CHAPTER 5 • VASCULAR FUNCTION
97
later in this chapter. Vascular compliance varies
depending on the state of venous smooth mus-
cle contraction, which is primarily regulated by
sympathetic nerves innervating the veins.
ARTERIAL BLOOD PRESSURE
Ejection of blood into the aorta by the left
ventricle results in a characteristic aortic pres-
sure pulse (Fig. 5.3). The peak pressure of
the aortic pulse is termed the systolic pres-
sure.
Shortly after the peak systolic pres-
sure, there appears a notch (dicrotic notch
or incisura) followed by the appearance of
a small increase in pressure (dicrotic wave)
prior to the pressure falling toward its mini-
mal value, the diastolic pressure. The dif-
ference between the systolic and diastolic
pressures is the aortic pulse pressure. If, for
example, the systolic pressure is 130 mm Hg
and the diastolic pressure is 85 mm Hg, then
the pulse pressure is 45 mm Hg. Therefore,
any factor that affects either systolic or dias-
tolic pressures affects pulse pressure.
The
systolic and diastolic pressures are those that
are measured with an arm blood pressure cuff
(sphygmomanometer).
While
these
values
are very important clinically, neither value is
the primary pressure that drives blood flow
in organs. That pressure is the mean arterial
pressure, which is the average pressure over
time. This pressure needs to be determined
when hemodynamic information is required
to assess vascular function.
Pulse Pressure = Psysto
lic- Pd
iasto
lic
■ FIGURE 5.3 Pressure pulse w ithin the aorta. The
pulse pressure is the difference between the maxi-
mal pressure (systolic) and the minimal pressure
(diastolic). The mean pressure is approxim ately
equal to the diastolic pressure plus one-third the
pulse pressure.
Mean Arterial Pressure
Because of the shape of the aortic pressure
pulse, the value for the mean pressure (geo-
metric mean) is less than the arithmetic
average of the systolic and diastolic pres-
sures as shown in Figure 5.3. At normal
resting heart rates, mean aortic (or arterial)
pressure (MAP) can be
estimated
from the
diastolic (Pdias) and systolic (Psys) pressures
by Equation 5-2:
Eq. 5-2
MAP = Pdias +
%
(Psys - Pdias)
For example, if systolic pressure is 120 mm Hg
and diastolic pressure is 80 mm Hg, the
mean arterial pressure will be approximately
93 mm Hg. At high heart rates, however, mean
arterial pressure is more closely approxi-
mated by the arithmetic average of systolic
and diastolic pressure because the shape
of the arterial pressure pulse changes (it
becomes narrower) as the period of diastole
shortens more than does systole. Therefore,
to determine mean arterial pressure accu-
rately, analog electronic circuitry or digital
techniques are used, usually in conjunction
with an indwelling arterial catheter.
No single value exists for normal mean
arterial pressure. In infant children, the mean
arterial pressure may be only 70 mm Hg,
whereas in older adults, mean arterial pres-
sure may be 100 mm Hg. With increasing
age, the systolic pressure generally rises more
than diastolic pressure; therefore, the pulse
pressure increases with age. Small differences
exist between men and women, with women
having slightly lower pressures at equivalent
ages. In adults, arterial pressure is considered
normal when the systolic pressure is <120 mm
Hg (but > 90 mm Hg) and the diastolic
pressure is <80 mm Hg (but > 60 mm Hg),
which represents a normal mean pressure of
<95 mm Hg. Abnormally low and elevated
arterial pressures are discussed in Chapter 9.
What
factors
determine
mean
arterial
pressure? As blood is pumped into the resist-
ance network of the systemic circulation,
pressure is generated within the arterial vas-
culature. The mean arterial (or aortic) pres-
sure (MAP) is determined by the cardiac
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