r a
■ FIGURE 5.21 Combined cardiac and systemic function curves: effects of exercise. Cardiac output
is plotted against right atrial pressure (PRA) to show the effects of altering both cardiac and systemic
function. Point
represents the normal operating point described by the intercept between the normal
cardiac and systemic function curves. Cardiac stim ulation alone changes the intercept from point
to B.
Cardiac stim ulation coupled w ith decreased venous compliance (CV) shifts the operating intercept to
point C. If systemic vascular resistance
also decreases, which is similar to w hat occurs during exer-
cise, the new intercept becomes point
can offer new understanding as to the way
cardiac and vascular function are coupled.
When the cardiac function and vascular
function curves are superimposed (Fig. 5.21),
a unique intercept between a given cardiac
and a given vascular function curve (point A)
exists. This intercept is the equilibrium point
that defines the relationship between cardiac
and vascular function. The heart functions at
this equilibrium until one or both curves shift.
For example, if the sympathetic nerves to the
heart are stimulated to increase heart rate
and inotropy, only a small increase in cardiac
output will occur, accompanied by a small
decrease in right atrial pressure (point B).
As previously discussed, cardiac stimulation
alone will not increase cardiac output appreci-
ably if right atrial pressure becomes negative.
If at the same time, however, the venous com-
pliance is decreased by sympathetic activation
of venous vasculature, cardiac output will be
greatly augmented (point C). If the decrease
in venous compliance is accompanied by a
decrease in systemic vascular resistance, car-
diac output would be further enhanced (point
D). These changes in venous compliance and
systemic vascular resistance, which occur
during exercise, permit the cardiac output to
increase. This example shows that for cardiac
output to increase significantly during cardiac
stimulation, there must be some alteration in
vascular function so that venous return is aug-
mented and right atrial pressure (ventricular
filling) is maintained. Therefore,
in the normal
heart, cardiac output is limited by factors that
determine vascular function.
In pathologic conditions such as heart fail-
ure, cardiac function limits venous return. In
heart failure, ventricular inot ropy is dimin-
ished, total blood volume is increased, and
systemic vascular resistance is increased (see
Chapter 9).The former two lead to an increase
in atrial and ventricular pressures and vol-
the Frank—Starling mechanism to partially
compensate for the loss of inotropy. These
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