Irregular Heart Beats

Although most people consider the heart to be purely a pump, it also has an electrical system. A normally functioning, healthy electrical system guarantees that the heart will beat in a regular fashion, gradually increasing its rate when more blood should be pumped (during exercise) and slowing when less blood is required (during sleep).

Arrhythmias - irregular heartbeats - occur when the electrical system is abnormal, diseased, or damaged. This can cause the heart to beat inappropriately slow or fast. In either case, the heart may not be able to pump enough blood efficiently. Severe arrhythmias cause very rapid or very slow heart rhythms, sometimes making it impossible for the heart to pump enough blood to maintain a blood pressure. Low blood pressure may cause dizziness or loss of consciousness.

How the Heart Beats The heart has four pumping chambers. The two upper chambers are called atria, and the two lower are called ventricles:

Atria. The atria serve as collection chambers for blood returning to the heart. When they contract, they force blood into the ventricles. The left atrium collects blood from the lungs and injects it into the left ventricle. The right atrium collects blood from the rest of the body and pumps it into the right ventricle.

Ventricles. The left ventricle is the main pumping chamber of the heart, delivering blood to the entire body. It has a big job to do and, therefore, has thicker muscle than the right ventricle or the atria. The right ventricle pumps blood only to the lungs. Because the pressures in the lungs are low, the right ventricle does not need such a muscular wall.

A normal heartbeat begins with the atria pumping blood into the ventricles. After a brief delay while the ventricles fill with blood, the ventricles contract and blood is pumped into the lungs and to the rest of the body. This regular beating pattern - atrial contraction, a delay for ventricular filling, ventricular contraction - is made possible by the heart's electrical system.

Although the muscle portion of the heart is responsible for the actual pumping of blood, the electrical system of the heart controls it. It tells the atria and the ventricles when to contract, and it determines the rate at which the heart will beat. All four chambers must beat in the proper sequence. When the control mechanism is damaged or diseased, the pumping function of the heart is also altered.

The electrical activity that initiates a heartbeat begins in a region of the electrical system called the sinoatrial or SA node. The SA node is located high in the right atrium. This electrical activity then spreads, like a wave, across the right and then the left atrium. As the wave of electrical activity spreads across the atria, muscular contraction occurs. The electrical wave then enters a region of the electrical system called the atrioventricular or AV node. The AV node is a special electrical fiber that acts as a conductor of impulses from the atria to the ventricles. Normally, this impulse conducts very slowly through the AV node. This slow conduction produces the delay that allows the ventricles to fill with blood after the atria have contracted. The electrical impulse then travels from the AV node through a specialized system of electrical tissue called the His-Purkinje system. This electrical conduction is rapid and results in the almost simultaneous contraction of the right and left ventricles.

Types of Arrhythmias

At rest, the heart beats 60 to 100 times per minute. Depending on the body's needs, the heart can normally beat slower or faster.

Bradycardia. Bradycardia, an abnormally slow rate, can be caused by disease in the SA node, in the AV node, or in the His-Purkinje system. Either electrical impulses are not formed fast enough by the SA node, or they are blocked from conducting through the AV node or His-Purkinje system. Some medications can produce bradycardia. Aging or scar tissue in the heart may cause sinus node dysfunction. The heart will beat too slowly; insufficient blood pumped to the brain or muscles can cause fatigue or fainting.

In heart block, the electrical signal in the atria does not reach the ventricle, causing the heart to beat too slowly.

Tachycardia. Tachycardias, abnormally fast rates, are caused by a part of the heart beating rapidly and out of control of the heart's electrical system.

When this abnormal beating originates in the atria, it is called supraventricular tachycardia. It usually stops and starts without warning, and this can be caused by an extra piece of electrical tissue connecting the atria with the ventricles. This "accessory pathway" can act like a short-circuit of the electrical system.

When tachycardia originates in the ventricles, it is called ventricular tachycardia. This condition is usually found in people who had a heart attack or other injury to the heart. It can be a life-threatening arrhythmia.

Fibrillation. Sometimes, the heart can beat so rapidly that the pumping function of the heart muscle is lost. The electrical activity is disorganized and out of control. This is called fibrillation. It can occur in the atria (atrial fibrillation) or the ventricles (ventricular fibrillation). Atrial fibrillation is rarely life-threatening, while ventricular fibrillation is always a life-threatening arrhythmia.

