224
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
Using the above relationship, if the valve ori-
fice area is reduced by 75%, the valve resist-
ance is increased 16-fold, which increases the
pressure gradient 16-fold if flow through the
valve remains unchanged. In reality, the for-
mation of turbulence increases the pressure
gradient across the valve even further. Tur-
bulence occurs because a reduced orifice area
leads to an increase in the velocity of blood
flow across the valve. Because flow (F) equals
the product of velocity (V) and area (A), the
velocity equals flow divided by area (V = F/A).
Therefore, if flow remains unchanged, a 75%
reduction in area causes a fourfold increase
in velocity, which increases turbulence and
produces a murmur. In summary, at a given
flow across a valve, a reduction in valve ori-
fice area increases the pressure gradient across
the valve that is required to drive the flow,
increases the velocity of flow, and increases
the turbulence.
AORTIC VALVE STENOSIS
In aortic valve stenosis, left ventricular pres-
sure is increased above normal during sys-
tole to eject blood across the narrowed valve
(Fig. 9.11, left panel). This leads to a large
pressure gradient across the valve during
ejection, the magnitude of which depends
on the degree of stenosis and the flow across
the valve. Increased flow velocity through
the stenotic valve causes turbulence and
a systolic murmur.
In moderate-to-severe
aortic stenosis, the aortic pressure may be
reduced because ventricular stroke volume
(and cardiac output) is reduced. The degree
of hypotension depends on the ability of
neurohumoral mechanisms to increase blood
volume
and
system ic vascular
resistance.
Because ejection is impeded by the increase
in ventricular afterload, more blood remains
in the heart after ejection, which leads to an
increase in left atrial volum e and pressure.
Changes
in
left
ventricular
pressure-
volume loops with moderate aortic steno-
sis are shown in Figure 9.11 (right panel).
Because left ventricular emptying is impaired
by the increased afterload (see Chapter 4),
the stroke volume is reduced, which leads
to an increase in end-systolic volume. With
chronic
aortic stenosis,
the left ventricle
hypertrophies.
This
decreases
ventricular
compliance, elevates end-diastolic pressure,
■ FIGURE 9.11 Changes in cardiac pressures and volumes associated w ith chronic aortic valve stenosis in
the absence of systolic failure. The le ft panel shows that during ventricular ejection, left ventricular pres-
sure (LVP) exceeds aortic pressure (AP) (the gray area represents the pressure gradient generated by the
stenosis); a systolic murm ur is present between S, and S2, and left atrial pressure
(LAP)
is elevated. Aortic
pressure may be reduced because of decreased stroke volume. The rig ht panel shows the effects of aortic
valve stenosis
(red loop)
on the left ventricular
(LV)
pressure-volume loop. The end-systolic volume is
increased, w ith little or no change in end-diastolic volume; therefore, stroke volume is decreased. Ven-
tricular hypertrophy reduces ventricular compliance, which elevates end-diastolic pressure at any given
end-diastolic volume.
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