212
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
mechanisms
counteract
the
compensatory
mechanisms and eventually lead to an addi-
tional reduction in arterial pressure.
Physiologic Basis for Therapeutic
Intervention
Treatment for hypotension depends upon
the underlying cause of the hypotension.
If hypotension is caused by hypovolem ia
owing
to
hemorrhage
or
excessive
fluid
loss (e.g., dehydration), increasing blood
volum e by administration of blood or flu-
ids becom es a treatment priority. Restoring
blood volum e increases preload on the heart
and thereby increases cardiac output, which
is reduced in hypovolem ic states. Adm in-
istration of fluids is occasionally accom -
panied
by
the
administration
of pressor
agents. These drugs increase arterial pres-
sure by increasing system ic vascular resist-
ance (e.g., a-adrenoceptor agonists such as
norepinephrine and phenylephrine; or vaso-
pressin) or by stim ulating cardiac function
(e.g., P-adrenoceptor agonists such as dobu-
tamine). Treatment of hypotension caused
by cardiogenic shock can include drugs that
stim ulate
the heart
(e.g.,
P-adrenoceptor
agonists such as dobutamine or dopamine,
or
cAM P-dependent
phosphodiesterase
inhibitors such as m ilrinone that inhibit the
degradation of cAMP); however, depending
upon the magnitude of the hypotension,
either pressor or depressor agents may be
used. Because the primary cause of hypo-
tension in cardiogenic shock is impaired
cardiac function, drugs such as phosphodi-
esterase inhibitors that stim ulate the heart
and dilate arterial vessels can improve car-
diac function by enhancing inotropy and
decreasing afterload on the heart. Systemic
vasodilators, however, cannot be used alone
if the hypotension is severe, because arte-
rial pressure may fall further. Hypotension
associated with septic shock results from
system ic vasodilation and, in its later stages,
cardiac
depression.
Therefore,
pressor
agents are com m only used with this form of
shock in addition to administration of fluid
and antibiotics.
HYPERTENSION
High blood pressure (hypertension) is a con-
dition that afflicts about one-third of Ameri-
can adults and is a leading cause of morbidity
and mortality. Hypertension is much more
than a “cardiovascular disease” because it can
damage other organs such as kidney, brain,
and eye. One-third of hypertensive people
are not aware of being hypertensive because
it is usually asymptomatic until the damaging
effects of hypertension (such as stroke, myo-
cardial infarction, renal dysfunction, visual
disturbances, etc.) are observed.
The term “hypertension” is applied to ele-
vations in diastolic or systolic pressures above
normal values. Normal arterial pressure is
defined as a systolic pressure <120 mm Hg (but
>90 mm Hg) and diastolic pressure <80 mm Hg
(but >60 mm Hg). Diastolic pressures of 80 to
89 mm Hg and systolic pressures of 120 to
139 mm Hg are considered prehypertension.
Hypertension is defined as diastolic or systolic
pressures >90 or 140 mm Hg, respectively.
Both diastolic and systolic hypertension have
been shown to be significant risk factors for
causing other cardiovascular disorders such as
stroke and myocardial infarction. Mean arte-
rial pressure is usually not discussed in the
context of hypertension because it is not nor-
mally measured in a patient.
Chronic
hypertension
is
caused
by
increases in systemic vascular resistance and
cardiac output. The elevation in cardiac out-
put is normally caused by an increase in blood
volume, which increases ventricular preload
and stroke volume. It is important to note
that to sustain a hypertensive state it is neces-
sary to increase blood volume through renal
retention of sodium and water. Evidence for
this comes from studies showing that eleva-
tions in arterial pressure produced by infus-
ing a vasoconstrictor drug for several days are
not sustained because of pressure natriuresis
in the kidneys. W hen renal artery pressure
is elevated by increasing systemic vascular
resistance, the kidneys respond by increasing
glomerular filtration and excretion of sodium
and water. The loss of sodium and water
decreases blood volume and restores pressure
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