Hemodynamic Measurements and Dynamics
of
Assisted Circulation
239
This increases mean diastolic pressure in the region of the coronary
arteries and augments coronary perfusion.
The rates of rise and fall of balloon inflation and deflation have been
shown both theoretically and experimentally to be crucial determinants
of IABP performance (Li et al., 1984). They result from the density and
viscosity of the driving gas, and the pressure of the gas source. The three
primary driving gases commonly that have been used are helium (He),
nitrogen
(Nz),
and carbon dioxide
(COZ).
The effects on global and
regional ventricular function have also been investigated. The use of
nitrogen has been commonplace until its replacement by helium. Carbon
dioxide was used because of its greater solubility and reduced risk of gas
embolism in the event
of
leakage. Helium use leads to faster rise and fall
times. The timing of IABP relative to the diastolic phase of the cardiac
cycle is also very important.
Experiments performed on dogs verified
theoretical predictions on these timing effects (Li et
al.,
1984; Zelano et
al., 1985). It appears that the optimal inflation time is a time period just
prior to the aortic valve closure or the dicrotic notch on the pressure
tracing.
A
short delay is necessary to take into account the balloon rise
time and the finite propagation time of the pressure pulse in the aorta.
Enhancement of cardiac output and mean diastolic pressure at greatly
reduced systolic loading and tension time index values can be achieved.
Regional contractile properties in the normal and ischemic border zones
are also improved (Fig. 8.2.2). Early inflation will decrease cardiac
output and increase myocardial oxygen consumption through an increase
in tension-time index. Late inflation will result in
a
lowering of mean
diastolic pressure and a decrease in cardiac output.
Since mean diastolic pressure is proportional to coronary blood flow,
coronary perfusion can be seriously compromised by improper timing of
balloon inflation. The time of deflation may vary depending upon the
desired hemodynamic effects. To minimize end diastolic aortic pressure,
it is necessary to deflate the balloon prior to ventricular ejection taking
into account the fall time of the gas in the balloon. Ideally, deflation
should be timed such that the minimum of the diastolic pressure
coincides with the onset
of
ventricular ejection. This will also minimize
ventricular af'terload.
