CHAPTER 4 • CARDIAC FUNCTION
77
EFFECTS OF AFTERLOAD
ON STROKE VOLUME
Afterload
is the “load” against which the heart
must contract to eject blood.
A major compo-
nent of the afterload for the left ventricle is
the aortic pressure, or the pressure the ventri-
cle must overcome to eject blood. The greater
the aortic pressure, the greater the afterload
on the left ventricle. For the right ventricle,
the pulmonary artery pressure represents the
major afterload component.
Ventricular
afterload,
however, involves
factors other than the pressure that the ven-
tricle must develop to eject blood. One way to
estimate the afterload on the individual cardiac
fibers within the ventricle is to examine ven-
tricular wall stress (a), which is proportional
to the product of the intraventricular pressure
(P) and ventricular radius (r), divided by the
wall thickness (h) (Equation 4-2). This rela-
tionship for wall stress assumes that the ven-
tricle is a sphere. The determination of actual
wall stress is complex and must consider not
only ventricular geometry, but also muscle
fiber orientation. Nonetheless, Equation 4-2
helps to illustrate the factors that contribute
to wall stress and therefore afterload on the
muscle fibers.
P ■
r
Eq. 4-2
a ~ —
h
Wall stress can be thought of as the aver-
age
tension that individual muscle fibers
within the ventricular wall must generate to
shorten against the developed intraventricu-
lar pressure. At a given intraventricular pres-
sure, wall stress is increased by an increase
in radius (ventricular dilation). Therefore,
afterload is increased whenever
intraven-
tricular pressures are elevated during systole
and by ventricular dilation. On the other
hand, a thickened, hypertrophied ventricle
will have reduced wall stress and afterload on
individual fibers. Ventricular wall hypertro-
phy can be thought of as an adaptive mecha-
nism by which the ventricle is able to offset
the increase in wall stress that accompanies
increased ventricular systolic pressures or
ventricular dilation.
Effects of Afterload on the
Velocity of Fiber Shortening
(Force-Velocity Relationship)
Afterload influences the contraction of cardiac
muscle fibers. Increased afterload decreases the
velocity of fiber shortening, whereas decreased
afterload increases the velocity of shortening.
This inverse relationship between afterload
and velocity of fiber shortening is basis for the
force-velocity relationship. To illustrate this,
a papillary muscle is placed in an in vitro bath,
set at a fixed initial length and passive ten-
sion (preload), and a load is attached to one
end (Fig. 4.13, left panel). When the muscle
is stimulated to contract, the fiber first gener-
ates active tension isometrically, that is, active
tension is developed with no change in length
(right panel,
a
to b). Once the developed ten-
sion exceeds the load imposed on the muscle,
the muscle fiber begins to shorten, and the ten-
sion remains constant and equal to the load that
is being lifted (
b
to
c
). The maximal velocity of
shortening (rate of shortening) occurs shortly
after the muscle begins to shorten. The muscle
continues to shorten until the muscle begins to
relax. When active tension falls below the load
(point
c
), the muscle resumes its resting length
and tension (i.e., preload) (point c). Active ten-
sion continues to fall isometrically (
c
to
d
) until
only the passive tension remains (point
d
).
If this experiment with the papillary muscle
were repeated with increasing loads, a decrease
would occur in both the maximal velocity of
fiber shortening (maximal slope of line) and the
degree of shortening, as shown in Figure 4.14.
Plotting the maximal velocity of shortening
against the load that the muscle fiber must
shorten against (i.e., the afterload) generates an
inverse relationship between velocity of short-
ening and afterload (force-velocity relation-
ship; Fig. 4.15). In other words,
the greater the
afterload, the slower the velocity of shortening
.
To further illustrate the force-velocity rela-
tionship, consider the following example. If a
person holds a 2-lb dumbbell at their side while
standing, and then contracts their biceps mus-
cle at maximal effort, the weight will be lifted at
a relatively high velocity as the biceps muscle
shortens. If the weight is increased to 20 lb, and
the weight once again is lifted at maximal effort,
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