216
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
involves modifying the factors that determine
arterial pressure by using drugs.
Because
hypertension
results
from
an
increase in cardiac output and increased
system ic vascular resistance, these are the
two physiologic m echanism s that are tar-
geted in drug therapy. In m ost hyperten-
sive patients, altered renal function causes
sodium and water retention. This increases
blood volum e, cardiac output, and
arte-
rial
pressure.
Therefore,
the
m ost
com -
m on treatment for hypertension is the use
of a diuretic to stim ulate renal excretion of
sodium and water. This reduces blood vol-
ume and arterial pressure very effectively
in many patients. In addition to a diuretic,
m ost hypertensive patients are given at least
one other drug. This is because decreasing
blood volum e with a diuretic leads to acti-
vation of the renin-angiotensin-aldosterone
system , w hich counteracts the effects of
the diuretic. Therefore, these patients may
be given an ACE inhibitor or angiotensin
receptor blocker (ARB) as well.
In addition to using diuretics, cardiac out-
put can be reduced using beta-blockers and
the
more
cardioselective
calcium
channel
blockers (e.g., verapamil). Beta-blockers are
particularly useful in patients who may have
excessive sympathetic stimulation caused by
emotional stress, and these drugs also inhibit
sympathetic-mediated release of renin.
In
combination
with
a
diuretic,
some
hypertensive
patients
can
be
effectively
treated with an a-adrenoceptor antagonist,
which dilates resistance vessels and reduces
systemic vascular resistance. Other drugs that
reduce systemic vascular resistance include
ACE inhibitors, ARBs, calcium channel block-
ers (especially dihydropyridines), and direct-
acting arterial dilators such as hydralazine.
Although pharmacologic intervention is
an important therapeutic modality in treat-
ing hypertension, improved diet and exercise
have been shown to be effective in reducing
arterial pressure in many patients. A proper,
balanced diet that includes sodium restriction
can prevent the progression of, and in some
cases reverse, cardiovascular changes asso-
ciated with hypertension. Regular exercise,
especially aerobic exercise, reduces arterial
pressure and has beneficial effects on vascular
function.
HEART FAILURE
Heart failure occurs when the heart is unable
to supply adequate blood flow and therefore
oxygen delivery to peripheral tissues and
organs, or to do so only at elevated filling pres-
sures. Heart failure most commonly involves
the left ventricle. Right ventricular failure,
although sometimes found alone or in asso-
ciation with pulmonary disease, more often
occurs secondary to left ventricular failure.
Mild heart failure is manifested as reduced
exercise capacity and the development of
shortness of breath during physical activity
(exertional dyspnea). In more severe forms of
heart failure, a patient may have virtually no
capacity for physical exertion and will experi-
ence dyspnea even while at rest. Furthermore,
the patient will likely have significant pulmo-
nary or systemic edema.
Causes of Heart Failure
Heart failure can be caused by factors origi-
nating from the heart (i.e., intrinsic disease
or pathology) or from external factors that
place excessive demands upon the heart. The
number-one cause of heart failure is coro-
nary artery disease, which reduces coronary
blood flow and oxygen delivery to the myo-
cardium, thereby causing myocardial hypoxia
and impaired function. A related common
cause of heart failure is myocardial infarc-
tion. Infarcted tissue does not contribute to
the generation of mechanical activity, and
noninfarcted regions must compensate for
the loss of function. Over time, the additional
demands placed upon the noninfarcted tissue
can cause functional changes leading to fail-
ure. Other heart conditions that can lead to
failure include:
Valvular disease and congenital defects,
which place increased demands upon the
heart
Cardiomyopathies (intrinsic diseases of the
myocardium) of known origin (e.g., bacterial
previous page 229 Cardiovascular Physiology Concepts  2nd Edition read online next page 231 Cardiovascular Physiology Concepts  2nd Edition read online Home Toggle text on/off