242
Dynamics
of
the Vascular System
derived mathematical relationships. Others have calculated the relative
ventricular work and aortic input impedance as a function of device
phasing for the fundamental frequency components and showed that
these parameters were minimized when the fundamental components of
aortic pressure and flow had a phase difference of 180" (Jaron et al.,
1983). Changes
in
the timing were shown to increase impedance and
ventricular work which could limit cardiac output and overload an
already weakened heart.
Several investigators have studied the ability of IABP to augment
hemodynamic performance dependence on cardiac state. No single
combination of device timing variables can simultaneously achieve the
best improvement for all hemodynamic parameters considered. Nerz et
al. (1979) and Cui et al. (1990) using a time-varying compliance model
of the left ventricle and an arterial system model, studied timing,
pumping rate, balloon placement in the aorta, and device stroke volume.
They indicated that increasing device pumping rate or rise and fall time
of the pumping pulse caused a greater increase in coronary blood flow
and cardiac output with a greater decrease in aortic end diastolic
pressure. Additional increases in coronary and aortic flows could be
obtained when termination of the device pulse occurred past end diastole.
The overall improvement in hemodynamic parameters was greater for
larger balloon volumes. Balloon inflation at the beginning of end systole
was most effective in enhancing hemodynamic parameters. Others have
shown that additional increases in coronary and aortic flows can be
obtained at the expense of systolic loading with balloon deflation later
than end diastole. Some of these model predictions have been verified in
experimental animal studies by Li et al. 1984) Model studies generally
can adequately predict short-term effects of IABP on the cardiovascular
system
(Dai
and
Li,
1995). But
the long-term effects on the
cardiovascular system will have to include physiological control aspects.
8.2.4:
Optimization
of
Intra-Aortic
Balloon
Pumping: Control Aspects
A real-time closed loop control scheme for IABP is necessary because of
the heart rate changes and the changes in the physiological state of the
cardiovascular
system. Dynamic timing
adjustments are therefore
