70
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
by the compliance of the ventricle, in which
compliance is defined as
the ratio of a change
in volume divided by a change in pressure.
Nor-
mally, compliance curves are plotted with vol-
ume on the Y-axis and pressure on the X-axis,
so that the compliance is the slope of the
line at any given pressure (i.e., the slope of
the tangent at a particular point on the line).
For the ventricle, however, it is common to
plot pressure versus volume (Fig. 4.5) and
to refer to this pressure-volume relationship
as the filling curve for the ventricle. Plotted
in this manner, the slope of the tangent at a
given point on the curve is the reciprocal of
the compliance. Therefore, the steeper the
slope of the pressure-volume relationship,
the lower the compliance. This means that
the ventricle becomes “stiffer” when the slope
of the passive filling curve is greater; there-
fore, compliance and stiffness are reciprocally
related.
The relationship between pressure
and
volume is nonlinear in the ventricle (as in
most biological tissues); therefore, compli-
ance decreases with increasing pressure or
volume. When pressure and volume are plot-
ted as in Figure 4.5, we find that the slope
of the filling curve (the EDPVR described in
Fig. 4.4) increases at higher volumes; that
is, the ventricle becomes less compliant or
“stiffer” at higher volumes.
Ventricular compliance is determined by
the physical properties of the tissues mak-
ing up the ventricular wall and the state of
ventricular relaxation. For example, in ven-
tricular hypertrophy, the increased muscle
thickness decreases the ventricular compli-
ance;
therefore,
ventricular
end-diastolic
pressure is higher for any given EDV. This
is shown in Figure 4-5, in which the filling
curve of the hypertrophied ventricle shifts
upward and to the left. From a different per-
spective, for a given end-diastolic pressure,
a less compliant ventricle will have a smaller
EDV (i.e., filling will be decreased). If ven-
tricular relaxation (lusitropy) is impaired, as
occurs in some forms of diastolic ventricular
failure (see Chapter 9), the functional ven-
tricular compliance will be reduced. This will
impair ventricular filling and increase end-
diastolic pressure. If the ventricle becomes
chronically dilated, as occurs in other forms
of heart failure, the filling curve shifts down-
ward and to the right. This enables a dilated
heart to have a greater EDV without causing a
large increase in end-diastolic pressure.
The length of a sarcomere prior to contrac-
tion, which represents its preload, depends on
■ FIGURE 4.5 Left ventricular compliance (or filling) curves. The slope of the tangent of the passive pres-
sure-volume curve at a given volume represents the reciprocal of the ventricular compliance. The slope
of the normal compliance curve is increased by a decrease in ventricular compliance (e.g., ventricular
hypertrophy), whereas the slope of the compliance curve is reduced by an increase in ventricular com pli-
ance (e.g., ventricular dilation). Decreased compliance increases the end-diastolic pressure
(EDP)
at a given
end-diastolic volume
(EDV),
whereas increased compliance decreases EDP at a given EDV.
LV,
left ventricle.
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