553
CHAPTER FIFTEEN
Cardiovascular System
DEFIBRILLATORS SAVE LIVES
two major risk factors are having had a previous myocardial infarction and a low
ejection fraction, which is the volume of blood pumped with each heartbeat.
Normal ejection fraction is 50% to 60%; low is below 30% to 40%. Scarring low-
ers the ejection fraction. An echocardiogram, which is an ultrasound scan of the
heart, can reveal the ejection fraction.
In a four-year study at seventy-six medical centers involving 1,232 patients
with the two risk factors, use of an ICD decreased death rate from sudden car-
diac arrest by 31%. Many nations use the device as a preventative. In the United
States, wider use of ICDs could save thousands of the 300,000 people who die
each year of sudden cardiac arrest.
A
man rushing to catch a F
ight at a busy airport stops suddenly,
looks about in confusion, and collapses. People congregate
around him, as a woman runs to a device mounted on a
nearby wall. It is an automated external de±
brillator (AED) and
looks like a laptop computer. The woman learned how to use
it in a cardiopulmonary resuscitation class. She brings it over to the man, opens
it, and places electrode pads over the man’s chest, as indicated in a drawing on
the inner cover of the de±
brillator. Then the device speaks. “Analyzing heart
rhythm,” it declares as a computer assesses the heart rhythm. After a short
pause, the device says, “charging, stand clear,” and then “push button.” The
woman does so, and the device delivers a shock to the man’s chest. It assesses
the heart rhythm again, and instructs the woman to deliver a second shock.
Soon, the man recovers, just as emergency technicians arrive.
The AEDs found in airports, malls, schools, health clubs, and other pub-
lic places can save the life of a person su²
ering sudden cardiac arrest. One
study conducted at Chicago’s O’Hare and Midway airports found that over a
ten-month period, AEDs saved 64% of the people on which they were used.
Without de±
brillation, only 5% to 7% of people survive sudden cardiac arrest.
Each minute, the odds of survival shrink by 10%, and after six minutes, brain
damage is irreversible. Sudden cardiac arrest can result from an accelerated
heartbeat (ventricular tachycardia) or a chaotic and irregular heartbeat (ven-
tricular ±
brillation). The electrical malfunction that usually causes these con-
ditions may result from an artery blocked with plaque or from build up of
scar tissue from a previous myocardial infarction (heart attack).
³or people who know that they have an inherited disorder that causes
sudden cardiac arrest (by having suffered an event and then had genetic
tests), a device called an implantable cardioverter de±
brillator (ICD) can be
placed under the skin of the chest in a one-hour procedure. Like the AED, the
ICD monitors heart rhythm, and when the telltale deviations of ventricular
tachycardia or ventricular ±
brillation begin, it delivers a shock.
ICDs have been so successful in preventing subsequent cardiac arrests
that they may soon be o²
ered to people at high risk for the condition. The
An implantable cardioverter de±
brillator delivers a shock to a heart whose
ventricles are wildly contracting, restoring a normal heartbeat.
15.1
INTRODUCTION
The heart pumps 7,000 liters of blood through the body each
day, contracting some 2.5 billion times in an average life-
time. This muscular pump forces blood through arteries,
which connect to smaller-diameter vessels, called arteri-
oles. Arterioles branch into the tiniest tubes, the capillaries,
the sites of nutrient, electrolyte, gas, and waste exchange.
Capillaries converge into venules, which in turn converge
into veins that return blood to the heart, completing the
closed system of blood circulation. These structures—the
pump and its vessels—form the
cardiovascular system.
The blood vessels form two circuits. The
pulmonary
(pul
mo-ner
e)
circuit
sends oxygen-depleted (deoxygen-
ated) blood to the lungs to pick up oxygen and unload carbon
dioxide. The
systemic
(sis-tem
ik)
circuit
sends oxygen-
rich (oxygenated) blood and nutrients to all body cells and
removes wastes. Without circulation, tissues would lack a
supply of oxygen and nutrients, and wastes would accu-
mulate. Such deprived cells soon begin irreversible change,
which quickly leads to death.
Figure 15.1
shows the general
pattern of blood transport in the cardiovascular system.
15.2
STRUCTURE OF THE HEART
The heart is a hollow, cone-shaped, muscular pump. It is in
the mediastinum of the thorax and rests upon the diaphragm.
Size and Location of the Heart
Heart size varies with body size. An average adult’s heart is
generally about 14 centimeters long and 9 centimeters wide
(f
g. 15.2)
.
The heart is bordered laterally by the lungs, posteriorly
by the vertebral column, and anteriorly by the sternum
(
f
g. 15.3
and reference plates 10, 16, 21, and 22). The
base
of the heart, which attaches to several large blood vessels,
lies beneath the second rib. The heart’s distal end extends
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