CHAPTER 4 • CARDIAC FUNCTION
71
the ventricular EDV This, in turn, depends
on the ventricular end-diastolic pressure and
compliance. Although end-diastolic pressure
and EDV are sometimes used as indices of
preload, care must be taken when interpret-
ing the significance of these values in terms
of how they relate to the preload of indi-
vidual sarcomeres. An elevated end-diastolic
pressure may be associated with sarcomere
lengths
that
are
increased,
decreased,
or
unchanged,
depending on the ventricular
volume and compliance at that volume. For
example, a stiff, hypertrophied ventricle may
have an elevated end-diastolic pressure with
a reduced EDV owing to the reduced compli-
ance. Because the EDV is reduced, the sar-
comere length will be reduced despite the
increase in end-diastolic pressure. As another
example, a larger than normal EDV may not
be associated with an increase in sarcomere
length if the ventricle is chronically dilated
and structurally remodeled such that new
sarcomeres have been added in series, thus
maintaining
normal
individual
sarcomere
lengths.
Effects of Preload on Tension
Development (Length-Tension
Relationship)
We have seen how ventricular EDV, which
is determined by ventricular end-diastolic
pressure and ventricular compliance, can
alter the preload on sarcomeres in cardiac
muscle cells. This change in preload will alter
the ability of the myocyte to generate force
when it contracts. The length-tension rela-
tionship examines how changes in the initial
length of a muscle (i.e., preload) affect the
ability of the muscle to develop force (ten-
sion). To illustrate this relationship, a piece
of cardiac muscle (e.g., papillary muscle) is
isolated and placed within an in vitro bath
containing an oxygenated, physiologic salt
solution. One end of the muscle is attached
to a force transducer to measure tension, and
the other end is attached to an immovable
support rod (Fig. 4.6, left side). The end that
is attached to the force transducer is mov-
able so that the initial length (preload) of
the muscle can be fixed at a desired length.
The muscle is then electrically stimulated to
contract; however, the length is not permit-
ted to change and therefore the contraction
is isometric.
If the muscle is stimulated to contract
at
a relatively short initial
length
(low
preload), a characteristic increase in tension
(termed “active” tension) will occur, last-
ing about 200 milliseconds (Fig. 4.6, right
side, curve a). By stretching the muscle to
a longer initial length, the passive tension
will be increased prior to stimulation. The
amount of passive tension depends on the
Tension
Transducer
Muscle
Resting
Length
L
Increased
Preload
t L
Fixed
■ FIGURE 4.6 Effects of increased preload on tension developm ent by an isolated strip of cardiac muscle.
The left side shows how muscle length and tension are measured in vitro. The bottom of the muscle strip
is fixed to an immovable rod, whereas the top of the muscle is connected to a tension transducer and
a movable bar that can be used to adjust initial muscle length
(L).
The right side shows how increased
preload (initial length) increases both passive and active (developed) tension. The greater the preload, the
greater the active tension generated by the muscle.
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