Arrhythmias - Atrial Fibrillation

 

ATRIAL FIBRILLATION
© Alan B. Schwartz, M.D. January1999/version 1.0

This sheet will review some important aspects of the heart rhythm disorder called ATRIAL FIBRILLATION. Atrial fibrillation is the most common heart rhythm disorder. To understand atrial fibrillation, knowledge of the basics of normal cardiac anatomy and normal electrical heart function is useful.

CARDIAC ANATOMY

The heart is divided into four chambers. There are two atria and two ventricles. There is one atrium and one ventricle on the right side of the heart and a similar configuration on the left side of the heart. The atria are the "upper" chambers of the heart and the ventricles are the "lower" chambers.

The right and left heart can be thought of as two, two-stage pumps. The right atrium contracts first, pumping venous (oxygen depleted) blood collected by the great veins from around the body into the right ventricle. A short time later, the right ventricle contracts pumping blood into the lungs where it picks up oxygen. The oxygen rich blood from the lungs then flows into the left atrium which pumps blood, in a similar manner, into the left ventricle. The left ventricle in turn pumps blood into the arterial circulation via the aorta.

The most powerful pumping takes place in the ventricles. In fact, when you feel your "heart beat", what you actually feel is the large muscular ventricles bumping against your chest wall, not the small thin walled atria. The atria only contribute about 10-15% to the total pump function of the heart. This small contribution is not important in people who have normal, undamaged ventricles but can be important if the ventricle has been impaired by a heart attack, a faulty heart valve or heart muscle disease.

ELECTRICAL SYSTEM

The cardiac electrical system originates in the right atrium where the natural pacemaker is located (called the sinus node). This collection of cells puts out an electrical signal 50 to 100 times per minute when a person is at rest. This electrical signal travels across the cells of the right and then the left atrial heart muscle like ripples moving across the surface of a pond. As it does this, the atrial muscle cells contract leading to a squeezing motion of the entire atrium, pushing the blood which has collected in that chamber to the next chamber, the ventricle.

Both atria are electrically insulated from the ventricles except for a "wiring system" which transmits the impulse to the ventricles. The beginning of this wiring system is called the AV node. The wiring system branches after the AV node into the right bundle and the left bundles. Each bundle transmits the impulse to its respective ventricle. The left bundle further divides into an anterior branch and a posterior branch. Therefore, there are three routes for the atrial pacemaker signal to reach the ventricle. As long as any one of these routes works, the ventricles will receive the proper signal so that they can initiate contraction.

ATRIAL FIBRILLATION

In atrial fibrillation, the native pacemaker stops working. The atrial muscle cells try to take over. However their efforts are uncoordinated and the net result is as if there were random firing of 400 to 600 atrial muscle cells per minute. This leads to the loss of an effective, coordinated atrial contraction. Instead, the atria develop a "quivering" motion. This means that in atrial fibrillation, the atria become a passive chamber that no longer pumps. Here, blood can stagnate and form clots which can then get expelled into the pulmonary or systemic circulation. These traveling blood clots can cause stroke or breathing difficulties.

Luckily, the AV nodal conducting system cannot accommodate the large number of atrial signals which bombard it during atrial fibrillation (400 to 600 beats per minute). However, anywhere from 150 to 200 beats a minute do get through the conducting system to the ventricles. These erratic atrial signals cause the ventricles to beat fast and irregularly even at rest. With exercise, the heart rate may increase even further. When the ventricles beat very fast, they do not have sufficient time to fill with blood so that when they contract, they pump less blood.

The mechanism of atrial fibrillation is thought to be due to 1) randomly wandering "wavlets" of electrical energy, 2) a rapidly firing single cluster cells or a 3) combination of 1 and 2. The mechanism is probably different depending on whether the atrial fibrillation is intermittent or constant. Based on the above, cathether ablation as been used to cure some times of atrial fibrillation.

