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 AF? (Incidence)
- Why Does it Happen? (Aetiology)
- Symptoms, Signs & Risks
- What Tests Should I have? (Investigations)
- Treatment options (success and complication rates)
- What is the Outcome? (Prognosis)
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