206
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
Pregnancy significantly alters the cardio-
vascular responses to exercise. Because cardiac
output at rest is substantially elevated, there is
less capacity for it to increase during exercise.
In addition, compression of the inferior vena
cava caused by an elevated intra-abdominal
pressure, particularly during the third trimes-
ter, limits venous return and thereby prevents
stroke volume from increasing as it normally
would during exercise. Compression of the
inferior vena cava, especially in the supine
position, can also diminish venous return at
rest, thereby reducing cardiac output and arte-
rial pressure (supine hypotensive syndrome).
HYPOTENSION
Causes of Hypotension
Hypotension is often defined clinically as
a systolic arterial pressure <90 mm Hg, or
a diastolic pressure <60 mm Hg. There are
many causes of hypotension as summarized
in Figure 9.4. Because arterial pressure is
the product of cardiac output and systemic
vascular resistance, a decrease in either will
reduce arterial pressure (see Chapter 5).
A reduction in systemic vascular tone or
impaired vasoconstrictor responsiveness to
baroreceptor reflexes can lead to hypoten-
sion. For example, septic shock (or Systemic
Inflammatory
Response
Syndrome,
SIRS),
which usually results from a bacterial infec-
tion in the blood, causes a loss of vascular
tone and hypotension. Septic shock is caused
by the release of bacterial endotoxins (e.g.,
lipopolysaccharide) that activate the inflam-
matory cascade. This leads to the produc-
tion of cytokines (e.g., tumor necrosis factor,
interleukins) and excessive amounts of nitric
oxide, causing systemic vasodilation. Severe
allergic reactions can lead to anaphylactic
shock. Another cause of vasodilatory circula-
tory shock is damage to the spinal cord sym-
pathetic tracts (neurogenic shock) resulting
in loss of vascular sympathetic tone. Systemic
vascular resistance can also be decreased if
autonomic dysfunction occurs. For example,
in
diabetic
individuals
having
autonomic
neuropathy,
baroreceptor-mediated
reflex
Hypotension
- hemorrhage
• Cardiogenic Shock
- dehydration
• Autonomic Dysfunction
• Volume Redistribution
- postural changes
- impaired venous return
• Arrhythmias
- atrial fibrillation
- tachycardia
■ FIGURE 9.4 Mechanisms and causes of hypotension. Ultimately, hypotension occurs because there is a
reduction in cardiac output, systemic vascular resistance, or both.
previous page 219 Cardiovascular Physiology Concepts  2nd Edition read online next page 221 Cardiovascular Physiology Concepts  2nd Edition read online Home Toggle text on/off