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Atrial fibrillation

The heart has two upper chambers and two lower chambers. The upper chambers are called atria and the lower chambers ventricles. Atrial fibrillation is a condition in which the upper chambers of the heart contract at a very high rate and in an entirely disorganised manner. Only the strongest impulses are passed down to the main lower pumping chambers (the ventricles), so the pulse, which is caused by the contraction of the left ventricle, is extremely irregular and of variable force.

Although established atrial fibrillation may sometimes appear in otherwise seemingly healthy people, research has shown that people with the condition are significantly more at risk of heart trouble and strokes than others. Fifteen per cent of all people who have had strokes have atrial fibrillation.

The upper and lower chambers of the heart have walls of almost pure muscle. When we talk of the heart ‘beating’, we are really referring to the sudden tightening of this muscle so that the chambers become smaller and the blood in them is squeezed out.

The control of the heartbeat starts with a small clump of muscle cells in the upper right chamber, called the sinoatrial node. This acts as the heart’s natural pacemaker by conveying electrical impulses to the atrioventricular node, which is located in between the upper and lower chambers.

This determines the rate of contraction of the lower chambers (ventricles) and the pulse rate. Atrial fibrillation occurs when the atrioventricular node receives more impulses than it can conduct and causes irregular squeezing of the ventricles.