Atrial Fibrillation

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Atrial Fibrillation 

What is Atrial Fibrillation?

The heart is a pump which is divided into two sections, the atria and ventricles, and into two halves the left and right. This means that the heart is divided into 4 chambers. The atria pump blood into the ventricles and thus have a relatively small job to do, and the ventricles pump blood around the body and thus take on most of the work. The atria are therefore thinner and smaller than the muscular ventricles. The right heart pumps blood through the lungs to get oxygen into it and the left heart pumps blood around the body including the brain.

The anatomy of the heart

Normally the atria pump blood into the ventricles before the ventricles then pump blood around the body. They therefore work in sequence. This sequence is controlled by an electrical system. A small bunch of cells situated at the top of the heart, the sinus node (SN) sends an electrical impulse out at about 60 to 70 times a minute (hence your heart rate is 60 to 70). This gets faster if you need your blood to move through your body more quickly, for example if you are exercising. This electrical impulse then washes like a wave across the atria from right to left being conducted from cell to cell and making the cells contract causing the atria to squeeze (video animation might be good here if we can get it commissioned). The atria and the ventricles are insulated from one another (black line) so when the electrical wave hits this insulation it dies out and the cells await the next impulse from the sinus node.

The atria and ventricle are electrically connected to one another by the atrioventricular node (AV node). This conducts the electrical wave slowly through from atria to ventricles giving the atria time to contract before it then sends the impulse out through the bundle branches (BB ? electrical ?motorways? that spread the electrical wave very quickly through the ventricle) and the ventricles causing them to contract. Once a heart cell has been excited by an electrical wave it takes a few tenths of a second to recover before it can be excited again so whatever area it passes through is unexcitable for a short time. The wave therefore washes through the heart from top to bottom before dying out, until the next impulse from the SN.

The conduction system

In AF rather than a single electrical wave emerging from the Sinus Node there are lots of smaller waves which are continuously wandering around the atrium. The path that these waves take and the number of waves that there are is limited by the amount of excitable tissue in front of each wave front. If one wave hits an area of the heart that another wave has just excited it cannot continue because the tissue has just been excited and is unexcitable. It therefore either changes direction, splits into 2 waves and passes around the unexcitable tissue or dies away. To see a recording of atrial fibrillation in a human click here. This is a recording made over a period of a 10th of a second with a mapping system called non-contact mapping and is of a human left atrium. The video shows 2 views of the left atrium (front and back) with electrical waves as white and coloured areas passing over the surface of the atrium (coloured purple when not being activated). In this recording, which is slowed down to help the viewer see a bit of what is going on, one can see how complex atrial fibrillation is.

Most often atrial fibrillation originates in the left atrium but will affect both the left and right atrium because they are in electrical continuity. Therefore in the film you have just seen the waves that appear from nowhere have probably just broken through from the right atrium to the left atrium.

The AF waves are therefore continuously moving around the atria which means that the atrial cells are contracting in a chaotic fashion. This means that the atria does not contract but appears to shiver or fibrillate. This has 2 important effects, firstly the atria do not squeeze blood out of the many cul-de-sacs that exist within the atria which may allow blood to stagnate or in high risk people, clot. If clots form in the left atrium then they may be dislodged and fly off into the circulation. One of the biggest destinations of blood from the heart is the brain, so if clots are carried away by the circulation they may reach the brain and cause a stroke. Stroke risk can be reduced by giving high risk patients warfarin and low risk patients aspirin.

For a list of patients at high risk of stroke and the level of anticoagulation they should get click here. The other effect that the lack of atrial contraction has is to reduce the efficiency of the heart. The atria can be likened to a turbo-charger on a car. If your car is sitting at the traffic lights it does not use the turbo. The effect of the atrial contraction is similarly minimal when someone is resting, because all the blood flows into the ventricle on its own without the atrium. However when the traffic lights go green and the driver puts their foot on the pedal they will notice immediately that the turbo is not working because the car will be much slower than normal. The same is true for the heart, particularly in active people. Many patients notice that they are in AF because they cannot do as much as they could when they were in normal rhythm. Obviously in people who normally do very little physical activity like the very elderly and the infirm, they will never use the atrial part of their heart function and so may not notice this effect. Because AF is a chronic condition, patients get used to this lack of function and forget what it was like when they were in normal rhythm. It is not uncommon for patients to be surprised at what they can do when restored to normal rhythm and feel worse if they return to AF because they are reminded what it is like to be in normal rhythm.

It is important to remember that the atrium is insulated from the ventricle so that ventricle is not directly affected by these waves. However there is a connection between the atria and ventricles, the AV node. When an AF wave passes across the AV node, if it is excitable (and hasn?t already just been activated by a wave), it will conduct to the ventricles and make them contract. Because this process is chaotic and random, because the AF waves follow a random path, activation of the AV node and thus the ventricles is random and irregular. Therefore one of the symptoms patient?s experiences is an irregular heart beat. The atria are too small for you to feel the effect of their contraction but the ventricle is strong enough for you to feel so when you feel your heart is irregular during AF it is the ventricles response to the atria that you are feeling, not the atria themselves. The AF waves are continuously passing around the atria rather than emerging once a second from the Sinus Node so the AV node is excited more often than normal so not only does the heart feel irregular but also faster than normal.

How common is Atrial Fibrillation (AF) ?

AF occurs in a approximately 1 in 100 people. It gets more common as one gets older and in the population over 75 years of age it occurs in 10% of the population.

Why does Atrial Fibrillation Happen?

