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.
Causes
The condition mostly
affects elderly people with high blood pressure,
coronary artery disease that has affected
the heart muscle, and heart valve disease,
especially abnormal narrowing of the valve
between the atrium and the ventricle on
the left side of the heart (mitral stenosis).
It may also affect middle-aged people who
apparently have no heart problems. In
people who do not apparently have structural
heart disease, atrial fibrillation may be
due to overactivity of the thyroid gland,
excessive alcohol intake, inflammation of
the bag surrounding the heart (pericarditis)
or inflammation of the heart muscle. There
is also a condition called sick sinus syndrome
in which there is a malfunction of the conducting
system of the heart at the atrial level.
This may feature fibrillation among other
forms of irregularity.
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Atrial Fibrillation
