CHAPTER 4 • CARDIAC FUNCTION
81
Effects of Inotropy on
Force-Velocity Relationship
Changes in inotropy also alter the force-
velocity relationship. If the inotropic state
of
the
myocyte
is
increased,
the
force-
velocity curve exhibits an upward parallel
shift, resulting in an increase in both V
max
(y-intercept)
and
maximal
isometric
force
(x-intercept) (Fig. 4.20). The increase in veloc-
ity at any given afterload
(a
to
b
to c) results
from the increased inotropy enhancing force
generation by the actin and myosin filaments and
increasing the rate of cross-bridge turnover. The
increase in V
represents an increased intrinsic
capability of the muscle fiber to generate force
independent of load. In contrast, changes in
preload do not alter Vmax (see Fig. 4-16).
Effects of Inotropy on Frank-
Starling Curves
The change in velocity of muscle shortening
associated with a change in inotropy results
in an increase in SV at any given preload and
afterload and therefore causes the Frank-
Starling curve to shift up or down (Fig. 4.21).
If, at a given preload, inotropy is enhanced,
SV will increase. Conversely, a decrease in
inotropy at a given preload will decrease SV.
Effects of Inotropy on Pressure-
Volume Loops
The increased velocity of fiber shortening
that occurs with increased inotropy causes an
increased rate of ventricular pressure devel-
opment
(dP/dt),
which
increases
ejection
velocity and SV and reduces ESV as shown
in Figure 4.22. When inotropy is increased,
the ESPVR is shifted to the left and becomes
steeper,
because
the
ventricle
can
gener-
ate increased pressure at any given volume.
The ESPVR sometimes is used experimen-
tally to define the inotropic state of the ven-
tricle. It is analogous to the upward shift that
occurs in the total tension curve in the length-
tension
relationship
(Fig.
4.19)
when
ino-
tropy increases. Conversely, a decrease in ino-
tropy
(decreased
ESPVR
slope)
decreases
the rate of ejection and SV, which leads to an
■ FIGURE 4.20 Effects of increasing inotropy
(parallel shift from curve a to c) on the force-
velocity relationship. Increased inotropy increases
the velocity of shortening at any given afterload
(vertical dashed line),
and increases Vmax (y-inter-
cept). Furthermore, increased inotropy increases
maximal isom etric force (x-intercept).
increase in ESV As described later in this chap-
ter, changes in inotropy in a normal, healthy
heart will also lead to secondary changes in
preload and afterload that are not shown in
Figure 4.22.
Changes in inotropy change the ejection
fraction, which is defined as the SV divided
by the EDV In Figure 4.22, this would be
represented by the ratio of the width of the
pressure-volume loop divided by the EDV
A normal EF is >0.55 (or 55%). Increasing
■ FIGURE 4.21 Effects of inotropy on Frank-
Starling curves. An increase in inotropy shifts the
Frank-Starling curve upward, whereas a decrease
in inotropy shifts the Frank-Starling curve dow n-
ward. Therefore, at a given preload
(vertical
dashed line),
increased inotropy increases stroke
volume, and decreased inotropy decreases stroke
volume.
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