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Cardiovascular Laboratory |
ECG> Cardiac
Arrhythmias |
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Here are a few traces that deviate from the
norm (cardiac "arrhythmias") : tachycardia or bradycardia, AV blocks,
premature contractions, and fibrillation. |
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1)
Sinus Tachycardia |
| Sinus tachycardia occurs
in adults when impulses
originate at the SA node at a rate greater than 100 per minute . It
may or may not be clinically significant, and is not in itself indicative of cardiac
disease. (For example, sinus tachycardia may be associated with fever or exercise.) |
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2)
Sinus Bradycardia |
| Sinus bradycardia occurs when impulses originate
at the SA node at a rate of less than 60 per minute. Sinus bradycardia is not
necessarily indicative of cardiac disease, and is often seen in athletes and during sleep. |
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3)
Atrioventricular (AV) block |
| Partial AV block occurs when AV
node damage prevents some atrial impulses from being transmitted to the ventricles.
In the particular case illustrated, every second P wave is not followed by QRS and T
waves, producing 2:1 AV block. It is said that there are "dropped beats"
of the ventricles. |
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Complete AV
block: When the condition that is causing poor conduction in the AV node
becomes severe, there is a complete block of the impulses from the atria to the
ventricles. In that case, a subsidiary pacemaker can arise in the
ventricles, which then paces the ventricular muscle. There is
then no synchronization between atrial and ventricular electrical
activity. The ventricles have "escaped" from atrial control, and are
beating at their own natural rate, which is typically much less than
the sinus rate. |
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4)
Premature Contractions |
| A premature contraction occurs when the heart
contracts prior to the time when normal contraction is expected.
Some premature
contractions are due to ectopic foci in the heart, which emit abnormal impulses at
abnormal times during the cardiac rhythm. The ectopic focus can be
situated anywhere in the heart. In the case of a premature
contraction, the PR interval is often shorter for the premature beat
than for the normal sinus beat. |
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5)
Ventricular Fibrillation |
| In ventricular fibrillation, multiple impulses
are simultaneously traveling in different directions through the ventricles. If left untreated,
ventricular fibrillation results in death within about 2-4 minutes. Although
electric shock can initiate ventricular fibrillation, a very strong electrical current
passed through the ventricles for a short period of time can actually stop
fibrillation ("defibrillation"). The defibrillation shock stimulates all parts of the ventricles simultaneously and puts
them in the same state of refractoriness, allowing the S-A node or some other part of the heart
to become the pacemaker when the heart starts to beat again. |
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