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8
Dynamics
of
the
Vascular
System
the designs of cardiac assist devices, whose primary use are to maintain
adequate cardiac output.
8.2.2
Optimization
of
Zntra-Aortic Balloon Pumping: Physiological
Considerations
The major hemodynamic aims and consequently the benefits of
IABP
are
the increase in coronary perfusion and the reduction of ventricular
afterload, hence an increase in cardiac efficiency. The extent of these
benefits is dependent upon a number of physical and physiological
parameters. These include the position
of
the balloon in the aorta, the
volume displacement and its geometric size relative to the aorta, the
driving gas, the rates and timing of balloon inflation and deflation, the
heart rate, the viscoelastic properties of the aorta, neural-humoral
influences, and the severity of the heart failure. Theoretically speaking,
the optimal position for the intra aortic balloon should be as close to the
aortic valve as possible, in order to generate the augmentation of mean
arterial diastolic pressure
(Pd)
to enhance coronary perfusion. But this
positioning obstructs flow to the aortic arch vessels. Thus, the ideal
position chosen is normally along the descending throracic aorta just
distal to the arch. Balloon volume determines the absolute magnitude of
changes in hemodynamic parameters. Experimental studies indicate that
diastolic pressure augmentation through volume displacement also
enables
one
to
effectively
lower peak ventricular
pressure and
myocardial oxygen consumption due to the rapid balloon collapse prior
to the following ventricular systole.
The
effects
of
balloon geometry indicate
that the greatest
augmentation of diastolic aortic pressure occurs at complete occlusion.
This condition however is not desirable, from the point of view of
afterload reduction, although there have been cases where occlusion up
to 95 percent have been shown to give good hemodynamic results.
Balloon configuration and its properties are also important. Nonuniform
inflation characteristics can cause preferential inflation at the terminal
segments of
the
balloon and this
results in ineffective
volume
displacement and pressure augmentation. Multiple segment chamber
balloons have been designed to eliminate this by causing inflation to
proceed from the distal end to the proximal end (closer to the heart).
