Hemodynamic Measurements
and
Dynamics
of
Assisted
Circulation
24
1
Both theoretical and experimental studies have shown that late
deflation
can
increase
coronary blood flow
and
stroke
volume
substantially. This is particularly beneficial to a failing heart with low
cardiac output. But late deflation will also increase the load on the heart
during ejection, hence increasing myocardial oxygen consumption. An
appropriate compromise would be to produce the maximum increase
in
mean diastolic
pressure and coronary
perfusion
from
systolic
augumentation (late deflation) and at the same time keep afterload low.
Deflation bordering on isovolumetric systole seems to be the best choice.
This choice tends to maximize the oxygen supply to demand ratio as well
as cardiac efficiency, which is defined as the ratio
of
external work
(stroke
volume
x mean arterial
pressure) to myocardial
oxygen
consumption. It is clear that the duration of the inflation-deflation
interval is critical to the ability of an IABP to maximize coronary
perfusion and to reduce the work requirements of the failing left
ventricle. While timing and inflation-deflation rates are important
determinants
of
IABP performance, other hemodynamic factors can also
exert their influences. These include the pressure-diameter relationship
of the aorta, peripheral resistance, heart rate, the severity of heart failure
and neural-humoral interactions. For instance, a pressure increase will
signal the baroreceptor reflexes to alter heart rate. The combined effects
of increased balloon occlusion and heart rate at lower pressure enhance
augmentation of mean diastolic pressure. The IABP cannot physically
pump blood like a cardiac bypass device, and must rely on the heart to
perform the necessary work. Thus the severity of heart failure is also a
critical determinant of the beneficial effects of IABP.
8.2.3:
Optimization
of
Intra-Aortic Balloon Pumping: Modeling
Aspects
Modeling studies are useful in providing detailed parameter analyses and
to obtain specific predictions which yield quantitative information
regarding the interaction of IABP with the cardiovascular system. Some
investigators have employed a windkessel model
of
the vascular system,
with a flow source model of the left ventricle, to explain observed
changes in left ventricular pressure and aortic flow during IABP using
