236
Dynamics
of
the
Vascular
System
Intra-aortic balloon pump and its control reflect an excellent example
of the dynamic interaction of the heart and the vascular system. The heart
pumps blood into the arterial system only during systole, while diastole
occupies the remaining period. Many investigators recognized that the
possibility of providing additional flow to vital organ vascular beds,
including the coronaries, exists during diastole. This observation enables
mechanical assistance designed to improve blood flow supplied by a
failing ventricle through proper timing. The intra aortic balloon pump
(IABP) was introduced to modify aortic pressure in a pulsatile manner,
first in dogs and later in humans in the 1960’s. An elongated balloon is
inserted via the superficial femoral artery and advanced
to the
descending aorta. Deflation in systole and inflation in diastole produces
in-series assistance. A variation is a device that directs blood from the
failing ventricle to the aorta, thus producing parallel assistance.
IABP
has
most widely been used in patients with either cardiogenic shock (Pierce
et al., 1981; Waksman et a1.,1993) after acute myocardial infarction
(Muller,
1994; Ohman et al., 1994; Ishihara et al., 1994) or left
ventricular failure after cardiac surgery. The beneficial aspects in terms
of hemodynamics are the decreased systolic afterload, augmented
diastolic
aortic
pressure,
increased
cardiac
output,
reduced
left
ventricular size, and improved myocardial metabolism.
Some of the direct and immediate beneficial effects of IABP have
been the reduction of epicardial segment elevation and limitation of the
spread of myocardial infarct, reduced S-T segment elevation and reduced
infarcted zone area. In addition, mortality and morbidity associated with
acute myocardial infarction might also be decreased by IABP. However,
when IABP
is
initiated in a delayed fashion, i.e. six hours after the onset
of symptoms of transmural myocardial infarction accompanied by acute
heart failure, IABP does not seem to alter myocardial infarct size, nor to
alter morbidity or mortality in patients.
Thus, the hemodyanmic and
cardiac electrophysiological beneficial effects and success rate of
mechanical assistance are higher, the earlier the application
of
such
devices.
Li et al. (1 984) and others have examined the hemodynamic effects of
IABP in terms of ultrasonic dimension gauges recorded cardiac muscle
shortening. These
miniature piezoelectric crystals
were
implanted
