Atrial
Fibrillation
How
Your Heart Beats
The
electrical heart conduction pathway must be
followed to ensure the heart pumps properly.
The
heartbeat starts in the right atrium when a
special group of cells (the sinus node or "pacemaker"
of the heart) sends an electrical signal.
The
signal spreads throughout the atria and to the
atrioventricular (A-V) node.
The
A-V node connects to a group of fibers in the
ventricles that conduct the electric signal.
The
impulse travels down these specialized fibers
to all parts of the ventricles.
Arrhythmias
(or dysrhythmias) are abnormal rhythms of the
heart which cause the heart to pump less effectively.
Normally,
as the electrical impulse moves through the heart,
the heart contracts - about 60 to 100 times a
minute. Each contraction represents one heartbeat.
The atria contract a fraction of a second before
the ventricles so their blood empties into the
ventricles before the ventricles contract.
Under
some conditions almost all heart tissue is capable
of starting a heartbeat, or becoming the pacemaker.
An arrhythmia occurs when:
- the heart's natural
pacemaker develops an abnormal rate or rhythm.
- the normal conduction
pathway is interrupted.
- another part of the
heart takes over as pacemaker.
The
electrical activity of the heart is measured by
an electrocardiogram (ECG or EKG). By placing
electrodes at specific locations on the body (chest,
arms, and legs), a graphic representation, or
tracing, of the electrical activity can be obtained.
Changes in an ECG from the normal tracing can
indicate arrhythmias, as well as other heart-related
conditions.
Almost
everyone knows what a basic ECG tracing looks
like. But what does it mean?
- The first little upward
notch of the ECG tracing is called the "P
wave." The P wave indicates that the atria
(the two upper chambers of the heart) are electrically
stimulated to pump blood to the ventricles.
- The next part of the
tracing is a short downward section connected
to a tall upward section. This next part is
called the "QRS complex." This part
indicates that the ventricles (the two lower
chambers of the heart) are electrically
stimulated to pump out blood.
- The next short flat
segment is called the "ST segment."
The ST segment indicates the amount of time
from the end of the contraction of the ventricles
to the beginning of the "T wave".
- The next upward curve
is the T wave. The T wave indicates the recovery
period of the ventricles.
When
your physician studies your ECG, he/she looks
at the size and length of each part of the ECG.
Variations in size and length of the different
parts of the tracing may be significant. The tracing
for each lead of a 12-lead ECG will look different,
but will have the same basic components as described
above. Each lead of the 12-lead is "looking"
at a specific part of the heart, so variations
in a lead may indicate a problem with the part
of the heart associated with the lead.
Atrial
fibrillation is a type of arrhythmia. With atrial
fibrillation, the electrical signals in the atria
(the two small chambers of the heart) are fired
in a very fast and uncontrolled manner. The atria
quiver instead of contract. The electrical signals
then arrive in the ventricles in an irregular
fashion. When atria do not contract effectively,
the blood may pool and/or clot. If a blood clot
becomes lodged in an artery in the brain, a stroke
(brain attack) may occur. About 15 percent of
strokes occur in persons with atrial fibrillation.
Aspirin, warfarin, and cardiac medications may
be used to treat atrial fibrillation.
According
to the latest recommendations from the American
College of Physicians and the American Academy
of Family Physicians, for most patients with atrial
fibrillation, slowing heart rate with atenolol,
metoprolol, diltiazem, or verapamil is the most
appropriate treatment. Patients with atrial fibrillation
should receive blood thinners to prevent stroke
unless they have a condition that would make anticoagulation
dangerous (such as alcoholism with frequent falls).
Patients
who choose conversion to normal heart rhythm instead
of rate control because of symptoms can select
electrical or medical cardioversion. For patients
who choose cardioversion, outcomes are similar
whether patients have immediate cardioversion
following a special test (transesophageal echocardiogram)
to make sure no blood clots exist or if they delay
cardioversion until blood thinners take effect.
Most patients should not take medications to maintain
normal rhythm after cardioversion, the two national
medical groups suggest.
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