Many patients try to find reasons why their AF has started and look for things they do that have triggered it. There is often no trigger that you have caused so do not blame yourself. Things that are strongly associated with AF are excessive alcohol, particularly binge drinking, other cardiovascular disease particularly heart valve disease and high blood pressure and thyroid gland disease. There is an almost infinite number of things that may trigger AF in individual patients but common things are exercise, reflux of acid into the oesophagus (heart-burn) and stress/anxiety. In patients with no heart disease or cause for their AF (common situation) it is called ?lone AF?.

Atrial Fibrillation Symptoms, Signs and Risks

Common symptoms are:

1. Palpitation ? a feeling that ones heart is racing or going faster than normal

2. Lethargy because the ?turbo-charger? of the heart is not working

3. Shortness of breath

4. Dizzy spells or fainting? because the heart can go too fast or too slow, preventing it from pumping blood efficiently and thus dropping the blood pressure

5. Chest pain

6. Symptoms of stroke ? transient or permanent weakness of an area limited to one side of the body

What are the signs of AF?

You may notice that your heart is continuously irregular or unexpectedly fast. The heart rate rises naturally when you exercise but should not be faster than 90 beats per minute if you have been resting for a while. Many patients may not realise they have AF until someone takes their pulse, for example at a routine medical examination.

What are the risks of AF?

Traditionally AF has been considered a benign heart rhythm, i.e. it is not dangerous. Unfortunately this is a belief still held by some doctors who have not kept up to date with research. AF has the following dangers associated with it:

1. Stroke - AF is the commonest cardiac cause of stroke for reasons explained under What is Atrial Fibrillation? above. Certain people are at high risk of stroke. This risk can be reduced by warfarin in high risk patients and aspirin in low risk patients. Many doctors under-prescribe warfarin because they feel that the elderly who are most at risk of stroke, may be at risk of falling and the bleeding because of the warfarin. Many patients refuse to take warfarin because they are concerned about the inconvenience of the regular blood tests and do not understand the risk of stroke and the devastation it can cause. There are newer drugs under trial at the moment that avoid the need for regular blood tests but there are currently problems with these because of potentially serious side effects. It will therefore be some time until they are fully tested and available.

2. Heart failure ? AF can cause heart failure by making the heart beat too quickly. If you were asked to run on a treadmill at a fast rate you would become tired pretty quickly. If you ask the heart to beat at a fast rate then it will also get tired. Good heart rate control both at rest and during exercise is therefore important. There is evidence emerging that heart failure can occur in some patients despite good rate control and that restoration of normal rhythm by catheter ablation can help the heart to recover.

3. Death ? no one truly understands why patients with AF die but a number of studies have shown that even when you correct for other diseases patients with AF are more likely to die than patients in normal rhythm. The largest study of this effect, the Framingham study showed that AF doubled peoples mortality rates. Because we do not know why this happens we cannot be sure that any treatment we give will prevent it so although it seems likely that restoration of normal rhythm will reduce the chances of death there is currently no proof that this is the case.

Benjamin EJ, Wolf PA, D'Agostino RB, Silbershatz H, Kannel WB, Levy D. Impact of atrial fibrillation on the risk of death: the Framingham Heart Study. Circulation. 1998 Sep 8;98(10):946-52

Atrial Fibrillation Investigations, What Tests Should I Have?

The most important thing is to confirm whether you do have AF or not. This is done by recording the electrical activity of the heart during your symptoms (or when your heart is fast and irregular if you do not have symptoms). Recording the electrical activity of the heart is called an electrocardiogram (ECG). This can be done either in a doctors surgery on a fixed machine, on a treadmill during an exercise ECG or with a device that you can carry around at home using either a Holter monitor or event recorder. It is also important that someone competent looks at the results. Even cardiologists who do not specialise in heart rhythm problems may get it wrong.

If AF has been confirmed then you should have some simple tests to check that there is not an underlying cause for it. You therefore should have had:

1. your blood pressure measured to exclude hypertension (high blood pressure)

2. your thyroid function measured with a blood test

3. an echocardiogram to exclude structural heart disease that is sometimes impossible to detect with an examination

Once other causes for AF have been excluded or treated then you must make a decision as to how you want your AF treated. If you opt to accept AF and simply have you heart rate controlled, recording the ECG during exercise or with a 24 hour Holter recording will give a much better idea of what your heart rate control is with just one examination at rest.


Atrial Fibrillation Treatment options

When deciding what treatment to have for your AF it is important to remember that no treatment is without risk associated with it. This includes tablets and operations. When doctors are advising on treatment we are trying help you assess what is the best option for you based on the risks and benefits of each therapy. Doctors are trying to predict how things may turn out for you based on statistics, research and their experience of other patients but inevitably until doctors can see into the future we will not always get it right. Obviously the more patients with AF we see and the more research we do the better our advice is likely to be.

Essentially there are 3 problems associated with AF that need treating:

1. stroke risk

2. fast heart rate

3. abnormal irregular rhythm

4. Catheter Ablation

For details go here

What is the Outcome of Atrial Fibrillation?

Contrary to popular belief AF is not benign. The mortality rate rises by up to 1.9 times in patients with AF, the reason for which we do not know. There have been a number of studies examining whether restoring sinus rhythm is better than just controlling rate in AF and these have all shown that controlling rate is a policy that is as good or better than controlling rhythm. HOWEVER these studies all examined conventional (drugs and cardioversion) methods of treatment and many of the patients in the rhythm control group failed to stay in sinus rhythm. Furthermore anticoagulation was stopped too soon and many patients who went back into AF suffered stroke. It has been shown that if the patient does stay in sinus rhythm they have a lower mortality than those in AF and there is early evidence that catheter ablation can reduce mortality and improve patient symptoms. Because this is a relatively new technique for treating AF more thorough evidence is yet to be collected to demonstrate whether catheter ablation eliminates the mortality risk of AF.


This material is used with the kind permission of The London Atrial Fibrillation Centre

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