AF is thought to be caused by a variety of cardiac and non-cardiac conditions such as hypertension, valvular disease, ischaemic cardiomyopathy, thyroid disease, diabetes mellitus and atrial fibrosis. Most people with AF suffers from one or more of these and is usually over 60 years of age.
Atrial fibrillation occurs because of an abnormality of the electrical signalling pathway. Instead of the signals following the regular pathway, the heart’s normal pulses generated by the sinoatrial node to provide rhythmic contractions, are overcome by randomly generated electrical pulses. These are produced in the atria and pulmonary veins and result in fibrillating activity.
Fibrillate means to contract very fast and irregularly
The rapid, irregular impulses are discharged at a rate of 300-600 beats per minute through the atrioventricular node. The AV node will struggle to filter the large number of fast irregular signals coming from the atria which will lead to poor emptying of the atria.
As the signals pass through the AV node into the ventricles, the ventricular rate will increase and begin to beat faster however the AV node cannot send the signals to the ventricles as fast as they arrive. Thus the atria and ventricles are no longer beating in coordination even though they are beating faster than normal, creating a fast and irregular heart rhythm.
During AF the ventricles contract around 150 times per minute and is much slower than the rate in the atria. This means the ventricles will not have enough time to completely fill with blood before the next contraction which will result in a decrease in the amount of blood that gets pumped by the ventricles because of their rapid rate of contraction and the lack of normal atrial contractions.