What causes Atrial Fibrillation (AF)?
Many patients try to find reasons why their AF has started and look for things they do that may have initiated it. The truth however is that it is rarely the fault of the patient – so do not blame yourself. AF is strongly associated with underlying cardiovascular problems such as high blood pressure, heart failure, coronary heart disease and valvular heart disease.
Non heart problems that can cause AF are excessive alcohol (particularly binge drinking) or an overactive thyroid gland disease. Finally in about 30% of people a cause is never found and as far as we can tell their heart is completely normal in all other respects, this is termed “lone AF”.
Trigger and substrate
No one is sure why AF happens but it is likely that there are two factors to consider, the AF trigger and the AF substrate.
AF is started by a trigger. Although the sinus node is usual pacemaker of the heart, all the cells in the heart have the potential to spontaneously activate. The normal heart cells are dominated by the sinus node and they activate only after they receive the electrical signal from the sinus node (link to electrical system of the heart). It is quite normal for other heart cells to occasionally activate spontaneously at the wrong time causing an ectopic (meaning in the wrong place) or extra heart 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 for much longer.
Occasional ectopic beats are experienced very commonly in people with normal hearts. In people who experience AF however groups of rogue atrial cells are producing ectopic beats rapidly and very frequently. These frequent ectopic beats are the trigger for AF. The rogue cells are usually grouped at the junction between the left atrium and the veins that drain blood from the lungs to the heart (pulmonary veins). It is not fully understood why the pulmonary veins are the usual location for the AF triggers but it may be because the atrial cells are abnormal within the veins. An example is in patients with high blood pressure, where the high pressure in the heart stretches the pulmonary veins altering and distorting the atrial tissue there. This makes them prone to fire 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 continually fibrillate for a prolonged period. This is called paroxysmal AF.
The substrate for AF describes an abnormality in the electrical properties of the atrial tissue itself. In order for the multiple chaotic fibrillation waves to continually circulate around the atria they need a certain amount of room to move around. In the absence of sufficient room in they will just collide into each other and stop. A large atrium in which the electrical waves move slowly provides the ideal conditions for these perpetual waves.
By travelling 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 ‘substrate’ is caused by many types of heart disease and is detectable on tests such as an echocardiogram as enlarged atria. If a patient’s heart is sufficiently abnormal then the substrate may allow the AF to be sustained without any need for the triggers described above. This is called persistent AF.
AF begets AF
It is a common story for patients to start with occasional paroxysmal AF however the attacks then become more frequent and last longer and longer until eventually the AF becomes persistent. This is because AF itself changes the electrical properties of the atria and causes the atria to dilate, in other words creates the substrate for persistent AF. Hence a term often used by doctors ‘AF begets AF’. The importance of this is that if AF is treated early its progression can be halted.