CHAPTER 4 • CARDIAC FUNCTION
79
■ FIGURE 4.16 Effects of increasing preload (shift
from curve
a
to c) on the force-velocity relation-
ship. At a given afterload (vertical dashed line),
increasing the preload increases the velocity of
shortening. Furthermore, increasing the preload
shifts the x-intercept to the right, which represents
an increase in isom etric force generation. Note
that y-intercept, which is the maximal velocity of
shortening (Vmax) extrapolated to zero load, does
not change w ith increasing preload.
discussed, changes in preload also affect the
velocity of fiber shortening (see Fig. 4.8). If
preload is increased, a cardiac muscle fiber will
have a greater velocity of shortening at a given
afterload. This occurs because the length-ten-
sion relationship requires that as the preload
is increased, there is an increase in active ten-
sion development. Once the fiber begins to
shorten, an increased preload with an increase
in
tension-generating
capability
causes
a
greater shortening velocity. In other words,
increasing the preload enables the muscle to
contract faster against a given afterload; this
shifts the force-velocity relationship to the
right (see Fig. 4.16). Note that increasing the
preload increases the maximal isometric force
(x-intercept) as well as the shortening velocity
at a given afterload (a to
b
to c). Changes in
preload, however, do not alter V
. Therefore,
max
an increase in preload on a cardiac myocyte can
help to offset the reduction in velocity that occurs
when afterload is increased.
Effects of Afterload on
Frank-Starling Curves
We have just seen how an increase in after-
load at a given preload decreases the velocity
and extent of fiber shortening. This being the
case, we should expect ventricular SV to be
LVEDP(mmHg)
■ FIGURE 4.17 Effects of afterload on Frank-
Starling curves. An increase in afterload shifts
the Frank-Starling curve downward, whereas a
decrease in afterload shifts the Frank-Starling
curve upward. Therefore, at a given preload
(verti-
cal dashed line)
increased afterload decreases
stroke volume, and decreased afterload increases
stroke volume.
reduced, and indeed, this is what occurs, as
shown in Figure 4.17. An increase in after-
load rotates the Frank-Starling curve down
and to the right. Therefore, at a given preload
(left
ventricular
end-diastolic
pressure
[LVEDP] in Fig. 4.17), an increase in after-
load decreases SV.
Conversely, decreasing
afterload shifts the curves up and to the left,
thereby increasing the SV at a given preload.
As discussed in Chapter 9, reducing ventric-
ular afterload in heart failure patients is an
important therapeutic approach to enhance
SV.
Effects of Afterload on
Pressure-Volume Loops
The
effects
of
afterload
on
ventricular
function can be depicted using ventricu-
lar
pressure-volume
loops
as
shown
in
Figure 4.17. Increasing afterload by increas-
ing aortic pressure at a constant preload
(EDV) causes a decrease in SV (width of the
loop) and an increase in ESV. The ventricle
will generate increased pressure to overcome
the elevated aortic pressure, but at the cost
of a reduced SV. A reduction in afterload
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