Causes Heart diseases that can result in arrhythmias include atherosclerosis, congestive heart failure, heart attack, cardiomyopathy, valvular heart disease, hypertension, congenital heart disease, ventricular aneurysm, cardiac tumors, and cardiac trauma. Reactions to medications and diseases of the body (systemic diseases) such as anemia, hypothyroidism, and hypothermia may also cause cardiac arrhythmias. Heavy use of tobacco, alcohol, and stimulants like caffeine (found in coffee, tea, chocolate, soft drinks, and medications) may cause arrhythmias. Once the effects of these stimulants leave the body, the arrhythmias can cease.

Symptoms

Palpitations, skipped heartbeats, chest pain, and a sense of fullness in the chest or neck may be caused by cardiac arrhythmias. Weakness, dizziness, lightheadedness, and fainting are also common. Other conditions may cause any of these symptoms, and a doctor should be consulted for a conclusive diagnosis.

Diagnosis

Each type of arrhythmia requires its own special treatment, so identifying the specific type of arrhythmia is essential. The following tests may be used to confirm that the patient is having arrhythmias of a particular type:

An electrocardiogram may be taken to evaluate the heart rate (fast or slow), rhythm (regular or irregular), and presence or absence of heart damage.

A 24-hour electrocardiogram, called Holter monitoring, may also be required.

Blood tests identify chemistry imbalances and inappropriate medication levels that may be contributing to arrhythmias.

How the heart responds to exercise may be evaluated by a stress test, in which an electrocardiogram is taken while the patient walks or jogs on a treadmill.

An echocardiogram is a painless test that uses high-frequency sound waves to show the heart valves and chambers. An echocardiogram may also be taken after exercise.

The resting gated blood pool scan, which uses a radioactive substance, shows abnormal movements in the heart chambers and measures how well the heart is pumping.

Cardiac catheterization reveals abnormalities in the heart valves or chambers. A long, narrow, flexible tube is threaded through an artery and/or vein to the heart. X-ray dye injected into the catheter makes the vessels show up like a road map. Blockage of the coronary arteries can also be detected.

Electrophysiology Studies. Electrophysi-ology studies help diagnose and guide therapy for almost all kinds of cardiac arrhythmias. Arrhythmias usually occur sporadically and unpredictably; if they don't occur during one of the previous tests, it may be difficult to identify the specific location in the heart that is causing the arrhythmia. The electrophysiology study is able to reproduce the patient's arrhythmia in a controlled, predictable fashion so the doctor can make the diagnosis.

During the procedure, insulated wires are advanced through the blood vessels to the heart, much like a cardiac catheterization. These wires record the electrical activity of the atria, the AV node, portions of the His-Purkinje system, and the ventricles. Using pacing techniques, the electrophysiologist attempts to trigger the arrhythmia. Diagnosis is made by interpreting these electrical signals.

Although testing the heart's electrical system sounds like a dangerous procedure, it is actually very safe. All necessary equipment for restabil-izing the heart rhythm is immediately available.

Treatment

Diet. Underlying diseases might be managed by diet in combination with other therapy. Tobacco, alcohol, and caffeine should be avoided.

Pacemaker. If a bradycardia is not caused by medication and if symptoms develop, then pacemakers are used. A pacemaker stimulates the heart to beat if it falls below a certain programmed rate. It guarantees that the heart will never beat more slowly than is safe.

Medication. Drugs that help regulate the heart's rhythm may be prescribed. Surgery or Implantable Devices. An automatic implantable cardioverter defibrillator (ICD) detects ventricular tachyarrhythmias and produces a countershock to control it. Catheter Ablation. During catheter ablation, the abnormal portion of the heart's electrical system is located with electrophysiology catheters. When the catheter tip is close to the abnormal electrical tissue, the tissue is heated by delivering radio frequency waves through the catheter tip. Depending on the type of arrhythmia, this procedure can be curative.

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Arizona Heart Institute
2632 N. 20th St.
Phoenix, AZ 85006
(602) 604-5123