106
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
CASE 5-1
A patient was found to have
S-T segment depression in his
electrocardiogram during an exercise
stress test, suggesting the presence of
coronary artery disease. A follow-up
coronary angiogram revealed that the
diameter of the left main coronary artery
(see Chapter 7, Fig. 7-6) was reduced by
50%. If this vessel normally contributes
to 1% of the total coronary vascular
resistance under resting flow conditions,
how much will this reduction in diameter
increase total coronary vascular
resistance? Assume no change in the
resistance of the vessels downstream
from the narrowed coronary artery.
increase.
REGULATION OF SYSTEMIC
VASCULAR RESISTANCE
Systemic
vascular
resistance,
which
is
sometimes called total peripheral resistance
(TPR), is the resistance to blood flow offered
by all of the systemic vasculature, excluding
the pulmonary vasculature. Systemic vascular
resistance primarily is determined by changes
in vascular diameters, although changes in
blood viscosity also affect systemic vascular
resistance. Mechanisms that cause generalized
vasoconstriction will increase systemic vas-
cular resistance, and mechanisms that cause
vasodilation will decrease systemic vascular
resistance. The increase in systemic vascular
resistance in response to sympathetic stimula-
tion, for example, depends on the degree of
sympathetic activation, the responsiveness of
the vasculature, and the number of vascular
beds involved.
Calculation of Systemic Vascular
Resistance
Systemic vascular resistance (SVR) can be
calculated if cardiac output (CO),
mean
arterial pressure (MAP), and CVP are known.
This
calculation
is
done
by
rearranging
Equation 5-3 as follows:
Eq. 5-10
SVR = (M AP ~ c v p )
CO
Although systemic vascular resistance can be
calculated from mean arterial pressure and
cardiac output, its value is not determined by
either of these variables (although its value
changes depending upon the pressureâ€”see
below). Systemic vascular resistance is deter-
mined by vascular diameters, length, anatomi-
cal arrangement of vessels, and blood viscosity.
Because vessels
are
compliant,
increasing
intravascular pressure expands the vessels,
thereby causing a small reduction in resist-
ance. Nonetheless, the decrease in systemic
vascular resistance that occurs when pressure
increases is not owing to the pressure directly
but rather is caused by passive increases in
vessel diameter. Mathematically, systemic vas-
cular resistance is the dependent (calculated)
variable in Equation 5-10; however, physi-
ologically, systemic vascular resistance and
cardiac output are the independent variables
normally, and mean arterial pressure is the
dependent variable; that is, mean arterial pres-
sure changes in response to changes in cardiac
output and systemic vascular resistance.
When calculating systemic vascular resist-
ance, it is customary to use the units of mm
Hg/mL â–
min-1
(peripheral
resistance
units,
PRU) or the units of dynes -s/cm5 (in which
pressure is expressed in dynes/cm2 instead of
mm Hg; 1 mm Hg = 1330 dynes/cm2) and flow
is expressed in cm3/s. When calculating resist-
ance in PRU, pressure has the units of mm Hg
and cardiac output is expressed in mL/min.
PROBLEM 5-3
Infusion of a drug is found to increase
cardiac output by 30% and decrease
mean arterial pressure by 10%. By what
percentage does this drug change
systemic vascular resistance? Is this
drug a vasodilator or vasoconstrictor?
Assume that CVP is 0 mm Hg and does
not change.
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