CARDIAC PACING AND DEFIBRILLATION - Design and Development of Medical Electronic Instrumentation - page 371

As a result, only a small amount of blood gets ejected with each beat, and cardiac output
drops signi cantly.

Arrhythmias result from disturbances to the normal pacing and/or conduction systems of
the heart. Sometimes the signal gets blocked along the way and the heart or a portion thereof
does not beat when it is supposed to since the cells do not receive a stimulus to contract. This
can cause slow heart rhythms, also known as bradycardia (from the Greek brady

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slow

cardia heart). Sometimes the electrophysiological parallel of a short circuit is formed,

causing the depolarization wave to take an abnormal route or to get stuck in a reentrant cir-
cular pathway. This can cause the heart to beat rapidly, in a condition known as tachycardia
(from the Greek, tachy

fast). Sometimes, a stimulus signal will arise from somewhere in
the heart besides the sinus node and cause the heart to contract prematurely in what is known
as a premature atrial contraction (PAC) or a premature ventricular contraction (PVC).

BRADYARRHYTHMIAS

The first obvious reason for bradycardia is that the natural pacing signal is generated by

the sinus node at a low rate. Sinus bradycardia happens when the sinus node

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fires at less
than 60 times per minute. However, sinus bradycardia is not necessarily abnormal. Healthy
people often have heart rates below 60 beats/minute during rest or sleep. Athletes will usu-
ally have rates below 60 beats/minute at even moderate levels of activity because their
training has made their hearts become very e

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cient at pumping. Sinus bradycardia is often
seen in the elderly as a result of medications taken for high blood pressure, angina, or
tachycardia. If there are no symptoms due to a sinus bradycardia, nothing needs to be done
about it. However, a medical condition by the name of sick sinus syndrome arises when
sinus bradycardia is associated with severe symptoms.
Sometimes the sinus node stops firing temporarily, and long pauses in the heart rhythm

may result. This condition may occur because of aging, ischemia, neurological imbalances,
and certain drugs. It causes symptoms of fatigue, lightheadedness, shortness of breath, and
syncope. Sick sinus syndrome is usually treated with a permanent pacemaker which sup-
plies a weak electrical signal to the atria that replaces the stimulus generated by the cyclic
depolarizations of the sinus node.
Another common condition that causes bradycardia is the AV node failing to conduct
atrial signals properly to the ventricles. This condition is known as AV block or heart block .
The severity of AV block varies from mild to life-threatening:

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•A first-degree AV block is an excessive delay (greater than 0.2 s) in the conduction of

the depolarization signal from the atria to the ventricles, but all the action potentials
are conducted. This benign condition is usually caused by disease in the AV node or
by certain cardiac medications (Ca -channel blockers and

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β -blockers). No treatment

is usually required at this stage.
• In second-degree AV block, some of the atrial signals fail to pass through the AV
node. Depending on how often atrial impulses are blocked, second-degree AV block
can cause symptoms that require a pacemaker implant.
• In third-degree AV block none of the atrial impulses can get through the AV node
because of severe disease in cardiac conduction system. Under these conditions, ven-
tricular contractions are initiated by autorhythmical (pacemaker) cells, whose pacemaker
behavior is usually inhibited by the higher rate of the SA node, as shown in Table 8.1.
• Complete AV block usually results in severe bradycardia, and occasionally, the ven-
tricles can stop beating, resulting in severe symptoms that may lead to death. The very
fi first pacemakers were designed to treat patients with complete heart block.