This means that at rest, when the heart rate should be between 50 and 100 beats per minute, the heart may beat chaotically at 150 to 200 beats per minute. The fast beating can cause fatigue and shortness of breath at rest or with exertion. It can lead to fluid retention in the lungs and legs. The irregular beating alone can be very disconcerting to some individuals even when the heart rates are in the normal range. This irregularity can be perceived as "fluttering", "rocking", "bumping" or "butterflies in the chest". The decrease in atrial pumping capability of the heart (seen primarily in people with poor heart function) can cause low blood pressure, cold extremities, extreme fatigue and tiredness.

PROGNOSIS

Atrial fibrillation is considered a relatively benign rhythm disorder and is not necessarily associated with any underlying heart problems. The prognosis of atrial fibrillation in the absence of underlying heart disease is good.

CLASSIFICATION

Episodic atrial fibrillation is called paroxysmal atrial fibrillation. Permanent atrial fibrillation is called chronic atrial fibrillation.

CAUSE

The most common known causes of atrial fibrillation are (1) high blood pressure, (2) Rheumatic mitral valve disease, (3) blockages in the arteries of the heart (coronary artery disease), and (4) an overactive thyroid. However in a large number of people who develop atrial fibrillation, no specific cause can be found.

The prevalence of atrial fibrillation increases with age occurring in over 10% of persons over the age of 80.

Persons in the "news" who have had atrial fibrillation include the basketball players Larry Byrd, Hakeem Alijawon, former Presidents George Bush and Richard Nixon, a Soviet Cosmonaut, Sophia Loren, Mike Ditka, Mike Wallace, Ted Williams, Ted Turner, Senator Bill Bradley, Prime Minister Tony Blair, Barry Bonds, Barry Manilow and the late Mother Theresa.

TREATMENT

Atrial fibrillation is not generally a "curable" condition. Fortunately, it is usually not considered a dangerous condition. The goal of treatment is to make the episodes infrequent and when they do occur, to make them very short-lived. Sometimes simple measures can reduce episodes of paroxysmal atrial fibrillation dramatically. Caffeine, nicotine, alcohol have been shown to make episodes of atrial fibrillation more frequent. Remember that caffeine is in tea and chocolate as well as coffee. Other triggers for atrial fibrillation are physical and emotional stress, a low potassium level and lack of sleep. Some forms of atrial fibrillation occur primarily at rest, others primarily during exercise.

The treatment strategies are individualized. Even among experts, there is no consensus for one "right way" to treat atrial fibrillation. These are current therapeutic options:

• Re-establishment of normal rhythm by medicines or a procedure known as cardioversion
• Control of the heart rate with medicines and treatment of the risk of stroke with blood thinners
• Prevention of some episodes of atrial fibrillation with a pacemaker alone
• Control of atrial fibrillation with a pacemaker in conjunction with a procedure known as catheter ablation of the AV conducting system
• Implantation of a "gizmo" known as atrial defibrillator
• A potentially curative cardiac surgical procedure called the "Maze" operation. This can be done at the time of a another clinically indicated cardiac surgery procedure (like bypass surgery or valve replacement) or by a "minimally invasive approach" as the sole reason for surgery. The cure rate using this approach is about 70%.
• A potentially curative catheter based procedure called "pulmonary vein isolation" or "Left atrial Catheter Ablation" or a combination of the two. The cure rate using this approach is also about 70%.

Your physician will discuss these options with you.

DIAGNOSTIC EVALUATION:

All persons with new onset atrial fibrillation should have (1) blood tests for thyroid disease, (2) an evaluation for heart function, left atrial size, and mitral valve disease by an ultrasound test, and (3) a test to exclude underlying coronary artery disease.

SUMMARY

From the above, there are four major effects of atrial fibrillation: (1) Loss of heart rate control at rest and during exercise, (2) irregular heart beating which may feel very uncomfortable, (3) loss of atrial pump function and (4) propensity to stroke. For people with normal hearts, the loss of atrial pump function is not important.

There are many treatment strategies for atrial fibrillation depending on the type of fibrillation you have, you age and your physical condition. Catheter ablation now offers the promise of a cure in some individuals.

This informational sheet is meant to give you an overview to atrial fibrillation, the most common heart rhythm disorder.

If you have any questions, please call or write Alan Schwartz, M.D. by telephone, e-mail or fax.

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