Aetiology

( 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”.

No one is sure why AF happens but it is likely that there are two factors that help it develop, the AF trigger and the AF substrate.

1. The trigger for AF is the thing that starts it in the first place. Although the sinus node is the rate controller or pacemaker of the heart all the cells in the heart have the ability to spontaneously activate it is just that they do it at a slower rate. This means that they are dominated by the sinus node. It is quite normal for other heart cells to fire off spontaneously at the wrong time and produce an ectopic (meaning in the wrong place) beat. This is experienced as a skipped heart beat which makes the heart feel irregular for a few seconds, as opposed to AF which makes the heart feel irregular from a few minutes at a time to many years. These ectopic beats are a quite normal way for the heart to function but if they happen very frequently it is possible for them to “irritate” the atria and cause them to fibrillate. The source of these ectopic beats is often the veins that drain blood from the lungs to the heart (pulmonary veins) and may be the result of abnormal atrial tissue contained within the veins. One of the reasons why patients with high blood pressure get AF may be that the blood pressure stretches the veins irritating the atrial tissue and making it fire-off ectopic beats. In patients who have otherwise normal atria the ectopic beats will initiate the AF but it soon stops on its own because the atria is not able to continue for any length of time. This is called paroxysmal AF.
   
   
2. The substrate for AF describes an abnormality in the electrical properties of the atrial tissue itself. In order for the waves to continue to circulate they need a certain amount of room to move around in otherwise they will just collide into each other and stop. So size is important. AF is common in race-horses because they have a big heart and uncommon in mice because they have small hearts. If the waves travel around slowly they leave plenty of time for the cardiac tissue to recover and become excitable so this means that the waves are more likely to always be moving towards excitable tissue. This may be why AF is more common as people get older because their hearts get slightly bigger and conduct more slowly. If a patient’s heart is sufficiently abnormal then the substrate may allow the AF to sustain or remain permanently. This is called persistent or permanent AF.

It is a common story for patients to present with paroxysmal intermittent AF (initiated by ectopic beats) which then lasts for longer and longer until eventually the AF becomes persistent or permanent. This is because AF begets AF. This means that AF in itself changes the electrical properties of the atria making them more likely to sustain AF.