heart arrhythmias

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Registered: 10-19-2005
heart arrhythmias
Thu, 01-26-2006 - 10:21am

Heart rhythm problems (arrhythmias) occur when the electrical impulses in your heart that coordinate your heartbeats don't function properly, causing your heart to beat too fast, too slow or irregularly.


Most people have experienced occasional, brief, usually harmless arrhythmias, such as the feeling of a skipped, fluttering or racing heartbeat. However, more than 4 million, mainly older Americans experience heart arrhythmias that may cause bothersome — sometimes even dangerous — signs or symptoms. These may include shortness of breath, fainting or even sudden cardiac death — an unexpected loss of heart function, breathing and consciousness that leads to death within minutes without emergency medical treatment.


Advances in medical technology have added new treatment methods to the array of procedures that doctors may use to try to control or eliminate arrhythmias. In addition, because troublesome arrhythmias are often made worse — or even caused — by a heart weakened or damaged by coronary artery disease (CAD), you may be able to reduce your arrhythmia risk by adopting a heart-healthy lifestyle.


Signs and Symptoms



Arrhythmias may not produce any symptoms. In fact, your doctor might detect them before you do during a routine examination. But often, abnormal heart rhythms cause noticeable signs and symptoms, which may include:



  • A fluttering in your chest
  • A racing heartbeat
  • A slow heartbeat
  • Chest pain
  • Shortness of breath
  • Lightheadedness
  • Dizziness
  • Fainting (syncope) or near fainting

Causes



Your heart is divided into four hollow chambers. Divided top to bottom, the chambers on either half of your heart form two adjoining pumps with an upper chamber (atrium) and a lower chamber (ventricle). During a single heartbeat, the pumps operate in a two-phase cycle. First, the smaller, less muscular atria contract and fill the relaxed ventricles with blood. A split second later, the powerful ventricles contract and discharge blood as the atria relax and fill. Efficient blood circulation requires rhythmic coordination of this priming and pumping system. In addition, it requires proper heart rate control, which, in a normal heart, is 60 to 100 beats a minute when the body is at rest. Orchestration of these two factors is performed by the heart's electrical system. Ideally, this system operates in the following three-step sequence:



  • Initiation. Each heartbeat normally originates within a specialized group of cells called the sinus node. Located in the upper-right atrium, the sinus node is your heart's natural pacemaker. It has the ability to spontaneously produce the electrical impulses that initiate heartbeats. Other cells within the heart have a similar ability, but they're normally inactive when the sinus node is doing its job of setting your heart's pace. Doctors refer to normal heart rhythm as normal sinus rhythm.


  • Propagation. From the sinus node, electrical impulses travel through the heart. As an impulse travels, the heart muscle contracts. In a normal heartbeat, the impulse first spreads across the right, then left atrium. After activating the atria from top to bottom, the impulse proceeds to the atrioventricular (AV) node, located at the center of the heart. The AV node normally is the only electrical path between the atria and ventricles. Within it, the impulse slows for a split second to allow the atria to fill the ventricles with blood. Exiting the AV node, the impulse is conducted along two electrical pathways (right and left bundles), which spread impulses throughout the right and left ventricles.


  • Relaxation and recharging. Each cell in the heart that helps to conduct the heart's electrical impulses has two electrical states — a poised (polarized) state and a relaxed (refractory) state. In a polarized state, heart cells are ready and able to conduct the electrical impulse that will cause a heartbeat. After a heartbeat, the cells are momentarily in a refractory state before recharging to a polarized state for the next heartbeat. While it's in the refractory state, a heart cell is unable to conduct an impulse.

In a healthy person with a normal, healthy heart, it's unlikely for a sustained arrhythmia to develop without some outside trigger such as an electrical shock or the use of illicit drugs. That's primarily because his or her heart is free from any abnormal elements such as a spot of scarred tissue. Scarring can result from numerous forms of disease — most commonly, from a previous heart attack — and may disrupt the initiation or conduction of electrical impulses. In addition, the inability of heart cells to conduct electrical impulses during the refractory period acts as a buffer, preventing the occasional offbeat electrical impulse from developing into an arrhythmia.


However, in a heart with some form of disease or deformity, the initiation or conduction of the heart's electrical impulses may be destabilized, which makes arrhythmias more likely to develop.


Conditions that may lead to arrhythmias
Any pre-existing structural heart condition can lead to arrhythmia development due to:



  • Inadequate blood supply. If blood supply to the heart is somehow reduced, it can alter the ability of heart tissue — including the cells that conduct electrical impulses — to function properly.


  • Damage or death of heart tissue. When heart tissue becomes damaged or dies, it can affect the way electrical impulses spread in the heart.

These pre-existing heart conditions may include:



  • Coronary artery disease. Although it has been linked to many arrhythmias, CAD is most closely associated with ventricular arrhythmias and sudden cardiac death. Narrowing of the arteries that occurs with CAD can progress until a portion of your heart dies from lack of blood flow (heart attack). An old heart attack leaves behind a scar. Electrical short circuits around the scar can prevent normal heart function by causing the heart to beat dangerously fast (ventricular tachycardia) or to quiver (ventricular fibrillation).


  • Cardiomyopathy. This occurs primarily when your ventricle walls stretch and enlarge (dilated cardiomyopathy) or when your left ventricle wall thickens and constricts (hypertrophic cardiomyopathy). In either case, cardiomyopathy decreases your heart's blood-pumping efficiency and often leads to heart tissue damage.


