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Cardiac arrhythmias can be classified by site
of origin:

- Sinus rhythms originate from the sinoatrial
node, or SA node

- Atrial rhythms originate from the atria
- Ventricular rhythms originate from the ventricles.

Sinus rhythm is the normal rhythm of the heart
set by its natural pacemaker in the SA node.

In a healthy heart, the SA node fires 60 to
100 times per minute resulting in the normal

heart rate of 60 to 100 beats per minute.
The most common variations of sinus rhythm
include:

- Sinus bradycardia: when the SA node fires
less than 60 times per minute resulting in

a slower heart rate of less than 60 beats
per minute.

and
- Sinus tachycardia: when the SA node fires

more than 100 times per minute generating
a faster heart rate of greater than 100 beats

per minute.
Sinus bradycardia and sinus tachycardia may
be normal or clinical depending on the underlying

cause.
For example, sinus bradycardia is considered
normal during sleep and sinus tachycardia

may be normal during physical exercises.
Cardiac arrhythmias that originate from other
parts of the atria are always clinical.

The most common include: atrial flutter, atrial
fibrillation and AV nodal re-entrant tachycardia.

These are forms of supraventricular tachycardia
or SVT.

Atrial flutter or A-flutter is caused by an
electrical impulse that travels around in

a localized self-perpetuating loop, most commonly
located in the right atrium.

This is called a re-entrant pathway.
For each cycle around the loop, there is one
contraction of the atria.

The atrial rate is regular and rapid - between
250 and 400 beats per minute.

Ventricular rate, or heart rate, however,
is slower, thanks to the refractory properties

of the AV node.
The AV node blocks part of atrial impulses
from reaching the ventricles.

In this example, only one out of every three
atrial impulses makes its way to the ventricles.

The ventricular rate is therefore 3 times
slower than the atrial rate.

This is an example of a “3 to 1 heart block”.
Ventricular rate in A-flutter is usually regular,
but it can also be irregular.

On an ECG atrial flutter is characterized
by absence of normal P wave.

Instead, flutter waves, or f-waves are present
in saw-tooth patterns.

Atrial fibrillation is caused by multiple
electrical impulses that are initiated randomly

from many ectopic sites in and around the
atria, commonly near the roots of pulmonary

veins.
These un-synchronized, chaotic electrical
signals cause the atria to quiver or fibrillate

rather than contract.
The atrial rate during atrial fibrillation
can be extremely high, but most of the electrical

impulses do not pass through the AV node to
the ventricles, again, thanks to the refractory

properties of the cells of the AV node.
Those do come through are irregular.
Ventricular rate or heart rate is therefore
irregular and can range from slow - less than

60 - to rapid -more than 100 - beats per minute.
On an ECG, atrial fibrillation is characterized
by absence of P-waves and irregular narrow

QRS complexes.
The baseline may appear undulating or totally
flat depending on the number of ectopic sites

in the atria.
In general, larger number of ectopic sites
results in flatter baseline.

AV nodal re-entrant tachycardia or AVNRT is
caused by a small re-entrant pathway that

involves directly the AV node.
Every time the impulse passes through the
AV node, it is transmitted down to the ventricles.

The atrial rate and ventricular rate are therefore
identical.

Heart rate is regular and fast, ranging from
150 to 250 beats per minute.

Ventricular rhythms are the most dangerous.
In fact, they are called lethal rhythms.
Ventricular tachycardia or V-tach is most
commonly caused by a single strong firing

site or circuit in one of the ventricles.
It usually occurs in people with structural
heart problems such as scarring from a previous

heart attack or abnormalities in heart muscles.
Impulses starting in the ventricles produce
ventricular premature beats that are regular

and fast, ranging from 100 to 250 beats per
minute.

On an ECG V-tach is characterized by wide
and bizarre looking QRS complexes.

P wave is absent.
V-tach may occur in short episodes of less
than 30 seconds and cause no or few symptoms.

Sustained v-tach lasting for more than 30
seconds requires immediate treatment to prevent

cardiac arrest.
Ventricular tachycardia may also progress
into ventricular fibrillation.

Ventricular fibrillation or v-fib is caused
by multiple weak ectopic sites in the ventricles.

These un-synchronized, chaotic electrical
signals cause the ventricles to quiver or

fibrillate rather than contract.
The heart pumps little or no blood.
V-fib can quickly lead to cardiac arrest.
V-fib ECG is characterized by irregular random
waveforms of varying amplitude, with no identifiable

P wave, QRS complex or T wave.
Amplitude decreases with time, from initial
coarse v-fib to fine v-fib and ultimately

to flatline.
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Cardiac Arrhythmias

1817 Folder Collection
alex published on October 14, 2017
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