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Bradycardia is characterized by an
abnormally slow heart rhythm. The weak
pace may mean the heart doesn’t beat often
enough to ensure blood flow.
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Each heart has it own normal
rhythm brought about by the
seamless flow of electrical
impulses that begins in the heart’s natural
“pacemaker” (sinus node). The
electricity flows through the upper
chambers (atria), crosses the bridge
between upper and lower chambers
(atrioventricular node) and travels to
the lower chambers (ventricles). This
passage of electricity culminates in
a carefully coordinated contraction
of heart muscle that pushes blood
throughout the human body.
Each day, a normal heart contracts
about 100,000 times, at a rate anywhere
from 60 to 100 times a minute.
Abnormally slow heart rates are typically
those below 60 beats a minute
and either can be relatively harmless
or life threatening.
Changes in rate brought about by
variations in activity, diet, medication
and age are normal and common. For
some people, such as athletes in top
condition, a resting heart rate of
below 60 can be normal. Similarly, at
certain times, such as sleep, a heart
rate may slow and still be normal.
When a heart slows for no apparent
reason, however, it can be a sign of
an abnormality in the electrical pathway
and is cause for evaluation. If
the problem is in fact a slow heart
rhythm (bradycardia), a pacemaker
is often the treatment of choice.
Symptoms
Some bradycardias do not produce
any symptoms; others do and warrant
treatment. When the heart beats
too slowly, not enough oxygen-rich
blood flows throughout the body. So,
it makes sense that the symptoms of
a slow heartbeat include:
| Fatigue, weakness |
| Dizziness |
| Lightheadedness |
| Fainting or near fainting |
| Shortness of breath |
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ABOUT HEART BLOCK/AV BLOCK
Heart block occurs when the electrical impulse that originates in the upper
chambers of the heart is unable to pass to the lower chambers of the heart
at the right pace. Heart block delays the impulse or may block it completely.
(Heart block is sometimes called AV block, because the impulse slows or
does not pass through the atrioventricular node that joins the upper and lower
chambers of the heart.) There are three kinds of heart block:
| | First-degree: The beats pass from the upper chambers to the lower
chambers, but conduction is slower than normal |
| | Second-degree: Not all of the beats pass from the heart’s upper to lower
chambers, so some are dropped |
| | Third-degree/Complete: The impulses cannot pass from the upper to the
lower chambers, so the lower chambers originate their own impulse. This
means they do beat and pump blood, but at a slower rate and more inefficiently
than if the impulse originated from the upper chambers of the heart |
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Types of Arrhythmias
Understanding some common terms
in the language of cardiac arrhythmias
helps sort through the types of rhythm
problems. An arrhythmia is any abnormality
in heart rhythm. Arrhythmias
are categorized in three main ways:
Rate
If the heart rate is slow, or less than
60 beats a minute, it is considered
bradycardia. Alternatively, a fast
heart rate, above 100 beats a minute,
is known as a tachycardia.
Location
The location of the problematic
electrical circuit helps define the
arrhythmia. For instance, a rhythm
is called supraventricular if it originates
above the ventricles (lower
chambers). So, the problem is most
likely in the upper chambers (atria). It
follows that a ventricular arrhythmia
is the result of a problem in the lower
chambers (ventricles.)
Irregular
The nature of the heartbeat, whether
it is steady or chaotic, is another key
to categorizing an arrhythmia. A rapid
beat that is irregular and chaotic may
be fibrillation, or a quivering beat.
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ABOUT SICK SINUS SYNDROME
Sick sinus syndrome (SSS) is a form of bradycardia in which the sinoatrial
node (the heart’s natural pacemaker) is not functioning as it should. This
means that the electrical signals that normally start a heartbeat do not all
leave the SA node (sinoatrial block) or that there are longer pauses in the
generation of the electrical signal (sinus arrest). SSS can cause tachycardia
(heart rates that are too fast) or bradycardia-tachycardia syndrome (heart
rates that fluctuate between being too slow and too fast).
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Causes
Slow heart rates can be the result
of certain medications, congenital
heart diseases or the degenerative
processes of aging.
The most common causes of bradycardia
are problems with the heart’s
electrical pathway. The problem can
occur at any point on the path, causing
an electrical “block.” Often it originates
in the heart’s natural “pacemaker”,
called the sinoatrial (SA) node.
This can lead to sick sinus syndrome.
Electrical pathway problems can
cause the heart to beat too slowly
all the time or only occasionally. In
either case, the heart may not pump
enough blood to meet the body's
needs. As the heart rate declines,
there is insufficient blood flow to
the brain, causing feelings of lightheadedness
and, sometimes, fainting.
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