  • Valvular heart diseases. Leaking or narrowing of your heart valves can lead to stretching and thickening of your muscle (myocardium). When the chambers become enlarged or weakened due to the added stress caused by the tight or leaking valve, there's an increased risk of developing arrhythmia.


Types of arrhythmias
Heart arrhythmias may occur when any phase in the heart's electrical system malfunctions. Doctors classify arrhythmias by where they originate (atria or ventricles) and by the speed of heart rate they cause. Tachycardia (tak-ih-KAHR-de-uh) refers to a fast heartbeat — a heart rate greater than 100 beats a minute. Bradycardia (brad-e-KAHR-de-uh) refers to a slow heartbeat — a resting heart rate less than 60 beats a minute. Not all tachycardias or bradycardias indicate disease. For example, during exercise, it's normal to develop sinus tachycardia as the heart speeds up to provide your tissues with more oxygen-rich blood. Athletes at rest often have a heartbeat less than 60 beats a minute because their hearts are so efficient.


Tachycardias originating in the atria include:



  • Atrial fibrillation. This fast and chaotic beating of the atrial chambers is the most common arrhythmia. It affects about 2 million, mainly older Americans. Your risk of developing atrial fibrillation increases past age 65 mainly due to the wear and tear that may affect your heart's function as you age. During atrial fibrillation, the electrical activity of the atria becomes uncoordinated. The atria beat so rapidly — as fast as 300 to 400 beats a minute — that they quiver (fibrillate). The electrical waves have the same chaotic activity that you would see if you threw a handful of pebbles into a quiet pond. Fortunately, not all of these atrial impulses reach the ventricles. The AV node between the atria and ventricles acts as a gatekeeper, usually letting only a portion of the impulses through. Still, extra impulses often get through the AV node. This may accelerate your pulse (ventricular contractions) during atrial fibrillation to 150 beats a minute or more. In addition, the atrial impulses that reach the ventricles often arrive at irregular intervals. This may cause an irregular heart rhythm. Atrial fibrillation can be intermittent (paroxysmal), lasting a few minutes to an hour or more before returning to a regular heart rhythm. It can also be chronic, causing an ongoing problem. Atrial fibrillation is seldom a life-threatening arrhythmia, but over time it can be the cause of more serious conditions such as stroke.


  • Atrial flutter. Although atrial flutter is less common than atrial fibrillation, the arrhythmias are in some ways similar. In fact, both can coexist in your heart, coming and going in an alternating fashion. The key distinction is that more-organized and more-rhythmic electrical impulses cause atrial flutter. These occur because atrial flutter, unlike atrial fibrillation, arises from a short circuit. In typical atrial flutter, this short circuit exists in the right atrium. This is an important distinction because typical right atrial flutter is more amenable to some forms of treatment, such as catheter ablation.


    • Supraventricular tachycardia (SVT). SVT is a broad term that includes many forms of arrhythmia originating above the ventricles (supraventricular). SVTs usually cause a burst of rapid heartbeats that begin and end suddenly and can last from seconds to days. These often start when the electrical impulse from a premature heartbeat begins to circle repeatedly through an extra pathway. SVT may cause your heart to beat 140 to 200 times a minute. Although generally not life-threatening in an otherwise normal heart, symptoms from the racing heart may feel quite strong.


    • Wolff-Parkinson-White syndrome (WPW). One type of SVT is known as Wolff-Parkinson-White syndrome (WPW). This arrhythmia may, rarely, run in families and is caused by an extra electrical pathway between the atria and the ventricles. This pathway may allow electrical current to pass between the atria and the ventricles without passing through the AV node, leading to short circuits and rapid heartbeats.
    • Ventricular tachycardia (VT). This fast, regular beating of the heart is caused by abnormal electrical impulses originating in the ventricles. Most often, these are due to a short circuit around a scar from an old heart attack and can cause the ventricles to contract more than 200 beats a minute. Most VT occurs in people with some form of heart-related problem such as scars or damage within the ventricle muscle from coronary artery disease or a heart attack. Sometimes, VTs last for 30 seconds or less (unsustained) and are usually harmless, although they cause inefficient heartbeats. Still, an unsustained VT may be a predictor for more serious ventricular arrhythmias such as longer-lasting (sustained) VT. An episode of sustained VT is a medical emergency. It may be associated with palpitations, dizziness, fainting, or possibly death. Without prompt medical treatment, sustained VT often degenerates into ventricular fibrillation. Rarely, VT occurs in an otherwise normal heart. In this setting, it's far less dangerous but the condition still needs the attention of a doctor.
    • Ventricular fibrillation. About 90 percent of sudden cardiac deaths, which claim the lives of about 300,000 Americans each year, are caused by this arrhythmia. With ventricular fibrillation, rapid, chaotic electrical impulses cause your ventricles to quiver uselessly instead of pumping blood. Without an effective heartbeat, your blood pressure plummets, instantly cutting off blood supply to your vital organs — including your brain. Most people lose consciousness within seconds and require immediate medical assistance such as cardiopulmonary resuscitation (CPR). Your chances of survival may be prolonged if CPR is delivered until your heart can be shocked back into a normal rhythm with a device called a defibrillator. Without CPR or defibrillation, death results in minutes. As with VT, most cases of ventricular fibrillation are linked to some form of heart disease. Ventricular fibrillation is frequently triggered by a heart attack. However, ventricular fibrillation may also be your first indication of heart problems.
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