Heart Rhythm Disorders
(Abnormal Heart Rhythms)
Medical Author: Benjamin C. Wedro, MD, FAAEM
Medical Editors: Daniel
Kulick, MD, FACC, FSCAI, and Melissa Conrad Stöppler, MD
How does the heart work?
The heart is a two stage electric pump whose job it is to circulate blood
through the body. The pump is dependent on the electric current that the cells
of the heart generate to have a coordinated squeeze so that the pump can pump.
The heart has four chambers, the right and left atria (singular= atrium) and
the right and left ventricles. The right side of the heart pumps blood to the
lungs while the left side pumps it to the rest of the body.
Blood from the body is collected in the right atrium and is pushed into the
right ventricle with a small beat of the upper chamber of the heart. The right
ventricle pumps the blood to the lungs to pick up oxygen, and the oxygenated
blood from the lungs returns to the left atrium, where the small atrial beat
pushes it to the left ventricle. The left ventricle is much thicker than the
right because it needs to be strong enough to send blood to the entire body.
There are special cells in the right atrium called the sino-atrial node (SA
node) that generate the first electrical impulse, allowing the heart to beat in
a coordinated way. The SA node is considered the "natural pacemaker" of the
heart. This pacemaker function begins the electrical impulse which
follows pathways in the atrial walls, almost like wiring, to a junction box
between the atrium and ventricle called the atrio-ventricular node (AV node).
This electric signal causes muscle cells in both atria to contract all at once.
At the AV node, the electric signal waits for a very short time, usually one to
two tenths
of a second, to allow blood to leave the atria and fill up the ventricles. The
signal then passes through electric bundles in the ventricle walls to allow
these chambers to contract, again in a coordinated way, and pump blood to the
body.
The SA node generates an electric beat about 60-80 times a minute, and each
should result in a heart beat. That beat can be felt as an external pulse. After a heart
beat, the muscle cells of the heart need a split second to get ready to beat
again, and the electrical system allows a pause for this to happen.
The heart and its electrical activity depend upon a relatively narrow normal
state for it to work. Fortunately, the body tends to protect the heart as best
as it can. Still, rhythm disturbances are frequent. Some disturbances are normal
physiologic responses, but some are potentially life threatening.
Every cell in the heart can act as a pacemaker. The SA node has an intrinsic
heart beat generation rate of 60-80. The AV node has a rate of about 40, and the
ventricles themselves can generate beats at a rate of about 20 and may kick in if the
cells of the upper chamber fail or if the electrical signals to the ventricle
are blocked.
Next: Premature atrial contractions (PACs) and premature ventricular contractions (PVCs) »
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