CARDIAC PACING AND DEFIBRILLATION - Design and Development of Medical Electronic Instrumentation - page 369

CARDIAC PACING AND DEFIBRILLATION

W ITH CONTRIBUTIONS BY F ERNANDO B RUM ,J ULIO A RZUAGA ,
P EDRO A RZUAGA , AND O SCAR S ANZ

The heart is a pump that normally beats approximately 72 times every minute. This adds
up to an impressive 38 million beats every year. The walls of the heart are made of mus-
cle tissue. When they contract, the blood is ejected from the heart into the arteries of the
body. As shown in Figure 8.1, the heart has four chambers, two on the left side and two on
the right side. Each side is divided further into a receiving chamber (atrium) and a pump-
ing chamber (ventricle). The atria and ventricles are separated by one-way valves that keep
the blood flowing in the proper direction. The right side of the heart pumps blood to the

lungs (via the pulmonary artery) and the left side of the heart pumps blood to the rest of
the organs (via the aorta). The amount of blood the left ventricle pumps into the aorta every
minute, known as cardiac output , is expressed in liters per minute. If cardiac output
decreases in a signi

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cant manner, the body's organs are starved for oxygen. In the case of
the brain, a very low cardiac output can cause lightheadedness, weakness, loss of con-
sciousness and even death.
The heart contains two specialized types of cardiac muscle cells. The majority (around
99%) are contractile cells responsible for the mechanical work of pumping the heart. The
second type of cardiac cells are the autorhythmic cells . Their function is to initiate and
conduct action potentials that are responsible for the contraction of the working cells.
Autorhythmic cells have pacemaker activity as opposed to a nerve or skeletal muscle cell
which maintains a constant membrane potential until stimulated. Cells that display pace-
maker activity have membranes that slowly depolarize between action potentials until
threshold is reached, at which time the cell undergoes active depolarization, initiating an
action potential. These action potentials, generated by the autorhythmic cardiac muscle
cells, will then spread throughout the heart, triggering rhythmic beating without any nerv-
ous stimulation.
The coordinated contraction of the various chambers of the heart is mediated through
an organized electrical conduction system within the heart. Disturbances within the elec-
trical conduction system are the cause of all arrhythmias (rhythm disturbances). The elec-
trical signal that initiates each normal heartbeat arises from a small structure located at the
top of the right atrium called the sinus node or sinoatrial node . In a normal heart, the sinus

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