinus node to the ventricles.
Second-degree A-V block, in which some heartbeat signals are lost between the atria and ventricles. Third-degree A-V block, in which no signals reach the ventricles, so the ventricles beat slowly on their own with no direction from above. Some causes of A-V block include cardiomyopathy, coronary artery disease, and medications such as beta blockers and digoxin.
Ventricular tachycardia VT - This is an abnormal heart rhythm that begins in either the right or left ventricle. It may last for a few seconds non-sustained VT or for many minutes or even hours sustained VT. Sustained VT is a dangerous rhythm and if it is not treated, it often progresses to ventricular fibrillation.
Ventricular fibrillation - In this arrhythmia, the ventricles quiver ineffectively, producing no real heartbeat. The result is unconsciousness, with brain damage and death within minutes. Ventricular fibrillation is a cardiac emergency.
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Ventricular fibrillation can be caused by a heart attack, an electrical accident, a lightning strike or drowning. Sinus node dysfunction - There may not be any symptoms, or it may cause dizziness, fainting and extreme fatigue. Supraventricular tachyarrhythmias - These can cause palpitations awareness of a rapid heartbeat , low blood pressure and fainting. Atrial fibrillation - Sometimes, there are no symptoms. This can cause palpitations; fainting; dizziness; weakness; shortness of breath; and angina, which is chest pain caused by a reduced blood supply to the heart muscle.
Some people with atrial fibrillation alternate between the irregular heartbeat and long periods of completely normal heartbeats. A-V block or heart block - First-degree A-V block does not cause any symptoms. Second-degree A-V block causes an irregular pulse or slow pulse. Third-degree A-V block can cause a very slow heartbeat, dizziness and fainting.
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VT - Non-sustained VT may not cause any symptoms or cause a mild fluttering in the chest. Sustained VT usually causes lightheadedness or loss of consciousness and can be lethal. Ventricular fibrillation - This causes absent pulse, unconsciousness and death. Your doctor will ask about your family history of coronary artery disease, cardiac arrhythmias, fainting spells or sudden death from heart problems. Your doctor also will review your personal medical history, including any possible risk factors for cardiac arrhythmias such as coronary artery disease, cardiomyopathy, thyroid disorders, and medications.
You will be asked to describe your specific cardiac symptoms, including any possible triggers for those symptoms. During the physical examination, your doctor will check your heart rate and rhythm, together with your pulses.
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This is because certain cardiac arrhythmias cause a mismatch of the pulse and the heart sounds. Your doctor also will check for physical signs of an enlarged heart and for heart murmurs, one sign of a heart valve problem. A test called an electrocardiogram EKG often can confirm the diagnosis of a cardiac arrhythmia.
However, because cardiac arrhythmias may come and go, a one-time office EKG may be normal. If this is the case, an ambulatory EKG may be required.
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During an ambulatory EKG, the patient wears a portable EKG machine called a Holter monitor, usually for 24 hours, but sometimes much longer. You will be taught to press a button to record the EKG reading whenever you experience symptoms. This approach is especially useful if your symptoms are infrequent. When a patient has ventricular fibrillation, it is an emergency.
The patient is unconscious, not breathing, and doesn't have a pulse. If available, electrical cardioversion must be administered as soon as possible. If not available, then cardiopulmonary resuscitation CPR should be started. How long a cardiac arrhythmia lasts depends on its cause. For example, atrial fibrillation that is caused by an overactive thyroid may go away when the thyroid problem is treated.
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Before you meet with your doctor: Write down questions to bring with you. Gather your medical records to share. Be ready to take notes to help you remember important points. Consider bringing a friend or family member to your appointment or procedure. Stay Informed Explore these resources to learn more about arrhythmia treatments.